Recent trends and economic significance of modified/functionalized biochars for remediation of environmental pollutants

The pollution of soil and aquatic systems by inorganic and organic chemicals has become a global concern. Economical, eco-friendly, and sustainable solutions are direly required to alleviate the deleterious effects of these chemicals to ensure human well-being and environmental sustainability. In recent decades, biochar has emerged as an efficient material encompassing huge potential to decontaminate a wide range of pollutants from soil and aquatic systems. However, the application of raw biochars for pollutant remediation is confronting a major challenge of not getting the desired decontamination results due to its specific properties. Thus, multiple functionalizing/modification techniques have been introduced to alter the physicochemical and molecular attributes of biochars to increase their efficacy in environmental remediation. This review provides a comprehensive overview of the latest advancements in developing multiple functionalized/modified biochars via biological and other physiochemical techniques. Related mechanisms and further applications of multiple modified biochar in soil and water systems remediation have been discussed and summarized. Furthermore, existing research gaps and challenges are discussed, as well as further study needs are suggested. This work epitomizes the scientific prospects for a complete understanding of employing modified biochar as an efficient candidate for the decontamination of polluted soil and water systems for regenerative development.

specific surface area, stable structure, high cation exchange capacity, and carbon content 1,2 .Its significance could be realized by the increasing number of published articles in the last ten years (Fig. 1).Biochar can amend the fertility of the soil and can sequester carbon; hence it can potentially lead to the mitigation of climate change [2][3][4][5] .To enhance soil fertility and carbon sequestration potential, biochar improves physical (moisture level, oxygen content, and capacity of water holding), chemical (sequestration of carbon and immobilization of pollutants), and biological (microbe's abundance, activity, and diversity) properties of soil.It also helps in the removal of various contaminants from soil and water systems [3][4][5][6] .Various conventional methods are used to remove organic, inorganic, and other emerging pollutants from the water and soil, such as coagulation/flocculation, chemical precipitation, and biochemical degradation 3 .These methods usually eliminate valuable contaminants from water and soil but have low efficiency with high operational and maintenance costs and massive waste production [2][3][4] .In contrast, adsorption using agricultural organic wastes is emerging as a cost-effective, user-friendly, and efficient method for removing various impurities from soil and water systems 5 .Adsorption is a key mechanism for biochar to eliminate organic and inorganic pollutants.The adsorption capability of biochar is directly linked to its physicochemical attributes such as functional groups, cation exchange capacity, distribution of pore size, and surface area, however, these attributes vary with the production conditions like nature of biomass utilized for biochar production, pyrolysis temperature, etc. 2,3 However, pristine biochar due to limited adsorption sites and low surface functionality does not display specific and high nutrient adsorption capability 4 .To enhance the adsorption capacity, biochars are modified/functionalized using multiple-modification agents including alkali, acids, metal oxides, and oxidizing agents, which manifest improved surface properties and novel structures after treatment 1,4,5 .Compared to pristine biochar, modified/functionalized biochar with enlarged surface area and abundant functional groups presents a new type of carbon-based material with enhanced adsorption potential for pollutants in water and soil systems 3,5 .Generally, functionalization techniques for biochar can be considered into three main modification types such as biological, chemical, and physical 6 .Chemical modification techniques include oxidizing treatment, soaking with acid and base, magnetization, loading of carbon nanomaterials, doping with clay minerals, organic surfactants, non-metallic elements, and layered double hydroxides 7,8 .These modifications not only improve the biochar's physical attributes but also influence its chemical characteristics such as surface functional groups, elemental distribution, zeta potential, electron transfer capacity, and cation exchange capacity due to their impact on porosity and enrichment of biochar surface with O-containing functional groups, especially carboxyl ones 6,7 .Physical modification such as activation by CO 2 /steam and microwave, and ball milling improves the particle size, pore structure, functional groups, and surface area of biochar 8,9 .It provides advantages over chemical techniques being less polluted in nature and economically more viable for biochar fabrication 8 .Moreover, chars can be functionalized via biological technique; which carries advantage of various microbes and biological-linked methods, and further assists in the elimination of toxic contaminants [9][10][11] .There have been few reviews focusing on diverse applications of biochar such as soil fertility and quality improvement, catalysis, and aqueous pollutant removal 10 .So far, various studies emphasize more on applying raw and modified biochar to eliminate pollutants from the water system 4,5,7,8,12,13 .Nonetheless, a comprehensive study including the use of multiple-functionalized biochar-based adsorbents in the removal of pollutants from soil and aquatic systems has scarcely been described.Furthermore, the compiled knowledge of multiple functionalization techniques for char/adsorbents, for example, doping of non-metallic heteroatom is scanty.Aiming to describe a thorough analysis of multiple-functionalized biochars for the remediation of environmental impurities, based on recently published literature, this updated study exhaustively outlined the novel approaches in multiple functionalized techniques for the biochars.Moreover, the reusability of modified/functionalized biochar as well as the economic perspective of the biochar production and application as compared to other expensive sorbents like activated carbon has also been discussed.This review could be helpful in the large-scale preparation and application of modified/functionalized biochars for managing polluted soil and aquatic systems and may ensure the sustainable protection of the environment.

Various modification techniques for the preparation of functionalized biochars and effects on the water system
Accessible modification techniques have been scrutinized in the published literature and are summed up in (Table 1) which can be distributed into 4 major classes, including physical and chemical modifications, magnetic modifications, and soaking with minerals (Fig. 2).Changes in biochar physiochemical properties after multiple modifications are detailed in (Table 2).

Chemical modification with acids and alkali
The purposes of acid and alkali modification are to introduce acidic binding sites (carbonylic lactonic, phenolic functional groups) and develop a better porous structure for contaminant removal 12 .Biochar surface properties have been improved by employing chemical treatment 13 .Numerous studies have discussed the influence of acid modification on functional groups, pore volumes, and specific surface area (SSA).Soaking with strong acids such as HCl, HNO 3 , H 3 PO 4 , and H 2 SO 4 has been examined for modification, which can increase the adsorbent surface acidities and improve the porous structure of biochar (Fig. 3).
After treatment with sulfuric acid, hydrochloric acid, citric acid, oxalic acid, or phosphoric acid, the resultant adsorbent generally possessed much higher surface area, pore volume, more hydrophobic and acidic groups for contaminant sorption [15][16][17][18][19][20] .The mechanism involves the improvement in the pore structure and specific area of the adsorbent, which has a significant effect on the physical sorption of pollutants 21 .Functional groups e.g.-COOH formed through acidic treatment also show a significant effect in the contaminant-sorption mechanism, therefore altering the removal ability of treated adsorbent 22 .Acid treatment modifies the physicochemical attributes of biochar to increase the adsorption capacities for the elimination of inorganic and organic pollutants from soil and wastewater.The pickling mechanism decreased the sludge-based adsorbent's micro-pore volume and enhanced the mesoporous volume, thereby improving the sorption capacity of biochar for antibiotics and heavy metals 23 .Compared to fresh biochar, H 3 PO 4 -treated eucalyptus-derived biochar exhibited higher removal efficiency of chromium hexavalent 24 .Citric acid-modified biochar showed the maximum sorption capacity of 12,109.4 and 2475 mg kg −1 for Pb and Cd in the soil, this capacity was greater than un-treated biochar 25 .H 3 PO 4 is a frequently used modifying agent for acid treatment and a more eco-friendly material than other hazardous and corrosive reagents e.g., zinc chloride 26 .H 3 PO 4 can decompose aromatic materials, aliphatic and lignocellulosic while creating polyphosphate and phosphate cross bridges to elude the shrinkage in the porosity enhancement mechanism 27 .More mineral acids including HCl, H 2 SO 4, and HNO 3 have also been extensively used for biochar activation.The modification with nitric acid has been exposed to cause micropore wall degradation owing to its corrosive property, subsequently in a reduction of the surface area 28 .Comparably, H 2 SO 4 modification caused a reduction in porosity from 12 to 40% and enhanced the distribution size of heterogeneous micropores in biochar.Organic acids e.g., oxalic acid increase the sorption of contaminants through proton-promoted and ligand mechanisms 29 .However pre-treatment with 10% H 2 SO 4 affected the O and C contents and a mixture application of 30% oxalic acid and H 2 SO 4 showed a 250fold enhancement of surface area than untreated biochar 30 .Similarly, the HCl-modified biochar of wheat straw showed more heterogeneous pores compared to un-treated biochar 31 .Indigenous metal/inorganic contaminants can also be efficiently eliminated by acid application 32 .Generally, it is documented that modification with acids can establish the various functional groups having acidic contents e.g.amine and carboxyl groups, thus increasing the metals removal capacity and affinity by surface complexation and cation exchange with these more active sites.
Dai et al. 29 applied H 2 SO 4 -modified bur cucumber adsorbent for sulfamethazine removal in soil.A high water-solid partition coefficient of 229 L kg −1 was noticed for loamy soil.Both chemisorption onto hemiacetal functional groups and chemical diffusion into pores were supposed to be retention processes.Table 3 sums up the chemical oxidation and acid/alkali modification approaches for biochars described.
The main objectives of alkaline modification are to improve the amount of O-comprising functional groups such as ether, carbonyl, carboxyl, and hydroxyl as well as the specific surface area of raw biochar, therefore enhancing the removal of several contaminants 11 .Alkaline activation is a mechanism including the basic (alkaline-nature) solution applied to change biochar structure at pre -or-post carbonization stages 33 .The most commonly used alkaline agents are sodium hydroxide (NaOH) and potassium hydroxide (KOH) 34,35 .Alkali activation of biochar using NaOH and KOH can enhance the surface basicity and O content while dissolving condensed organic matter (such as cellulose, and lignin) and ash to aid subsequent modification 36 .After alkaline activation, blocked pores are cleansed, causing higher porosity 24 .Biochars with very large SA have been observed after being functionalized by NaOH and KOH 37 .Potassium species such as K 2 CO 3 and K 2 O may be generated during modification due to the intercalation of potassium ions in the crystallite layer that creates condensed carbon structures.K 2 CO 3 and K 2 O may diffuse into the internal structure of the adsorbent matrix expand available pores and form new different pores of product 38 .
Chemical activation of adsorbent may increase its pollutant sorption capability by forming abundant and additional sorption-sites on improved SA, providing biochar surface more conducive to surface precipitation, surface-complexation, and electrostatic attraction, and providing higher affinity for sorption and strong sorption capacity via sturdy interactions with ether, carbonyl, carboxyl, and hydroxyl functional groups 39 .Figure 4 and Table 4 encapsulate the mechanism and improved ability of the chemically activated adsorbent for contaminants  www.nature.com/scientificreports/sorption.However, most surface attributes prevailed unaltered after the KOH-soaking, Cd 2+ , and Cu 2+ sorption was notably increased which may be owing to a large number of oxygen-containing functional groups and higher surface area 40 .Moreover, amination and carboxyl groups modification of adsorbent also greatly increased Cu 2+ adsorption by powerful surface complexation with NH 2 -functional groups, which were greatly selective and slightly affected via competing cation 32 .KOH-treated biochar significantly increased its adsorption capacity to As (V) due to the enhancement of pore volume and surface area and the alternation of surface functional groups 23 .Elimination of organic contaminants could be increased through π-π EDA interaction between aromatic rings of pollutants and biochar 13 .Another study reported the removal capacity of tetracycline (58 mg g −1 ) via KOH-treated adsorbent was markedly higher than reported in other studies (5 to 54 mg g −1 ), while the noticed capacity for the Chloromycetin elimination via NaOH-treated char was remarkably greater than un-activated adsorbent 22,24,30,41 .Conversely, urea treatment could produce N-enrich functional groups as well as enhance the basic nature of the surface, therefore increasing π-π dispersion forces for carbolic acid sorption 27 .He et al. 42 presented that oxalic and sulfuric acid-modified biochar delivered a better result of 183 to 229 L kg −1 sulphadimethyl pyrimidine removal in various kinds of soil, maybe due to enhancement of surface areas and surface functional groups in activated adsorbent (Table 1).

