Weathered Sand of Basalt as a Potential Nickel Adsorbent

The natural mineral, weathered sand of basalt (WSB), was utilized to investigate whether nickel can be removed. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis were performed to characterize WSB. The effects of various conditions, i.e., contact time, pH, WSB dosage, particle size of WSB, and temperature were analyzed. The experimental data were analyzed by two widely used equations, i.e., Langmuir and Freundlich isotherms. The results obtained revealed that the WSB adsorption process was more consistent with the Langmuir model than the Freundlich equation. The kinetics data fitted well into the pseudo-second-order model. The findings of the present study indicate that WSB could be used for removing nickel from aqueous solution. Moreover, its concentration can be reduced from 1.0 mg/L to ND (not detected, below the device limit <0.01 μg/L) under the optimal condition. Therefore, WSB is considered to be usable as one of the adsorbents for nickel removal in water. In addition, since heavy metals are often present in low concentrations in water, it is considered that WSB can be applied as one of the effective alternatives for removing low-concentration nickel.


Introduction
Heavy metal is one of the important pollutants in water and wastewater, and has possible impacts of public health concerns because of its toxic and persistent nature [1]. The heavy metals, like nickel, aluminum, etc., are common pollutants present in the environment from various sources [2]. Heavy metal, if not treated will bring toxicity to biological systems and a continuous threat to our environment [2,3].
The heavy metal removal methods that have been studied and widely used in the environmental field include electrochemical precipitation [4,5], physical treatments, such as membranes [6], and adsorption [7][8][9][10]. However, in the case of chemical treatment, it is difficult to effectively treat when the concentration of metal is very low. In addition, when the amount of chemical used increases, the amount of sludge generated increases, resulting in sludge treatment problems [1,4,5]. Ion exchange and membranes have been also widely used for removal of metals from aqueous environments, however, they are expensive [1,6]. The solvent extraction method has a disadvantage in that it is limited to more concentrated solutions, considering the cost-effective aspect [9]. Reverse osmosis is also available but it is a cost-prohibitive process, as the membranes get easily spoiled requiring frequent replacement [1].
Among the various methods for removing heavy metals, the adsorption process has been widely used because it is an inexpensive and ecofriendly technique. Therefore, research has been continuously conducted to find a cheaper adsorbent with better heavy metal removal efficiency. In the case of natural minerals, they are commonly found, and so can be easily obtained and are inexpensive. In addition, inexpensive. In addition, natural minerals may be applied to water treatment without additional pretreatment. Therefore, if only the treatment effect on pollutants can be proven, it is believed that natural minerals can receive attention as an effective adsorbent [9,11]. For example, previous studies have been reported in which mesoporous silica [12] and montmorillonite clay [13] were used as effective adsorbents for mercury removal.
Therefore, this study investigated the feasibility of weathered sand of basalt (WSB) being used as a natural mineral for the removal of nickel from aqueous solution. To achieve this purpose, the effects of pH, contact time, WSB dosage, and particle size were studied to determine the effect on adsorption capacity, etc. Moreover, X-ray diffraction (XRD) and scanning electron microscope (SEM) analysis were performed to understand the material properties and information of WSB.

Preparation of the Weathered Sand of Basalt
WSB for the experiment was collected from Jeju island in Korea. The sand was washed 20 times with third distilled water to remove the impurities. The washed WSB was soaked in tertiary distilled water for a day. After that, the WSB was dried in an oven at 104 ℃. Then, the WSB was classified by particle size using an American Society for Testing and Materials (ASTM) standard sieve. The composition ratio of WSB by particle size is as follows. That is, 4% for 46 μm or less, 12% for 46~75 μm, 11% for 75~150 μm, 14% for 150~180 μm, 16% for 180~251 μm, 15% for 251~355 μm, 12% for 355~426 μm, and 426~850 μm consists of 5% over 850 μm. Sieved WSB was used as it was, without any additional processing or modification.
The scanning electron micrograph (SEM, Tescan, Brno, Czech Republic) and X-ray diffraction (XRD, Bruker, Leipzig, Germany) of the WSB are given in Figure 1a,b, respectively. The most common elements and compounds in WSB are SiO2, Al2O3, CaO, and FeO.

