Effects of biochar-based materials on nickel adsorption and bioavailability in soil

The potential for toxic elements to contaminate soil has been extensively studied. Therefore, the development of cost-effective methods and materials to prevent toxic element residues in the soil from entering the food chain is of great significance. Industrial and agricultural wastes such as wood vinegar (WV), sodium humate (NaHA) and biochar (BC) were used as raw materials in this study. HA was obtained by acidizing NaHA with WV and then loaded onto BC, which successfully prepared a highly efficient modification agent for nickel-contaminated soil, namely biochar-humic acid material (BC-HA). The characteristics and parameters of BC-HA were obtained by FTIR, SEM, EDS, BET and XPS. The chemisorption of Ni(II) ions by BC-HA conforms to the quasi-second-order kinetic model. Ni(II) ions are distributed on the heterogeneous surface of BC-HA by multimolecular layer adsorption, which accords with the Freundlich isotherm model. WV promotes better binding of HA and BC by introducing more active sites, thus increasing the adsorption capacity of Ni(II) ions on BC-HA. Ni(II) ions in soil are anchored to BC-HA by physical and chemical adsorption, electrostatic interaction, ion exchange and synergy.

Preparation of the remediation materials. The preparation of humic acid is composed of sodium humate and wood vinegar in a mass ratio of 1:3. At this time, the sodium humate is acidified by wood vinegar to become black-brown precipitate humic acid. Biochar-humic acid materials were prepared with a mass ratio of 1:1 biochar and humic acid 36 , the two solutions were uniformly mixed, dried at room temperature, ground and passed through a 0.15 mm sieve to obtain biochar-Humic acid material, namely BC-HA. Biochar powder was added into a certain proportion of sodium humate solution, and the material was filtered and dried to obtain BC-NaHA. Under the same conditions, the biochar-humate sodium material (BC-NaHA) was composed of sodium humate (50.0 wt%) and biochar (50.0 wt%), and the biochar material (BC) was only composed of biochar powder (100 wt%).

Adsorption of different forms of nickel in soil by biochar-based materials.
(1) The content of available nickel in soil extracted by CaCl 2 .
After culturing the remediation materials and polluted soil in the same environment for 60 days, the above experimental group and control group were sampled by the five-point sampling method. The content of available nickel in 1.0 g soil samples before and after treatment with three remediation materials was extracted with 0.01 M CaCl 2 (1:10 w/v) 37 .
(2) Four chemical forms of nickel in soil were extracted by BCR sequential extraction.
Soil samples treated with BC-HA-1, BC-HA-2, BC-HA-3, BC-HA-4, BC-HA-5, BC-HA-6 were air-dried and ground, and passed through a 0.125 mm sieve. The content of four chemical forms of nickel in contaminated soil was extracted by BCR sequential extraction method, as shown in Table 1.
All metal extracts and filtrate concentrations were determined by atomic absorption spectrometer (AAS). Three replicates were set up for each sample, and the average value is displayed.

Sorption properties of remediation materials. Adsorption kinetics. A Ni solution was prepared with
an initial concentration of 200 mg·L −1 (with 0.01 mol·L −1 NaNO 3 as the background) in 1000 mL-beakers. Under the temperature of 25 °C and the initial pH of 5, stirring with a thermostatic magnetic stirrer, 0.5 g of BC-HA is added into the Ni(II) solution. At the adsorption time of 5, 10, 30, 60, 120, 180, 240, 360, 480, 600, 720,  , the concentrations of the  nickel solutions are set as 10, 20, 40, 60, 80, 100, 120, 150, 180, 200 mg·L −1 . The centrifuge tube is oscillated at 200  r·min −1 for 24 h in a shaking incubator with a constant temperature of 15 °C, 25 °C and 35 °C, respectively. After oscillation, the sample is centrifuged using a centrifuge at 8000 r·min −1 for 10 min. The supernatant solution is passed through a 0.22 μm filter membrane, and the concentration of nickel in the filtrate is measured by the AAS, and the adsorption capacity of BC-HA is calculated.