Modification by an oxidant (chemical oxidation)
Oxidant treatment can enhance the content of O-containing functional groups, stimulating the complexation of heavy metals such as Cd, Zn, Cu, Pb, etc. 43 .H 2 O 2 activation of manure biochar enhanced the carboxyl contents (101%) and oxygen contents (63%) of treated biochar, while the content of ash was reduced by 42% after modification.The oxidant-modified biochar could remove Zn, Cu, Cd, and Pb efficiently, which was due to the shifting of the adsorption process from precipitation to complexation.Nonetheless, H 2 O 2 treatment was inefficient in methylene blue sorption 44 .After hydrogen peroxide activation of pinewood chip biochar, the sorption capacity of methylene blue decreased as the O-rich groups weakened the forces of delocalized π-interaction which was the core process for methylene blue sorption 38 .Apart from potassium permanganate which has a direct impact in enhancing cation-π interaction and O-containing functional groups in modified biochar 43,44 , hydrogen peroxide could also be applied as an activation agent 45 .The effectiveness of this technique depends mainly on the target pollutant type and pollutant elimination process.It is hypothesized that this technique is The hydrophilicity and functional groups of adsorbent can be activated with chemicals to fit the explicit necessities of environmental safety including the elimination of pollutants from soil and water 46 .In general, biochar derived at low temperatures has more C-H and C=C functional groups 27 .Chemical treatment using H 3 PO 4 , NHO 3 , H 2 O 2, and KMnO 4 and a mixture of H 2 SO 4 /HNO 3 can generate acidic-content functional groups (e.g., phenolic, lactonic, carbonyl, and carboxyl) on C surface at comparatively low temperatures 47 .A substantial amount of oxygen-enrich functional groups were formed via chemical activation using nitric acid compared to potassium permanganate, showing a resilient oxidizing ability of nitric acid 32 .In addition, H 2 O 2 treatment was able to enhance the carboxylic group from 2 to 8% by oxidizing the carbonized structure of the adsorbent 22 .
N-rich functional groups (such as pyridinic, pyrrolic, lactam, imide, and amide) and oxygen-comprising functionalities play a significant role in the environmental implication for their potent complexation attractions, particularly for the base metal cations e.g.Cd, Zn, Cu 48 .Formation of the N-bearing functionalities could be achieved through nitration followed by dwindling on the carbon surface 20 .Nitro-groups are consequently decreased to amino groups on the surface by applying Na 2 O 4 S 2 (reducing agent).Surface amination leads to the formation of amino-based groups, which stimulate basic attributes and strong attractions to impurities 28 .Applying chitosan as a modifying agent, established amine functionalities on the surface of biochar to enhance its adsorption capacity and affinity to inorganic pollutants 49 .Chitosan loading on the surface of biochar can also enhance its effectiveness as a soil rectification, as well as chitosan-loaded adsorbent, may be applied as an efficient, eco-friendly, and low-cost adsorbent to decontaminate the pollutants from the environment 50 .H 2 O 2 -modified biochar derived from peanut hull surface enhanced oxygen-enrich functional groups, particularly the carboxyl group which accelerates the metals removal capacity and affinity of adsorbent 9 .Another study found that amino-treated char with enhanced functional groups e.g., C-N, N-H, C-O, and CH 2 could efficiently eliminate copper from wastewater because copper was intensely complexed with amino-functionalities on the surface of adsorbent 18 .Besides, char activated through KOH enhanced oxygen-comprising functionalities such as COOH, C=O, C-O, and O-H and consequently increased the tetracycline adsorption capacity 40 .At pH 7, oxygen-rich functional groups on the alkali-treated biochar accelerated the formation of hydrogen bonding with tetracycline molecules thus increasing its removal capacity 41 .Modification of biochar with hydrogen peroxide has been depicted to contain a great amount of oxygen-comprising functional groups and efficiently eliminate heavy metals such as Cd, Ni, Cu, and Pb 26 .

Organic modification/activation by organic solvents
It is conceivable to promote the types and quality of functional groups in biochars by mixing biochars with an organic substance containing a large number of functional groups 27 .The sorption capability also can be improved by elevating the number of sorption sites 51 .Out of various substances, chitosan has been applied in many studies.Chitosan is a natural polysaccharide that is produced from the Crustacea shell and is rich in -O,-OH, -NH 2 functionalities.The removal capability of contaminants can be raised via doping chitosan onto the char's surface 51 .Boraah et al. 52 examined how the sole utilization of textile residue biochar coated with chitosan influenced cadmium-contaminated soil on cadmium distribution in horseradish trees.Plant-available cadmium in the soil showed the best efficiency in reducing cadmium concentration in the soil by 58% than the control sample.Braghiroli et al. 53 stated that the composite of biochar and chitosan showed a great removal ability of ammonium, with a maximum range of 149.25 mg g −1 , comparatively higher than raw biochar.Moreover, reported that the amount of surface functional groups can be improved via activated chars with macromolecules e.g.cyclodextrin, Polyaziridine, lignin, and humic acid, these provide better elimination capacity for impurities 52 .Figure 5 displays how β-cyclodextrin coated rice biochar was produced appropriately by employing a microwave-based one-pot method.The coated composite was used for concurrently removing Pb (II) and bisphenol-A 53 .Microwave irradiation could ascertain surface alteration in 15 min and the generated biochar microwave-assisted-β-cyclodextrin composite (BCMW-β-CD) showed higher sorption efficiency with a theoretical monolayer uptake of 240 mg g −1 for lead and a heterogeneous removal volume of 209 mg g −1 for bisphenol A in the monocomponent system.Carboxyl-group activation also can be achieved by applying water-soluble methanediimine and esterification by methanol 11 .The application of methanol for carboxyl alteration is low-cost.For instance, biochar derived from rice soaked with sodium hydroxide and consequently treated with methanol exhibited an improvement in surface attributes.Chemical mechanisms involved in char activation by methanol are esterification and then direct reaction between biochar carbonylic functional properties with methanol 38 .Methanol treated-adsorbent was enriched in hydroxyl and ester groups than un-activated char, which contributes to the EDA interaction formation between adsorbent surface and pollutants (pharmaceutic manufacturing wastewater, contaminated water, and soil) 54 .Jin et al. 55 presented that methanol-treated adsorbent was more efficient in TC elimination compared to un-treated char owing to functional group modification as well as enhancement of oxygen-enrich functionalities on the adsorbent.The process proposed by the researchers was the EDA interaction generation between hydroxyl groups or ionized moiety of activated char and electron-depleted sites.

