Efficient Removal of Cd(II) Using SiO2-Mg(OH)2 Nanocomposites Derived from Sepiolite.

The pollution of Cadmium (Cd) species in natural water has attracted more and more attention due to its high cumulative toxicity. In the search for improved removal of cadmium from contaminated water, we characterized uptake on a recently identified nanomaterial (SiO2-Mg(OH)2) obtained by subjecting sepiolite to acid-base modification. The structural characteristics of SiO2-Mg(OH)2 were analyzed by means of SEM-EDS, Fourier Transform Infra-Red Spectroscopy (FTIR) and Powder X-ray Diffraction (PXRD). Static adsorption experiments were carried out to evaluate the effect of contact time, temperature, amount of adsorbent, and pH-value on the adsorption of Cd(II) by SiO2-Mg(OH)2. The results show that the pore structure of SiO2-Mg(OH)2 is well developed, with specific surface area, pore size and pore volume increased by 60.09%, 16.76%, and 43.59%, respectively, compared to natural sepiolite. After modification, the sepiolite substrate adsorbs Cd(II) following pseudo-second-order kinetics and a Langmuir surface adsorption model, suggesting both chemical and physical adsorption. At 298 K, the maximum saturated adsorption capacity fitted by Sips model of SiO2-Mg(OH)2 regarding Cd(II) is 121.23 mg/g. The results show that SiO2-Mg(OH)2 nanocomposite has efficient adsorption performance, which is expected to be a remediation agent for heavy metal cadmium polluted wastewater.


Introduction
In recent years, due to the high toxicity, non-degradability, enrichment of heavy metals and the potential threat to human health through water pollution, the problem of heavy metal pollution has attracted more and more attention [1,2]. The heavy metal cadmium(Cd) is a non-essential element of the human body and is widely used in electroplating, batteries, smelting and chemical production processes, resulting in a considerable amount of cadmium in wastewater, waste gas and waste residue [3]. After entering the environment, it cannot be biodegraded. It is enriched and transferred in the body through the food chain. When the concentration of cadmium reaches a certain level, cadmium poisoning will occur, and it can even cause various diseases of the human body [4,5]. Most of the cadmium in water exists in the form of metal cation Cd(II).At present, common methods for removing Cd(II) are ion exchange [6], chemical precipitation [7], membrane separation [8], and adsorption [9]. Adsorption method is widely used because of its advantages such as simple operation, simple process, and abundant sources of adsorbent. Natural clay minerals have good internal pore structure, large surface area, strong chemical adsorption, and abundant reserves, so they are expected to become cheap adsorbents for water treatment [10].

Adsorption Equilibrium and Kinetic Experiments
Dissolved the cadmium powder with concentrated nitric acid to make a mother liquor of 1 g/L, pH 3. In the experiment, the simulated wastewater containing different concentrations of cadmium was diluted by the mother liquor multiples. Sepiolite was added to the simulated cadmium wastewater and oscillating perform adsorption at a rate of 150 r/min according to different experimental requirements and conditions. After passing through a 0.45 µm filter, the concentration of Cd(II) in the filtrate was measured using an atomic absorption spectrophotometer (Ultra TAS-990, Purkinje General Instrument Co., Ltd., Beijing, China). Cd(II) adsorption efficiency η (%) and equilibrium adsorption capacity (q e ) are calculated by the following formulas: where C 0 and C e are the initial and equilibrium concentrations (mg/L) of Cd(II) respectively, m is the mass of the adsorbent (g), and V is the volume of the solution (L). For the kinetic studies, 50 mL Cd(II) solutions (50, 100 and 150 mg/L) were added to a 100 mL capped Erlenmeyer flask with HCl (0.1-1 mg/L) and NaOH (0.1-1 mg/L) to adjust the pH value to 7.0, put it in a constant temperature air bath shaker, and add 0.5 g/L of SiO 2 -Mg(OH) 2 when the temperature reaches 45 • C. After shaking for a certain time (5-720 min) at constant temperature the adsorption of SiO 2 -Mg(OH) 2 for heavy metal cation Cd(II) was recorded. The adsorbed amount is plotted as a function of the adsorption time.