Preparation of Nickel Aqueous Solutions and Reagents
Ni standard solution (10 mg/L) from AccuStandard (AccuStandard, New Haven, CT, USA) was used. The concentration of Ni in the aqueous solution was prepared by diluting STD (10 mg/L, AccuStandard, New Haven, CT, USA) with tertiary distilled water. The pH of the solutions was adjusted using 0.1 M of HCl (FUJIFILM Wako Pure Chemical Coporation, Osaka, Japan) and 0.1 M of NaOH (Yakakuri Pure Chemicals, Kyoto, Japan). All chemicals were of reagent grades.

Experimental Procedures
The experiments were performed using a shaking incubator (IS-971R, JEIO TECH, seoul, Korea) at 100 rpm.
Samples were taken at 0~120 min. At the end of the equilibrium time, they were centrifuged at 3000 rpm for 15 min to remove WSB from the suspension by centrifugation (Combi-514R, HANIL

Preparation of Nickel Aqueous Solutions and Reagents
Ni standard solution (10 mg/L) from AccuStandard (AccuStandard, New Haven, CT, USA) was used. The concentration of Ni in the aqueous solution was prepared by diluting STD (10 mg/L, AccuStandard, New Haven, CT, USA) with tertiary distilled water. The pH of the solutions was adjusted using 0.1 M of HCl (FUJIFILM Wako Pure Chemical Coporation, Osaka, Japan) and 0.1 M of NaOH (Yakakuri Pure Chemicals, Kyoto, Japan). All chemicals were of reagent grades.

Experimental Procedures
The experiments were performed using a shaking incubator (IS-971R, JEIO TECH, Seoul, Korea) at 100 rpm.
Samples were taken at 0~120 min. At the end of the equilibrium time, they were centrifuged at 3000 rpm for 15 min to remove WSB from the suspension by centrifugation (Combi-514R, HANIL Science Industrial, Seoul, Korea). The supernatant liquid was analyzed for the residual Ni using ICP-MS (Agilent Technologies, 7900, Santa Clare, CA, USA).
The Ni removal properties of WSB were evaluated according to various experimental variables. Among the various factors affecting the adsorption process, this study analyzed the effects of pH, sorbent dosage, contact time, temperature, and particle size.
To identify the nickel ion adsorption capacity of WSB, the reaction time was tested for 120 min and pH was tested in the range of 2.5 to 12. The removal rate of nickel ion according to WSB particle size was assessed between 46 µm and 850 µm. Furthermore, the temperature effect on the adsorption of nickel ions between 10 and 30 • C was examined.

Effect of Contact Time
Since it is possible to understand the time to reach the parallel state and the optimum reaction time, contact time is one of the key operating parameters for effective use of the WSB. Therefore, samples were taken at appropriate time intervals for 120 min to establish the equilibrium time. The experiment was conducted 30 times to ensure reliability of the results.
Variation of the Ni uptake with the adsorption time is shown in Figure 2. The Ni removal rate is largely divided into three sections, as shown in Figure 2. As shown in Figure 2, the Ni removal characteristics according to the reaction time are as follows: a first stage (up to 10 min) in which Ni is abruptly removed, a second stage (between 10 and 60 min) in which there is gentle Ni removal, a third stage (60 min or more) in which the equilibrium state is reached. From this result, therefore, 60 min has been selected as optimum contact time in the following experiments. Science Industrial, seoul, Korea). The supernatant liquid was analyzed for the residual Ni using ICP-MS (Agilent Technologies, 7900, Santa Clare, CA, USA). The Ni removal properties of WSB were evaluated according to various experimental variables. Among the various factors affecting the adsorption process, this study analyzed the effects of pH, sorbent dosage, contact time, temperature, and particle size.
To identify the nickel ion adsorption capacity of WSB, the reaction time was tested for 120 minutes and pH was tested in the range of 2.5 to 12. The removal rate of nickel ion according to WSB particle size was assessed between 46 μm and 850 μm. Furthermore, the temperature effect on the adsorption of nickel ions between 10 and 30 ℃ was examined.