Nickel ion adsorption under different pH conditions. 0.2 g BC-HA is added to a 50 mL polyethylene centrifuge tube, then 20 mL Ni(II) solution (with 0.01 mol·L −1 NaNO 3 as the background) is added to the tube. After that, the initial pH of the nickel solutions are set as 2, 3, 4, 5, and 6. The centrifuge tube is oscillated at 200 r·min −1 for 24 h in a shaking incubator with a constant temperature of 25 °C. After oscillation, the sample is centrifuged using a centrifuge at 8000 r·min −1 for 10 min. The supernatant solution is passed through a 0.22 μm filter membrane, and the concentration of nickel in the filtrate is measured by the AAS, and the adsorption capacity of BC-HA is calculated.
FT-IR, SEM/EDS, BET and XPS measurements. All BC-HA, BC-NaHA, BC and their immobilization samples are rinsed three times with ultrapure water before and after immobilize metal ions to remove any physisorbed metal ions and dried under vacuum overnight before all measurements 38 .
Using the KBr disc technique, the KBr and the sample were uniformly ground and pressed into thin slices in a 200:1 ratio, and the samples were analyzed by a VECTOR-22 Fourier transform infrared spectrometer (Bruker, Germany). A Q45 SEM(FEI, USA) was used to characterize the micromorphology of all BC-HA, BC-NaHA, BC and their immobilization samples. Appropriate amount of samples to be tested are uniformly pasted on the conductive adhesive surface and fixed on the sample table. After spraying gold, the instrument is used for testing. BET analysis is run using a Surface Area and Porosity Analyzer (MICROMERITICS ASAP 2460, USA). BC-HA, BC-NaHA, and BC samples were vacuum-dried at 120 °C for 5 h before testing. XPS analysis is run using an XPS spectrometer (AXIS Supra type, Kratos, UK). The spectra of all BC-HA, BC-NaHA, BC and their immobilization samples are obtained over the range of 0 to 1200 eV, with a slit width of 1.9 mm into the analyzer and an energy of 300 eV. The binding energy is calibrated using the C 1 s peak as 284.60 eV. Data analysis. Adsorption capacity. The adsorption amount of metals is calculated according to the following formula: where Q e is the adsorption amount of metals, mg·g −1 ; C 0 and C e are the initial and equilibrium concentration of Ni(II), mg·L −1 ; V is the volume of the solution, mL; m is the mass of the soil sample, g.

Adsorption kinetic. Pseudo-first-order kinetic equation
Pseudo-second-order kinetic equation

Elovich dynamics equation
where Q e is the adsorption amount at equilibrium, mg·g −1 ; Q t is the adsorption amount at time of t, mg·g −1 ; t is the reaction time, min; K 1 is the Pseudo-first-order rate constant, min −1 ; K 2 is the Pseudo-second-order rate constant, min −1 ; A is the diffusion rate constant, mg·g −1 ; K t is the reaction rate constant, mg·g·min −0.5 .

Adsorption isotherm. Langmuir adsorption isotherm
Freundlich adsorption isotherm where C e is the concentration at adsorption equilibrium, mg·L −1 ; Q e is the adsorption amount at equilibrium, mg·g −1 ; Q m is the maximum adsorption amount, mg·g −1 ; K L is related to the size of the adsorption energy Constant, L·mg −1 ; K F is a constant related to the adsorption strength, mg·g −1 ; n is the heterogeneity of the adsorbent. The factor, when n > 1, indicates that there is a strong force between the adsorbate and the adsorbent; R is the gas constant, which is 8.314 J·mol −1 ·K −1 ; T is the Kelvin temperature, K; b is the Temkin constant, J·mol −1 ; K T is the equilibrium constant at the maximum adsorption capacity, L· mg −1 . All data are analyzed by the single-factor analysis of variance (one-way ANOVA) using SPSS24.0 (SPSS Inc. Chicago, USA), the data is the mean ± standard deviation, and lowercase letters indicate significant differences between different groups under the same percentage (p < 0.05).

Results and discussion
The effect of biochar-based materials on the availability of nickel in soil. The availability of metals generally refers to the degree of absorption, accumulation or toxicity of metals in the ecological environment 39 . The distribution of heavy metals in various chemical forms is affected by soil aging, and the type of metal determines the degree of redistribution of metal forms. The redistribution of heavy metal forms in soil is characterized by rapid preservation at the beginning and gradual transition later 40 . In this study, CaCl 2 was used to extract the available content of nickel in soil.
