Studies on the Removal of Cadmium Toxic Metal Ions by Natural Clays from Aqueous Solution by Adsorption Process

Laboratory of Physical Chemistry of Materials LCPM, Faculty of Sciences Ben M’Sik, Hassan II University of Casablanca, B.P.7955, Bd CdtDriss El Harti, Casablanca, Morocco Laboratory of Organic, Organometallic and +eoretical Chemistry, University Ibn Tofail, Faculty of Science, B.P. 133, Kenitra 14000, Morocco Laboratory of Physico-Chemistry of Inorganic and Organic Materials (LPCIOM), Materials Science Centre (MSC), Mohammed V University, Ecole Normale Supérieure (ENS), Rabat, Morocco Regional Center for Education and Training Occupations, Casablanca Anfa, Bd Bir Anzarane, Casablanca, Morocco


Introduction
Cadmium is selected as a highly toxic inorganic pollutant to the environment or other living organisms. e environmental problem of cadmium is a consequence created by a human through their activities such as alloy in the automotive industry; as a pigment; as a stabilizer for plastics, in battery manufacturing, and nuclear reactors [1][2][3]. Several studies have shown that long-term use of water contaminated with this element causes adverse effects on human health [4,5]. Considering the toxicity and adverse effects of cadmium on human health, the World Health Organization (WHO) fixed the maximum concentration of cadmium in drinking water at 0.03 mg/L [6]. e elimination of these types of pollutants is always a major challenge. Numerous studies have developed several treatment processes to reduce the amount of cadmium in aquatic environments [7][8][9][10][11]. e adsorption process is a better advantage rather the other methods due to their simplicity, low cost, and efficiency in removal of contaminants of a different nature in particular heavy metals [12,13]. In addition, research and development of new natural adsorbents which are abundant, economically profitable, and efficient for the treatment of ecosystems is a great challenge. In this context, the application of natural clay minerals is of great scientific interest. e adsorption of different pollutant by clays minerals has three types: the first is physical and nonionic adsorption, the second is ion exchange with electrostatic interaction and exchange reaction, and the third is zeolite action [14][15][16][17]. e objective of this work was to optimize the adsorption performance of cadmium by natural clays minerals, without any purification step, through its casting as adsorbent to replace expensive adsorbents used in the treatment of cadmium-polluted water. In particular, this study was focused on the elimination of cadmium in aqueous solution by retention onto local natural clays QC-MC and QC-MT collected from the Middle Atlas in the region of Khenifra, Morocco, and their ability to regenerate and be used again. QC-MC and QC-MT were selected based on their different physical and chemical properties such as chemical composition, nature of the oxygen and hydrogen surface atoms, defect sites, layer charge, and the type of exchangeable cations. Moreover, they were also selected according to their originality. QC-MC and QC-MT have never been the subject of a structural and mineralogical study allowing the knowledge of their nature and their performance over the adsorption of cadmium ions. erefore, this work ensures the determination of the physicochemical properties of QC-MC and QC-MT. Hence, we studied the performance of cadmium adsorption selected as an inorganic pollutant. e removal of cadmium using QC-MC and QC-MT was carried out by varying several parameters such as adsorbent mass, pH of the solution, initial concentration of cadmium, contact time, and temperature. e kinetic, isotherm, and thermodynamic studies were investigated. e regeneration study was also examined by using different acids.

Reagents.
All chemical reactants were of high analytical quality and were applied without further distillation, which was obtained from the LaboChimie Society. All solutions were prepared with high purity deionized water. In addition, all glassware was washed by soaking in 10% HCl and rinsed three times with deionized water. e Cd (II) stock solution was prepared from cadmium chloride (CdCl 2 .H 2 O).

Preparation and Characterization of Adsorbents.
QC-MC and QC-MT were collected from the Middle Atlas Mountains of Morocco and were used as adsorbents to remove cadmium cations. QC-MC and QC-MT were ground and sieved, and the fraction contains 500 µm and was used in this study without any further pretreatment. e X-ray diffraction (XRD) detected the mineralogical composition using BRUKER-Binary V4, the Fourier transform infrared (FTIR) was used to determine the grouping structural of adsorbents which was executed by SHIMADZU8400S, the inductively coupled plasma mass spectrometry (ICP-MS) allows the determination of a wide range of inorganic elements and elementary impurities, in one go, with extreme reliability and precision, the scanning electron microscopy (SEM) was used to provide the detailed high-resolution image with Zeiss model, the cation exchange capacity (CEC) parameter has been made at the free pH of the suspensions by the cobalt hexamine method, the porosity was affected by the mercury intrusion porosimeter method recorded on Quantachrome Mercury POREMASTER, and the point of zero charges (PZC) was determined using a drift method.

