Preparation of a Novel Activated Carbon from Cassava Sludge for the High-Efﬁciency Adsorption of Hexavalent Chromium in Potable Water: Adsorption Performance and Mechanism Insight

: Particularly, because of the leakage risk of metal elements from sludge carbon, little attention has been focused on using sludge activated carbon as an adsorbent for the removal of Cr (VI) from contaminated water sources. Herein, a novel sludge carbon derived from dewatered cassava sludge was synthesized by pyrolysis using ZnCl 2 as an activator at the optimal conditions. The prepared sludge activated carbon possessed a large BET surface (509.03 m 2 /g), demonstrating an efﬁcient removal for Cr (VI). Although the time to reach equilibrium was extended by increasing the initial Cr (VI) concentration, the adsorption process was completed within 3 h. The kinetics of adsorption agreed with the Elovich model. The whole adsorption rate was controlled by both ﬁlm and intra-particle diffusion. The Cr (VI) removal efﬁciency increased with elevating temperature, and the adsorption equilibrium process followed the Freundlich isotherm model. The adsorption occurred spontaneously with endothermic nature. The removal mechanism of Cr (VI) on the prepared sludge activated carbon depended highly on solution pH, involving pore ﬁlling, electrostatic attraction, reduction, and ion exchange. The trace leakage of metal elements after use was conﬁrmed. Therefore, the prepared sludge activated carbon was considered to be a highly potential adsorbent for Cr (VI) removal from contaminated raw water.


Introduction
Hexavalent chromium (Cr (VI)) has been widely applied in petroleum, oil refining, electroplating, tanning, metallurgy, fertilizer, and other industrial processes [1,2]. Over the past two decades in many developing countries, raw water sources have been frequently contaminated by wastewater and solid waste containing Cr (VI) due to abnormal discharge [3]. The total Cr (VI) in potable water has been limited within the range of 0.05 mg/L by WHO due to its carcinogenicity, teratogenicity, and mutagenicity [4]. For raw water contaminated by Cr (VI) due to sudden pollution accidents, traditional drinking water treatment processes, comprising coagulation, sedimentation, filtration, and disinfection, are poorly effective for Cr (VI) removal. For example, local potable water treatment plants were closed for nearly ten days due to water pollution caused by Cr (VI) in Qujing city, China [3]. Hence, it is essential to search for a high-efficiency treatment technology to remove Cr (VI) from contaminated raw water sources.
For eliminating Cr (VI) from industrial wastewater, many researchers have reported different technologies, such as bioremediation [5], reduction-coagulation (chemical or (NaOH), hydrochloric acid (HCl), and Zinc chloride (ZnCl 2 ) were obtained from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Only ZnCl 2 was of analytical reagent grade, and the other reagents were all in guaranteed grade.

Preparation Process of ACDCS and Parameters Optimization
The preparation of ACDCS was achieved by chemical activation using DCS as a raw material and zinc chloride (ZnCl 2 ) as an activator. The preparation process was as follows. A certain weight of DCS particles with different sizes (20-50, 50-100, 100-200, and >200 mesh) was immersed into a certain volume of ZnCl 2 solution with different concentrations (10%, 20%, 30%, 40%), subsequently activated at 333 K for a period (6, 12, 18, 24 h) in an Electro-thermostatic Water Cabinet (HWS24, Shanghai Yiheng Scientific Instrument Co., Ltd., Shanghai, China). Then, the activated DCS was pyrolyzed and carbonized in a muffle furnace (KSL-1200X, Hefei Kejing Material Technology Co., Ltd., Hefei, China) at the preset temperature (673, 773, 873, 973 K) for the desired retention time (30, 60, 90, 120 min). After cooling to room temperature, the obtained samples were fully pickled with 1.0 mol/L HCl solution, and then repeatedly rinsed with ultrapure water until turned neutral. After washing, the samples were dried, crushed, and ground into powder. The desired products were labeled as ACDCS, where (a) was DCS mesh, (b) was the activation time, (c) was the weight concentration of the ZnCl 2 solution, (d) was the carbonization temperature, and (e) was the carbonization time. For comparison, the unpretreated DCS, using Zinc chloride, produced sludge activated carbon according to a similar process as above, and the product was recorded as CDCS.
The orthogonal test method can optimize the process parameters of producing ACDCS by simplifying the multi-factor and multi-level experiment in this study. Based on literature and the preliminary experimental study, process parameters, including DCS particle size, activation time, weight concentration of ZnCl 2 solution, carbonization temperature, and carbonization time, play a key role in the performance of ACDCS . Consequently, the parameters above were chosen as factors, each factor was set at four value levels, and an L 16 (5 4 ) orthogonal table was designed. Furthermore, the value of iodine adsorbed on the prepared ACDCS (named as iodine value in this study) as a response, was used as an index for evaluating the performances of the prepared ACDCS.