The effects of different application amounts of BC, BC-NaHA and BC-HA on available nickel in soil with aging time are shown in Fig. 1a-c, while the control group is nickel-contaminated soil without any remediation materials. It can be seen from the figure that with the increase of aging time, compared with the control group, after adding BC, BC-NaHA and BC-HA, the content of available nickel in soil decreased to varying degrees. The decreasing degree of soil available nickel content by substances was BC-HA > BC-NaHA > BC.  www.nature.com/scientificreports/ In Fig. 1a-c, compared with the control group, after aging (63d), 3.5 g·kg −1 BC reduced the content of available nickel in soil by 31.27%, and 2.5 g·kg −1 BC-NaHA reduced soil available nickel content by 37.16%. 2.5 g·kg −1 BC-HA significantly reduced the soil available nickel content by 48.17%, which was 38.85% lower than the control group. The results confirmed that 2.5 g kg −1 BC-HA material had the maximum effect on reducing the effective Ni content in soil, which was better than the effect of humus as a cleaning agent to remove Ni in polluted soil reported in the literature (35.4% ~ 46.1%) 35 . Therefore, it is feasible to desorb and fix nickel in soil with humic acid-modified biochar, and the fixation effect of humic acid and biochar composite on nickel is much greater than that of single material. Compared with BC and BC-NaHA, BC-HA has a stronger capacity for Ni immobilization/stabilization during incubation.
Since the content of available metals in soil cannot describe the distribution of chemical forms of metals in soil in detail, the BCR sequential extraction method was used to study the binding form and migration ability of metals in soil, so as to determine the size of the biological availability of metals. Studies have shown that the accumulation process of metals in soil to plants has a good correlation with the bioavailable concentration of metals in soil 39 . Figure 1d showed the distribution of nickel in soil supplemented with BC-HA after aging culture (63d). It can be seen from the figure that the acid-soluble content of nickel was significantly reduced, which was attributed to the adsorption and precipitation of metals on the adsorbent surface, which was consistent with the literature results 41 . Compared with the control group, the exchangeable and reducible components of Ni(II) in the soil treated with 2.5 g BC-HA decreased by 10.11% and 9.29%, respectively, while the oxidizable and residual components of Ni(II) increased by 3.30% and 16.10%, respectively. The reason was attributed to the combination of Ni(II) with carbonate and Fe/Mn oxides after the addition of BC-HA material, resulting in a significant reduction of Ni content in weak acid extraction and reduction states, forming insoluble stable complexes 42 . The increased content of nickel oxidizable state components can be attributed to the formation of nickel complexes with organic functional groups present in the sorbent 43,44 . In addition, the largest immobilization of Ni in soil was the transformation from the acid-soluble fraction of Ni to residual Ni 44,45 . Therefore, the conversion of acidsoluble nickel components into residual components is an effective way for nickel to be solidified in soil. The results showed that when BC-HA was added into soil, the weak acid extract and reduced nickel components in soil were transformed into oxidizable and residual components, which were not easy to be absorbed by plants and had low environmental mobility. Therefore, BC-HA was a good curing agent for stabilizing and repairing nickel-contaminated soil.
Adsorption kinetics. The experimental data were fitted using the Pseudo-first-order kinetic model (Eq. 2), the Pseudo-second-order kinetic model (Eq. 3) and the Elovich (Eq. 4) model. Generally, the Pseudo-first-order kinetic model considers the adsorption mechanism to be physical adsorption, the Pseudo-second-order model considers the adsorption mechanism to be monolayer adsorption through chemisorption, and the Elovich kinetic model considers chemisorption on a heterogeneous surface.
