Sodium salt-assisted low temperature activation of bentonite for the adsorptive removal of methylene blue

The sodium salt-assisted low temperature activation of bentonite (BB) was attempted. The unique features of the raw bentonite and BB were characterized with respect to the morphological, functional, and textural analysis. The adsorptive behaviour was evaluated by adopting methylene blue (MB) as the model pollutant via batch adsorption experiment. The experimental data were fitted to the non-linear isotherm equations (Freundlich, Langmuir, and Temkin), while the adsorption modelling was interpreted by the pseudo-first order, pseudo-second order and Elovich models. The adsorptive mechanism was ascertained according to intraparticle-diffusion and boyd models. The intercalation of sodium salt into the bentonite surface give rise to the specific surface area and total pore volume from 120.34 to 426.91, m2/g and 0.155 to 0.225 cm3/g, respectively, indicating a large proportion of the newly formed surfaces may be connected to new pore walls, associated with the silanol (≡SiOH), and aluminol (≡AlOH), and hydroxyl (–OH) groups for the possible entrapment MB onto the adsorbent. The equilibrium data was satisfactory described by the Langmuir isotherm and pseudo-second order model, with a monolayer adsorption capacity for MB of 318.38 mg/g, while the thermodynamic study verified spontaneous, feasible, and endothermic of the adsorption process.


Methodologies and materials
Functionalized bentonite. Raw bentonite applied in this work was commercially acquired from Sigma Aldrich, Malaysia. The activation step was performed by mixing the raw bentonite with NaCl solution at 70 ℃ for 1 h, with the impregnation ratio (IR) given by: where W bentonite and W NaCl are the dry mass of bentonite (g) and NaCl (g). The activated bentonite (BB) was collected by centrifugation, subsequent by washing extensively with deionized water (DW) until the washing solution reached to a neutral pH before it was dried and stored in a sealed bottle.
Model pollutant. Methylene blue (MB), a model pollutant with low biodegradability in nature was adopted.
The chemical structure, molecular weight and λ max of MB are derived as C 16 H 18 C·N 3 S 3 .H 2 O, 373.91 g/mole and 663 nm, respectively. It could be dissociated into a MB + cation and a Cl − anion in the aqueous medium, with the dehydrated features of 14.3 Å in width, 6.1 Å in depth, 4 Å in thickness, and the molecular volume and molecular diameter of 241.9 cm 3 /mole and 0.8 nm, respectively. An appropriate amount of MB was dissolved in DW for preparation of standard stock solution, and working solutions was acquired by successive dilutions.
Adsorption experiments. The adsorption experiments were performed in the conical flasks containing 200 mL of adsorbate solution and 0.2 g of BB at the pre-determined concentrations from 50 to 500 mg/L. The solution mixture were agitated at 130 rpm and 30 °C for 24 h, and the concentration of the supernatant solution was analyzed at 663 nm using a UV-Vis spectrophotometer (Shimadzu-1800). The experiments were conducted in triplicates, and the equilibrium uptake, q e (mg/g), was determined by: where C 0 , and C e , (mg/L) are the initial and equilibrium liquid-phase concentration of MB, and V and W is denoted to volume of adsorbate solution (L), and dry mass of BB (g), respectively.
To better correlate the experimental data with the isotherm equations, the non-linearized Langmuir 12 , Freundlich 13 and Temkin 14 isotherm models have been adopted: where Q 0 (mg/g), K L (L/g) and K F (mg/g) (L/mg) 1/n are the Langmuir isotherm constants for adsorption capacity, adsorption energy, and Freundlich isotherm constant respectively, where 1/n is a measure of adsorption intensity, and B = RT/b T , with b T (J/mole), R (8.314 J/mole K), T (K) and A (L/mole) are the heat of sorption, universal gas constant, absolute temperature and equilibrium binding constants, respectively. www.nature.com/scientificreports/ For the assessment of the non-linear isotherm models, a trial-and-error procedure was performed to maximize the coefficient of determination R 2 , between the experimental data and isotherms in the solver add-in with Microsoft's spreadsheet, Microsoft Excel, defined as: where q e,calc , q e,calc and q e,meas are the predicted, average mean and measured methylene blue concentration at equilibrium deduced from the isotherm model. The experimental data then was validated by the root-meansquare deviation (RMSD), given by: where n indicating the total data points, while the experimental and predicted adsorption capacity are represented by corresponding q exp (mg/g) and q p (mg/g), respectively.