Effect of Adsorbent Dosage and pH-Value
Under the experimental conditions of a temperature of 35 • C, a solution volume of 50 mL, an initial concentration of Cd(II) of 100 mg/L, an initial pH value of 6 and a oscillation adsorption time of 240 min, the SiO 2 -Mg(OH) 2 was investigated. When the adsorbent dosage (0.2-2.5 g/L) is different, it will affect the adsorption of heavy metal Cd (II) respectively.
Varying the pH value between 2 and 7 by keeping the other parameters constant, the effect of the pH value on Cd(II) adsorption is evaluated.

Adsorption Isotherms
Adsorption isotherm experiments at three different temperatures (25 • C, 35 • C and 45 • C) were carried out by changing the initial concentration of Cd(II) from 30 to 200 mg/L. The procedure was like the procedure above except for vibrating for 4 h. Isotherm models such as the Freundlich, the Langmuir and the Sips isotherm were employed to interpret the adsorption isotherm data.

Results and Discussion
3.1. Characterization of SiO 2 -Mg(OH) 2 Nanocomposites 3.1.1. SEM-EDS Analysis Figure 1a,b shows the SEM-EDS images of the sepiolite and SiO 2 -Mg(OH) 2 . It can be seen from Figure 1a that the structure of the sepiolite material is a thin and straight rod-like fiber structure. In Figure 1b, a scanning electron microscope image of the treated sepiolite-SiO 2 -Mg(OH) 2 material after being treated with hydrochloric acid and ammonia is shown. It can be seen that compared to the original the surface of the composite material is uneven, and shows broken nanofibers in a messy block like structure, showing agglomeration and increased pore volume, indicating that the sepiolite-like magnesium octahedron structure is destroyed, the silicon-oxygen tetrahedron is retained, and SiO 2 colloids are shown formed [29]. The EDS in Figure 1 confirms the main constituent elements of the sepiolite as well as of the SiO 2 -Mg(OH) 2 material, which are O, Si, Mg. The signal for magnesium in SiO 2 -Mg(OH) 2 is stronger than for pure sepiolite due to the relative enrichment during alkali treatment to form a magnesium-containing compound on the surface.

N 2 Adsorption-Desorption Isotherm and Pore Size Distribution
The N 2 adsorption-desorption isotherm and pore size distribution for sepiolite and the SiO 2 -Mg(OH) 2 composite is shown in Figure 1c,d. Both materials show type IV adsorption [30] and their pore size distribution is concentrated in 2-50 nm diameter, which indicates that there is a large number of mesopores in the sample, which is conducive to adsorption. The specific surface area of the SiO 2 -Mg(OH) 2 composite material (458.29 m 2 /g) is significantly higher than that of sepiolite (301.66 m 2 /g), and the average pore diameters of the SiO 2 -Mg(OH) 2 composite material and sepiolite calculated by BJH are 3.97 nm and 3.40 nm, respectively, the pore volumes are 0.56 and 0.39 cm 3 /g. Compared with the original sepiolite, they have increased by 60.09%, 16.76%, and 43.59%. After modification, surface properties have been greatly improved due to removal of Mg 2+ from the structure by continuous alkali treatment, from which Mg(OH) 2 , becomes attached to the acidified sepiolite. Because of this, the specific surface area and pore volume of sepiolite are further increased.

FTIR Analysis
In Figure 1e the infrared spectrum of sepiolite and SiO 2 -Mg(OH) 2 composite material are overlain. The wave numbers range between 1300-900 cm −1 , it is a wide and strong stretching vibration band produced by Si-O-Si [31,32]. The peak at 3569 cm −1 corresponds to the stretching of OH connected to Mg 2+ . The absorption peak at 485 cm The vibration band in the range of 700-600 cm −1 is different from that of SiO 2 , which may be related to the brucite subsequently formed.