The relationship between the adsorption amount of Ni(II) adsorbed by BC-HA and the adsorption time is shown in Fig. 2. The adsorption capacity of BC-HA for Ni(II) increased rapidly in the first 240 min, was in the slow adsorption stage in 240 ~ 720 min, and reached the equilibrium state in 1440 min, with the equilibrium adsorption capacity of 18.04 mg/g. The reason is that sufficient adsorption sites are provided on the surface of BC-HA in the initial stage, and Ni(II) occupy the sites quickly. However, with the progress of the adsorption process, the active sites on the surface of BC-HA were reduced, and the adsorption rate gradually decreased. With the increase of the adsorption time, the amount of Ni(II) adsorbed by BC-HA increased continuously. The reason was that the driving force of the concentration difference existing when the solution was at a high concentration made the Ni(II) sneak into the interior of BC-HA, expanding the Utilization of adsorption sites. Since the kinetic model assumes that the adsorption amount increases in the initial adsorption stage, the adsorption rate slows www.nature.com/scientificreports/ down as the adsorption process progresses, and finally approaches the adsorption equilibrium. Therefore, the above results were consistent with the kinetic model. The pseudo-first-order kinetic equation was well fitted (R 2 = 0.992) ( Table 2), indicating that the adsorption process of BC-HA on Ni (II) may be affected by surface physical adsorption. However, The adsorption kinetic data of Ni(II) onto BC-HA could be well-described by the Pseudo-second-order model, showing high regression coefcients of R 2 = 0.997 (Table 2). Meanwhile, the theoretical Q e values ftted by the Pseudo-second-order model were 18.82 mg/g for Ni(II) ion, which was very close to the experimental result of 18.04 mg/g ( Table 2). The better ftting results of the Pseudo-second-order model than the Pseudo-first-order and Elovich model indicated that chemical adsorption was mainly responsible for the immobilized of Ni(II) ion by BC-HA 46 .
Adsorption isotherms. The Langmuir (Eq. 5), Freundlich (Eq. 6) and Temkin (Eq. 7) isotherm models widely adopted for evaluation of adsorption behaviors were used to fit the isotherm data (Fig. 3). The ftting parameters for each isotherm model are compiled in Table 3. In general, the Langmuir model assumes that all sites have the same affinity for pollutants, and when the solid surface is saturated, a monolayer adsorption is formed; the Freundlich model assumes that the adsorption of the molecular layer occurs on a non-uniform surface, and the adsorption mechanism is Multilayer adsorption; the Temkin isotherm model, which assumes   www.nature.com/scientificreports/ that the heat of adsorption (a function of temperature) of all molecules in the adsorption layer decreases linearly with increasing coverage area, mainly describes the chemisorption process as an electrostatic interaction 47 .
As can be seen from Fig. 3, the initial concentration of the solution is the same, and the increase of temperature is conducive to the increase of the adsorption capacity of Ni(II) by BC-HA, and eventually tends to the adsorption equilibrium. The Langmuir model has a good fit, indicating that Ni(II) may be physically adsorbed on the BC-HA material by the monolayer. The Freundlich model has the greatest fitting degree, indicating that the adsorption of Ni(II) was mainly multi-layer adsorption on the surface of the adsorbent, which was consistent with the results reported in the literature 47,48. The maximum adsorption amount of Ni(II) obtained by the Langmuir model is 19.78 mg/g (Table 3), which was better than the maximum adsorption amount of Ni (12.41 mg/g) by rice husk humic acid in the literature 49 . The results showed that the added BC-HA material with WV has an enhanced ability to adsorb Ni(II).
At temperatures of 288.15 K, 298.15 K and 308.15 K, the K L values in the Langmuir model ranged from 0.082 to 0.121 L/mg ( Table 3).The K L value is related to the adsorption capacity. The larger the K L value, the stronger the adsorption capacity, indicating that the increase of temperature was beneficial to reduce the interaction between the adsorbent and the solvent surface and promote the adsorption of Ni(II) on BC-HA. The n value in the Freundlich model varies from 2.933 to 3.400 (Table 3), and was between 1 and 10, indicating that there was a strong force between the adsorbate and the adsorbent 50 . The results showed that BC-HA had a strong ability to adsorb Ni(II) ion. Figure 4 shows the variation of Ni(II) ion adsorption onto BC-HA with diferent initial solution pH. Solution pH significantly affects the adsorption capacity of heavy metals by affecting the complexation behavior of surface-active functional groups (hydroxyl, carboxyl, and amino groups) 51 . When the pH value of the solution is greater than 6, nickel ions form hydroxide precipitation on the surface of the adsorbent 52 , so the pH value of the solution was set in the range of 2 to 6 in the experiment.

Effects of solution pH on adsorption.