Effect of Contact Time
Since it is possible to understand the time to reach the parallel state and the optimum reaction time, contact time is one of the key operating parameters for effective use of the WSB. Therefore, samples were taken at appropriate time intervals for 120 minutes to establish the equilibrium time. The experiment was conducted 30 times to ensure reliability of the results.
Variation of the Ni uptake with the adsorption time is shown in Figure 2. The Ni removal rate is largely divided into three sections, as shown in Figure 2. As shown in Figure 2, the Ni removal characteristics according to the reaction time are as follows: a first stage (up to 10 minutes) in which Ni is abruptly removed, a second stage (between 10 and 60 minutes) in which there is gentle Ni removal, a third stage (60 minutes or more) in which the equilibrium state is reached. From this result, therefore, 60 min has been selected as optimum contact time in the following experiments.

Effect of pH
One of the important factors that influences Ni ions' adsorption into WSB is pH, which affects the functional groups, the binding sites electrical properties, and the Ni chemistry. In order to investigate the contribution of pH, the effect of pH values (ranging from 2.5 to 12.0) on the adsorption capacity of WSB was measured at a nickel concentration of 1.0 mg/L. The results are presented in Figure 3, WSB dose 2 g/L, WSB particle size 46~75 μm, adsorption time 60 min, and temperature 30 ℃. The experiment was conducted 30 times to ensure reliability of the results.
As shown in Figure 3, the adsorption capacity of WSB reached maximum value in the alkaline region above pH 8, which may be due to the easier adsorption under basic conditions, since nickel is generally present in the divalent cation oxidation state. However, at the pH in the acidic region, the values are decreased drastically. The reason for these phenomenon is that as the pH of the solution decreases, the H + in the solution competes with Ni (generally present as Ni 2+ ) to occupy the adsorption sites. Moreover, the higher the pH, the lower the zeta potential, and the more sites the

Effect of pH
One of the important factors that influences Ni ions' adsorption into WSB is pH, which affects the functional groups, the binding sites electrical properties, and the Ni chemistry. In order to investigate the contribution of pH, the effect of pH values (ranging from 2.5 to 12.0) on the adsorption capacity of WSB was measured at a nickel concentration of 1.0 mg/L. The results are presented in Figure 3, WSB dose 2 g/L, WSB particle size 46~75 µm, adsorption time 60 min, and temperature 30 • C. The experiment was conducted 30 times to ensure reliability of the results.
As shown in Figure 3, the adsorption capacity of WSB reached maximum value in the alkaline region above pH 8, which may be due to the easier adsorption under basic conditions, since nickel is generally present in the divalent cation oxidation state. However, at the pH in the acidic region, the values are decreased drastically. The reason for these phenomenon is that as the pH of the solution decreases, the H + in the solution competes with Ni (generally present as Ni 2+ ) to occupy the adsorption sites. Moreover, the higher the pH, the lower the zeta potential, and the more sites the Ni ion binds to the negatively charged WSB surface. These results have been reported in many previous studies for removing metal ions using minerals as adsorbents. Huang et al. [14] explained this result as follows, i.e., the metal ion uptake was limited in this acidic medium, and this can be attributed to the presence of H + ions which compete with the Ni ions for the adsorption sites. Ni ion binds to the negatively charged WSB surface. These results have been reported in many previous studies for removing metal ions using minerals as adsorbents. Huang et al. [14] explained this result as follows, i.e., the metal ion uptake was limited in this acidic medium, and this can be attributed to the presence of H + ions which compete with the Ni ions for the adsorption sites. Nickel adsorption capacity was the highest at 100 μg/g in the range of pH 8-10. As the pH decreased to 6, 4, and 2.5, the amount of Ni absorption effectivity decreased to 96.3, 28.7, and 22.0 μg/g, respectively. Therefore, the optimal pH condition was judged to be 8. Since the optimal pH condition is the pH range of natural water, it is a great advantage that pH adjustment is not necessary when removing Ni with WSB.
In addition, when nickel in the aqueous solution is removed by adsorption, the chemical surface properties of WSB play an important role, depending on the pH. The pHZPC of WSB is one of the important factors affecting adsorption. pHZPC represents the pH of an aqueous solution with a surface charge of 0. If the pH of the aqueous solution is lower than that of WSB, the surface of WSB has a positive charge, and in the opposite case, the surface of WSB has a negative charge. Therefore, the nickel ion in the aqueous solution is in an advantageous state because the higher the pH in water than the pHZPC of WSB, the higher the adsorption that takes place on the surface of WSB. As a result of the analysis, pHZPC was measured to be 5.8.