Characterizations of ACDCS
The iodine number of ACDCS was determined according to the national standard testing method for wood activated carbon-Determination of iodine number (GB/T 12496. . The BET surface area and pore characteristics of ACDCS were measured by nitrogen adsorption using a specific surface area and porosity analyzer (ASAP 2460, Micrometrology, Norcross, USA) at 77K. The surface morphologies of the prepared ACDCS were evaluated using SEM (JSM-6490LV, Akishima-shi, Japan) to evaluate the ACDCS elemental constitution. The functional groups on the surface of each sample were analyzed before and after modification in the frequency range of 4000-400 cm −1 by Fourier transform infrared spectroscopy (Nicolet 6700, Nicolet, Madison, MI, USA). In addition, the zeta potentials of the ACDCS were analyzed by a Malvern Zetasizer Nano ZS90 (Malvern Instruments Ltd., Malvern, UK). X-ray photoelectron spectroscopy with the Al K-Alpha spectrometer (Axis Ultra DLD, Shimadzu-Kratos, Manchester, UK) was utilized to reveal the valence of elements and compositions of the samples' surface. The metal contents in leachate solution from the DCS and ACDCS were detected by an inductively coupled plasma-atomic emission spectrometer (ICP-AES, ThermoFisher, Waltham, MA, USA).

Adsorption Experiment
A definite amount of K 2 Cr 2 O 7 (2.829 g) was dissolved in 1 L ultrapure water to prepare Cr (VI) stock solution (1000 mg/L). Cr (VI) solutions with the desired concentration used as model water samples in this study were obtained by diluting stock solution. A certain amount of ACDCS was added into a series of Erlenmeyer flasks containing 500 mL of Cr (VI) solution with the desired concentration. These flasks were sealed and shaken at 150 r/min for 180 min in a water bath shaker. After adsorption, the mixtures were filtered with 0.45 µm micro-membrane, and then the final concentration of Cr (VI) in each filtrate was determined using a UV-visible spectrophotometer at 540 nm. All the experiments were repeated three times.
The effects of adsorbent dosage (0.2-2.5 g), pH (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13), contact time (0-180 min), and solution temperature (283-323 K) on Cr (VI) removal were investigated. Kinetics studies were carried out by varying initial Cr (VI) concentrations (1, 5, 10 mg/L), and a water sample was drawn at regular time intervals for the concentration of Cr (VI) measurements. Isotherm experiments were carried out at three temperatures (283, 293, 303 K) using varying initial Cr (VI) concentrations (1-15 mg/L). The removal percentage of Cr (VI), E (%), and the amount of Cr (VI) adsorbed on adsorbent at time t, q t was calculated by using the following expressions: where C 0 and C t (mg/L) are the initial concentration and the final concentration of Cr (VI) at contact time t (min), respectively; R (%) is the removal efficiency of Cr (VI); q t (mg/L) is the adsorption capacity and at time t (min); W (g) is the weight of ACDCS; and V (L) is the volume of Cr (VI) solution.

Optimization of Preparation Parameters of ACDCS
The preparation conditions of ACDCS were optimized by 16 groups of orthogonal experiments, and the results are shown in the supplementary material (demonstrated in Table S1). The iodine values of the prepared ACDCS varied from 274.52 mg/g to 557.10 mg/g at various preparation conditions. The results showed that the different preparation parameters had a considerable influence on the adsorption performances of the prepared ACDCS.
The R value (response at the average of the difference between the extreme values of Ki) can be applied to evaluate the significance of the influence of various factors on the performance of the prepared ACDCS, and the larger the R value, the more considerable the influence of the factor [20]. Based on the viewpoint above, the impact degree of the selected factors in this study was in the order DCS particle size > carbonization temperature > ZnCl 2 weight concentration > activation time > carbonization time. In addition, it was found that the R values of the DCS particle size and carbonization temperature were much higher than those of the other three factors. This indicated that sludge size and carbonization temperature were the key factors affecting the performances of the prepared ACDCS. DCS particle size had the greatest impact on the iodine value. This was because the contents of organic matter and ash varied remarkably for different-sized DCS sludge particles (demonstrated in Table S2). For raw materials used to produce activated carbon, a high content of organic matter and low ash content could significantly improve the adsorption properties of prepared activated carbon [21]. Accordingly, the organic matter content of the raw material was a key factor for producing high-quality activated carbon. Furthermore, carbonization temperature was another crucial parameter for influencing the properties of ACDCS. This was because the carbonization temperature affected the pyrolysis reaction process of sludge and ZnCl 2 , thereby improving or decreasing the pore structure of the prepared ACDCS [22].
For a factor, the K i value (sum response at the ith level) can be applied to recognize the optimal level, and the maximum value of K i denotes the optimal level [23]. Based on the theory above, the optimal preparation conditions for ACDCS in this study are as follows: DCS particle size 20-50 mesh, ZnCl 2 weight concentration 20%, activation time 12 h, carbonization temperature 773 K, and carbonization time 60 min. The iodine number adsorbed on the prepared ACDCS at the optimal process parameters added to 557.10 mg/g, in contrast to other adsorbents [22,24,25]. The prepared ACDCS at the optimal conditions was applied to the subsequent adsorption experiments and was recorded as ACDCS optimal .