PXRD Analysis
X-ray diffraction was used to test the internal crystal structure of sepiolite and SiO 2 -Mg(OH) 2 composites. During the test, the 2θ angle range was 5 • to 60 • . It can be seen from the Sep spectrum in Figure

PXRD Analysis
X-ray diffraction was used to test the internal crystal structure of sepiolite and SiO2-Mg(OH)2 composites. During the test, the 2θ angle range was 5° to 60°. It can be seen from the Sep spectrum in Figure

Adsorption Kinetics
Non-linear fitting was performed using pseudo-first-order (3) and pseudo-second-order (4) equations. The formulae are used as follows: where t is the adsorption time (min); K1 is the pseudo-first-order kinetic equation rate constant (min −1 ); K2 is the pseudo-second-order kinetic equation rate constant (g•mg −1 •min −1 ); qt and qe are the Cd(II) adsorption amount (mg•g −1 ) at time t and adsorption equilibrium, respectively.
As shown in Figure 3, the adsorption capacity of SiO2-Mg(OH)2 for Cd(II) first increased rapidly and then stabilized: within 1 h, the adsorption rate of the material is very large; within 1-3 h, the adsorption rate of the material gradually decreased and reached equilibrium in about 4 h. The maximum adsorption of Cd(II) at an initial concentration of 150 mg•g −1 is 92.76 mg•g −1 ,which is much larger than the original sepiolite adsorption capacity for Cd(II) [33]. This is because the SiO2-Mg(OH)2 composite has a larger specific surface area, so the number of surface active adsorption sites after the modification of the original sepiolite increases, and the adsorption capacity is enhanced. The initial rapid adsorption might be due to the external diffusion of Cd(II) transfer into the porous structure fast enough. This process is mainly ion exchange and physical adsorption. However, with the increase in time, the free active sites on the surface of the adsorbent for Cd(II) adsorption decreased, the concentration gradient of Cd (II) in the solution and the surface of the adsorbent decreased, and the driving force for adsorption decreased, so for Cd(II) the resistance of adsorption by diffusion into the adsorbent increases, and the adsorption rate gradually decreases [34].
From Table 1, it can be seen that the R 2 value of SiO2-Mg(OH)2 for Cd (II) adsorption by quasi-second-order kinetics is above 0.99, which is greater than the R 2 value of quasi-first-order kinetics. On this basis it can be inferred that the adsorption of Cd (II) by SiO2-Mg(OH)2 is dominated by chemisorptions, and true chemical bond formation between the adsorbent and Cd (II) [35].

Adsorption Kinetics
Non-linear fitting was performed using pseudo-first-order (3) and pseudo-second-order (4) equations. The formulae are used as follows: where t is the adsorption time (min); K 1 is the pseudo-first-order kinetic equation rate constant (min −1 ); K 2 is the pseudo-second-order kinetic equation rate constant (g·mg −1 ·min −1 ); q t and q e are the Cd(II) adsorption amount (mg·g −1 ) at time t and adsorption equilibrium, respectively. As shown in Figure 3, the adsorption capacity of SiO 2 -Mg(OH) 2 for Cd(II) first increased rapidly and then stabilized: within 1 h, the adsorption rate of the material is very large; within 1-3 h, the adsorption rate of the material gradually decreased and reached equilibrium in about 4 h. The maximum adsorption of Cd(II) at an initial concentration of 150 mg·g −1 is 92.76 mg·g −1 , which is much larger than the original sepiolite adsorption capacity for Cd(II) [33]. This is because the SiO 2 -Mg(OH) 2 composite has a larger specific surface area, so the number of surface active adsorption sites after the modification of the original sepiolite increases, and the adsorption capacity is enhanced. The initial rapid adsorption might be due to the external diffusion of Cd(II) transfer into the porous structure fast enough. This process is mainly ion exchange and physical adsorption. However, with the increase in time, the free active sites on the surface of the adsorbent for Cd(II) adsorption decreased, the concentration gradient of Cd (II) in the solution and the surface of the adsorbent decreased, and the driving force for adsorption decreased, so for Cd(II) the resistance of adsorption by diffusion into the adsorbent increases, and the adsorption rate gradually decreases [34].