The changing solution pH on the adsorptive performance was conducted at the solution pH from 1 to 12, at the BB dosage of 0.2/200 mL, initial concentration at 500 mg/L and operating temperature of 30 °C. The pH adjustment was conducted by using 0.1 M hydrochloric acid (HCl) and/or sodium hydroxide (NaOH), and the measurement was ascertained using a pH meter.
Kinetic modelling. For the interpretation of kinetic analysis, the solution samples were measured at prescribed time interval, and the adsorptive uptake at time t, q t (mg/g), was determined by: where C t (mg/L) is denoted as the liquid-phase concentration of MB dye at time, t.
It describes the adsorbate uptake, mass transfer, and the equilibrium time of an adsorption process. In this study, the pseudo-first order 15 , pseudo-second order 16 and Elovich kinetic 17 equations were adopted for the effective simulation of the equilibrium data, defined as: where k 1 (1/h) and k 2 (g/mg h) is the adsorption rate constant for the pseudo-first and pseudo-second order kinetic equation respectively, while a (mg/g h) is the initial sorption rate and b (g/mg) is related to the extent of surface coverage, and activation energy for chemisorption of the Elovich equation. The suitability of the kinetic models was validated by the correlation coefficient, R 2 , and the normalized standard deviation, Δq (%): where the data points, the experimental and calculated adsorption capacity are represented by n, q e,exp (mg/g) and q e,cal (mg/g), respectively. Adsorption mechanism. The kinetic study explains solute-adsorbent surface interaction during the adsorption process, but does not elucidate the solutes particles diffusion mechanism onto the adsorbent surface. Therefore, the equilibrium data was analyzed according to the Fick's second diffusion law, the Weber and Morris intra-particle diffusion 18 and the Boyd film diffusion models 19 : where the diffusion rate constant and thickness of the boundary layer could be devoted as k pi (mg/g h 0.5 ) and C i , respectively, whereas B t is a mathematical function of fractional attainment of equilibrium F, at any time t, given by: (q e,meas − q e,calc ) 2 (q e,meas − q e,calc ) 2 + (q e,meas − q e,calc ) 2 (q e,exp − q e,cal )/q e,exp 2 n − 1 × 100  20 . The Gibbs free energy change, ΔG° (kJ/mole) represents the spontaneity assessment of a chemical reaction, and both changing entropy and energy must be critically identified. Accordingly, the negative ΔG° indicates a high spontaneity and energy of a reaction, specific at a temperature, and the adsorption equilibrium constant K L could be derived by: with R (8.314 J/mole K) is the universal gas constant, and T (K) is the absolute temperature. The relation of the equilibrium constant with the changing operating temperature could be acquired from the differential equation proposed by Ho et al. 21 : The integrated Eq. (17)  where ΔH° and ΔS° are the standard enthalpy and standard entropy change, respectively.
Physio-chemical characterization. The morphological structure of the functionalized adsorbent was examined by using the scanning electron microscope (Supra 35 VP, Germany) equipped with W-Tungsten filament, with MnK α as the energy source. The porosity measurement was conducted by the nitrogen adsorption isotherm using the Micromeritics ASAP 2020 analyzer. The Fourier transform infrared spectroscopy (FTIR) analysis was conducted according to the KBr method using the Perkin-Elmer Spectrum GX infrared spectrometer in the scanning range of 4000-400 cm −1 . The point of zero charge, pH pzc was justified by adjusting the solution pH of 200 mL of 0.01 M NaCl solution to a value between 1 and 12. The final pH was measured after 2 days agitation. The point where pH initial − pH final = 0 is pH pzc .