Effect of WSB Dosage
The effect of WSB dosage on Ni ions' adsorption capacity was investigated by different WSB dosage (500~2000 mg/L). The results are shown in Figure 4, at initial levels of Ni 1.0 mg/L, pH 8, WSB particle size 46~75 μm, adsorption time 60 min, and temperature 30 ℃. The experiment was conducted 30 times to ensure reliability of the results.
At WSB dosage of 500 mg/L, the Ni ions absorption capacity was the highest, at 394 μg/g. As the WSB dosage increased to 1000, 1500, and 2000 mg/L, the amount of Ni ions absorbed effectivity decreased to 198, 133, and 100 μg/g, respectively.
However, Ni removal efficiency was the highest (not detected (ND), i.e., below the device limit <0.00001 mg/L) at 2000 mg/L of WSB dose, and the removal efficiency was decreased to 99.8, 99.2, and 98.6% at 1500, 1000, and 500 mg/L, respectively.
In the WSB dosage range (500~2000 mg/L) applied in this experiment, the removal efficiency was higher as the dosage increased. However, Ni removal per WSB dosage showed the highest removal at 500 mg/L with the lowest WSB dosage. The reason being that there is almost no difference in removal rate within the range of WSB dosage in this experiment (98.6% to 100% (ND)). Therefore, it is assumed that the amount of Ni removal is large when the injection amount is small. After all, if you want to treat Ni removal rate to 98.6% (0.014 mg/L as concentration), this means that you do not need to inject more WSB than necessary. Nickel adsorption capacity was the highest at 100 µg/g in the range of pH 8-10. As the pH decreased to 6, 4, and 2.5, the amount of Ni absorption effectivity decreased to 96.3, 28.7, and 22.0 µg/g, respectively. Therefore, the optimal pH condition was judged to be 8. Since the optimal pH condition is the pH range of natural water, it is a great advantage that pH adjustment is not necessary when removing Ni with WSB.
In addition, when nickel in the aqueous solution is removed by adsorption, the chemical surface properties of WSB play an important role, depending on the pH. The pH ZPC of WSB is one of the important factors affecting adsorption. pH ZPC represents the pH of an aqueous solution with a surface charge of 0. If the pH of the aqueous solution is lower than that of WSB, the surface of WSB has a positive charge, and in the opposite case, the surface of WSB has a negative charge. Therefore, the nickel ion in the aqueous solution is in an advantageous state because the higher the pH in water than the pH ZPC of WSB, the higher the adsorption that takes place on the surface of WSB. As a result of the analysis, pH ZPC was measured to be 5.8.

Effect of WSB Dosage
The effect of WSB dosage on Ni ions' adsorption capacity was investigated by different WSB dosage (500~2000 mg/L). The results are shown in Figure 4, at initial levels of Ni 1.0 mg/L, pH 8, WSB particle size 46~75 µm, adsorption time 60 min, and temperature 30 • C. The experiment was conducted 30 times to ensure reliability of the results.
At WSB dosage of 500 mg/L, the Ni ions absorption capacity was the highest, at 394 µg/g. As the WSB dosage increased to 1000, 1500, and 2000 mg/L, the amount of Ni ions absorbed effectivity decreased to 198, 133, and 100 µg/g, respectively.
However, Ni removal efficiency was the highest (not detected (ND), i.e., below the device limit <0.00001 mg/L) at 2000 mg/L of WSB dose, and the removal efficiency was decreased to 99.8, 99.2, and 98.6% at 1500, 1000, and 500 mg/L, respectively.
In the WSB dosage range (500~2000 mg/L) applied in this experiment, the removal efficiency was higher as the dosage increased. However, Ni removal per WSB dosage showed the highest removal at 500 mg/L with the lowest WSB dosage. The reason being that there is almost no difference in removal rate within the range of WSB dosage in this experiment (98.6% to 100% (ND)). Therefore, it is assumed that the amount of Ni removal is large when the injection amount is small. After all, if you want to treat Ni removal rate to 98.6% (0.014 mg/L as concentration), this means that you do not need to inject more WSB than necessary.