As can be seen from Fig. 4, the zero point charge pH ZPC value of BC-HA is about 3.4. When pH < pH ZPC , a large number of H + ions in the solution occupy the adsorption active site of BC-HA, leading to the increase of electrostatic repulsion between positively charged BC-HA and Ni (II) ions. On the contrary, when pH > pH ZPC , the concentration of H + ions decreased, and a large number of negatively charged active groups on the surface   www.nature.com/scientificreports/ of BC-HA were exposed, which promoted the coordination complexation reaction between -OH and -COOH and Ni (II) ions, resulting in the enhanced ability of BC-HA to adsorb Ni (II) ions. The results showed that the maximum adsorption capacity of Ni(II) by BC-HA was 15.60 mg/g at pH 6. Similar results showed that the maximum adsorption of Ni(II) was achieved in the pH range of 5-6 53 . Therefore, all adsorption experiments were performed at pH 6.
Immobilization mechanism. FTIR analysis. The FTIR spectra of BC, BC-NaHA and BC-HA prior to and after Ni(II) adsorption are presented in Fig. 5a,b. The main absorption bands displayed were assigned as follows [54][55][56] , the broad peak in the 3448 ~ 3455 cm −1 band is the stretching vibration peak of the hydroxyl group (-OH) of phenols; the 1633 ~ 1636 cm −1 band is the aromatic C = C and C = O stretching vibration peaks; the 1022-1024 cm −1 band region is the stretching vibration peaks of C-O in alcohols, ethers and esters. The FTIR results confirmed that the intensities of the C = C, C = O and C-O peaks of the BC-HA material were much greater than the intensities of the corresponding peaks on the BC and BC-NaHA materials, so the ability of the material to adsorb nickel was BC-HA > BC-NaHA > BC. The results showed that the introduction of WV increased the number of oxygen-containing functional groups on the adsorbent surface and promoted the formation of more metal complexes between BC-HA and Ni(II). After Ni(II) was adsorbed by BC, BC-NaHA and BC-HA, the stretching vibration peaks of C = O and C-OH were both weakened, indicating that the carboxyl group and hydroxyl group are closely related to the Ni(II) reaction 57 . Compared with BC-NaHA and BC, BC-HA has more hydroxyl and carboxyl groups to react with Ni(II). According to related literature 58 , the complexation of oxygen-containing functional groups on the surface of biochar with Ni was a mechanism for Ni immobilization in soil.
SEM/EDS analysis. The SEM images of BC, BC-NaHA and BC-HA before and after adsorption of Ni(II) were shown in Fig. 6 at a magnification of 4000 times. It can be seen from the figure that compared with BC and BC-NaHA, BC-HA presents a more complex, rough and porous surface structure with a large number of adsorption pores. Similarly, after adsorption of Ni(II) by BC, BC-NaHA, and BC-HA, the surface of BC-HA exhibited more irregular and heterogeneous morphologies. It was confirmed that the WV in the adsorbent increased the adsorption surface area and porosity, provided more adsorption active sites for Ni(II), and promoted the adsorption of more Ni(II) by BC-HA.  Fig. 7a,b. After BC, BC-NaHA and BC-HA adsorbed Ni(II), the percentages of nickel elements in the unit adsorption section were 0.64%, 3.06%, and 5.11%, respectively. Therefore, BC-HA has the greatst adsorption capacity for Ni(II). In addition, the content of other cations such as Na(I), Ca(II), Al(III), Mg(II) changed significantly, which was attributed to ion exchange of Ni(II) with other cations on biochar surface 59 .
