Kinetic and Isotherm Studies of Ni 2+ and Pb 2+ Adsorption from Synthetic Wastewater Using Eucalyptus camdulensis —Derived Biochar

: The production of biosorbents by waste biomass has attracted considerable attention due to the low cost and abundance of the raw materials. Here biochar produced from Eucalyptus camdulensis sawdust ( EU -biochar) via pyrolysis at 600 ◦ C was used as a potential biosorbent for Ni 2+ and Pb 2+ metal ions from wastewater. Characterization experiments indicated the formation of C- and O-bearing functional groups on the EU -biochar surface, while shifts and changes in the shape of C–H bands suggested the adsorption of Ni 2+ and Pb 2+ onto EU -biochar by interacting with surface carboxylic groups. Pb 2+ was adsorbed more quickly than Ni 2+ , indicating a faster and stronger interaction of Pb 2+ with EU -biochar compared to Ni 2+ . As the initial concentrations of both metal ions increased, the percentage removal decreased, whereas increasing the EU -biochar dose improved the percentage removal but impaired the adsorption capacity for Ni 2+ and Pb 2+ . The adsorption capacity could only be improved without affecting the percentage removal of both ions by increasing the pH of the metal solutions. The sorption efﬁciency of EU -biochar and the removal mechanism of Ni 2+ and Pb 2+ were further explored using non-linear and linear forms of kinetic and isotherm models.


Introduction
Rapid industrialization and urbanization in the last century have increased the consumption of global freshwater by six times [1], leading to a shortage of fresh drinking water for the majority of the world's population [2]. It is estimated that almost 3.9 billion people will suffer from water scarcity by 2050 [3]. Anthropogenic activities have also increased the levels of unwanted contaminants in freshwater resources worldwide [4]. For instance, industrial effluents carrying various organic and inorganic pollutants contribute significantly to the contamination and depletion of freshwater reservoirs [5]. In particular, metal and metalloids, such as nickel (Ni 2+ ) and lead (Pb 2+ ), are critical ecological threats, as they are widely used in plating and tanning, mineral, machinery, battery manufacturing, and other industries [6], while the Ni 2+ -and Pb 2+ -rich industrial effluents are discharged into adjacent water resources, posing a serious threat to human and animal health. In order to address the current challenges of increasing freshwater needs, depletion of available freshwater supplies, and deterioration of clean water resources due to contamination, numerous studies have focused on developing novel technologies for removing threatening pollutants from wastewater streams.
Specifically, several methods have been developed for the removal of metal and metalloids from wastewater streams, but most of them are ineffective or expensive. Even advanced methods, such as catalytic degradation, oxidation, solvent extraction, membrane filtration, steam stripping, ion exchange, chemical precipitation, electrocoagulation, microbial degradation, and electrocoagulation, proved to be ineffective in completely removing contaminants from water [7][8][9][10][11][12]. In contrast, adsorption is a widely accepted technology for the removal of metal and metalloids from contaminated wastewater, as it is a cost-effective, highly efficient, and easy-to-use method that generates little residue and allows the recovery and recycling of the adsorbent unlike most traditional wastewater treatment methods [13,14]. Therefore, great effort has been made to develop effective, inexpensive, and environmentally friendly adsorbents that are suitable for each type of contaminant [15][16][17][18]. To date, several adsorbents, including clay, biomass, nanomaterials, layered double hydroxide, graphene, activated carbon, chitosan, and fly ash, have been successfully used for the removal of metal/metalloids from contaminated wastewater [19][20][21][22][23][24][25][26][27]. Furthermore, the production of biosorbents by reusing or recycling waste biomass and feedstock, such as agricultural waste and wood sawdust, has attracted considerable attention due to the low cost, abundance, and environmental friendliness of the raw materials [28][29][30][31]. Similarly, biochar has been produced from agricultural waste and used as an efficient adsorbent.
Biochar is a solid, black, and stable porous carbonaceous material produced by the pyrolysis of waste materials, such as wood, grass, manure, sludge, etc. [32], with little or no air at 300-800 • C [33,34]. Several studies have shown that biochar is one of the most effective adsorbents for the removal of metal or metalloids from industrial effluents due to its large specific surface area, water insolubility, richness in carbon, abundant pore structure, and active functional groups on its surface [35]. Moreover, the use of agricultural or fruit waste as the raw material not only reduces the overall costs, but also limits the pollution of surface water through the reuse of solid waste. To date, various waste materials, such as rice husk, wheat straw, wood waste biomass, and fruit waste, have been used to produce biochar to remove metals or metalloids from contaminated wastewater [29,[36][37][38][39][40].
Based on the existing data, we considered that Eucalyptus camdulensis (EU) wood biomass could serve as a low-cost raw material for the production of biochar, as its extensive use in the manufacture of furniture and other purposes generates a large amount of sawdust waste. Therefore, in this study, we used EU sawdust to produce biochar via pyrolysis (EU-biochar), which was further evaluated as a potential adsorbent of Ni 2+ and Pb 2+ in synthetic contaminated water. In addition, we applied a series of kinetic and isotherm models to study the adsorption efficiency of EU-biochar and to elucidate the removal mechanism of Ni 2+ and Pb 2+ from water.