Effect of WSB Particle Size
In order to investigate the removal characteristics of Ni according to the particle size difference, WBS was separated and the experiment was conducted for each size section. WBS particle size used in this study was found to vary from 46~850 μm and the ratio of each particle size is in the range of 13 ± 2%.
Experiments were performed at pH 8, WSB dose 2 g/L, adsorption time 60 min, and temperature 30 ℃. Ni removal efficiencies against various particle size, i.e., 46~75 μm, 150~180 μm, 251~355 μm, and 426~850 μm were recorded. Since the particle size experimented on in this study accounts for 91% of the total WSB particle size, it is possible to analyze the Ni removal characteristics by all of the particle size of WSB.
As shown in Figure 5, as the WSB particle size increased to 46~75 μm, 150~180 μm, 251~355 μm, and 426~850 μm, the adsorption effectivities of Ni tended to decrease to not detected (below the device limit <0.00001 mg/L), 99.9, 99.8, and 99.6%, respectively. Therefore, it is more effective to use as small particle a WSB as possible to increase the removal rate of Ni. This might be explained by that as the particle size decreases, the removal efficiency increases due to the increase in the surface area that WSB and nickel can contact. Nevertheless, all of the particles used in this study showed a high Ni removal rate of over 99.6%. In addition, since all of the particle sizes of WSB showed a removal rate of 99.8% or more, it is not necessary to select a specific particle size for removal of Ni. That is, even if the WSB existing in the natural state is used as it is, a sufficient removal (>99.8%) effect can be expected.

Effect of WSB Particle Size
In order to investigate the removal characteristics of Ni according to the particle size difference, WBS was separated and the experiment was conducted for each size section. WBS particle size used in this study was found to vary from 46~850 µm and the ratio of each particle size is in the range of 13 ± 2%.
Experiments were performed at pH 8, WSB dose 2 g/L, adsorption time 60 min, and temperature 30 • C. Ni removal efficiencies against various particle size, i.e., 46~75 µm, 150~180 µm, 251~355 µm, and 426~850 µm were recorded. Since the particle size experimented on in this study accounts for 91% of the total WSB particle size, it is possible to analyze the Ni removal characteristics by all of the particle size of WSB.
As shown in Figure 5, as the WSB particle size increased to 46~75 µm, 150~180 µm, 251~355 µm, and 426~850 µm, the adsorption effectivities of Ni tended to decrease to not detected (below the device limit <0.00001 mg/L), 99.9, 99.8, and 99.6%, respectively. Therefore, it is more effective to use as small particle a WSB as possible to increase the removal rate of Ni. This might be explained by that as the particle size decreases, the removal efficiency increases due to the increase in the surface area that WSB and nickel can contact. Nevertheless, all of the particles used in this study showed a high Ni removal rate of over 99.6%. In addition, since all of the particle sizes of WSB showed a removal rate of 99.8% or more, it is not necessary to select a specific particle size for removal of Ni. That is, even if the WSB existing in the natural state is used as it is, a sufficient removal (>99.8%) effect can be expected.