Modification with surfactant
Surfactants are categorized into Gemini, non-ionic, anionic, and cationic surfactants rendering to a hydrophilic nature.Generally, surfactants are employed as additives in washing detergents and industrial fabrication in environmental cleaning, as well as recently extensively used as chemical agents to alter the surface attributes of several solid substances e.g.zeolite, bentonite 56 .Owing to the surface negatively charged of biochar, cationicsurfactant can smoothly capture through biochar via exchange with ample exchangeable cations (K + , Na + , and Mg 2+ ), and electrostatic attraction in the char-matrix, and consequently, a surfactant-char composite/complex is generated 57 .For instance, the adsorption of cationic surfactant 1-hexadecyl pyridinium chloride on granular charcoal was mostly by ion exchange at a lower level.The partial monomolecular layer may be established with an enhancement of the cetylpyridinium chloride amount in the solution.Further, it enhanced cetylpyridinium chloride level, hydrophobic interaction among hydrophobic chains of cetylpyridinium chloride, and char improved sorption of cetyl pyridinium chloride 58 .Kumar et al. 59 described the influences of cationic surfactants on pentachlorophenol adsorption by biochar and activated carbon.Cationic-surfactant cetrimonium bromide was merely mixed in a solution comprising pentachlorophenol, and the char-activation mechanism through CTAB occurred thru ion exchange concurrently with adsorption of pentachlorophenol on biochar-CTAB composite.Therefore, cationic-surfactants can be applied as an efficient material to alter char to improve the elimination of the anionic contaminants.Non-ionic surfactants also can be removed through charcoal by physisorption mechanism as showed through low free energy alterations in adsorption.Around 300 mg g −1 of Triton X-100 was doped on the charcoal 60 .Labanya et al. 61 also observed a fixed degree of adsorption of nonionic surfactant Triton X-100 onto the adsorbent.Contrastingly, owing to electrostatic repulsion, both micellar and mono-molecular anionic surfactants are not fluently sorbed on the biochar surface.For instance, the fragile removal of anionic surfactants on charcoal was noticed 62 .However, substantial removal of anionic surfactant sodium dodecyl sulfate was recorded on modified charcoal 48 .Labanya et al. 61 observed that after CTAB treatment of biochar, pentachlorophenol removal capacity of activated adsorbent reduced with enhancing aqueous  19 .(B) The interaction mechanism between magnetic biochar and nanoplastics 15 .
CTAB concentrations.This may be because of the hindrance of the hydrophobic sorption site via CTAB sorption.Contrastingly, solubilization and mobilization of pentachlorophenol by CTAB in the solution might contribute to the reduction of pentachlorophenol adsorption onto the adsorbent.The existence of cationic-surfactant CTAB greatly reduced the adsorption capacity of thiodiphenylamine onto modified char.Apart from, the sorption-sitehindrance mechanism; competition of cationic surfactant with cationic thioridazine hydrochloride interdicted the adsorption of thioridazine hydrochloride on char.Nonetheless, the occurrence of anionic surfactant only marginally reduced thioridazine hydrochloride sorption, while non-ionic surfactant Triton X-100 improved thioridazine hydrochloride sorption 22 .

Doping of biochars
Recently, doping of biochars by metal-oxides has been applied to enhance the characteristics of biochars and consequently improve their removal performance.Doped biochar can be obtained by biochar mixed with clays, carbonaceous materials (carbon nanotubes, graphene oxide), and metal oxides, to change the surface traits of biochar.Biochar doping is distinguished from chemical modification because it involves the formation of totally new surface functional groups that were not previously present on biochar surfaces.

Doping with metal oxides
The aim of this method to produce doped biochar with metal oxides is to confirm a homogenous spread of metal over the surface of biochar.The biochar is used as a porous carbon support upon which metal oxides precipitate to enhance the surface area of the metal oxide.In general, doping with metal oxides of biochar is performed via soaking biochars in solutions of metal chloride and nitrate.The most used doping agents in literature are MgCl 2 , Fe (NO 3 ) 3 , Fe, and FeCl 3 63-65   .After mixing biochar with metal oxides or salt solutions, the mixture is heated under oxygenated conditions at temperatures 100-400 °C to allow chlorides or nitrates to be driven off as Cl 2 and NO 2 gases and alter the metal ions to metal oxides 9 .Fu et al. 66 prepared Fe-and cobalt-soaked bamboo biochar for the elimination of the metal from wastewater.Charcoal of bamboo was mixed in a 100 mL solution comprising iron (III) chloride, cobalt nitrate, and 9 M nitric acid, followed via carbonized by microwave at 640 W for some minutes.Moreover, Mg(OH) 2 -loaded wheat-straw adsorbent was produced by applying NaOH and MgCl 2 solutions 63 .The iron-impregnated char had markedly enhanced hydroxyl-functional groups than un-coated char due to the creation of Fe oxides on the adsorbent surface 64 .Chen et al. 65 reported that soaking municipal waste and rice husk adsorbents in FeCl 3 , iron powder, and Cao before pyrolysis generated Fe 3+ , Fe 0 , and Ca-coated biochars.These modifications improved the removal capacity of biochar for Cr (IV) and As(V) from wastewater.Most metal oxide loading results in a decrease in the surface area of biochar owing to pores blockage by metal oxide precipitates 63 .Bamboo biochar-coated with tetrabutylammonium bromide Fe 3 O 4 , FeCl 3 and used for the removal of polybrominated diphenyl ethers.The findings showed that coated biochar was more efficient for polybrominated diphenyl ether removal compared to other uncoated biochar 67 .Besides, Fe 3+ -loaded char was synthesized using ferric chloride salt.Doping adsorbent/char by Fe 3+ significantly improved As(III) and As(V) removal capacities 68 .The removal capacity of Magnesium oxide-loaded biochar for the anionic dye was greatly enhanced compared to untreated biochar by around 5 times.This was due to the surface being positive in the solution after MgO-loading, which increased the removal of anionic dye.Moreover, a large number of functional groups were observed on coated biochar, which aided the elimination of dye 66 .Thakur et al. 51 described a substantial enhancement of Cr(III)-oxyanion sorption via cobalt-loaded bamboo biochar rather than unmodified biochar.The cobalt-loaded showed higher pore volume and surface area compared to uncoated biochar, which led to greater sorption capability.Nonetheless, microwave heating and nitric acid were also involved during the preparation process of cobalt-loaded composite 66 .Coated biochar was synthesized with the oxides of Mg, Mn, and Al and observed increasing sorption of metal cations (Pb) and oxyanions (P and As) and metal cations (Pb) 58 .Fu et al. 66 prepared a biochar with great removal capacity for P and As from wastewater via modification with aluminum chloride to form an adsorbent-AlCOOH composite.Hemavathy et al. 43 modified the biochar with MnCl 2 and noticed that the treated biochar showed significantly increased adsorption capacity for Pb and As compared to pristine biochar.