Results and discussions
Chemical impregnation ratio, IR. A feasible approach to enhance the adsorptive performance of the clay mineral is the alteration of surface chemistry, in which activation agents play a decisive role. During the activation process, the pore system of the clay minerals could be modified, leading to the changing catalytic, adsorptive and environmental functions of the minerals with the improvement on the crystalline structure of the minerals via structural ion dissolution or reorganization of the interior porosity. The effect of IR for the equilibrium uptake of MB onto BB is depicted in Fig. 1. It can be clearly found from Fig. 1 that increasing the IR www.nature.com/scientificreports/ from 1:1 to 1:3 indicated a steadily rise of the adsorption uptake from 205.67 to 252.59 mg/g, with the best IR recorded at 1:3. The phenomena may be ascribed to the rising ionic force with the incorporation of sodium salt into the clay structure, that has induced a double layer compression to assist the process of approximation and association of the clay structure. Therefore, the dye molecules which were initially bonded as the aggregates or monomers onto the external surface of the clay minerals during the association process, could be re-located into the internal region. Cione et al. 5 further stated that, the electrostatic repulsion between the negative charged of the clay particles would be screened in the presence of salt, that in turn reduced the repulsive interaction, favouring the approximation between the tactoids, resulting in the entrapment of MB as the monomers or aggregates molecules.
In contrast, the subsequent increase in the IR ratio, beyond the optimum value showed a gradually decrease in the adsorptive uptake of MB. The excessive presence of sodium salt would promote vigorous reaction within the clay particles, which destroyed the clay framework, resulting in a dramatic reduction of the accessible surface binding sites. The adsorption mechanism, which has been reported to occur partly by ion exchange releasing cations in the interlayer and basal plane surfaces, and partially via non-columbic interaction between the bentonite surface neutralized site and the adsorbed cation, is greatly governed by the changing surface polarity, compression layer, or ionic strength of the negatively charged adsorbent. In this sense, the excessive deposition of sodium would introduce a repulsive force to the cationic MB molecules, resulting in the reduction of the adsorptive uptake.

Effect of initial concentration and contact time. The effect of initial concentration and contact time
for the adsorptive uptake of MB onto BB at 30 °C is displayed in Fig. 2. It could be observed that the adsorption process increased rapidly at the initial time intervals, before the transitional phase took place, and reached to a plateau. At the initial stage, the adsorption uptake rate increased significantly, signalling the presence of readily accessible surface binding sites 22 . As the equilibrium approached, the process turned slower, where the maximum adsorptive uptake under the operating conditions reflects the equilibrium uptake. From the present result, it can be inferred that the rising adsorptive uptake from 49 to 318 mg/g with the increasing initial MB concentration from 50 to 500 mg/L, may ascribed to a higher concentration gradient to overcome the mass transfer resistance between the aqueous MB solution and the solid BB.
Furthermore, in the MB-BB suspension system, the adsorptive uptake of MB dye molecules to the external surface of BB would result in the rising local concentration of MB dye, with the formation of dye aggregates 23 . These MB molecules would further migrate to the interlamellar region, with the disaggregation of MB aggregates, and restoration of the MB monomers 24 . At the higher loading of MB, these agglomerates of dye molecules are predicted to be dominant, while these monomers or dimers would be practically absent in the MB-BB complexes. Additionally, it can be found that a longer contact time was required for a higher MB concentration to attain the equilibrium. The obtained findings could be depicted by the diffusion of MB molecules across the boundary layer, into the internal structure of adsorbent, and to the binding sites 25 , suggested a smooth and monolayer coverage of MB dye onto BB.
Effect of solution pH. The changing solution pH remains one of the most influencing parameters determining the performance of the adsorption process. The response is particularly due to influence of the hydrogen ions which may affect the surface charges and ionization of the functionalities of the solid adsorbent 26 . Figure 3 shows the changing adsorptive uptake of MB onto BB as a function of solution pH. In the aqueous medium, cationic dye including MB tend to produce the reduced form of CH + and C + cations 27 . The negatively surface With the pH zpc of 9, the BB surface turned positive at the pH below 9, and showed a net negative charge at the pH above the pH zpc , which supported the adsorption of cationic MB onto the negatively charged BB via electrostatic attraction after the modification process as described in Eq. (21).
Specifically, the potential surface charge of the clay-aqueous system is mainly governed by the activity of the ions or the newly formed ion complex with H + and OH − derived as: In this study, the maximum adsorptive uptake of MB onto BB was found to be at the basic medium, where an extremely high cation exchange capacity was recorded, and driven by the basal oxygen surface of the tetrahedral sheets with the MB solution, which contain excess hydroxyl ion. The highly saline solution at the alkaline environment was expected to contribute to the protonation/deprotonation of the surface hydroxy site (M-OH), not limited to the alumina group of the BB, which gave rise to the more negative charges.