Preparation and Characterization of Eucalyptus camdulensis (EU)-Biochar
EU-biochar was prepared from EU sawdust collected from different places in Riyadh, Saudi Arabia. The collected sawdust was first washed with tap water to remove dirt particles, followed by pre-drying under sunlight for a few days, and final drying in an ordinary oven at 60 • C for 6 h. Small pieces of the oven-dried sawdust was then crushed to achieve an average particle size of about 0.5 mm. The very small pieces of EU sawdust were finally pyrolyzed in a box furnace (Nabertherm, B−150, Lilienthal, Germany) at 600 • C for 3 h to obtain EU-biochar with an average particle size of 50 µm. The specific surface area of EU-biochar was measured by Brunauer-Emmett-Teller analysis, while the carbon and volatile contents were estimated by proximate and ultimate analysis using an elemental analyzer (Thermo Fisher Scientific, Berlin, Germany). The morphology and chemical composition of the EU-biochar surface were analyzed by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and Fourier-transform infrared (FTIR) spectroscopy.

Materials
The stock solutions (1.0 g L −1 ) of Ni 2+ and Pb 2+ were prepared by dissolving 1.0 g of Ni(NO 3 ) 2 ·6H 2 O (analytical grade, Tianjin Benchmark Chemical Reagent Co., Ltd., Tianjin, China) or Pb(NO 3 ) 2 (analytical grade, Tianjin Benchmark Chemical Reagent Co.), respectively, in 1 L double distilled water. The stock solutions were further diluted with double distilled water to prepare metal solutions with known initial concentrations. Depending on the requirements of each batch experiment, the initial pH of the metal solutions was adjusted using sodium hydroxide (0.1 M) or hydrochloric acid (0.1 M).

Batch Experiments and Adsorption Performance
For the batch experiments, each metal solution (50 mL) was homogeneously mixed with the required amount of EU-biochar for a predetermined time in 100 mL glass funnels in a temperature and humidity controlled shaker. The samples were then filtered, and the residual metal concentration (C t , mg L −1 ) was measured using an ultraviolet-visible (UV-Vis) spectrophotometer, while the amount of metal uptake (q t , mg g −1 ) and the percentage removal (R, %) by EU-biochar were estimated using the following equations: where C in (mg L −1 ) is the initial metal concentration in the solution with volume V (L) and M (g) is the mass of EU-biochar. Equation (1) was also used to measure the equilibrium adsorption capacity (q e , mg g −1 ) against the equilibrium metal concentration (C e ; mg L −1 ) in the solution. Each batch test was repeated in triplicate to reduce any potential experimental error (Figures 3 and 4). The efficiency of the batch process was optimized using a contact time in the range of 1-300 min, an adsorbent dose in the range of 0.05-0.8 g, a solution pH in the range of 2.0-10.0, and an initial metal concentration in the range of 5-100 mg L −1 (Figures 3 and 4).