Non-metallic heteroatom doping (emerging technique)
Premarathna et al. 69 reported that the non-metallic heteroatom loading is an advanced approach to modifying char by influencing its electronic characteristics, therefore increasing its catalytic sorption ability for pollutant remediation.Previous studies involving non-metallic materials-loaded-biochar and related environmental implications and mechanisms are detailed in Table 5.As reported, the most frequently employed non-metallic elements for modification of biochar include iodine, phosphorus, boron, sulfur, and nitrogen, and can be used to eliminate the aqueous pollutants 70 .According to Chen et al. 71 nitrogen is extensively used heteroatom in char modification.The nitrogen-doping technique can increase the electrochemical functioning of char by creating amine-N, pyridinic-N, and pyrrolic-N, graphitic-N species during the modification mechanism [72][73][74][75] .Cheng et al. 72 reported that graphitic-N could stimulate the transfer of electrons within the C skeleton, therefore increasing the catalytic capability of Nitrogen-loaded char for the persulfate-activation.Petrovic et al. 73 presented that the pyridinic-N and pyrrolic-N bonds can perform as electron donors and concurrently create N-defects which render more active sites in nitrogen-loaded char.Qiu et al. 74 demonstrated that the amine-N could work as a binding site for metals-ions through chelation results.Thus, nitrogen-doped/loaded biochars have been used as efficient catalysts in the elimination of heavy metals, dyes, oxybenzene, and antibiotics in the aquatic system (Table 5).For example, Cheng et al. 72 produced nitrogen-doped biochar and it exhibited high sorption capacity for methyl orange (906 mg g −1 ) and methylene blue (326 mg g −1 ).Theoretical calculations and spectroscopic studies verified that introduced pyridine-N and pyrrole-N had significant effects on dyes elimination and suggested mechanisms e.g., H-bonding, electrostatic attractions as well as π-π stacking .In addition, N-doped/loaded biochar exhibited high potential in the catalytic capability for contaminant removal [72][73][74][75] .For instance, Deng et al. 75 described that the N-doped biochar exposed high N content (13%) and large surface www.nature.com/scientificreports/area (738.50 m 2 g −1 ), and exhibited a strong catalytic capacity for per-oxymoron-sulfate activation to eliminate cotrimoxazole, with an elimination rate of 90% in a half hour.As observed through electron paramagnetic resonance spectra and quenching tests, 1 O 2 was noticed to be the leading reactive species favoring sulfamethoxazole degradation and non-radical oxidation involving electron transfer and 1 O 2 was the leading removal mechanism 76 .Rangabhashiyam et al. 77 reported that Sulfur doping provides a modified adsorbent with additional functional groups including C-S-O, C=S, and C-S-C, C-S as well as sulfur rings.In particular, sulfur-containing groups can reinforce the spin density of surrounding C atoms, consequently enhancing the catalytic performance of the char 43 .Moreover, the C-S-O structure in char can prompt the nucleophilic addition of per-oxymoron-sulfate to create copious 1 O 2 78 .After S-doping, modified biochar has exposed a great affinity towards inorganic elements.For instance, Rangabhashiyam et al. 77 manufactured a hierarchical Fe-maize straw biochar composite, and then Mn 2+ and S 2− were concurrently introduced, creating a ternary Fe-Mg-sulfur-biochar composite.This composite was examined for Pb 2+ removal from an aqueous solution, and they concluded that the Sulfur-doping increased the removal of Pb 2+ through PbS precipitation.Dinh et al. 79 reported that the Sulfur-loading could introduce Phenoxytamol radicals (C-O•) and vacancy defects on the acacia-derived adsorbent, which expedited peroxymonosulfate activation to degrade 90% of bisphenol A in half an hour.Raman analyses and scavenging experiments confirmed that Sulfur-loaded char prompted the creation of surface-bound •OH and 1 O 2 which led to the efficient elimination of BPA Diphenylolpropane 79 .
Boron (B) is another excellent material heteroatom that can restrain electron dispersion modify the surface characteristics of biochar and offer additional defect sites 80 .Doping of maize straw-biochar with boron enhanced the O 2 -enrich functional groups and SSA (890 m 2 g −1 ), which eventually enhanced its capability to adsorb Fe 2+81 .The contact mechanisms involved between boron-treated biochar and Fe 2+ were co-precipitation, ion exchange, and chemical complexation.Respecting the catalytic influence of boron-treated biochar, Murtaza et al. 82 applied the boron-doped biochar derived from wheat to activate peroxy-disulfate for sulfamethoxazole elimination and degradation of sulfamethoxazole significantly (up to 90%) was found in 120 min.Theoretical and experimental results explained that the introduced boron species could perform as Lewis acid sites to increase the surface affinity towards peroxydisulfate and that the char-facilitated electron transfer process was mainly accountable for the non-radical route 82 .In another study, B-doped biochar (B-BC) was prepared using boric acid.The modified biochar had more porosity and SSA up to 897.97 m 2 g −1 .Among the modified biochars prepared, the maximum adsorption of Fe 2+ (132.78 mg g −1 ) was noted at 55 °C using 800B-BC 1:2 due to chemisorption, co-precipitation, and ion exchange 18 .A spontaneous endothermic physical adsorption process was recorded in the case of neonicotinoid adsorption while using boron-doped porous biochar prepared through hydrothermal carbonization.Electrostatic and hydrophobic interactions were noted between acetamiprid and porous biochar used with a maximum adsorption capacity of 227.8 mg g −1 for acetamiprid 83 .
Moreover, several studies reported that co-doping of two materials on the char.Copper and nitrogen co-doped char synthesized and was applied for tetracycline degradation, and composite biochar showed better performance compared to raw biochar, Cu-doped, and N-doped biochar 72 .In another study, Singh et al. 49 prepared three kinds of doped biochar (co-doping of S and N, S-doped and N-doped) biochar produced from bamboo.Conversely, they observed that the degradation amount of antibiotics via sulfur and nitrogen-co-doped biochar (70%) was less than sulfur-doped biochar (89%) and nitrogen-doped (90%).The main cause for this mechanism was that nitrogen-doped biochar had the maximum amount of •OH and SO4• − , higher defects, and higher SSA 32 .Although substantial advances in non-metal component-loaded biochars have been attained, it is still in infancy with great ability for melioration in environmental decontamination.The non-radical process behind persulfate activation is still unclear, which demands more research.Moreover, complex sample fabrication, poor reusability, and the high cost of this kind of biochar require to be accurately addressed in future exploration.

Modification by carbonaceous nanomaterial
Coating with carbon nanotubes Biochar in combination with carbon nanotubes, comprising functional groups can generate resilient bonds with contaminants and biochar surface 85 .Han et al. 86 reported that the carbon nanotube shows significant physicochemical characteristics such as greater π-π interactions, large surface area, magnificent thermal conductivity, superior electron mobility, and higher mechanical strength.These properties are helpful for the sorption of several contaminants and work as a perfect catalytic supplement for the removal of impurities.Thus, carbon nanotube shows a substantial potential to be applied in the processes of remediation.Jiang et al. 87 produced a composite of carbon nanotube with sludge biochar to remove sulfamethoxazole.Compared with pristine biochar, the composite exhibited a higher surface area (119 m 2 g −1 ), and maximum sorption capacity.The study of physicochemical characteristics, kinetics, thermodynamics, isotherms, and various environmental factors showed that its remarkable removal efficiency was mainly ascribable to pore filling, π-π conjugation as well as the interaction of functional groups 87 .Chen et al. 58 presented the methylene blue adsorption capacities of carbon nanotube doped biochar and untreated bagasse and hickory biochar.The maximal sorption capacities of nano-tube biochar composite and untreated biochar (5.5 and 2.4 and mg g −1 respectively) were about twice the time higher than untreated biochar.Methylene blue took up greater affinity binding sites within the carbon nanotube.Moreover, the char exhibited the capability to eliminate Phenothiazin-5-ium via itself, when sorptionsites of carbon nanotube were fulfilled.The electrostatic interaction was the leading process for Phenothiazin-5-ium sorption and microporous diffusion governed its sorption amount 30 .Jiang et al. 88 observed that the SA of carbon nanotube-doped biochar derived from Giant cane was very low compared to un-doped biochar, but a large amount of acidic-functional groups were noticed on the surface of carbon nanotube-doped adsorbent.These acidic functional groups may smoothly interact with lead and generate a firm type to immobilize it on a coated-biochar surface, which improves the adsorption capacity of lead.Jin et al. 55 observed that the cobalt-doped biochar of bamboo showed higher pore volume and surface area than the un-doped adsorbent, which led to greater chromium hexavalent removal capacity.Similarly, a substantial increase in pore volume and surface area was also found with an enhancement of Fe quantity on coated biochar 50 .Loading of surfactant for carbon nanotube dispersion during the preparation of carbon nanotube doped-biochar resulted in a greatly higher sorption capacity of carbon nanotube-loaded biochar for pollutants (lead and sulfapyridine) than without surfactant owing to the magnificent distribution and dispersion of carbon nanotube on the adsorbent surface 11 .Captivatingly, no noticeable competition was found between sulfapyridine and lead, signifying the site-specific sorption of both pollutants on carbon nanotube-doped adsorbent surface 15 .Further, pollutant sorption and hydrogen storage capability were assessed on multi-walled carbon nanotubes that were loaded on bamboo biochar using microwave plasma to improve chemical vapor deposition.Nonetheless, only little enhancement was noticed compared with un-doped biochar because of the lower hydrogen storage ability of multi-walled carbon nanotubes than pristine biochar 89 .However, carbon nanotubes are very efficient for pollutant elimination due to their nanostructure and large surface area, high cost, and inconvenience for engineering applications limiting their use.Thus, biochar could aid as a mesoporous/microporous carrier of carbon nanotubes to develop new recyclable and effective sorbents for wastewater and polluted soil treatment.

Using graphene for modification
Graphene modification has attracted both engineers and scientists after its discovery for its special two-dimensional structure and novel traits, such as electrical and thermal conductivity, surface area, and mechanical strength 90 .Compared to carbon nanotubes, difficult recovery and separation for reuse limit the extensive application of graphene in wastewater and polluted soil remediation.To overcome these drawbacks, graphene-based composite covering particles are produced and biochar is one of the promising materials as a carrier of graphene.Production of graphene-doped adsorbent also typically follows the two-step dip-coating process as above, e.g.peanut shell-derived feedstock was soaked in graphene solution to absorb graphene and then pyrolyzed by slow pyrolysis in an N 2 environment 74 .Ghanim et al. 91 observed the sorption enhancement of methylene blue and phenol via graphene-doped biochar.Higher pore volume (0.55 cm 3 g −1 ) and surface area (11.30m 2 g −1 ) after coating graphene on cotton-biochar may be the major reasons for enhanced sorption.Moreover, π-π bonding between methylene blue or phenol graphene sheets contributed to enhancing sorption capacity.Hafeez et al. 92 found a substantial enhancement of methylene blue on graphene soaked-biochar (almost 20 times greater), and strong π-π bonding between methylene blue and graphene on biochar surface was believed to be the leading mechanism for the improvement of methylene blue sorption via graphene-doped biochar.Graphene loading on biochar introduces a large amount of oxygen-enrich functional groups such as carboxyl, hydroxyl, and carbonyl creating the binding between biochar surface graphene.For the recover and regeneration of graphene adsorbents, simple desorption processes were sufficient using ethanol and deionized water as eluents.However, the regenerative properties of graphene-based adsorbents have not been rigorously investigated and are required to be explored in future studies for the sustainable circular economy.

Physical modification
Generally, mechanical/physical modification techniques are usually economically feasible and simple but are less efficient compared to chemical modification.The physical modification method uses various oxidizing agents e.g., CO 2 , air, and stream.Physical modification has been considered an effective method to improve biochar functionality by influencing hydrophobicity, polarity, and surface functional groups of biochar 44 .Nonetheless, the drawbacks of physical activation techniques include a long time (~ 4 h) for activation and more energy consumption (10.6 to 58.0 kcal) 89 .For these drawbacks, chemical activation is considered the primary option for biochar engineering.Future studies should focus on filling the research gap to address the drawbacks of physical activation.