Effect of ACDCS optimal Dosage
The effects of the dosage of DCS (dewatered cassava sludge), CDCS (carbonized dewatered cassava sludge without activation), and ACDCS optimal on the removal of Cr (VI) were studied, and the results are presented in Figure 1A. Compared with DCS and CDCS, the removal efficacy of Cr (VI) adsorbed on ACDCS optimal was much better at the same dosage. The SEM images of DCS, CDCS, and ACDCS optimal can be found in the supplementary material (demonstrated in Figure S2). The surface of DCS was smooth without obvious pores. Although a few folds appeared on the surface of the CDCS, the surface was poor in the pore structure. However, the surface of the ACDCS optimal was rich in pore structure, resulting in much larger specific surface areas (509.03 m 2 /g, shown in Table S2) compared with DCS and CDCS. This could be the reason for causing the better removal of the Cr (VI) adsorbed on ACDCS optimal . Moreover, this indicates that activation with a proper activator (ZnCl 2 ) during the process of producing activated carbon can obviously elevate the performance of prepared activated carbon [26].
5 adsorbed on the prepared ACDCS at the optimal process parameters added to 557.10 mg/g, in contrast to other adsorbents [22,24,25]. The prepared ACDCS at the optimal conditions was applied to the subsequent adsorption experiments and was recorded as AC-DCSoptimal.

Effect of ACDCSoptimal Dosage
The effects of the dosage of DCS (dewatered cassava sludge), CDCS (carbonized dewatered cassava sludge without activation), and ACDCSoptimal on the removal of Cr (VI) were studied, and the results are presented in Figure 1A. Compared with DCS and CDCS, the removal efficacy of Cr (VI) adsorbed on ACDCSoptimal was much better at the same dosage. The SEM images of DCS, CDCS, and ACDCSoptimal can be found in the supplementary material (demonstrated in Figure S2). The surface of DCS was smooth without obvious pores. Although a few folds appeared on the surface of the CDCS, the surface was poor in the pore structure. However, the surface of the ACDCSoptimal was rich in pore structure, resulting in much larger specific surface areas (509.03 m 2 /g, shown in Table S2) compared with DCS and CDCS. This could be the reason for causing the better removal of the Cr (VI) adsorbed on ACDCSoptimal. Moreover, this indicates that activation with a proper activator (ZnCl2) during the process of producing activated carbon can obviously elevate the performance of prepared activated carbon [26]. As shown in Figure 1A, the removal efficiency of Cr (VI) rapidly increased from 15.32% to 95.44% with an increasing dosage of ACDCSoptimal from 0.2 to 2.5 g/L. This was because the adsorption sites available increased with the increasing adsorbent dosage. Nevertheless, the amount of Cr (VI) adsorbed on ACDCSoptimal declined from 7.66 to 3.82 mg/g with the increasing adsorbent dosage. This was attributed to the overlapping of active sites at high dosage, causing a decrease in effective contact surface [27]. In addition, at a high dosage, the excessive active sites were not utilized effectively for a certain Cr (VI). This may be another reason causing the decrease in the amount of Cr (VI) adsorbed As shown in Figure 1A, the removal efficiency of Cr (VI) rapidly increased from 15.32% to 95.44% with an increasing dosage of ACDCS optimal from 0.2 to 2.5 g/L. This was because the adsorption sites available increased with the increasing adsorbent dosage. Nevertheless, the amount of Cr (VI) adsorbed on ACDCS optimal declined from 7.66 to 3.82 mg/g with the increasing adsorbent dosage. This was attributed to the overlapping of active sites at high dosage, causing a decrease in effective contact surface [27]. In addition, at a high dosage, the excessive active sites were not utilized effectively for a certain Cr (VI). This may be another reason causing the decrease in the amount of Cr (VI) adsorbed on ACDCS optimal with an increase in the adsorbent dosage. Accordingly, the prepared ACDCS optimal at the Water 2021, 13, 3602 6 of 17 optimal level was applied in subsequent tests, and the dosage of ACDCS optimal was 1.0 g/L in this study, unless otherwise stated.

Effect of pH
The pH value of the solution, affecting the surface properties of the adsorbent and the chemical form of the adsorbate, is an important parameter of the adsorption process [28]. Figure 1B shows the relationship between the solution's pH and the removal efficiency of Cr (VI) adsorbed on ACDCS optimal . The maximum amount of Cr (VI) adsorbed on ACDCS optimal was found to be 9.84 mg/g, and the removal efficiency of Cr (VI) was 98.43% at pH 2.0. The amount of Cr (VI) adsorbed dwindled from 9.84 mg/g to 0.15 mg/g with the increase in pH from 2.0-10.0. This result suggests the adsorption removal of Cr (VI) is a highly pH-dependent process.
Depending on the pH of the solution and the Cr (VI) concentration, Cr (VI) exists in different forms (H 2 CrO 4 , HCrO 4 − , CrO 4 2− , Cr 2 O 7 2− ) (shown in Figure 1C) in aqueous solution [29]. Moreover, the surface charge of ACDCS optimal (given in Figure 1B) was affected by aqueous solution pH as well [28]. The pH at the point of zero charges (pH pzc ) for ACDCS optimal was found to be 4.8, indicating that the surface of ACDCS optimal was charged positively at pH < 4.8 and, conversely, the surface of ACDCS optimal showed a negative charge at pH > 4.8. At pH = 2, Cr (VI) mainly exists in the anion form of HCrO 4 -. The surface of ACDCS optimal was loaded with a lot of positive charges due to the protonation, and the strong electrostatic attraction between the positively charged surface of ACDCS optimal and the negatively charged anion HCrO 4 − achieved the highest removal of Cr (VI) at pH = 2.0. The electrostatic attraction decreased gradually due to the fade of the protonation with the increase in pH from 2 to 4.8, leading to a decline in the removal of Cr (VI). After pH > 4.8, the surface of the ACDCS optimal was charged negatively, and the electrostatic attraction between ACDCS optimal and HCrO 4 − vanished. The reduction of Cr (VI) to Cr (III), and the ion exchange of Cr (III) with adsorption sites at acidic conditions, might play key roles in the removal of Cr (VI) [30]. With an increase in pH from 4.8 to 7.0, the reduction and ion exchange were weakened gradually, resulting in a decline in the removal of Cr (VI). After pH > 7.0, the van der Waals forces probably contributed to ACDCS optimal adsorption of Cr (VI) removal because of the vanishing of the reduction and electrostatic attraction [31]. CrO 4 2− ions were confirmed as the dominant species of Cr (VI) in an aqueous solution at pH > 7.0 (seen in Figure 1C). In comparison with HCrO 4 − , the stronger repulsion forces between the surface of ACDCS optimal and CrO 4 2− ions decreased the removal of Cr (VI) with the increase in pH from 7.0 to 10.0 [32]. Furthermore, the competition between the excess OHand CrO 4 2− for the adsorption sites might be another reason causing the decline in the Cr (VI) removal with the increase in pH from 7.0 to 10.0 [31].