Adsorption Kinetic and Equilibrium Isotherm Models
The pseudo-first-order (PFO), pseudo-second-order (PSO), Elovich, and intraparticle diffusion of Weber and Morris (ID-WM) kinetic models and the Langmuir, Freundlich, Temkin, Halsey, Dubinin-Radushkevich (D-R), Harkins-Jura (H-J), and Jovanovic isotherm models were used in their linear and non-linear forms to elucidate the adsorption properties and adsorption mechanism of EU-biochar ( Table 1). The values obtained for the corresponding parameters in each model using linear and non-linear regression analysis ( Figures 5 and 6) are presented in Tables 2 and 3.
The adsorption data of both metal ions were analyzed using the non-linear and linear forms of four different kinetic models (Table 1), and the correlation coefficient (R 2 ) of each model was determined to compare their suitability in fitting the adsorption data. The parameters of the non-linear form were calculated in the OriginPro 8.5 software, while straight-line curve fitting in Microsoft Excel was used for the linear form. In order to model the adsorption data, four different initial concentrations of each metal ion with an interval of 10 mg L −1 were selected (Ni 2+ : 10-40 mg L −1 and Pb 2+ : 20-50 mg L −1 ). Table 3 presents the variables calculated for each kinetic model for the selected metal ion concentrations, i.e., 20 and 40 mg L −1 . The EU-biochar dose used for the adsorption of Ni 2+ and Pb 2+ was kept at 0.4 and 0.2 g, respectively, while the pH of all metal solutions was maintained at 6.0 ± 0.2. Table 1. Linear and non-linear equations of kinetic and isotherm models.

Model Non-Linear Form Linear Form
Elovich Freundlich Temkin Jovanovic ln q e = ln q m − k j C e q e = q m 1 − exp k j C e

Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray (EDX) Spectroscopy Analysis
SEM analysis revealed that the outer surface of EU-biochar was rough and consisted of abundant pore structures, which may favor the adsorption of Ni2+ and Pb2+ (Figure 1a). In contrast, very small particles and some brighter zones were observed onto the adsorbent surface after the metal ion adsorption (Figure 1b,c), which could be assigned to Pb2+ and Ni2+ particles [41]. The variations in the elemental compositions of EU-biochar before and after the adsorption of Ni2+ and Pb2+ were further observed by EDX spectroscopy (Figure 1). The C content of EU-biochar increased from 63.05% to 88.14% and 73.51% after the adsorption of Ni 2+ and Pb 2+ , respectively, whereas its O content decreased from 12.85% to 10.76% and 3.54%, respectively. Additionally, the adsorption of Ni 2+ and Pb 2+ onto the adsorbent was confirmed by the presence of 0.78% Ni 2+ and 19.71% Pb 2+ . These results suggested that complexes bearing C-and O-containing functional groups were generated on the biochar surface, successfully contributing to the sorption of Ni 2+ and Pb 2+ .

Fourier Transform Infrared (FTIR) Spectroscopy Analysis
The functional groups and the chemical composition of the free EU-biochar surface were investigated by FTIR spectroscopy. The FTIR spectra of Ni 2+ -and Pb 2+ -loaded EUbiochar were also recorded to further investigate the binding interactions between the adsorbent and the metal ions ( Figure 2). Interestingly, no noticeable and sharp bands were observed in the recorded FTIR spectra, probably due to the high pyrolysis temperature used for the production of EUbiochar (600 • C). It has been previously reported that pyrolytic temperatures ≥500 • C can destroy most functional groups of the biochar surface due to thermalization [42]. Nevertheless, a vibrational band with very low intensity was identified at 877 cm −1 in the FTIR spectrum of free EU-biochar, which was attributed to aromatic C-H bonds. After Ni 2+ adsorption, a slight shift from 877 to 876 cm −1 was observed, whereas no similar bands were detected in the spectrum of Pb 2+ -loaded EU-biochar, suggesting that the surface carboxylic groups were involved in the adsorption of Ni 2+ and Pb 2+ . Moreover, after the adsorption of Ni 2+ and Pb 2+ , very weak stretching vibration bands appeared at 1534 and 1403/683 cm −1 , respectively, which corresponded to carboxylate and aromatic C-H groups. A weak stretching vibration peak was also detected in the spectrum of free EU-biochar at 2355 cm −1 corresponding to C=N (or CO 2 impurity), which shifted to 2353 cm −1 after Pb 2+ adsorption, but remained at the same position after Ni 2+ adsorption. Taken together, the adsorption of Ni 2+ and Pb 2+ onto EU-biochar was achieved through their complexation with the carboxylic groups of the biochar surface.