Adsorption Kinetics
Non-linear fitting was performed using pseudo-first-order (3) and pseudo-second-order (4) equations. The formulae are used as follows: where t is the adsorption time (min); K1 is the pseudo-first-order kinetic equation rate constant (min −1 ); K2 is the pseudo-second-order kinetic equation rate constant (g•mg −1 •min −1 ); qt and qe are the Cd(II) adsorption amount (mg•g −1 ) at time t and adsorption equilibrium, respectively.
As shown in Figure 3, the adsorption capacity of SiO2-Mg(OH)2 for Cd(II) first increased rapidly and then stabilized: within 1 h, the adsorption rate of the material is very large; within 1-3 h, the adsorption rate of the material gradually decreased and reached equilibrium in about 4 h. The maximum adsorption of Cd(II) at an initial concentration of 150 mg•g −1 is 92.76 mg•g −1 ,which is much larger than the original sepiolite adsorption capacity for Cd(II) [33]. This is because the SiO2-Mg(OH)2 composite has a larger specific surface area, so the number of surface active adsorption sites after the modification of the original sepiolite increases, and the adsorption capacity is enhanced. The initial rapid adsorption might be due to the external diffusion of Cd(II) transfer into the porous structure fast enough. This process is mainly ion exchange and physical adsorption. However, with the increase in time, the free active sites on the surface of the adsorbent for Cd(II) adsorption decreased, the concentration gradient of Cd (II) in the solution and the surface of the adsorbent decreased, and the driving force for adsorption decreased, so for Cd(II) the resistance of adsorption by diffusion into the adsorbent increases, and the adsorption rate gradually decreases [34].
From Table 1, it can be seen that the R 2 value of SiO2-Mg(OH)2 for Cd (II) adsorption by quasi-second-order kinetics is above 0.99, which is greater than the R 2 value of quasi-first-order kinetics. On this basis it can be inferred that the adsorption of Cd (II) by SiO2-Mg(OH)2 is dominated by chemisorptions, and true chemical bond formation between the adsorbent and Cd (II) [35].   From Table 1, it can be seen that the R 2 value of SiO 2 -Mg(OH) 2 for Cd (II) adsorption by quasi-second-order kinetics is above 0.99, which is greater than the R 2 value of quasi-first-order kinetics.
On this basis it can be inferred that the adsorption of Cd (II) by SiO 2 -Mg(OH) 2 is dominated by chemisorptions, and true chemical bond formation between the adsorbent and Cd (II) [35].

Effect of SiO 2 -Mg (OH) 2 Dosage and pH on Cd (II) Removal
The study of the effect of the adsorbent dose and pH on the adsorption performance and the maximum adsorption effect of the adsorbent on different metal ions has obvious practical significance for the industrial application of the adsorbent. It can be seen from Figure 4 that as the amount of SiO 2 -Mg(OH) 2 added increases, the removal rate of Cd (II) also increases, but the rate of increase slows and finally stabilizes. When the dose is 2 g/L, the removal rate reaches 90.88%. At the same time, with the increase of the adsorbent, the adsorption capacity showed a trend of first increase and then decrease. When the amount of adsorbent is 0.4 g/L, the maximum adsorption amount reaches 71.88 mg/g. This is because with the increase of the amount of adsorbent added, the number of adsorption sites is increasing, so that the removal rate increases rapidly at the beginning. However, when the amount of the adsorbent exceeds a certain concentration, some of the adsorption sites of the adsorbent cannot be completely occupied. As a result, the amount of Cd (II) adsorbed by the adsorption unit site is lower, which is shown by a decrease in the adsorption unit.