Effect of Initial Cr (VI) Concentration and Adsorption Time
Adsorbate concentration and adsorption time are significant parameters for developing cost-effective adsorption procedures [33]. Figure 2A shows the relationship of the amount of Cr (VI) adsorbed on ACDCS optimal and adsorption time at different initial Cr (VI) concentrations.
removal of Cr(VI) was because there were more vacancies on the ACDCSoptimal surface; and, because the concentration gradient between the bulk solution and the ACDCSoptimal surface was larger, the driving force was higher. [34]. However, with the continuous decrease of the density of vacant active centers, the adsorption rate gradually slowed down [35]. At nearly 180 min, the amount of Cr (VI) adsorbed showed no obvious change, indicating that equilibrium was achieved [33]. Moreover, It was also observed that as the initial concentration of Cr(VI) increased from 1.0 mg/L to 10 mg/L, the amount of Cr (VI) adsorbed increased. The active adsorption sites, including the external surface sites and the internal pore sites, were fixed at the ACDCSoptimal dosage of 1 g/L in this study. At a low Cr (VI) concentration (for example, 1.0 mg/L), the active sites of ACDCSoptimal were sufficient for the contents of Cr (VI), and all the Cr (VI) ions were almost adsorbed by the external surface sites. As the initial Cr (VI) concentration gradually increased, the external surface active sites of ACDCSoptimal were completely occupied, and the higher Cr (VI) concentration gradient promoted the continued diffusion of the Cr (VI) ions into the internal pore sites [27]. The sufficient utilization of active sites of ACDCSoptimal might be the reason causing the increase in the amount of Cr (VI) adsorbed with the increase in the initial concentration of Cr (VI) solution [33]. Moreover, Figure 2A demonstrates that the initial concentration of Cr (VI) solution was directly proportional to the time required to reach equilibrium. Compared with the internal pore adsorption at the higher Cr (VI) concentration, the external surface adsorption was much faster at the low Cr (VI) concentration. This could be because as the initial concentration of Cr(VI) increased, it needed to take longer to reach equilibrium.

Kinetics Models
Adsorption kinetics is of major significance for the study of reaction pathways and mechanisms [36]. In order to investigate the adsorption kinetics of Cr (VI) on ACDCSoptimal, the pseudo-first-order model (PFO), pseudo-second-order model (PSO), and Elovich It can be easily observed that the adsorption process of Cr (VI) on ACDCS optimal included three stages: rapid adsorption (0-20 min), slow adsorption (20-150 min), and reaching adsorption equilibrium (180 min). In the initial stage of adsorption, the rapid removal of Cr(VI) was because there were more vacancies on the ACDCS optimal surface; and, because the concentration gradient between the bulk solution and the ACDCS optimal surface was larger, the driving force was higher. [34]. However, with the continuous decrease of the density of vacant active centers, the adsorption rate gradually slowed down [35]. At nearly 180 min, the amount of Cr (VI) adsorbed showed no obvious change, indicating that equilibrium was achieved [33].
Moreover, It was also observed that as the initial concentration of Cr(VI) increased from 1.0 mg/L to 10 mg/L, the amount of Cr (VI) adsorbed increased. The active adsorption sites, including the external surface sites and the internal pore sites, were fixed at the ACDCS optimal dosage of 1 g/L in this study. At a low Cr (VI) concentration (for example, 1.0 mg/L), the active sites of ACDCS optimal were sufficient for the contents of Cr (VI), and all the Cr (VI) ions were almost adsorbed by the external surface sites. As the initial Cr (VI) concentration gradually increased, the external surface active sites of ACDCS optimal were completely occupied, and the higher Cr (VI) concentration gradient promoted the continued diffusion of the Cr (VI) ions into the internal pore sites [27]. The sufficient utilization of active sites of ACDCS optimal might be the reason causing the increase in the amount of Cr (VI) adsorbed with the increase in the initial concentration of Cr (VI) solution [33]. Moreover, Figure 2A demonstrates that the initial concentration of Cr (VI) solution was directly proportional to the time required to reach equilibrium. Compared with the internal pore adsorption at the higher Cr (VI) concentration, the external surface adsorption was much faster at the low Cr (VI) concentration. This could be because as the initial concentration of Cr(VI) increased, it needed to take longer to reach equilibrium.