These potential determining ions are represented by H + and OH -, and this aforementioned electrostatic force between of the negatively charged BB and the positively charged MB would result in a greater uptake of cationic dyes.
Isotherm modelling. The modelling is the development of representative equation of the adsorption system, which could be applied for the design purposes 30 . It is well known that, the estimation of isotherm parameters from the non-linear isotherm modelling is preferred in minimizing the data distribution errors between the experimental and predicted parameters or fit distortions of the linearization models 31 . Three non-linear equilibrium isotherms, specifically Langmuir, Freundlich, and Temkin isotherms have been adopted. Each theoretical plots of the modelling have correlated with the experimental data, and further evaluated by the correlation coefficients (R 2 ) and Root Mean Square Deviation (RMSD). The isotherm parameters with the correlation R 2 and RMSD are tabulated in Table 1. The presented data was most suitable described by the Langmuir isotherm, with the highest R 2 value of 0.998, and the lowest RMSD value of 1.70. The best fit to the Langmuir isotherm model suggested the homogeneous nature, energetically equivalent and identical of the surface binding sites, with the corresponding Q 0 and K L of 318.38 mg/g and 0.29 L/mg, respectively.
The equilibrium data was further analyzed with respect to the separation factor (R L ), a dimensionless constant proposed by: www.nature.com/scientificreports/ whereby R L is a quantitative verification on the favourability of the adsorption system where R L = 0 indicates irreversible, 0 < R L < 1 represents favorable, R L = 1 is linear and R L > 1 corresponds to unfavourable interaction. From Supplemental Fig. S1, it has been found that the calculated R L ranged between 0.01 and 0.1 at the initial concentration range of 50-500 mg/L, indicating favourable of the tested adsorption system. Adsorption kinetics and mechanism study. Adsorption kinetic represents a variable tool elucidating the residence time and controlling step of the adsorptive interaction. The equilibrium data was simulated by pseudo-first order, pseudo-second order and Elovich kinetic equations by adopting the linear plots of ln (q e -q t ) against t, t/q t versus t, and q t against lnt, respectively. The adsorption data given in Supplemental Table S1 showed a good compliance with the pseudo-second order equation, with the highest R 2 and lowest Δq (%) of 0.999 and 1.08-5.32%, respectively. This suggested that chemisorption may be the rate-limiting step, with electrons sharing between MB cations and the hydrophilic site of BB. However, the experimental q e results were deviated significantly from the theoretical value with the pseudo-first order and Elovich kinetic models, implying more than one-step may be involved in the adsorption MB molecules onto BB. Similar correlation has been recorded by previous researches for the adsorption of MB dye onto different functionalized adsorbents 23,42 . The diffusion mechanism was further accessed using the Weber and Morris 18 intraparticle diffusion model, which describes the time-dependent diffusion of adsorbate components of the adsorption process. The model implies that particle diffusion is the controlling step if the relationship between the adsorbed adsorbate per unit mass of adsorbent (q t ) versus the square root of time (t 1/2 ) provide a straight line, and passes through the origin 43 . Specifically, external diffusion, surface diffusion or pore diffusion have been identified to be the major steps, driven by the ion exchange, complexation, precipitation or a combination of their interactions 44 . www.nature.com/scientificreports/ The instantaneous adsorption of the first 40 min may be corresponded to the mass transfer of adsorbate molecules from the bulk solution onto the adsorbent surface, subsequent by gradual adsorption stage of the external mass transfer resistance, that was mainly intraparticle diffusion controlled. The alteration is related to the coupling between solid and liquid phases, or the initial and boundary of the interactive system 45 . The final equilibrium stage is the last stage, where the adsorption process started to slow down, and reached to a plateau. From the presented findings, the greater adsorption rate was observed at the initial stage, and it reduced gradually as the equilibrium approached. This changing equilibrium behaviour could be resulted from the dimerization of the MB molecules at the higher initial concentration range or ionization of the MB molecules, with one or more reactions took place during the adsorption interaction 28 .
The results summarized in Supplemental Table S2 provide good agreement that intraparticle diffusion represents the rate controlling step of the adsorption process, supported by the plots q t versus t 1/2 plots (Fig. 4a) that moved beyond the origin, and the linear straight lines yields the intercepts C i . This intercept C i reveals the growing thickness of the boundary layer, with larger the C i value, indicates the greater effect 46 . If the diffusion could be identified, the boundary layer may be seen as a viscous drag between the adsorbate and BB over the bulk surface, and the higher adsorptive uptakes could be represented by the rising C i values.