Adsorption Equilibrium and Optimization of Contact Time, Initial Metal Ion Concentrations, EU-Biochar Dose, and Solution pH
The changes in the uptake of both metal ions (for selected initial concentrations) and the removal efficiency of EU-biochar were investigated in relation to the contact time (0-300 min) using samples with four different initial metal concentrations that were collected at regular intervals. During these experiments, the solution pH was kept constant at 6.0 and the EU-biochar dose for the adsorption of Ni 2+ and Pb 2+ was set at 0.4 and 0.2 g, respectively.
Upon immediate contact of the sample solutions with EU-biochar, the percentage removal and metal uptake increased rapidly due to the abundant active sites on the EUbiochar surface that allowed the development of numerous interactions [43], while a gradual increase was observed in both parameters as the contact time increased from 10 to 30 min (Figure 3). At this point and when the initial metal concentration was 20 mg L −1 , the percentage removal for Ni 2+ and Pb 2+ was 73% and 95%, respectively, corresponding to an adsorption capacity of 36 and 129 mg g −1 . However, after 30 min, the adsorption performance remained stable with insignificant (p = 0.01) or no changes in the percentage removal or the metal ion uptake by EU-biochar due to the increasing interference of solutesolute interactions. Therefore, we considered that the adsorption equilibrium was reached at 30 min. Similar changes were also observed when other initial metal concentrations were used (Figure 3). For instance, when the metal ion concentration was 40 mg L −1 , the percentage removal for Ni 2+ and Pb 2+ reached 48% and 73%, respectively, corresponding to adsorption capacities of 47 and 183 mg g −1 . Although the half dose of EU-biochar (0.2 g) was used for the adsorption of Pb 2+ compared to Ni 2+ (0.4 g), the percentage removal of Pb 2+ was higher and more uptake points were identified, suggesting a faster interaction between Pb 2+ and the biochar surface, as well as a stronger deposition of Pb 2+ on the adsorbent surface compared to the interactions between Ni 2+ and EU-biochar. At the adsorption equilibrium (contact time, 30 min) and a constant solution pH of 6.0, the percentage removal decreased linearly from 92% to 27% for Ni 2+ and from 95% to 37% for Pb 2+ as the initial metal ion concentrations increased from 5 to 80 mg L −1 and from 20 to 100 mg L −1 , respectively (Figure 4a). This reduction could be attributed to the unavailability or saturation of the active sites of EU-biochar with increasing metal ion concentration and to the reduction in the ratio of fixed number of active sites to initial number of metal ions [44][45][46][47]. Moreover, the adsorption capacities showed an increasing trend with increasing initial concentrations and reached a maximum of 54 and 200 mg g −1 for Ni 2+ and Pb 2+ , respectively, at an initial concentration of 80 mg L −1 . This change was attributed to the increased metal ion uptake, which resulted from the high mass transfer and the reduced ionic strength of the solution at high initial concentrations. However, the uptake of metal ions started to decrease after Ni 2+ and Pb 2+ concentrations of 70 and 80 mg L −1 , respectively, suggesting that these values were the optimal concentrations for efficient adsorption performance of EU-biochar. Due to its direct impact on the adsorption performance, the EU-biochar dose for the batch experiments was optimized using ranges of 0.1-0.8 g for Ni 2+ and 0.05-0.3 g for Pb 2+ to enhance its removal efficiency (Figure 4b). In addition, the solution pH was set at 6.0, and the initial concentrations of Ni 2+ and Pb 2+ were set at 20 and 40 mg L −1 , respectively, and the samples were shaken for 30 min. Our results indicated that, with increasing EU-biochar dose from 0.1 to 0.8 g, the adsorption capacity of 20 mg L −1 Ni 2+ decreased by 59% (from 59 to 24 mg g −1 ), while the percentage removal increased from 30% to 100%. In contrast, the adsorption capacity of EU-biochar for 40 mg L −1 Pb 2+ was reduced by about 64% (from 364 to 132 mg g −1 ) as the EU-biochar dose increased from 0.05 to 0.3 g, while the percentage removal increased from 46% to 100%. Therefore, 0.6 g and 0.15 g were the optimal EU-biochar doses to achieve the optimal percentage removal and adsorption capacity for Ni 2+ and Pb 2+ , respectively. We also reasoned that a high amount of EU-biochar would provide a larger surface area and a higher number of active sites for the selected metal ion concentrations, resulting in less significant changes in the percentage removal.
A pH range of 2.0-10.0 and 2.0-8.0 was also selected to determine its effect on the adsorption performance after mixing 20 and 40 mg L −1 Ni 2+ and Pb 2+ with 0.4 and 0.2 g EU-biochar, respectively, for 30 min. In particular, as the pH of the Ni 2+ solution increased from 2.0 to 10.0, the adsorption capacity of EU-biochar for Ni 2+ increased from 9 to 48 mg g −1 and the percentage removal increased from 17% to 99% (Figure 4c). The low initial adsorption capacity and percentage removal were attributed to the excess amount of positively charged H + (at low pH values), which competed with the positively charged metal ions for adsorption on the negatively charged biochar surface [48,49]. Similarly, the adsorption capacity of EU-biochar for Pb 2+ increased from 34 to 196 mg g −1 with increasing pH, while the percentage removal showed the same trend as that of Ni 2+ due to the improved electrostatic interactions between the negatively charged biochar surface at high pH values and the positively charged Pb 2+ (Figure 4c) [49][50][51]. A comparison of the adsorption capacities of EU-biochar with that of other adsorbents for the studied metal ions is presented in Table 2.