Batch Adsorption Studies.
A batch test was conducted to investigate Cd (II) adsorption at several effects pH of the solution, concentration, mass, contact time, and temperature. e pH was adjusted by adding HCl (1M) or NaOH (1M). e solution of adsorption Cd (II) was shaken at 450 rpm. After the adsorption reaction, the suspension was centrifuged for 10 min at 3 500 rpm and then filtered. e Cd (II) concentration in an aqueous solution was revealed by ICP-MS using the PerkinElmer model. e quantity of cadmium (II) adsorbed is intended by the following equation: where Q e is the capacity of Cd (II) adsorbed at equilibrium (mg/g), C o is the initial concentration of cadmium (mg/L), C e is the concentration of cadmium in equilibrium (mg/L), m is adsorbents dose (g), and V is solution volume (L).

Isotherm
Modeling. Adsorption isotherms are experimental curves that represent variation in adsorbed metal ions by adsorbent as a function of concentration. ey are generally expressed in the form of mathematical equations. e equilibrium isotherm analysis was affected by applying two models: Langmuir and Freundlich [18].
e Langmuir isotherm theory supposes that adsorption is a monolayer and occurs at homogeneous sites specific to the adsorbent. e linear equation from the Langmuir equation is expressed as where K L is the equilibrium constant comparative to the Langmuir model and Q max is the maximum amount adsorbed of Cd (II) (mg/g). e Q max and K L are determined from the drift. e Freundlich isotherm presumes that adsorption is multilayer and the adsorbent surface is heterogeneous. It can be confirmed by the following equation: where K F (L/g) and n are the Freundlich constants representing the adsorption and intensity, respectively.

Kinetic
Modeling. e kinetic study permits us to examine the impact of contact time on its retention. Usually, kinetic results allow us to determine the rate and mechanism of adsorption. In this study, two models were applied to describe the mechanism of kinetic adsorption of Cd (II) on adsorbents: the pseudo-first-order and the pseudo-secondorder [19].
e pseudo-first-order kinetic model is presented by Lagergren in the equation: where Q t (mg/g) and Q e (mg/g) are the capacities of cadmium ion adsorbed at time t and equilibrium, respectively. K 1 (min −1 ) is the pseudo-first-order constant. e pseudo-second-order model is defined by the supposition that the correlation of occupation of adsorption sites is corresponding to the square of the number of unoccupied sites and is indicated in its linear form as follows: where K 2 (g/mg.min) is the pseudo-second-order constant.

Intraparticle Diffusion.
Weber and Morris presented this model in equation (6); K w and constant C values can be resolute from the graph and of order at the origin of the Q t curve as a function of t t1/2 : 2.7. ermodynamic Study. e determination of thermodynamic parameters is very important to understand the effect of temperature on adsorption. It also ensures in principle to predict the strength of the bonds between the adsorbent and the adsorbate. e thermodynamic parameters including the free energy (∆G°), the standard enthalpy (∆H°), and the standard entropy (∆S°) can be calculated by the following equations: where R is the perfect gas constant (8.314 J/mol. K), T is the temperature in Kelvin (K), and K d is the distribution coefficient of the solute in the adsorbent and the solution.

Regeneration of QC-MC and QC-MT.
Adsorbent regeneration is an important process for low-cost treatment of adsorbents to recycling again. In this study, the contaminated QC-MC and QC-MT were regenerated by three different acids including nitric acid (3M), hydrochloric acid (3M), and sulfuric acid (3M). e mixture was shaken for one hour at 450 rpm. e desorption process was analyzed for four cycles.
e QC-MC contains rich elements such as iron, silicon, magnesium, potassium, aluminum, calcium, and sodium, but also low-grade elements, for example, strontium, selenium, thallium, vanadium, and manganese. On the other hand, silicon, magnesium, potassium, aluminum, calcium, and sodium are also enriched in QC-MT; other elements contained in QC-MT are low concentrations, such as iron, strontium, selenium, thallium, vanadium, and manganese.
ese results confirm those found by XRD and FTIR.

Physicochemical Properties of Adsorbents.
e physicochemical properties of the samples determined in this study are cation exchange capacity (CEC), humidity, and porosity. e principle of measurement is to move all adsorbed cations to the exchange sites and to saturate the sites by a single cation; the cationic exchange capacity is defined as the number of monovalent cations capable of substituting the 100 g cations of the solid. Large CEC values of 148.8 me/100 g and 145.6 me/100 g were detected for the QC-MC and QC-MT, respectively, which signified the swelling character. Moisture consists of detecting the mass of water removed by drying in a moisture analyzer, and the results of the moisture are 2.89% and 1.75% for QC-MC and QC-MT, respectively. Porosity is a macroscopic quantity that allows quantifying of the unoccupied volume by the solid, the QC-MC, and QC-MT, defined by a medium porosity of 65% and 70%, respectively.