Effect of SiO2-Mg (OH)2Dosage and pH on Cd (II) Removal
The study of the effect of the adsorbent dose and pH on the adsorption performance and the maximum adsorption effect of the adsorbent on different metal ions has obvious practical significance for the industrial application of the adsorbent. It can be seen from Figure 4 that as the amount of SiO2-Mg(OH)2 added increases, the removal rate of Cd (II) also increases, but the rate of increase slows and finally stabilizes. When the dose is 2 g/L, the removal rate reaches 90.88%. At the same time, with the increase of the adsorbent, the adsorption capacity showed a trend of first increase and then decrease. When the amount of adsorbent is 0.4 g/L, the maximum adsorption amount reaches 71.88 mg/g. This is because with the increase of the amount of adsorbent added, the number of adsorption sites is increasing, so that the removal rate increases rapidly at the beginning. However, when the amount of the adsorbent exceeds a certain concentration, some of the adsorption sites of the adsorbent cannot be completely occupied. As a result, the amount of Cd (II) adsorbed by the adsorption unit site is lower, which is shown by a decrease in the adsorption unit.
In the pH range from 2 to 7 (Figure 4), the amount adsorbed and removal of Cd (II) increased with the increase of pH. The adsorption effect is poorest at pH 2, because the lower the pH is, the higher the H + content, the more intense the competition between H + and Cd 2+ for SiO2-Mg(OH)2 surface adsorption sites. The optimal pH was 7.0, and the maximum adsorption amount and removal rate were 69.48 mg/g and 34.74%, respectively. When pH > 7, OHin the solution will bind to Cd 2+ and exist as Cd(OH) + or Cd(OH)2, which will cause precipitation and inhibit adsorption [36].

Adsorption Isotherm
Three adsorption models ( Figure 5) were used to study uptake data: Langmuir (5), Freundlich (6), and Sips (L-F) (7) were used for nonlinear fitting of adsorption isotherms at different temperatures (25 °C, 35 °C, and 45 °C).  In the pH range from 2 to 7 (Figure 4), the amount adsorbed and removal of Cd (II) increased with the increase of pH. The adsorption effect is poorest at pH 2, because the lower the pH is, the higher the H + content, the more intense the competition between H + and Cd 2+ for SiO 2 -Mg(OH) 2 surface adsorption sites. The optimal pH was 7.0, and the maximum adsorption amount and removal rate were 69.48 mg/g and 34.74%, respectively. When pH > 7, OHin the solution will bind to Cd 2+ and exist as Cd(OH) + or Cd(OH) 2 , which will cause precipitation and inhibit adsorption [36].
where q e is the equilibrium adsorption amount (mg/g); C e is the equilibrium concentration (mg/L); Q m is the saturation adsorption amount (mg/g); K L (L·mg −1 ), K F [mg·g −1 · (L·mg −1 ) 1/n ] and K S [(L·mg −1 ) β ] are Langmuir adsorption constant, Freundlich adsorption constant, and Sips adsorption constant, respectively; 1/n is a constant for adsorption strength, which varies with the heterogeneity of the material; β is the Sips isotherm index [37]. where qe is the equilibrium adsorption amount (mg/g); Ce is the equilibrium concentration (mg/L); Qm is the saturation adsorption amount (mg/g); KL (L•mg −1 ), KF [mg•g −1 • (L•mg −1 ) 1/n ] and KS [(L•mg −1 ) β ] are Langmuir adsorption constant, Freundlich adsorption constant, and Sips adsorption constant, respectively; 1/n is a constant for adsorption strength, which varies with the heterogeneity of the material; β is the Sips isotherm index [37]. It can be seen from Table 2 that the R 2 values of Langmuir and Sips isotherms are higher than the R 2 values of the Freundlich isotherms at different temperatures, and both can better describe the adsorption process of Cd (II). The Sips model has the best correlation. Langmuir isotherm is suitable for monolayer adsorption with uniformly distributed adsorption sites on the surface of the adsorbent and there is no interaction between adjacent sites and adsorbent particles, while the Freundlich adsorption isotherm model is an empirical formula for multi-layer adsorption [38,39]. Therefore, it can be speculated that the adsorption of Cd(II) at different temperatures is mainly a uniform monolayer adsorption process.  It can be seen from Table 2 that the R 2 values of Langmuir and Sips isotherms are higher than the R 2 values of the Freundlich isotherms at different temperatures, and both can better describe the adsorption process of Cd (II). The Sips model has the best correlation. Langmuir isotherm is suitable for monolayer adsorption with uniformly distributed adsorption sites on the surface of the adsorbent and there is no interaction between adjacent sites and adsorbent particles, while the Freundlich adsorption isotherm model is an empirical formula for multi-layer adsorption [38,39]. Therefore, it can be speculated that the adsorption of Cd(II) at different temperatures is mainly a uniform monolayer adsorption process. The Sips isothermal model is a combination of Langmuir and Freundlich equations, and is suitable for describing the adsorption of monolayer heterogeneous surfaces on heterogeneous adsorption systems under various pressures. At lower metal ion concentrations, it is closer to the Freundlich isotherm; while at high concentrations, it describes monolayer adsorption in a similar way to the Langmuir isotherm. The higher the Sips constant K s value, the stronger the bond between the adsorbate and the active site of the adsorbent [33]. It can be seen that as the temperature increases, the adsorption capacity of SiO 2 -Mg(OH) 2 becomes higher. An increase of the adsorption capacity with increasing temperature is typical for chemisorption processes.
According to the Sips model, the maximum adsorption capacity of Cd(II) by SiO 2 -Mg(OH) 2 is 121.23 mg/g at 25 • C. According to Table 3, it can be seen that SiO 2 -Mg(OH) 2 has compared with similar adsorbents very good adsorption performance.