Evaluation of the Adsorption Data through Kinetic Fitting Models
Very low R 2 values (0.2-0.36) in the linearized PFO kinetic model reflected a poor fitting of the model to the adsorption data of both metal ions, confirming its applicability only for the initial stage (contact time) of adsorption [57,58]. The non-linear form of the PFO model showed a better fitting to the adsorption of Ni 2+ than that of Pb 2+ , especially at high initial concentrations (30 and 40 mg L −1 ). Moreover, the estimated adsorption capacities (q e cal ) were considerably lower than the experimental values in the linear PFO model, whereas the q e cal values in the non-linear form were slightly lower than the experimental values except for the adsorption of Pb 2+ at 40 mg L −1 . In addition, the rate constant k 1 was significantly lower in the linear than in the non-linear form of the PFO model with the Pb 2+ adsorption showing higher values than Ni 2+ ( Table 3).
The adsorption data of all tested Ni 2+ and Pb 2+ concentrations were perfectly fitted to the linear form of the PSO kinetic model with R 2 values of~1.0 (Figure 5a). Similar results were obtained for the adsorption of Ni 2+ (R 2 = 0.94-0.99) using the non-linear form of the PSO model, while the adsorption data of Pb 2+ were good fitted (R 2 = 0.81-0.88), confirming the chemisorption of both metal ions onto EU-biochar, consistent with previous studies [59][60][61]. The rate constant k 2 in the PSO kinetic model decreased with increasing initial concentrations of both metal ions, but a reverse trend was observed for the linearized fitting to the adsorption data of Pb 2+ . However, the correlation of the initial adsorption rate (h) with the initial metal ion concentrations could not be predicted easily. In particular, the h value was higher for 20 mg L −1 Ni 2+ than for 40 mg L −1 Ni 2+ , but an opposite trend was observed for Pb 2+ using both forms of the PSO kinetic model (Table 3). Moreover, the q e cal value was closer to the experimental value only in the non-linear PSO model, but deviated from the experimental value by about 7% for 40 mg L −1 Pb 2+ in both model forms.  Both the linear and non-linear forms of the Elovich kinetic model yielded good R 2 values (0.80-0.90) for all tested initial concentrations of Ni 2+ and Pb 2+ , suggesting that both metal ions were adsorbed via activated chemical adsorption without desorption (Figure 5b) [43]. However, in most cases, the relationship of the initial adsorption rate constant (α) with the initial concentrations could not be established by estimating its highest values at low initial concentrations. Moreover, based on the calculated β values in linearized fitting, the number of sites available for adsorption [62] and the activation energy of chemisorption [63] increased with increasing initial metal ion concentrations, whereas non-linear fitting showed an opposite trend corresponding to the decreasing β values (Table 3).
Furthermore, the ID-WM kinetic model yielded high values of parameter C, an indicator of the boundary layer thickness, suggesting a greater boundary layer effect on the adsorption process at high initial metal ion concentrations. The model rate constant K ip also increased as the initial metal ion concentrations increased in both model forms. However, the R 2 values ranged between 0.50-0.63 (Table 3) and did not allow the prediction of multilayer adsorption of Ni 2+ and Pb 2+ onto EU-biochar.