Effect of WSB Particle Size
In order to investigate the removal characteristics of Ni according to the particle size difference, WBS was separated and the experiment was conducted for each size section. WBS particle size used in this study was found to vary from 46~850 μm and the ratio of each particle size is in the range of 13 ± 2%.
Experiments were performed at pH 8, WSB dose 2 g/L, adsorption time 60 min, and temperature 30 ℃. Ni removal efficiencies against various particle size, i.e., 46~75 μm, 150~180 μm, 251~355 μm, and 426~850 μm were recorded. Since the particle size experimented on in this study accounts for 91% of the total WSB particle size, it is possible to analyze the Ni removal characteristics by all of the particle size of WSB.
As shown in Figure 5, as the WSB particle size increased to 46~75 μm, 150~180 μm, 251~355 μm, and 426~850 μm, the adsorption effectivities of Ni tended to decrease to not detected (below the device limit <0.00001 mg/L), 99.9, 99.8, and 99.6%, respectively. Therefore, it is more effective to use as small particle a WSB as possible to increase the removal rate of Ni. This might be explained by that as the particle size decreases, the removal efficiency increases due to the increase in the surface area that WSB and nickel can contact. Nevertheless, all of the particles used in this study showed a high Ni removal rate of over 99.6%. In addition, since all of the particle sizes of WSB showed a removal rate of 99.8% or more, it is not necessary to select a specific particle size for removal of Ni. That is, even if the WSB existing in the natural state is used as it is, a sufficient removal (>99.8%) effect can be expected.

Thermodynamic Study
In order to evaluate the effect of temperature on the adsorption, thermodynamic parameters were investigated. The thermodynamic parameters, entropy change (∆S o ), Gibbs free energy change (∆G o ), and enthalpy change (∆H o ) were determined using the following van 't Hoff plot [15].
where K L is the equilibrium constant at the temperature T, T is absolute temperature and, R is the gas constant. The van 't Hoff plot for this experiment is shown in Figure 6 and Table   In order to evaluate the effect of temperature on the adsorption, thermodynamic parameters were investigated. The thermodynamic parameters, entropy change (∆S o ), Gibbs free energy change (∆G o ), and enthalpy change (∆H o ) were determined using the following van 't Hoff plot [15].
where KL is the equilibrium constant at the temperature T, T is absolute temperature and, R is the gas constant. The van 't Hoff plot for this experiment is shown in Figure 6 and Table    To continue, the issue that the equilibrium constant of adsorption should be dimensionless in the thermodynamics calculation is discussed, and some alternatives to solve this problem are suggested. In order to apply the dimensionless equilibrium constant of adsorption, Lima et al.   To continue, the issue that the equilibrium constant of adsorption should be dimensionless in the thermodynamics calculation is discussed, and some alternatives to solve this problem are suggested. In order to apply the dimensionless equilibrium constant of adsorption, Lima et al. (2019) proposed Equations (3) and (4) [18]. In addition, Equations (5) and (6) where K e (dimensionless) is the equilibrium constant of an ideal solution, γ (dimensionless) is the coefficient of activity of adsorbate, K e o (dimensionless) is the thermodynamic equilibrium constant, k g is obtained from the isotherm and (Adsorbate) • is the standard concentration of the adsorbate (mol/L) [19].
where K d (dimensionless) is the distribution constant, V is the volume of adsorbate solution (L), m is the mass of adsorbent (g), q e is the sorption capacity (mg/g) at the equilibrium, and C e is the equilibrium concentration (mg/L). [20][21][22].
where K C (dimensionless) is the equilibrium constant, C e is the adsorbate equilibrium concentration in the supernatant phase (mg/L), C o is the initial adsorbate concentration (mg/L), and C s is the concentration is the solid phase (mg/L) [21,[23][24][25].
The dimensionless equilibrium constant values calculated by Equations (4)-(6) are shown in Table 2.

Adsorption Kinetics
The adsorption kinetics of Ni onto WSB are shown in Figures 7 and 8. Pseudo-first-order kinetic model and pseudo-second-order kinetic model were considered for experimental data.
The pseudo-first-order kinetic model (Figure 7) can be applied for the adsorption of solid/liquid systems. The integrated linear form is given by log(q e − q t ) = logq e − (k 1 t/2.303) (7) where q t is the adsorption capacity at time t (mg/g) and k 1 (min −1 ) is the rate constant.
The pseudo-second-order kinetic model (Figure 8) is based on the assumption that the adsorption follows second order chemisorption [26]. The integrated linear form is given by t/q t = 1/(k 2 q e 2 ) + 1/q e t (8) where k 2 is the pseudo-second-order rate constant (g/mg min). k 2 and q e can be calculated from the intercept and slope of the plot of t/q t against t. where Kd (dimensionless) is the distribution constant, V is the volume of adsorbate solution (L), m is the mass of adsorbent (g), qe is the sorption capacity (mg/g) at the equilibrium, and Ce is the equilibrium concentration (mg/L). [20][21][22].
where KC (dimensionless) is the equilibrium constant, Ce is the adsorbate equilibrium concentration in the supernatant phase (mg/L), Co is the initial adsorbate concentration (mg/L), and Cs is the concentration is the solid phase (mg/L) [21,[23][24][25].
The dimensionless equilibrium constant values calculated by Equations (4)-(6) are shown in Table 2.