Since BC-HA had more oxygen-containing groups than BC-NaHA and BC, the ability of BC-HA to adsorb Ni(II) was stronger than that of BC-NaHA and BC, which was consistent with the FT-IR results. In addition, the above results also confirmed that wood vinegar changed the number of oxygen-containing functional groups in humic acid during the composting process of heavy metal-contaminated soil 60 . The composted humic acid forms insoluble metal complexes with heavy metal ions, thereby reducing the bioavailability of heavy metals in the soil.  www.nature.com/scientificreports/ BET analysis. The N 2 adsorption-desorption isotherms and pore size distributions of BC, BC-NaHA and BC-HA at 77.35 K were shown in Fig. 8. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, the isotherms of BC, BC-NaHA and BC-HA were classified as Type IV with a hysteresis loop of H4 type 61 . Type IV isotherm is a characteristic of mesoporous adsorbent materials, and the hysteresis loop is related to narrow slit-like pores in BC-HA 62 . The peak value of pore size distribution of BC-HA was less than 2 nm, indicating that there were a large number of micropores in BC-HA. BC, BC-NaHA and BC-HA all have some pore structure at 2-50 nm, mainly mesoporous structure. In addition, when the relative pressure (P/P 0 ) > 0.80, the adsorption capacity of the three materials increased rapidly, which might be responsible for multilayer adsorption 63 . The specific surface area (SSA) and pore properties of BC, BC-NaHA and BC-HA are listed in Table 4. Compared with BC and BC-NaHA, BC-HA had the largest SSA (1.93 m 2 /g), Barrett Joyner-Halenda (BJH) pore volume (0.015 cm 3 /g), and average pore size (30.18 nm). These results indicated that the pores in BC, BC-NaHA and BC-HA mainly existed in the form of mesopores, and the addition of wood vinegar promoted BC-HA to provide abundant binding active sites for nickel ions. This conclusion also confirms the results of SEM. XPS analysis. The XPS spectra of BC, BC-NaHA and BC-HA prior to and after the adsorption of Ni(II) are presented in Fig. 9. After BC-HA and BC-NaHA materials were combined with Ni(II), the peaks of Na1S and Ca2P disappeared, which was attributed to the fact that Ni(II) had replaced Na(I) and Ca(II) to complex with oxygen-containing functional groups of the materials through ion exchange.
The high-resolution spectra of C1s before and after adsorption of Ni(II) by BC, BC-NaHA and BC-HA are shown in Fig. 10. According to related literatures 42  www.nature.com/scientificreports/ respectively, and the contents after adsorption of Ni(II) were 69.49%, 17.94%, and 12.57%, respectively. The above results showed that compared with BC, BC-NaHA and BC-HA, the total oxygen content of BC, BC-NaHA and BC-HA after adsorption of Ni(II) decreased by 2.46%, 1.16% and 3.89%, respectively. In addition, before BC, BC-NaHA and BC-HA adsorb Ni(II), the BC-HA material has the highest total oxygen content among the three materials, followed by BC-NaHA and BC. This was attributed to the modification of NaHA by WV into HA with more oxygen-containing functional groups, which promoted the formation of metal complexes between the hydroxyl and carboxyl groups on BC-HA and Ni(II), making the BC-HA material more conducive to the adsorption of Ni(II). The high-resolution O1s spectra of BC, BC-NaHA and BC-HA before and after the adsorption of Ni(II) were shown in Fig. 11. According to related literatures 52,53 , the O1s spectra of BC, BC-NaHA and BC-HA had two binding energy peaks at 531.70 eV and 533.31 eV, which were the characteristic absorptions of C-OH and O-C = O peak, respectively. The contents of C-OH and O-C = O before and after BC adsorption of Ni(II) were 45.68% and 54.32%, 67.13% and 32.87%, respectively. The contents of C-OH and O-C = O before and after the adsorption of Ni(II) by BC-NaHA were 67.33% and 32.67%, 60.67% and 39.33%, respectively. The contents of C-OH and O-C = O before and after adsorption of Ni(II) by BC-HA were 71.98% and 28.02%, 53.31% and 46.69%, respectively. The above results confirmed that the hydroxyl or carboxyl group on the adsorbent reacted  www.nature.com/scientificreports/ with Ni(II) to form metal complexes, which reduced the migration and transformation of heavy metal ions in the environment 45 The results have proved that more carboxyl groups were bound to Ni(II) in BC materials, while more Ni(II) were bound to hydroxyl groups on the surface of BC-NaHA and BC-HA materials, which was the same as the results reported in the literature 46 . This result was consistent with the results of FT-IR, SEM, EDS, BET analysis.
Summary of mechanism of adsorption of nickel ions by BC-HA. The preparation procedure of BC-HA material and its adsorption mechanism for nickel ions are shown in Fig. 12. The addition of BC-HA effectively reduced the migration and transformation capacity of nickel in soil and environmental risks, which can be attributed to the coordination reaction between organic functional groups in the adsorbent and Ni(II)