The experimental data were further fitted to the Boyde's equation 47 for predicting the rate controlling phase of the adsorption process. Accordingly, the rate limiting step is film-diffusion or chemical reaction controlled if the plot is linear or non-linear, but do not pass through the origin. Conversely, intraparticle diffusion is the rate controlling step if a straight line passes through the origin is produced. As demonstrated by Fig. 4b, the plots with a linear profile but did not pass through the origin, verified that external transport to be the major rate limiting step, due to the major governance by film diffusion, and external transports at the surface could be more prominent than the internal transport, with the assumption that the controlling mechanism at the surface of BB was a result of the chemical interaction.
Thermodynamic study. The energy change in term of thermodynamic consideration represents an important marker for the practical application, and provides additional knowledge underlying the inherent energetic www.nature.com/scientificreports/ different during the adsorptive interaction 48 . The Gibbs free energy change, ΔG°, indicates the spontaneity of a reaction and therefore justifies the viability of the adsorptive system. The ΔG° for the adsorption system at the tested temperature range were computed by Eq. (20), as depicted in Table 3. Increasing the adsorption temperature from 30 to 50 °C, recorded a steadily decrease of ΔG° from − 6.38 to − 9.58 kJ/mol, illustrating greater feasibility of the adsorption at the higher temperature range, indicative of spontaneous nature and low activation energy of the adsorptive interaction. The results were supported by the rising Q 0 from 318.38 to 357.14 mg/g. At the low temperatures region, the competition between the MB and water molecules took place within the interlamellar surface, resulted from the strong hydrophilic feature of the clay layer, and an intensive electrostatically interaction. In this sense, the high charged trimmers (MB + ) 3 may involve in ion pairing mechanism for the intensive interaction surface within the active sites of the clay minerals at the low temperature region. The rising temperature however could facilitate the formation of well-defined adsorbed molecules around the exchange sites, driven by delamination of the clay particles, mainly attributed to the greater interactions and changing swelling properties of the bentonite surface. The physical or chemical interactions depends on the magnitude of ΔG°, in which −20 kJ/mole corresponded to the spontaneous physical processes, and the ΔG° between − 80 to − 400 kJ/mole is denoted to the chemisorption process 30 . The acquired ΔG° for the adsorption system ranging from − 6.38 to − 9.58 kJ/mole, revealed that it is dominated primarily by the physical adsorption mechanism. This similar trend has been supported by Alhumaimess 49 , Oukil et al. 50 , and Uyar et al. 51 for the adsorption of MB onto vermiculate, modified HUSY zeolite and an amorphous mixture of γ-alumina and silica, and alginate-clay quasi-cryogel beads, respectively. The values of ΔS° derived from the plot of ΔG° versus T was found to be 159.55 J/mole K. The positive standard entropy (ΔS°) may be related to the hydration of dye cations, reorientation, and restructuring of the clay platelets, and rocketed the high affinity of BB for MB molecules, with structural changes in dyes and BB surface 52 . This positive entropy change was ascribed to the increasing disorder, driven by the delamination of BB, with the formation of an ordered system.
Morphological structure and surface characteristics. The fundamental physical properties and morphological structure of the functionalized adsorbents was accessed in term of Scanning Electron Microscopy (SEM), as presented in Fig. 5. The smooth appearance of the raw bentonite surface could be closely related to the packed flakes structures. In contrast, BB demonstrated a well developed meso and microstructural porosity. The incorporation of accumulated sodium salt crystals implied a detrimental effect on bentonite structure, to generate a siliceous skeleton with wide quantity of open-air voids, characterized by a broad range of mutual bonds and interfacial zones. The smectite leaflets have been completely de-structured after the chemical modification, with the collapse of the interlayer spaces within the raw bentonite.