Activation by steam
Activation by steam is a common modification process used to improve the structural porosity of biochar and eliminate contaminations e.g. products of incomplete combustion.This method helps to enhance the surface area upon which sorption can proceed.Activated poultry manure biochar was produced at 700 °C followed by steam modification at 800 °C with a range of water-flow rates and durations.Greater flow rates and longer treatment times enhanced the sorption of Zn, Cu, and Cd on the surface of the adsorbent 93 .However, activation with steam increases the porosity and surface area of the biochar, found in a study conducted by Ghassemi-Golezani and Rahimzadeh 94 the copper removal capacity of slow pyrolysis-derived biochar of Miscanthus was not notably changed by modification with steam at 800 °C.They noticed that while steam activation of treated biochar enhanced the surface area, many functional groups were reduced, alongside a rise in aromaticity.Likewise, Ghazimahalleh et al. 95 noticed that sawdust-derived biochar activated by steam activation enhanced the surface area but had a slight effect on the properties of functional groups.The steam activation did not affect the removal capability for phosphate owing to electrostatic repulsion via the negatively charged surface of the adsorbent.Thus, concerning inorganic contaminant removal, activation with steam looks to be more efficient when employed before a second modification step that forms functional groups, as the steam only improves the surface area of the adsorbent.

Gas purging
Biochar modification with high-temperature CO 2 -ammonia mixture application has been examined to adsorb gases (CO 2 ) obtained NH 3 and CO 2 treated-biochar under a range of creation temperatures 96 .After the manufacture of biochar from the cotton stalk, it was slowly heated up to a specific temperature (500-900 °C) in a quartz reactor with N 2 purging and then NH 3 or CO 2 was purged.The ammonification could introduce Nitrogen-comprising groups onto biochar and enhance Nitrogen content up to about 3.90 wt% in CO 2 -ammonia treated biochar, although CO 2 application could play a substantial role in the pore formation and amend the micro-porous structure of adsorbent accelerating gas sorption capabilities of the biochar 23 .Krerkkaiwan et al. 96 presented that the pore volume and surface area of the CO 2 -treated biochar were higher than the untreated biochar.CO 2 could react with the carbon of biochar to form CO, thereby forming microporous structures.At ambient temperature, the gas sorption capability of CO 2 -treated biochar was markedly higher than that of untreated biochar 97 .The removal capacity of treated biochar exhibited a linear relationship with micro-pore volume and the process of CO 2 sorption was recognized as physical sorption 18 .

Ball milling
Ball milling is a low-cost and eco-friendly technique that has been used for biochar modification through the improvement of key attributes such as pore structure, specific surface area, and enhancement of various surface functional groups than to the un-modified biochar 38 .Previously published literature has reported that several operational parameters could influence the physicochemical and catalytic/adsorptive characteristics of resultant biochar such as media mass ratio: biochar, dry or wet milling (without or with solvent), reaction time, and solvent characteristics, milling temperature and speed, reaction atmosphere and ball size distribution 98 .Furthermore, the grinding media shape (cubes, ellipsoids, and spherical balls) may affect the milling mechanism but spherical balls are the most reliable grinding media 99 .Recently, remediation of pollutants from the environment has been examined by ball-milled modified biochars.For example, Haider et al. 100 reported that the ball milling technique expedited the mixing of Al/Mg layered double hydroxide into the matrix of biochar, and ball-milled Al/Mg layered double hydroxide-biochar composite exhibited a higher adsorption capability for Cd 2+ (119 mg g −1 ).Kasera et al. 89 reported that the ball-milled modified-biochar has also shown prominent potential for the remediation of organic pollutants such as phenols, dyes, and antibiotics, and other inorganic pollutants including phosphate and ammonia as well as heavy metals and metalloids.The main mechanisms behind this modification included pore structure, specific surface area, and enhancement of various surface functional groups.In addition, emerging applications employing ball-milled modified-biochars have focused on soil remediation and thermal/ photocatalysis 13 .However, the ball-milling approach is a new research field to synthesize modified biochar; it is still in its beginning.Thus, further research is required to determine critical research directions and control or reduce the existing challenges.

Microwave activation/modification
Pokharel et al. 68 reported that Microwave modification is an advanced technique based on high-frequency electromagnetic radiation with frequencies (from 0.03 to 300 GHz).Microwave irradiation creates dipole rotation at an atomic scale, therefore producing heat energy within the constituents/materials 47 .This activation technique allows both outer and inner biochar surfaces to be heated concurrently without direct interaction at low temperatures and results in the production of microwave-modified biochar with a larger surface area and various functional groups than raw or inactivated biochar 101 .Microwave modification resulted in rough and lamellar morphology of the modified biochar, enhanced fraction of micropore volume from 9.01% at 400 °C to 60.25% at 900 °C, and BET surface area from 19 to 1722 m 2 g −1 .The modified biochar had somewhat irregular mesopores (2.3-11.9nm), and abundant functional groups such as β-CD, − OH, and -COOH.Ma et al. 102 stated that microwave-modified biochar was more efficient in reducing the phytotoxicity of PAHs and heavy metals in peppergrass compared to pristine biochar.Moreover, microwave modification for biochar also accelerated the removal of Congo red and methylene blue from wastewater.As an instance, Maaoui et al. 103 described that the combined application of steam and microwave activation created highly effective modified biochar with a greater surface area (570 m 2 g −1 ), and resulted in an efficient performance in the methylene blue removal with a maximum sorption volume of 38.5 mg g −1 .Nevertheless, the environmental utilization of microwave-modified biochar is still scanty, which is mostly ascribed to the expensive equipment operation and maintenance 103 .

Impregnation with clay mineral and mineral oxides
As a novel idea, modified biochar has been produced by biochar doped with minerals 104 .Various minerals can accelerate biochar efficiency, among which clay minerals such as vermiculite, montmorillonite, and attapulgite have attracted great attention (Table 6) because of their great ion exchange capability and large pore structures for different contaminants under aquatic ecosystem 105 .Clay minerals have been widely used for pollutant elimination due to their composition, mineralogical structure, surface charge, and cation exchange capacity.The clay-loaded biochar obtained from a biomass mixture with montmorillonite and kaolinite showed lower surface areas than untreated biochar, this was because of pores blockage in biochar through clay minerals 106 .Kaolinite, gibbsite, and montmorillonite are the most frequently used clay minerals, as a cost-effective adsorbent 107 .Lv et al. 108 carried out a study on, the biochar of hickory chips, bagasse, and bamboo doped with clay particles (kaolin and montmorillonite) to improve its functionality.They observed the mineral-doped biochar composite showed a higher porous structure after modification.Lyu et al. 109 observed that the surface complexation between hydroxyl groups and Pb (II) provided via montmorillonite stimulated the sorption efficiency.Sonowal et al. 50produced porous magnesium oxide-doped biochar with thick flakes of polycrystalline magnesium oxide from several feedstock biomasses such as peanut shells, sugar beet, cottonwood, sugarcane bagasse, and pinewoods.These biomasses were dipped in magnesium dichloride solutions and ensued dry-mixture of magnesium dichloride incorporated biomass was heated at 10 °C min −1 at 600 °C.The use of N 2 treatment is necessary to eliminate by-product gases such as hydrochloric acid and thus accelerate the generation of magnesium oxide particles in the biochar matrix.Additionally, the direct application of Ca/Mg-contained tomato tissues is another novel method of creating Mg-comprised biochar, which enriches Mg(OH) 2 -particles and nano-sized magnesium oxide within the biochar matrix 110 .Besides, Magnesium oxide and birnessite doped biochars derived from pine were produced via two modification ways to enhance sorption capacity for Pb(II) and As (III) 111 .Pre-dripping pine wood feedstock in Mn(II) Chloride solution and resulting pyrolysis yielded magnesium oxide doped biochar, although birnessite doped biochar was prepared by soaking of pine-wood char with birnessite by precipitation adopting pyrolysis process 112 .The potassium permanganate mixing with biochar markedly changed the pore volumes and surface area of the biochar.A significant reduction in surface area was found, whereas the width of pores enhanced from 23 to 92 nm with enhancing potassium permanganate (KMnO 4 ) doping 113 .These structural alterations of modified biochar may be because of the demolition of nano-pore structures and deformation from nano-pores into macropores/mesopores by potassium permanganate, which typically performs as an impregnable oxidizing agent 113 .Murtaza et al. 114 observed the enhancement of oxygen-enrich functional groups in magnesium oxide-doped biochar.In addition, most of the surface oxygen in magnesium oxide doped-biochar was bonded to magnesium (Mn) in the form of Mn-OH and Mn-O (26.3%, and 63.9% respectively).XPS examination showed the presence of hydroxyl, Mn-O, Mn-OH, and C-OH on the surface and chemisorbed water on the surface of biochar 114 .Modification-caused alterations in the elemental composition of the biochars are detailed in Table 6.Biochar doped with kaolinite and montmorillonite significantly improved the iron and aluminum contents than un-doped biochar, although the carbon, hydrogen, nitrogen, sulfur, and oxygen contents were significantly higher than un-doped biochar 115 .On the other hand, the surface oxygen content of magnesium oxide-doped biochar was greatly higher (40%) compared to un-doped biochar (15%), showing the enrichment of oxygen-comprising functionalities 116 .MnOx-doped biochar has greater thermal stability compared to untreated biochar, owing to the transition of MgO through the heating mechanism 117 .The magnesium oxide-doped biochar showed much greater sorption capability to copper (II) (160 mg g −1 ), which was higher than the un-doped biochar (19 mg g −1 ).Generation of inner-sphere complexes with magnesium oxide and oxygen-enrich groups, cation-π-bonding, and cation exchange are the main processes involved in improving the sorption of copper (II) on the magnesium oxide-doped biochar 118 .