Evaluation of the Adsorption Data through Isotherm Fitting Models
Several two-parameter isotherm models were also applied to the adsorption data of Ni 2+ and Pb 2+ (Table 1), and the adsorption performance of Ni 2+ (5-80 mg L −1 ) and Pb 2+ (20-100 mg L −1 ) onto 0.4 and 0.2 g EU-biochar, respectively, at a solution pH of 6.0 ± 0.2 was assessed using both the linearized and non-linear forms of the models ( Table 4). The regressions of the linear and non-linear forms of isotherms were solved using the OriginPro 8.5 and Curve Expert Professional software, respectively. Based on the experimental data, the maximum adsorption capacities were achieved when the initial concentrations of Ni 2+ and Pb 2+ were 80 and 100 mg L −1 , respectively. Therefore, the maximum adsorption capacities at 80 mg L −1 were selected to compare the theoretical calculated values of both metal ions in each model.
The Langmuir model proved to be promising in explaining the monolayer adsorption of both metal ions due to the high R 2 values (0.96-0.99) and nearly perfect fitting using both regression forms (Figure 6a). The estimated maximum adsorption capacity of the model (q m ) was close to the experimental values for both metal ions (54 mg g −1 for Ni 2+ and 200 mg g −1 for Pb 2+ ) with a slightly better agreement of the non-linear form (Table 4). Lower values of the Langmuir constant (K L ) were also found in the non-linear form for both metal ions (Table 4), while K L was almost double for Pb 2+ (0.81 L mg −1 ) than for Ni 2+ (0.41 L mg −1 ), implying a higher affinity of Pb 2+ for EU-biochar compared to Ni 2+ . In addition, the dimensionless separation factor, R L [(1 + K L C 0 ) −1 ], had almost the same value for both the linearized and non-linear Langmuir model, suggesting a similar isotherm shape for both model forms [64]. This similarity was, however, not observed for the adsorption of Ni 2+ onto EU-biochar.
The q m values of the non-linear and linear Freundlich isotherm model were by 4% and 8% higher than the experimental values for the adsorption of Pb 2+ , respectively, while an overestimation of 17% and 10% was observed for Ni 2+ (Table 4). However, the adsorption data of Ni 2+ were well fitted to the model with R 2 values ranging between 0.94-0.95 (Figure 6b), while a reasonable fit to the data of Pb 2+ was also achieved with R 2 of 0.85 and 0.81 for the linear and non-linear form, respectively. The suitability of the model was further confirmed by the dimensionless factor (n), which was >1.0 for both metal ions, suggesting the surface heterogeneity of EU-biochar (1/n = 0-1.0) [65]. Moreover, the model constant K F was higher in the non-linear form for both metal ions, but deviated significantly from the favorable range [1-20 (mg g −1 ) (L mg −1 ) 1/n ], except for the adsorption of Ni 2+ in the linear form ( Table 4), suggesting that the Freundlich isotherm model could not be used to predict metal ion adsorption. Compared to the Freundlich isotherm model, the Temkin model provided a better fit to the adsorption data of both metal ions with a perfect fit to the adsorption of Ni 2+ (R 2 = 0.99), reflecting the heterogeneous adsorption of Ni 2+ onto EU-biochar with uniform distribution of binding energies [66]. In addition, a considerably lower binding constant at equilibrium (K T ) and a higher heat of adsorption (H ads ) were observed for the adsorption of Ni 2+ compared to the adsorption of Pb 2+ ( Table 4).
The Halsey model yielded exactly the same R 2 values as the Freundlich model with higher R 2 for the adsorption of Ni 2+ than Pb 2+ and slightly better results in the linear than the non-linear form. The q e cal values of the non-linear form were closer to the experimental values compared to the linear form. In addition, the model constant and exponent (K H and n H , respectively) were higher for the adsorption of Ni 2+ than for the adsorption of Pb 2+ , while the linearized fitting yielded higher K H values than the non-linear fitting (K H = 0) for both metal ions ( Table 4).
The D-R isotherm provided good R 2 values (0.81-0.86) for both metal ions, while linear fitting yielded slightly higher R 2 values than the non-linear fitting. However, the theoretical q e cal of the non-linear form agreed more with the experimental values compared to that of the linear form for both metal ions (Table 4). Considering also the calculated values of the mean free energy of adsorption [E = (2K DR ) −1/2 , <8 kJ mol −1 ] resulting from the significantly low values of the model constant (K DR~0 , Table 4), the D-R model suggested that both metal ions were physisorbed onto EU-biochar. A reasonable fit to the adsorption data of Ni 2+ was also estimated by the non-linear H-J isotherm with R 2 = 0.86 (Table 4), supporting its multilayer adsorption onto EU-biochar with heterogeneous pore distribution, similar to the Freundlich and Halsey models. The model constant A HJ was much higher in the linear than in the non-linear form for both metal ions, while A HJ and B HJ (model constant) were significantly higher for the adsorption of Pb 2+ than for the adsorption of Ni 2+ . In addition, the B HJ values in the non-linear H-J isotherm were almost double the values in the linear isotherm for both metal ions.
The linear form of the Jovanovic isotherm was not satisfactory due to the very low R 2 values (0.55 and 0.46 for Ni 2+ and Pb 2+ , respectively) and the great deviation of the theoretical from the experimental q m values. The model constant K j was higher for the adsorption of Ni 2+ than for the adsorption of Pb 2+ in the linear form of the model, while a reverse trend was observed in the non-linear form (Table 4). In addition, the non-linear Jovanovic model provided satisfactory results with R 2 values of 0.91 and 0.87 and slightly underestimated theoretical q m values of about 6% and 8% compared to the experimental values for Ni 2+ and Pb 2+ , respectively. Thus, similar to the Langmuir model, the nonlinear Jovanovic model supported the localized monolayer adsorption of both metal ions onto EU-biochar without lateral interactions, suggesting the development of mechanical interactions between the adsorbed and desorbed molecules [67,68].