The FTIR spectra of the raw bentonite and BB are given in Supplemental Fig. S2. The detected OH stretching of the adsorbed water could be observed within the multiple broad band of 3400-3650 cm −1 , implied the presence of two types of OH groups related to the hydrogen bonding and isolated OH-group. The shifted from 3622 to 3624 cm −1 , and 3429 cm −1 to 3436 cm −1 of adsorption band after the modification could be attributed to the structural OH groups of BB. The aliphatic hydrocarbons of the raw bentonite is ascribed to the sharp peaks at 2925 cm −1 and 1431-1433 cm −153 , while the OH group and Al-Al-OH bending resulting from the dioctahedral montmorillonite 2:1 layer were detected at 1638-1639 cm −1 and 916/917 cm −154 . The strong absorption band at 1035 cm −1 and the sharp peak at 796-797 cm −1 are due to the Si-O, and the quartz admixture of the clay based-adsorbent, while the intensity at 695-694 cm −1 is driven by the deformation of the Si-O bond. The tetrahedral bending modes of Si-O-Al and Si-O-Si could be illustrated at 523 and 466-467 cm −1 . From the presented analysis, the substitution of the sodium salt of the raw metal ions in the interlayer clay structure, or located into the Si-O sheets hexagonal cavities induced changes in the Si-O vibration modes of the bentonite have led to the reconstruction of the tetrahedral sheets either in the hexagonal holes, or in the previously vacant octahedral sites. The result was supported by Jawad and Abdulhameed 55 , who reported the potential of silanol (≡SiOH), aluminol (≡AlOH, and hydroxyl (-OH) groups as the potential active sites within the mineral edges. Accordingly, electrostatic attraction is considered to be one of the most impactful interaction between the MB dye molecules with the clay based adsorbent. Other governing mechanisms for sorption process are H-bonding between the H atom available on the surface of BB, and the N within atom in the MB dye structure, and n-π between the delocalization of the lone pair electron of O atoms into the π orbital of the dye aromatic rings 56 .
Impenetrability of nitrogen to the interlayer space of clay minerals represents an appropriate method in describing the chemical transformation of the external surface of the functionalized materials. The detailed of porosity structures of the raw bentonite and BB are summarized in Supplemental Table S3. It was evident that a greater porosity development was found at BB, with the higher BET surfaces area of 426.91 m 2 /g, Langmuir surface area of 539.02 m 2 /g, and total pore volume of 0.225 cm 3 /g, respectively, as compared with the raw bentonite. The greater porosity structure could be ascribed to the considerable amount of trapped pores in the range of micro and medium mesopores (20-400 Å) of BB, and this ion valence behaviour is expected to be more important in improving the textural properties of the clay derivatives 57 . The porosity development is interrelated to the www.nature.com/scientificreports/ stacking level of different elementary layers, to support the formation of outer sphere and adsorption process. The intercalation of the Na + cations during the modification has reduced the number of macropores and large mesopores (> 400 Å), resulting in the dramatic improvement of the textural properties of the raw bentonite. Pore size distribution (PSD) is a model of adsorbent internal structure, elucidating the complex void spaces within the functionalized adsorbent, and the fraction of pore surface of a given shape and size. It is classified into micropores (d < 2 nm), mesopore (d = 2-50 nm) and macropore (d > 50 nm) according to the classification of International Union of Pure and Applied Chemistry (IUPAC) pore dimensions. From the presented pore size given in Supplemental Fig. S3 justified by the Density Functional Theory model, intensive peaks of the pore diameter (40-70 Å) were detected at a vast majority of the mesoporous region, which are capable to support the overall adsorption process. The rising pore volume under NaCl treatment could be driven by the production of finely scattered Si oxides, destruction and leaching of ion mineral, removal of silica compound or X-ray amorphous aluminium, blocking the interlamellar spaces and surface porosity by the voids, cracks or decrease in the mineral particle sizes, in which a larger proportion of the newly formed surfaces may be connected to new pore walls.

Conclusion
In this work, the sodium salt induced activation of bentonite derived adsorbent at low temperature has been attempted. The specific surface area, and total pore volume was identified to be 426.91 m 2 /g and 0.225 cm 3 /g, respectively. Adopting methylene blue as the model adsorbate, the best chemical impregnation ratio at 1:3 resulted in a monolayer adsorption capacity of 318.38 mg/g, well described by the Langmuir isotherm and pseudo-second order kinetic equations. The adsorption process was feasible, endothermic and spontaneous in nature, supported by different surface active sites, notably silanol, aluminol, and hydroxyl groups. The presented results provide a new insight into the sodium salt assisted activation of high-quality clay-based adsorbents, with a reliable, simple, efficient and economical approach.