Electrochemical modification
To generate modified biomass/biochar adsorbents, an easy-to-use and promising alternative method to chemical solution-based methods is electrochemical modification.Electrochemical modification is a rapid and simple technique aimed at acquainting the specific functional groups and soaking chemicals onto the pristine biochar surface.Typically, this technique uses a two-electrode electrochemical cell where the modifying agent doesn't need to be added from an outside chemical source thanks to a sacrificial anode like Fe.The first step in the modification process is to run an electric current between the electrodes in an electrolyte solution while stirring the biomass and/or biochar components around in the solution.The anode dissolves to release modifying agent ions (Fe 2+ ), which then proceed through a sequence of reactions to deposit the appropriate modifying agent (iron oxide, for example) on the biomass/biochar surface 54 .
For target pollutants like As the electrochemical modification process parameters must be optimized to provide an adsorbent with the maximum adsorption capacity.For instance, it has been noted that the density of sites on the adsorbent surface and the particle structure affect the affinity of iron oxides, such as goethite, magnetite, and hematite, for arsenate, As(V) 119 .Moreover, altering the preparation procedure's pH and temperature can alter the structure of the iron oxide particles produced during the modification process 120 .Another important factor that is directly related to the concentration of solution iron that may be deposited on the surface of biomass or biochar is the length of the applied direct current.
A simple and unique electrochemical alteration that can simultaneously improve porosity and surface functioning has been developed more recently to find alternative methods 54 .For instance, biochar produced by aluminum electrode-based electro-modification followed by pyrolysis demonstrated superior phosphate adsorption capability over other chemically modified adsorbents due to an increase in the formation of a crystalline structure on the surface with nano-sized AlOOH 115 .Although powdered biochars have a remarkable potential for phosphate adsorption, their low hydraulic conductivity makes them unsuitable for continuous fixed-bed column adsorption systems and makes them difficult to collect and separate from an aqueous solution at the post-adsorption step 121 .
Islam et al. 44 synthesized a magnetic biochar of corn straw under an electric field produced via an electrode.The external electric field facilitates the rod-like crystalline iron oxide (Fe 3 O 4 ) nano-particles to disseminate rigorously into inner pores of biochar, resulting in an enhancement of pore diameter and a slight reduction of SSA 44 .Electrochemical modification techniques can also be applied for magnesium oxide (MgO) impregnation 28 .Using graphite as the electrode and magnesium dichloride as the electrolyte, magnesium dichloride nano-particles were disseminated and enhanced on the surface of marine macroalgae derived-biochar.The resultant biochar/ magnesium oxide composite was exhibited to remove phosphate efficiently, achieving a higher removal capacity of about 90% 28 .MgO-loaded biochars are efficient for the soil metal stabilization mechanisms 122 .

Magnetic modification
Magnetic biochars have been extensively used as a sorbent for soil and wastewater remediation.After activation with ferromagnetic elements such as Ni, Fe, and Co their oxides, biochars can be recycled easily by an external magnetic field, making the cleaning and regeneration procedures much easier.Many studies have scrutinized the sorption mechanisms and re-usability of magnetic adsorbents 123 .The multi-functional attributes of biochar indicate its potential as an efficient sorbent for pollutants in soil, wastewater, and water systems, thus modified biochar has been manufactured through a magnetic loading process to improve its sorption capacity of anionic pollutants 124 .Production approaches, sorption efficiency, and reusability of the magnetic adsorbent have been studied elsewhere 72 .Due to the ability to produce reactive oxygen species including hydroxyl radical, sulfate radical, and hydrogen peroxide magnetic adsorbent can be applied for catalytic degradation of organic pollutants.For example, Li et al. 119 produced a magnetic biochar of pine needles with Mn-Fe binary oxides and scrutinized its capability for the degradation of naphthalene in polluted water.The redox potential of Mn(III)/(II) is greater than that of Fe(III)/(II), signifying that in this Fenton system, Mn(III) could be decreased via Fe(II) efficiently, thus ceasing the limitation of hydrogen peroxide fabrication on Mn(III) reduction.Thus, the transfer of electrons in this system can be elevated, resulting in more than 80 times greater naphthalene decomposition capacity compared to untreated biochar 119 .The surface areas of magnetic modified biochar produced via chemical coprecipitation of Fe 2+ /Fe 3+ were lower than non-magnetic biochar, although the average pore diameter of modified biochar was higher compared to un-modified biochar 125 .This is due to magnetic loaded-biochar containing a substantial amount of ferric oxide, which has abundant transitional pores and low surface areas 125 .The hybrid adsorption characteristic of this magnetic-loaded biochar accelerates the effective elimination of phosphate and organic contaminants simultaneously.The stability and presence of magnetic in magnetic char for a long period supported the possibility of magnetic separation after utilization, which is a main benefit for the wastewater remediation process 120 .Rajput et al. 126 prepared a magnetic biochar via thermal pyrolysis of iron (III) chlorideactivated biomass.The resulting modified biochar has nano-sized (< 5-60 nm) or colloidal maghemite particles embedded in a porous biochar matrix and therefore showed excellent ferromagnetic attributes with a saturation magnetization of 69.2 emu g −1 .Additionally, sorption results exhibited that magnetic loaded-biochar has a higher sorption capability to arsenic (III) in solution.Due to its ferromagnetic characteristic, the spent/exhausted biochar can be smoothly collected and separated via magnetic separation 126 .The introduction of ferric oxides facilitated the creation of inner-sphere complexes, resulting in reduced bioavailability and mobility of the heavy metals 127 .Besides plant growth, metal stabilization can also be accelerated, but the processes involved in this mechanism remain unclear.It is proposed that magnetic loaded-biochars could also be applied for the degradation/adsorption of organic pollutants in contaminated soil due to the formation of reactive oxygen species 128 .

Photocatalytic modification
Photocatalytic degradation of organic pollutants could be attained by biochars after doping with metal oxidebased semiconductors e.g.Titanium dioxide, cuprous oxide, copper (II) oxide, and Zinc oxide 121 .Metal oxide doping on biochar can be accomplished in several methods, for example, sol-gel, hydrothermal, and hydrolysis 129 .Pan et al. 130 synthesized Titanium dioxide-doped biochar using a low temperature; the resultant TiO 2 -supported biochar attained great phenol degradation efficiency 130 .Biochar could not only assist as a host substance for metal semiconductors but also accelerate the process of electron transfer.For example, the mesoporous structure of biochar derived from walnut shells confirmed the dispersion of Titanium dioxide nanoparticles 131 .Another research carried out by Premarathna et al. 69 used TiO 2 -Zn doped biochar for the degradation of sulphamethoxazole under visible light.Due to the electro-negativity of the adsorbent, intermediates, and sulphamethoxazole come into interaction with the photo-catalyst more smoothly, and the produced electron, can then transfer to the biochar surface without the recombination of the electron-hole pairs, consequently stimulating the photocatalytic mechanism.In particular, biochar performs as an electron pursuer in the conduction band of semiconductors that can promote the separation of electron-hole pairs and the process of electrons 132 .
In comparison to the Cu2O-CuO sample, the Cu2O-CuO@BC-1.0 composite showed lower values of band gap energy for CuO (1.70 eV) and Cu 2 O (2.10 eV).Due to the high surface area and small band gap, the Cu2O-CuO@BC-1.0 composite performed better photocatalytically than its separate equivalents, BC and Cu2O-CuO.After 90 min of photocatalysis, the Cu2O-CuO@BC-1.0 composite was able to degrade RO29 with a 94.12% efficiency at the starting concentration of 20 mg/L and pH 8.9.After 90 and 180 min of treatment, 47.31% and 79.62% COD elimination efficiencies were reported, respectively 111 .