Kinetics Models
Adsorption kinetics is of major significance for the study of reaction pathways and mechanisms [36]. In order to investigate the adsorption kinetics of Cr (VI) on ACDCS optimal , Water 2021, 13, 3602 8 of 17 the pseudo-first-order model (PFO), pseudo-second-order model (PSO), and Elovich model were used employed to describe the adsorption process of Cr (VI) on ACDCS optimal in this work [37], and they were expressed by Equations (3)- (5): q t = k 2 q 2 e t/ 1 + k 2 q e t (4) where q t (mg/g) is the adsorption capacity at time t; k 1 and k 2 (mg/(g min)) are the equilibrium rate constants of the PFO model and the PSO model, respectively; q e represents the equilibrium adsorption capacity; α is the original adsorption rate constant (mg/(g min)); and β is concerned with the extent of surface coverage and activation energy for chemisorption (mg/g). The relevant parameters of the three kinetic models with the correlation coefficients at diverse initial solution concentrations of Cr (VI) are presented in Table 1 and Figure 2B. The results showed that the correlation coefficients (R 2 ) of the Elovich model were larger than those of the PFO model and PSO model, and the SEE values of the Elovich model were the smallest among the three models. This indicated that the Elovich was the best representation for adsorption of Cr (VI) on ACDCS optimal . Further, adsorption of Cr (VI) on ACDCS optimal might be chemisorption by transferring electrons [37].

Diffusion Mechanism
Although the adsorption process of Cr (VI) on ACDCS optimal was fitted well by the Elovich kinetic model, it still could not confirm the diffusion mechanism and the ratecontrolling step in the adsorption process, such as film diffusion and intra-particle diffusion. To identify the step of actual rate-controlling in the adsorption process for Cr (VI), the Weber-Morris model was used [38]. q t = k id t 0.5 + C i (6) where q t (mg/g) is the amount of Cr (VI) adsorbed at time t (min), k id is the intra-particle diffusion rate (mg/(g min 0.5 )), and C i represents the diffusion effects of the boundary layer. Based on the Weber-Morris model, the fitting result of q t to t 0.5 is linear, and the line passes through the origin if the intra-particle diffusion process is a rate-determining step. Otherwise, the adsorption process could be affected by more steps [39]. The fitted plots of q t against t 0.5 at different initial Cr (VI) concentrations are shown in Figure 2C, and the parameters of k id and C i were calculated according to the model and are listed in Table 1. Obviously, the adsorption data were fitted very well by the three straight lines, suggesting that three stages occurred during Cr (VI) adsorption on ACDCS optimal [40]. Firstly, the linear section had a higher slope, indicating that Cr (VI) was transferred to the external surface of ACDCS optimal from an aqueous solution [38]. Then, the slope of the straight section decreased gradually, suggesting that Cr (VI) diffused from the external into the interior pore structure of ACDCS optimal [27]. Finally, the slope of the straight section approached zero, indicating that the adsorption equilibrium was reached [40]. In addition, the values of C i at all the stages were greater than zero, indicating that the process was not only intra-particle diffusion but also some other steps that might control the rate of the kinetic process [38]. Further, the Boyd model (given in Equation (7)) was used to check whether the film diffusion was involved in the adsorption process, and the results are represented in Figure 2D and Table 1, respectively.
where Bt is the time constant; q t (mg/g) is the amount of Cr (VI) adsorbed at time t (min); and q e is the equilibrium adsorption capacity. Based on the Boyd model, the fitting result of B to t −1 is not linear, or the linear does not pass through the origin of the film diffusion process, which is a rate-determining step [39]. It was found that the plots fitted by the Boyd model demonstrated good linearity with a high R 2 value (0.9833, 0.9624, 0.9809), and the fitting lines were the extended model and did not pass through the origin at different initial Cr (VI) concentrations. Those results confirmed that film diffusion occurred during the adsorption process of Cr (VI) on ACDCS optimal [39]. To sum up, the adsorption process of Cr (VI) on ACDCS optimal was mainly affected and controlled by film diffusion and intra-particle diffusion. Firstly, Cr (VI) ions diffused from the bulk solution to the external surface of ACDCS optimal , and this process was controlled by film diffusion. Then, the next process was dominated by intra-particle diffusion, in which Cr (VI) ions entered the internal pores to load into the internal active adsorption sites until adsorption equilibrium.