Adsorption Kinetics
The adsorption kinetics of Ni onto WSB are shown in Figures 7 and 8. Pseudo-first-order kinetic model and pseudo-second-order kinetic model were considered for experimental data.
The pseudo-first-order kinetic model (Figure 7) can be applied for the adsorption of solid/liquid systems. The integrated linear form is given by log(qe − qt) = logqe − (k1t/2.303) (7) where qt is the adsorption capacity at time t (mg/g) and k1 (min −1 ) is the rate constant.
The pseudo-second-order kinetic model (Figure 8) is based on the assumption that the adsorption follows second order chemisorption [26]. The integrated linear form is given by t/qt = 1/(k2qe 2 ) + 1/qet (8) where k2 is the pseudo-second-order rate constant (g/mg min). k2 and qe can be calculated from the intercept and slope of the plot of t/qt against t.     Table 3 shows the parameters of the two models utilized. From the fitting results and adsorption capacities, the theoretical adsorption capacity of the pseudo-first-order kinetic model and pseudo-second-order kinetic model were calculated as 0.07 mg/g and 0.20 mg/g, respectively.
In addition, as shown in Table 3, pseudo-second-order kinetic model shows correlation coefficients of 0.9994 i.e., nearly equal to 1. In contrast, the pseudo-first-order kinetic model shows a lower value, i.e., 0.9004. Therefore, based on the correlation coefficient (R 2 ), the experimental kinetics data for Ni ions' adsorption onto WSB is in better agreement with the pseudo-second-order kinetics model than with the pseudo-first-order kinetic model. In addition, the calculated qe value obtained from the pseudo-second-order kinetic model also shows good agreement with the experimental qe value. Table 3. A comparison of the pseudo-first-order and pseudo-second-order rate constants (pH 8.0, initial Ni concentration 1.0 mg/L, WSB dose 2 g/L, adsorption time 60 min, WSB particle size 46~75 μm, and temperature 30 ℃).

Adsorption Isotherms
The sorption isotherms of Ni ions for WSB were calculated. Langmuir and Freundlich models were used to analyze the equilibrium adsorption of Ni onto WSB in this study. The Langmuir model can be reasonably applied when ions adsorbed on the mineral surface are adsorbed only in a single layer, and when the adsorption site and the mutual attraction between the adsorbed ions are always the same regardless of the adsorbed amount. However, the Freundlich model is based on the assumption that adsorption is not the same because it has a high adsorption coefficient on some surfaces, and is initially covered only by molecules. On the surface, the heat generated by adsorption is higher. The heat of adsorption is logarithmically dependent on the surface range. The Freundlich model is applied for the adsorption of heterogeneous surfaces of the multilayer [27]. Figure 9 depicts the Langmuir isotherm model linear relation that was used to fit the experimental data at equilibrium. The experimental data nearly fits the Langmuir isotherm model isotherm. However, the Freundlich model ( Figure 10) presented poor linearity. The parameters for Langmuir and Freundlich isotherms and R 2 are summarized in Table 4. As can be seen from the R 2 , it was found that the WSB sorption process was more consistent with the Langmuir model. That is, it is estimated that Ni is removed through monolayer adsorption onto WSB.  Table 3 shows the parameters of the two models utilized. From the fitting results and adsorption capacities, the theoretical adsorption capacity of the pseudo-first-order kinetic model and pseudo-second-order kinetic model were calculated as 0.07 mg/g and 0.20 mg/g, respectively.
In addition, as shown in Table 3, pseudo-second-order kinetic model shows correlation coefficients of 0.9994 i.e., nearly equal to 1. In contrast, the pseudo-first-order kinetic model shows a lower value, i.e., 0.9004. Therefore, based on the correlation coefficient (R 2 ), the experimental kinetics data for Ni ions' adsorption onto WSB is in better agreement with the pseudo-second-order kinetics model than with the pseudo-first-order kinetic model. In addition, the calculated q e value obtained from the pseudo-second-order kinetic model also shows good agreement with the experimental q e value. Table 3. A comparison of the pseudo-first-order and pseudo-second-order rate constants (pH 8.0, initial Ni concentration 1.0 mg/L, WSB dose 2 g/L, adsorption time 60 min, WSB particle size 46~75 µm, and temperature 30 • C).