Iron modification
Iron modification or activation technique for biochar is projected for two main purposes: i) strengthening the separation efficacy to reuse and recycle the biochar, and (ii) increasing the decontamination capability of biochar by the interaction between the target contaminants and loaded Fe 39 .Iron-activated biochar has been synthesized using iron oxides such as goethite and hematite, nano zero-valent iron, and iron sulfide 133 .Iron-activated biochar can be created by precipitation, ball milling technique, thermal reduction, and co-pyrolysis 8 .Considering previous studies, the utilization of Iron-activated biochar can be classified into three types including a catalyst, reductive agent, and adsorption material.Hematite and goethite are the most commonly employed Fe minerals to increase the adsorption efficiency of biochar for heavy metals such as Hg, As, Cr, Cu, Pb, and Cd 134 .Lima et al. 135 produced the goethite-modified biochar and observed maximum sorption capacities for As 3+ and Cd 2+ were 78 mg g −1 and 63 mg g −1 , respectively.Baser et al. 40 synthesized three types of Fe-activated biochars including Fe 3 O 4 -modified biochar, Fe 2 O 3 -activated biochar, and Fe-treated biochar via one-step ball milling of magnetite, ferric oxide, and iron powder, respectively.In the elimination trial for antibiotics, Fe 3 O 4 -activated biochar exhibited the highest sorption capacities for tetracycline 90 mg g −1 and carbamazepine 60 mg g −1 .Fe-activated biochar has also exposed great efficiency in redox reactions of metals such as U(VI), Cr(VI), and As(III) and organic pollutants to reduce their toxicity 136 .Biochars modified via FeOOH, FeS, and nZVI have been examined for reductive degradation since they can provide reductive agents such as Fe (II), S (II), and Fe 0 species.Gautam et al. 64 observed that the nZVI-activated biochar exhibited effective sorption ability (54 mg g −1 ) for Cr(VI).The decline of toxic Cr 6+ to less toxic Cr 3+ was also observed, which highlighted the significant role of Fe 0 .The excellence of nZVI-activated biochar over un-activated biochar was confirmed for sulfamethazine removal because nZVI-activated biochar expedited the free radicals generation, which favored the sulfamethazine degradation 14 .Fan et al. 33 stated that peanut hull biochar activated/engineered via starch and FeS minimized labile U(VI) to non-labile U(IV) species and the main role of S(II) and Fe 0 in the reduction mechanism was highlighted through the XPS study.Another dynamic direction for the utilization of Fe-treated biochar is in the organic pollutant's degradation, particularly in persulfate and Fenton-like activation systems.Oxidant activation such as ozone, permanganate, persulfate, H 2 O 2 , and via Fe-treated biochar has received great attention 137 .Due to great electron shuttling capability, abundant oxygen-enrich functional groups, and persistent free radicals (PFRs), Fe-activated biochar has been confirmed for its potential as a Fenton-like catalyst in catalyzing hydrogen peroxide to create hydroxyl radicals to remove organic pollutants in water and soil systems 138 .Moreover, PFRs, Fe 2+ on Fe-activated biochar can act as efficient activators to create reactive oxygen species such as SO 4 •− , 1 O 2 , and •OH which can efficiently degrade the several types of organic contaminants such as phthalate esters, metronidazole, bisphenol A and tetracycline 139 .For example, with the assistance of Electron spin resonance analysis, Fan et al. 34 verified that the application of Iron-activated biochar derived from sugarcane bagasse favored the formation of reactive oxygen species in a Fenton-like system and •OH radical quenching trials exhibited that the elimination efficiency reduced by around 85%, indicating the leading role of •OH in metronidazole degradation.Furthermore, iron activation can result in a slenderer band gap of biochar, which supports the photocatalysis/adsorption for aqueous methylene blue, F − , Pb 2+ , and Cr 6+140 .Compared to other metal salts and oxides, iron is the most abundant element on the earth, more economical, and less toxic for the environment.Thus, Fe-activated biochar is suggested as an amendment in the remediation of polluted soil systems owing to such advantages.Fe-modification/ activation has been the most widely practical and advanced tailoring method in the remediation of environmental pollutants by biochar at large-scale utilization.

Biological modification/bacterial inoculation of biochar
Compared to chemical and physical modification techniques, biological techniques for biochar modification are less researched, mainly because of their more complex functioning properties.Initially, authors employed anaerobic bacteria to transform biomass into biogas and applied the resulting digested residue for biochar synthesis 141 .Biochar produced from anaerobically digested biomass residues shows superior characteristics such as higher SSA, high pH, and resilient ion exchange capacity and has exposed greater removal efficacy for heavy metals, organic contaminants, phosphate, etc. 142 .Furthermore, a comparatively novel method employing the biological post-treatment technique to immobilize the microbes on biochar has attracted recently 143 .Biochar has been suggested as a superior carrier for microbe inoculation due to its simple preparation, economical, nutrient enrichment, and high porosity 144 .Biofilm theory proposes that living cells can release a wide range of polymers and attach themselves to the surface of biochar, creating a microbial biofilm surrounded via an extracellular matrix that accelerates the pollutant's degradation/ adsorption 145 .Dou et al. 32 examined the elimination potential of Bacillus cereus-inoculated biochars derived from pharmaceutical wastes for chlortetracycline.The maximal elimination rate of chlortetracycline was found 85%.The microbial inactivation test exposed that the chlortetracycline removal mechanism was mainly governed via chemisorption and microbial degradation through biochar 48 .Increased bioremediation of diesel oil was described by Labanya et al. 61 , they used Vibrioloaded biochar to clean the diesel oil-polluted seawater, and they found superior removal efficiency as compared to the control.Menzembere et al. 146 verified that biochar-inoculated with three strains including Citrobacter sp., Bacillus cereus, and Bacillus subtilis could efficiently immobilize Cd (II) and U (VI) in the polluted soil.Relative to the control, the diethylenetriaminepentaacetic acid-extractable concentration of Cd and U in the soil reduced by 56% and 69%, respectively and bacteria-modified biochar decreased metal uptake, thus stimulating the growth of celery.

Inorganic contaminants (metalloids/metals)
Owing to the extensive anthropogenic actions, the agricultural/farming soils are contaminated with a wide range of inorganic contaminants such as metalloids/metals and organic contaminants 147 .Engineered/modified biochar is believed as an economical and efficient soil-remediation candidate to reduce the environmental stress of polluted soils 142 .The remediation process includes the indirect process (enhanced soil characteristics via engineered biochar) and the direct process (degradation/ immobilization of pollutants via modified biochar) in multipart terrestrial systems 148 .The various engineered/modified biochars and their remediation efficiency for metalloids/ metals in soils are documented in (Table 7).Sarkar et al. 149 amended a cadmium-contaminated soil (20 mg kg −1 ) with 5% chitosan and fabric waste biochar combined application of fabric waste biochar and chitosan (1:1 w/w) and chitosan-coated fabric waste biochar, respectively.They observed that the chitosan-biochar application exhibited the greatest efficacy in reducing cadmium levels in soil (up to 57%), and cadmium uptake in plant root (up to 50%) and shoot (up to 70%) compared to the control.Chitosan-biochar rendered negative sites to bind Cd (II) ions effectively by cation exchange, precipitation, and surface complexation.The elevated soil pH also sturdily influenced the phytoavailability and mobility of cadmium.Higher soil pH may surge the soil's negative charges, which may accelerate the cadmium immobilization via electrostatic attraction.Moreover, elevated soil pH expedited the generation of hydroxyl-bound species of cadmium 34 .In another study, Aoulad El Hadj et al. 27 described that 2% goethite-doped biochar treatment could decrease arsenic uptake in Oryza sativa grains (up to 77%) compared to pristine biochar.The goethite-treated biochar enhanced the content of Fe oxide in soils, which stimulated the Fe-plaque formation in the root and ultimately reduced the arsenic content in Oryza sativa grains 150 .Several studies have also been carried out utilizing other kinds of biochar involving various engineering/ modifying techniques such as ultraviolet activation, sulfur power, Al-Mg LDH, polyethyleneimine treatment, and CTAB doping to remediate metals/metalloid polluted soils 113 .
Nonetheless, agricultural/farming soils are often concurrently polluted with several metalloids.Thus, many authors have also stated the influence of engineered/modified biochars in the immobilization of several metalloids in co-polluted soils.Mishra et al. 151 synthesized MnFe 2 O 4 -engineered biochar produced from tea and investigated the effect of its addition with different rates such as 0.1, 1%, and 2% on the removal of Cd and Sb from the polluted soil.They found that MnFe 2 O 4 -engineered biochar treatment at 2% simultaneously reduced the amount of bioavailable Cd and Sb in soil by 76.0%, and 43.5%, respectively.Nonetheless, the raw biochar only decreased the bioavailable cadmium amount in soil (12%-33%).Goswami et al. 98 assessed the effect of HCl and ultrasonication-activated coconut shells on the availability of Zn, Ni, and Cd in multi-metal polluted soils.After the incubation of 63 days, the addition of modified biochar at a 5% rate reduced the amount of bioavailable Zn, Ni, and Cd by 12%, and 57%, respectively relative to the un-modified biochar.The modified biochar contained abundant active surface functional groups such as C = O, -OH, and -COOH, and its addition enhanced the soil CEC and pH, which consequently reduced the metal ions mobility in the soil by cation exchange, surface complexation, and electrostatic attraction.