Effect of Temperature
In a solid-liquid adsorption system, solution temperature plays a key role [36]. The equilibrium tests of Cr (VI) adsorbed on ACDCS optimal were investigated at 283, 293, and 303 K, and the results are shown in Figure 3A. It was observed that the equilibrium adsorption capacity of Cr (VI) on ACDCS optimal increased slightly with the increase in solution temperature from 283 to 303 K, indicating that the increasing temperature of the solution was conducive to the adsorption removal of Cr (VI). This was because the increase of temperature promoted the thermal movement of Cr (VI) ions in the solution and facilitated the diffusion of Cr (VI) ions into the internal active adsorption sites of ACDCS optimal [27].
Water 2021, 13, x FOR PEER REVIEW 10 of 17 was conducive to the adsorption removal of Cr (VI). This was because the increase of temperature promoted the thermal movement of Cr (VI) ions in the solution and facilitated the diffusion of Cr (VI) ions into the internal active adsorption sites of ACDCSoptimal [27].

Adsorption Isotherm Models
The adsorption isotherm showed a specific relationship between its accumulation on the surface of the adsorbent and the concentration of adsorbate [39]. Three isotherm models were used to fit the equilibrium experimental data of Cr (VI) on ACDCSoptimal. These isotherm models included the Langmuir model, Freundlich model, and Temkin, and they were expressed by Equations (8)-(10) [41]. The fitted curves and model parameters with R 2 are presented in Figure 3B-D and Table 2, respectively.
where qe is the equilibrium adsorption capacity; qm is the maximum single-layer adsorption capacity of the adsorbent; KL is the Langmuir constant; Ce is the equilibrium concentration of Cr (VI) solution; KF is the Freundlich adsorption constant; n is the adsorption strength; BT is the heat of adsorption; and AT is the equilibrium constant. It was evident that the Freundlich isotherm model showed a better fitting to the equilibrium adsorption data in terms of the coefficient of determination (R 2 > 0.98). The Freundlich isotherm model is suitable for highly heterogeneous surfaces, and the lack of a saturated platform for Freundlich isotherm may indicate a multilayer adsorption mechanism [39]. Based on this viewpoint, the adsorption process of Cr (VI) on ACDCSoptimal was a multilayer adsorption process over a heterogeneous surface. Moreover, the parameter n was very important for the Freundlich isotherm model. The adsorption type is considered favorable when n is more than 1.0, and the adsorption process occurs very easily when n −1

Adsorption Isotherm Models
The adsorption isotherm showed a specific relationship between its accumulation on the surface of the adsorbent and the concentration of adsorbate [39]. Three isotherm models were used to fit the equilibrium experimental data of Cr (VI) on ACDCS optimal . These isotherm models included the Langmuir model, Freundlich model, and Temkin, and they were expressed by Equations (8)-(10) [41]. The fitted curves and model parameters with R 2 are presented in Figure 3B-D and Table 2, respectively. q e = (q m K L C e )/(1 + K L C e ) where q e is the equilibrium adsorption capacity; q m is the maximum single-layer adsorption capacity of the adsorbent; K L is the Langmuir constant; Ce is the equilibrium concentration of Cr (VI) solution; K F is the Freundlich adsorption constant; n is the adsorption strength; B T is the heat of adsorption; and A T is the equilibrium constant. It was evident that the Freundlich isotherm model showed a better fitting to the equilibrium adsorption data in terms of the coefficient of determination (R 2 > 0.98). The Freundlich isotherm model is suitable for highly heterogeneous surfaces, and the lack of a saturated platform for Freundlich isotherm may indicate a multilayer adsorption mechanism [39]. Based on this viewpoint, the adsorption process of Cr (VI) on ACDCS optimal was a multilayer adsorption process over a heterogeneous surface. Moreover, the parameter n was very important for the Freundlich isotherm model. The adsorption type is considered favorable when n is more than 1.0, and the adsorption process occurs very easily when n −1 is less than 0.5. The fitted results confirmed the adsorption process of Cr (VI) on ACDCS optimal was very easy to carry out.
3.6. Energy Changes during the Adsorption Process 3.6.1. Adsorption Thermodynamics Adsorption thermodynamics can reflect the energy changes during the process of target pollutants adsorbed on the adsorbent, and can be used to analyze and explain the adsorption mechanism from the perspective of thermodynamics [25]. The thermodynamic parameters of the adsorption of Cr (VI) onto ACDCS optimal , including the Gibbs free energy change (∆G • , kJ/mol), changes of adsorption heat (∆H • , kJ/mol), and changes of adsorption entropy (∆S • , J/mol/K), were determined by the van't Hoff approach using Equations (11)-(13) [29]: where C 0 and C t (mg/L) are the initial concentration and the final concentration of Cr (VI) at contact time t (min), respectively; R (8.314 J/(mol·K)) is the universal gas constant; T (K) is the absolute temperature; W (g) is the weight of ACDCS; V (L) is the volume of Cr (VI) solution; and K D (L/g) is the distribution coefficient of Cr (VI) in ACDCS optimal and solution.
The plot of ln (K D ) against 1/T is shown in Figure S1. The parameters of ∆H • and ∆S • were determined through the slope and intercept value, respectively. The calculated values of ∆H • , ∆S • , and ∆G • for the adsorption of Cr (VI) on ACDCS optimal are given in Table 3. Table 3. Adsorption thermodynamic parameters of Cr (VI).