Conclusions
EU-biochar produced via thermal degradation from EU sawdust was used as a potential adsorbent for Ni 2+ and Pb 2+ contaminants in synthetic wastewater. EDX and FTIR spectroscopy demonstrated that the metal ions were successfully adsorbed by interacting with the carboxylic groups on the biochar surface. Further experiments on the uptake of both metal ions and the removal efficiency of EU-biochar indicated that the adsorption equilibrium was reached at 30 min, while Pb 2+ was removed faster than Ni 2+ , suggesting the development of stronger and faster interactions between Pb 2+ and the EU-biochar surface. Additional studies by varying the initial metal ion concentration, the adsorbent dose, and the pH of the metal solutions indicated that the simultaneous improvement of the adsorption capacity of EU-biochar and the corresponding percentage removal for Ni 2+ and Pb 2+ could only be achieved by increasing the pH of the metal solutions from 2.0 to 10.0 and from 2.0 to 8.0, respectively.
A series of kinetic and isotherm fitting models were further employed to study the removal mechanism of Ni 2+ and Pb 2+ . The adsorption data fitted well to the linear and nonlinear forms of the PSO and Elovich kinetic models, suggesting activated chemisorption as the removal mechanism of both metal ions. Among the applied isotherm models, the Langmuir model could adequately explain the monolayer adsorption of both metal ions, while the Temkin model fitted perfectly the adsorption data of Ni 2+ , suggesting its heterogeneous adsorption onto EU-biochar. The non-linear D-R isotherm also suggested that both metal ions are adsorbed onto EU-biochar via physisorption, while the nonlinear H-J isotherm supported the multilayer adsorption of Ni 2+ with heterogeneous pore distribution. Finally, the non-linear Jovanovic model supported the localized monolayer adsorption of both metal ions onto EU-biochar without lateral interactions.