Organic contaminants
In addition to metals/metalloids immobilization, biochars activated/modified by ball milling, CO 2 /steam activation, iron materials, oxidizing, bacteria loading, organic surfactants, LDH for the removal of several organic contaminants such as phenols, PAHs, plasticizer, antibiotics and pesticides in soil have been stated and are documented in (Table 8).Especially, a sulfidation-nZVI doped biochar created via 152 , the modified at a rate of 1% exhibited greater nitrobenzene degradation with an elimination rate (98%) within 24 h.They verified that the solubilization influence via biochar and reduction through FeSx were the leading mechanisms for the removal of nitrobenzene.Moreover, sulfidation-nZVI-doped biochar also had superior antioxidant capability and kept a great removal performance for nitrobenzene (up to 70%) after aging for 98 days, which advocated that this engineered biochar had great potential for field trials.Pan et al. 153 produced biochar from olive residues at 400 °C, and KMnO 4 (0.025 M) was employed as an oxidizing agent to synthesize the engineered biochar with great redox ability for the pentachlorophenol remediation.KMnO 4 -loaded biochar showed a higher remediation rate under anaerobic (2.4 μg PCP g −1 soil d −1 ) and aerobic (3.7 μg PCP g −1 soil d −1 ) conditions, which was much greater than that of fresh biochar 153 .Furthermore, Bacillus siamensis loaded biochar was produced to reduce dibutyl phthalate contamination in farming soils 84 .The bacterial-modified biochar could increase the biodegradation of dibutyl phthalate by simultaneously elevating the rate of degradation constant (from 0.20 to 0.24 d −1 ) and half-life (from 2.31 to 2.11 d −1 ).A substantial reduction of dibutyl phthalate uptake via leafy vegetables was also noticed, which could be accredited to the increased degradation/adsorption through Bacillus siamensis strain T 7 and biochar 84

Reusability of modified/engineered biochar
As with any biochar, consumed biochar gives a challenge, which requires suitable management.Depending on the kind of contaminants, and material cost, spent biochars are typically regenerated or become waste and incinerated or disposed of 154 .Consequently, adsorption pollutants are immobilized via the biochars, but the mechanism is typically reversible, particularly in the case of physical sorption.Therefore, the disposal of spent biochar with adsorbed contaminants in dumpsites takes the hazard of environmental pollution of groundwater, soil, and surface water, also creating additional costs 155 .Incineration of spent biochar contributes to the release of noxious gases, and ash creation possibly with risky elements, and needs money 156 .Regeneration and recovery of the spent biochars can decrease the cost and quantity of dumped waste.Chemical, microwave irradiation, and thermal regeneration with an inorganic or organic solvent regeneration are the techniques usually applied in biochar regeneration 157 .Nonetheless, in non-thermal approaches, occurring in the solution, the contaminant can desorb without degrading, thus there is a further problem with its elimination 60 .Therefore, the novel method for biochar regeneration is centered on catalytic oxidation such as the Fenton reaction leading to adsorbate degradation 158 .The efficiency of these approaches depends on several factors of which the most vital are the kind of biochar and interactions among adsorbate and adsorbent 159 .The regeneration through higher temperatures and chemicals affects the biochar characteristics such as porosity, structure of biochar, and functional groups and ultimately alters the catalytic functionality and sorption capacity of biochar and the resultant loss of their effectiveness 160 .This is particularly needed for the regeneration of engineered/modified biochar, which is typically characterized by the presence of some additives (catalytic compounds in doped biochar) and functional groups.Thus, it is substantial to have a case-by-case method depending on the kind of adsorbate-biochar system verified.Another alternative to spent biochar usage is the reuse of biochar with a bound contaminant in other several areas of life.Modified biochars used for the elimination of PO 4 3− can be applied as a soil fertilizer and conditioner 161 .Studies show that biochars can be utilized in energy, e.g., an additive in biogas production and as a solid fuel 162 .Therefore, the use of exhausted biochars for these intentions seems to be possible.Nonetheless, there is no relevant information about this area, which should be examined more intensely in the future.

Economic importance of biochar and its application
Pan et al. 163 reported that the production cost of biochar from various biomasses has seldom been enlisted in previous literature.The cost of biochar depends on various factors including availability, collection, and transportation of raw biomass and scale, production technology, handling, and supply.Given biochar fabrication factors and transportation is the most significant parameter.They noticed the economic feasibility of making two kinds of biochar in three states and exposed that the net present cost of biochar increases with the reduction in movement of mobile pyrolysis unit.Figure 6 displays the economic paybacks of biochar utilization.Another significant factor of interest is the cost of labor in biochar fabrication, which varies globally as comparatively high in the USA and UK (about 5000 $(USD)/t) and less in India and the Philippines ($90/t) 112 .In general, biochar cost varies in several states of the globe, where, Shen et al. 164 and Bhavani et al. 47 figured $50 to 682.54/t of the biochar.Qiu et al. 74 also noticed that the biochar was sold for about $200/t to farmers and suggested the price of biochar as ($300 to 500/t).Different kinds of biochar costs such as coconut coir, wood waste, and yard waste, $775/t, $329/t, $775/t, and $69/t respectively have been valued in previous studies 165,166 .Contrastingly, the cost of wood and yard waste biochar is greatly less than other materials such as (activated carbon $1500/t), and (zeolites $600/t) as well as other biochar ($50 to 2000/t), thus these could be utilized as a cost-effective and efficient material for nutrients restores from contaminated water 167 .Additionally, food waste feedstock is a profitable choice because of its low cost and easy availability.These characteristics limit the biochar fabrication cost from food waste to $50/t than that of conventional feedstocks ($2200/t) 168 .Biochar production is gaining attention because of its auspicious potential in the environment and energy.Gupta et al. 99 presented the economic supposition results for the biochar production in Selangor at $532/year and the total income from the sale of biochar was $8012/ year.Therefore, the net present cost for biochar fabrication, which was measured via the investment amount and the net income, exhibited positive outcomes of the economic feasibility of biochar 99 .Cost-effectuality of biochar fabrication depended on its retailing price, with a break-even of around $280/t for pyrolysis at 450 °C and approximately $220/t for pyrolysis at 300°C 169 .Panwar and Pawar 170 revealed that yard waste was confirmed to be auspicious feedstock for biochar creation with a net margin of $16 and $69 for the low and high-income scenario of carbon dioxide equivalents (CO 2 e), such as horse and cattle manures.Thus, the production of biochar can be tempting if the proceeds of the above costs offset the economic prices of elevating, hauling, harvesting, and storing the feedstock, besides those of applying pyrolysis, transportation, and application of the biochar.As reported from the examination, the net margin of biochar production could be enhanced with low-cost feedstock and an auspicious processing approach 171 .

Conclusions and future perspectives
Based on the above discussion, multiple modifications/engineering strategies have been employed to improve the physicochemical and biological properties of biochar.The multiple-functionalized biochars have been successfully utilized for the remediation of soil and aquatic systems contaminated by various pollutants.The specific attributes of multiple-modified biochar including the appearance of new functional groups, enhanced surface area, and greater electron transport capability are amongst the main factors affecting the remediation efficiency in multifaceted applications.Generally, functionalized biochar is an environmentally friendly catalyst/adsorbent that can be applied to address various environmental concerns.Nonetheless, some concerns remain unresolved and the following strategies need to be considered to attain a sustainable future for multiple-functionalized biochar in environmental applications.The superiority and remediation efficiency of multiple engineered/modified biochars is affected by the feedstock type, pyrolysis parameters, and modification techniques.A combined method following the modeling and experimental results should be employed to make standards for biochar fabrication, characterization as well and life cycle assessment (LCA) procedures.The presence of PFRs, PAHs, and heavy metals in biochar has been stated.Furthermore, some modification methods may introduce new hazardous substances.The stability and ecotoxicity of such potentially hazardous biochars should be assessed from an ecotoxicological perspective, including the toxic chemicals released over a long period.Engineered/modified biochars undergo long-term weathering via biotic and abiotic aging when exposed to environmental circumstances.Nonetheless, little knowledge is available about aged biochar remediation performance.Future research is required to investigate the stability of its decontamination potential with aging procedures and mechanisms influenced by various functionalization approaches.Chemical and physical modification techniques have often been employed to produce functionalized biochar.The production of functionalized biochar employing biological modification techniques involving microorganisms needs to be studied in detail about its significance in the decontamination of organic pollutants in soil and water systems.Advanced spectroscopic exploration techniques including XAFS (synchrotron-based X-ray absorption fine structure spectroscopy) and computational methods based on DFT (density functional theory) and MD (molecular dynamics) calculations should be explored to elucidate the remediation mechanisms for several contaminants.Machine learning and artificial intelligence should be applied as effective methods to enhance the development of functionalized biochar.Another advantage of modification is to attain easy removal of consumed biochar after contaminated-water treatment, for instance, magnetization, and the practical feasibility for recycling magnetic biochar demands to be examined at a pilot scale.Furthermore, there is scarce literature on the safe disposal of exhausted modified biochar after the sorption of toxic contaminants.Thus, related technology should be established to recover the exhausted functionalized biochar, i.e., use of specific solvents to effectively desorb the targeted pollutant.Additionally, non-renewable exhausted modified biochar should also be employed for energy creation from cost-effective and environmental perspectives.In the context of carbon neutrality worldwide, biochar as a carbon-negative technology has received wide attention.However, quantitative estimation methods of carbon sequestration by engineered biochar are missing, and the potential carbon sequestration value of engineered biochar has not been effectively verified and developed.

Figure 1 .
Figure 1.The number of papers (a) research + review and (b) review articles published in the last 10 years.

Figure 3 .
Figure 3. SEM images, FTIR, and XRD spectra of walnut shell biochar (WSC) and wood powder biochar (WPC) and modified ones with different acids and alkalis (After Liu et al. 14 ).

Figure 6 .
Figure 6.Economic benefits of biochars prepared from different feedstocks.

Table 2 .
Properties of modified biochars obtained from various treatments.

Table 3 .
Chemical oxidation and base/acid modification process of the biochar.

Table 4 .
The mechanisms and enhanced performance of chemically oxidized and base/acid modified biochars.

Table 5 .
Removal of contaminants and associated mechanisms through non-metallic heteroatom-doped biochar.

Table 6 .
Recent advances in biochar modification by minerals and their effectiveness in environmental application.

Table 7 .
Summary of various engineered/modified biochars and their immobilization efficiency for metals/ metalloids in the soil system.

Table 8 .
Summary of various engineered/modified biochars and their immobilization efficiency for organic contaminants in the soil system.