T (K)
∆G The negative ∆G • indicates that the process of Cr (VI) adsorbed on ACDCS optimal was spontaneous. The positive values of ∆S • indicate that the adsorption process is entropydriven rather than enthalpy. Furthermore, the positive ∆H • confirms the endothermic nature, and that the increasing temperature favored the adsorption process. Moreover, the values of ∆G • indicate the physical or chemical nature of adsorption [25]. The values of ∆G • were <−10 kJ/mol in this study, implying that physisorption was one of the mechanisms for the removal of Cr (VI) adsorbed on ACDCS optimal .

Average Free Energy
The calculation of average free energy is a method based on the fitting analysis of the Dubinin-Radushkevich (D-R) isotherm model to determine the reaction type [42]. The isotherm model and the average free energy calculation formula were as follows: where q max is the maximum adsorption capacity of Cr (VI); q e is the equilibrium adsorption capacity; R (8.314 J/(mol·K)) is the universal gas constant; T (K) is the absolute temperature; Ce is the equilibrium concentration of Cr (VI) solution; K dr (mol 2 /J 2 ) is a model parameter related to the average adsorption energy; and E is the average free energy.
The fitted results and model parameters with R 2 are presented in Table 2. It was evident that the Dubinin-Radushkevich isotherm model showed a good fitting to the equilibrium data in terms of coefficients of determination (R 2 > 0.87). The average free energy (AFE) for the adsorption of Cr (VI) on ACDCS optimal at 283 K, 293 K, and 303 K were 3.14 kJ/mol, 3.33 kJ/mol, and 4.44 kJ/mol, respectively. The values of AFE were less than 8 kJ/mol at a different temperature, indicating that the physisorption occurred during the process of Cr (VI) adsorbed on ACDCS optimal [43].

Adsorption Activation Energy
The activation energy can be used to judge the difficulty of the adsorption process [31]. The Arrhenius equation was used to calculate the adsorption activation energy, expressed by Equations (13): where E a is adsorption activation energy (kJ/mol); A is frequency factor; R is the gas constant (8.314 J/(mol·K)); T (K) is the absolute temperature; and k is the reaction rate constant. The data of ln k versus 1/T were plotted and are shown in Figure S1. The correlation coefficient (R 2 = 0.97) showed that there was a good linear relationship between a value and 1/T. The weak van der Waals force is the cause of physical adsorption, and its activation energy is lower than 4.18 kJ/mol [31]. In this study, the value of E a was 3.37 kJ/mol, indicating that the adsorption process of Cr (VI) on ACDCS optimal was easy to carry out and that physical adsorption was involved.

Pore Filling
The excellent adsorption performances of carbon material are based on its developed pore structure and high specific surface areas, resulting in the fact that pore filling is the most basic adsorption mechanism in the removal of pollutants [28]. The prepared fresh ACDCS optimal in this study possessed rich pores, and the total pore volume added to 0.375 cm 3 /g (demonstrated in Table S2). Furthermore, Table S2 demonstrates that the pore sizes were mainly distributed in the range around the micropores and mesopores, leading to a high BET surface area (509.03 m 2 /g). This was favorable for the removal of Cr (VI) adsorbed on ACDCS optimal . In addition, the surface pores were covered after adsorption, showing that Cr (VI) ions were adsorbed on the pores (demonstrated in Table S2 and Figure S2). The Cr (VI) ions could diffuse into the internal pores due to the smaller size, compared with the sizes of micropores and mesopores [28]. The decreased BET surface areas and total pore volume (demonstrated in Table S2) after adsorption indicate that the Cr (VI) ions were removed on the internal active sites. The results above confirm that pore filling was one of the mechanisms for the removal of Cr (VI) adsorbed on ACDCS optimal .

Electrostatic Attraction
As discussed in Section 3.3, the ACDCS optimal exhibited a positively charged surface resulting from the -OH 2 + functional groups because of the protonation at pH < 4.8. In addition, Cr (VI) mainly existed in the form of HCrO 4 − ion. The strong electrostatic attraction was formed between -OH 2 + and HCrO 4 − , facilitating the removal of Cr (VI). The electrostatic attraction disappeared after pH > 4.8 due to the ACDCS optimal surface having negative charges, although Cr (VI) still appeared in the anions (HCrO 4 − or CrO 4 2− ).