Thermodynamic Parameters
Thermodynamic parameters (the standard free energy ∆G • , standard enthalpy ∆H • and standard entropy ∆S • ) at three different temperatures were calculated by Equations (8) and (9), respectively.
where K 0 is the distribution coefficient obtained from the Langmuir isotherm (L/mol). With lnK 0 plotted against 1/T, the intercept and slope obtained from the linear regression analysis can be used to calculate ∆S • and ∆H • , respectively. And ∆G • can be calculated directly from Equation (8).
As shown in Table 4, when the temperature increases from 298 K to 318 K, the adsorption of Cd(II) on SiO 2 -Mg(OH) 2 has ∆G • < 0 and ∆H • > 0, indicating that the adsorption is a spontaneous endothermic reaction. ∆H • is mainly physical adsorption between 0-80 kJ·mol −1 , and chemical adsorption between 80-800 kJ·mol −1 [43]. In this study, the ∆H • of SiO 2 -Mg(OH) 2 was 29.71 kJ·mol −1 between 298 and 318 K, therefore the adsorption process of Cd(II) belongs to physical adsorption. The adsorption mechanism may be electrostatic force and pore filling. Combining with the results of kinetic adsorption, it can be seen that there is physical and chemical adsorption of SiO 2 -Mg(OH) 2 for Cd(II), but mainly chemisorption. lnK 0 increases with increasing temperature, indicating that temperature rise is conducive to the progress of adsorption.

Conclusions
The SiO 2 -Mg(OH) 2 nanocomposite based on sepiolite has developed pores and a large specific surface area and can be used as a good material for adsorbing Cd(II). The adsorption was dependent mainly on the initial solution pH, whereas the optimal Cd(II) adsorption was achieved at pH 7. Adsorption kinetics followed a pseudo-second-order kinetic model. The adsorption isotherm data of the nanocomposite adsorption were successfully described by the Sips isotherm model with a maximum adsorption capacity of 121.23-141.49 mg/g at 25-45 • C.
Therefore, this kind of nanocomposite material can effectively remove Cd(II) in water, and has great application potential in remediation of heavy metal cadium polluted wastewater.