Adsorption Isotherms
The sorption isotherms of Ni ions for WSB were calculated. Langmuir and Freundlich models were used to analyze the equilibrium adsorption of Ni onto WSB in this study. The Langmuir model can be reasonably applied when ions adsorbed on the mineral surface are adsorbed only in a single layer, and when the adsorption site and the mutual attraction between the adsorbed ions are always the same regardless of the adsorbed amount. However, the Freundlich model is based on the assumption that adsorption is not the same because it has a high adsorption coefficient on some surfaces, and is initially covered only by molecules. On the surface, the heat generated by adsorption is higher. The heat of adsorption is logarithmically dependent on the surface range. The Freundlich model is applied for the adsorption of heterogeneous surfaces of the multilayer [27]. Figure 9 depicts the Langmuir isotherm model linear relation that was used to fit the experimental data at equilibrium. The experimental data nearly fits the Langmuir isotherm model isotherm. However, the Freundlich model ( Figure 10) presented poor linearity. The parameters for Langmuir and Freundlich isotherms and R 2 are summarized in Table 4. As can be seen from the R 2 , it was found that the WSB sorption process was more consistent with the Langmuir model. That is, it is estimated that Ni is removed through monolayer adsorption onto WSB.     Table 5 summarizes the Ni removal efficiencies of various adsorbents. Due to the different initial concentrations and operating conditions, it is difficult to directly compare the literature results with the results of this study. However, compared with other literature studies, some operating conditions are similar, and even with very low nickel concentrations of 1 mg/L, the results show very high removal efficiencies. Therefore, it can be concluded that WSB may be sufficiently available as a potential adsorbent for Ni removal.       Table 5 summarizes the Ni removal efficiencies of various adsorbents. Due to the different initial concentrations and operating conditions, it is difficult to directly compare the literature results with the results of this study. However, compared with other literature studies, some operating conditions are similar, and even with very low nickel concentrations of 1 mg/L, the results show very high removal efficiencies. Therefore, it can be concluded that WSB may be sufficiently available as a potential adsorbent for Ni removal.    Table 5 summarizes the Ni removal efficiencies of various adsorbents. Due to the different initial concentrations and operating conditions, it is difficult to directly compare the literature results with the results of this study. However, compared with other literature studies, some operating conditions are similar, and even with very low nickel concentrations of 1 mg/L, the results show very high removal efficiencies. Therefore, it can be concluded that WSB may be sufficiently available as a potential adsorbent for Ni removal.

Conclusions
Experimental investigation revealed that WSB could be used as a possible adsorbent, for the removal of nickel from aqueous solution. The experimental conditions, i.e., contact time, pH, WSB dosage, particle size of WSB, and temperature were found to have a significant effect on the adsorption process. The experimental equilibrium revealed that the WSB adsorption process followed the Langmuir equation. The kinetic studies found that the WSB adsorption process fitted well the pseudo-second-order model. The optimal conditions for adsorption of nickel ions on each parameter were pH 8, WSB dose of 2 g/L, adsorption time 60 min, WSB particle size 46~75 µm, and temperature 30 • C.
We revealed the maximum nickel ion adsorption capacity 100 µg/g, the maximum nickel removal efficiency was not detected (below the device limit <0.01 µg/L).
Compared with the removal of nickel ion adsorption experiments, using various adsorbents, it could be used as an available adsorbent for nickel removal. Above all, WSB can be used as an adsorbent that can effectively remove nickel in water and wastewater, and it is expected that it can be particularly applied to the removal of low-concentration nickel.