Adsorption-Combined Reduction
The Cr (VI) adsorbed on ACDCS optimal might be reduced by the functional groups on the carbon surface at the acidic conditions (shown in Equation (14)). Compared with the fresh ACDCS optimal , two new peaks at 576.51 eV and 585.88 eV, attributed to Cr2p 3/2 and Cr2p 1/2 , respectively, were found in the XPS wide spectra of ACDCS optimal after adsorption (demonstrated in Figure S3). This indicated chromium was adsorbed on ACDCS optimal [27]. The two peaks at 579.17 eV and 587.60 eV were ascribed to Cr (VI), and the peaks at 585.88 eV and 576.51 eV were considered as Cr (III) in the high-resolution spectrum of Cr2p [32]. Those results confirmed that the Cr (VI) adsorbed on the ACDCS optimal was partially reduced to Cr (III) by the rich functional groups on the surface of ACDCS optimal , such as the -OH and C-H groups (shown in Equations (15) and (16)) [39].
In comparison, the atomic ratio of graphitic carbon after adsorption (C-C and/or C-H) was decreased to 73.88%, and the contents of the C=O and C-O bond increased to 7.58% and 18.53%, respectively [27]. This showed that some graphitic carbon was oxidized by Cr (VI) (shown in Equations (15) and (16)) [39]. The existence of chemical coordination between Cr (VI) and the C-O functional groups was due to a slight shift at the energy of 285.95 eV.
The FTIR spectra of ACDCS optimal before and after adsorption was also used to analyze the functional groups involved in the redox reaction. The peak, at approximately 3428 cm −1 , was associated with the -OH functional groups, and the peak slightly shifted because of Cr (VI) adsorbed. Furthermore, the peaks at 2923 and 2857 cm −1 slightly decreased because of -C-H stretching [25]. These results indicate that the -OH functional groups and graphitic carbon (-C-H) were oxidized because of the reduction of Cr (VI) to Cr (III) [44]. Further, the peak at 1617 cm −1 shifted to 1565 cm −1 , which could be associated with stretching vibrations of -COO − [35]. These results suggest that the carboxyl functional groups increased after Cr (VI) removal, and chromium was chemically immobilized on these oxygen-containing groups [39]. Hence, Cr (III), as a reductive product of Cr (VI), was removed by ion exchange with the -COOH functional groups as well pore filling. The characterizations of the above studies are presented in the form of Supplementary Materials (demonstrated in Figure S3).
Based on the discussion above, the removal of Cr (VI) with ACDCS optimal was dependent on the pH of the solution, and the mechanism was considered to comprise the following three sections, as illustrated in Figure S4: (i) the reduction reaction and ion exchange were involved in the removal of Cr (VI), as well pore filling and electrostatic attraction at pH < 4.8; (ii) pH = 6.5, with the electrostatic attraction disappearing due to deprotonation of the oxygen-containing groups, pore filling, reduction reaction, and ion exchange, contributed to the removal of Cr (VI) at 4.8 < pH < 6.5; (iii) pore filling played the main role for the removal of Cr (VI) on ACDCS optimal at the alkaline conditions.

Comparison with Other Adsorbents for Cr (VI)
Although it is difficult to compare with various adsorbents due to different test conditions, the maximum adsorption capacity (q max ) is still used as a parameter for comparison. The results in the literature are listed in Table 4. Table 4 demonstrates that it was comparable for the prepared ACDCS optimal in this study with the shorter adsorption time and the higher maximum adsorption capacity. In addition, the trace metal leakage (demonstrated in Table S3) indicated that the prepared ACDCS optimal was a potential adsorbent for the removal of Cr (VI) from the contaminated raw water.

Conclusions
A novel sludge activated carbon was produced successfully by pyrolysis using dewatered cassava sludge as the raw material and ZnCl 2 as the activator. The removal performances and mechanisms of Cr (VI) on the prepared sludge activated carbon were systemically investigated. The optimal preparation parameters were as follows: DCS particle size 20-50 mesh, ZnCl 2 weight concentration 20%, activation time 12 h, carbonization temperature 773 K, and carbonization time 60 min. The specific surface areas and iodine value for the prepared sludge activated carbon (named as ACDCS optimal ) at the optimal process parameters added to 509.03 m 2 /g and 557.10 mg/g, respectively. The initial Cr (VI) concentration affected the time of reading equilibrium, but the adsorption could be fulfilled within 3 h. The kinetics could be fitted well by the Elovich model. The whole adsorption rate was controlled by both film and intra-particle diffusion. The adsorption equilibrium could be described well by the Freundlich isotherm model. The maximum equilibrium capacity was 8.01 mg/g. The adsorption was a spontaneous and endothermic process. Elevating temperature facilitated the removal of Cr (VI) adsorbed on ACDCS optimal . The removal of Cr (VI) on ACDCS optimal was dependent on the pH of the solution. The pore filling, electrostatic attraction, reduction, and ion exchange were involved in the removal mechanism of Cr (VI) adsorbed on ACDCS optimal . The metal leakage contents were traced. Accordingly, the prepared ACDCS optimal was a potential adsorbent for the removal of Cr (VI) from contaminated raw water.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/w13243602/s1, Figure S1: Van't Hof plot for the adsorption of Cr (VI) on ACDCS optimal (A); Fitting curve of activation energy for Cr (VI) adsorption on ACDCS optimal (B). Figure S2: SEM images of DCS (A), fresh CDCS (B), fresh ACDCS optimal (C) and ACDCS optimal with Cr (VI) adsorbed (D). Figure S3: (A) Survey XPS spectra of the fresh ACDCS optimal and ACDCS optimal with Cr (VI) adsorbed, (B) high resolution XPS spectra of the Cr2p peaks of ACDCS optimal with Cr (VI) adsorbed, and (C) high resolution XPS spectra of the C1s peaks of ACDCS optimal with Cr (VI) adsorbed, and (D) FTIR spectrum of fresh ACDCS optimal and ACDCS optimal with Cr (VI) adsorbed. Figure S4: Adsorption mechanism of Cr (VI) removal by ACDCS optimal . Table S1: Results of the orthogonal test for ACDCS preparation and range analysis. Table S2: Elemental analysis and porosity characteristics of sample.

Conflicts of Interest:
The authors declare no conflict of interest.