Point of Charge Zero.
e PZC is an important parameter that represents an estimate of its acidobasicity; generally, it is used to describe variable-charge surfaces of solids. e activities of potential ions (H + and OH -) and the electrolyte concentrations are influenced by the surface charge of adsorbents. In this study, the PZC was determined at the point of intersection of the titration drift at different  [20][21][22]. erefore, the difference between acid and basic pH is very large for QC-MC and QC-MT. at signified a high electrostatic attraction present between the OHions and the cationic adsorbates. erefore, it can be concluded that QC-MC and QC-MT are capable of fixing Cd (II) on their surface at a pH greater than pH pzc .

Effect of Time.
e effect of contact time on the elimination rate of Cd (II) ions has been studied on a range of 1 min to 180 min and the variation on adsorption capacity as shown in Figure 3 with an initial concentration of Cd (II) ions is 10 mg/L, and an adsorbents dose of 1.5 g at room temperature (25°C) and pH of the solution is 5 for QC-MC and QC-MT. e results obtained show that the elimination rate increases rapidly to reach equilibrium in 45 min for QC-MC; the removal rate of Cd (II) ions on QC-MT was increasing during the 30 min and then remains almost constant, equivalent to the adsorption capacity of 4.17 mg/g for QC-MC and 5.49 mg/g QC-MT. erefore, the slow increase of the Cd (II) ions elimination rate up to the equilibrium time indicates that there is an internal mass transfer of the adsorbent; this mostly relates to a phenomenon of diffusion in the internal porosity of the adsorbent.

Effect of Mass.
e influence of adsorbent mass was explored between 0.1 and 4 g. e concentration initial of Cd (II) ions is 10 mg/L, and the contact time is 45 min for QC-MC and 30 min for QC-MT at pH of solution 5 for QC-MC and QC-MT. e curve in Figure 4 shows that a mass of 1.5 g of QC-MC and QC-MT is capable of setting maximum Cd (II) ions of the order of 3.98 mg/g for QC-MC and 5.21 mg/g for QC-MT. To ensure an equivalent number of adsorption sites, the quantities of Cd (II) ions fixed must be following the doses of adsorbent in the solution. Beyond a certain mass, the retention rate remains constant slightly indicating the saturation of the adsorbent surface. erefore, it is useful to work with doses of adsorbent 1.5 g and an ineffective overdose is avoided.       Journal of Chemistry

Effect of pH.
e pH of the medium conditions the state of the surface charge as well as the adsorbent adsorbate. e initial concentration of cadmium was fixed at 10 mg/L and the adsorbents dose is 1.5 g in 45 min for QC-MC and 30 min for QC-MT of contact time at room temperature 25°C. Figure 5 shows a high removal of Cd (II) ions in pH � 5 for QC-MC and QC-MT, with the values of rate increases of 4.15 mg/g for QC-MC and 5.36 mg/g for QC-MT. However, at alkaline medium, the concentration of Cd 2+ decreases due to the formation of Cd (OH) 2. . As a result, it was obvious that the best adsorption of cadmium onto QC-MC and QC-MT is favorable at an acid medium.

Effect of Initial Concentration of Cd (II).
For analysis of the effect of Cd (II) concentration on adsorption capacity, the contact time of Cd (II) ions is 45 min for QC-MC and 30 min for QC-MT, the pH of the solution is 5, and the adsorbent dose is 1.5 g. e process was performed with different initial Cd (II) concentrations between 5 and 120 mg/L. As shown in Figure 6, adsorption efficiency increases when the initial high Cd (II) concentration reaches a constant value of 4.15 mg/g for QC-MC at 5.75 mg/g for QC-MT. All Cd (II) ions present in the adsorption medium may interact with the binding sites on the adsorption surface, such as at higher concentrations; adsorption yield is stable due to saturation of adsorption sites.

Isotherm Adsorption.
e tracing of the retention isotherms gives information on the maximum adsorption capacity and the adsorption mechanism.
e study was carried out with different initial concentrations at 25°C temperature, the contact time 45 min for QC-MC and 30 min for QC-MT at adsorbents dose is 1.5 g, and the pH of the solution is 5 for QC-MC and QC-MT, respectively. e drifts show the change in the amount adsorbed at equilibrium as a function of the equilibrium concentration Q e � f(C e ) (Figure 7). e corresponding isotherm adsorption parameters are summarized in Table 2, which was deduced from Langmuir and Freundlich curves (Figure 7). e numerical value of n indicates that adsorption of Cd (II) ions is favorable for both adsorbents, and the correlation coefficient values for Freundlich were higher than those of Langmuir for QC-MC and QC-MT. e Freundlich equation can best describe the removal of Cd (II) using QC-MC and QC-MT, which suggests that the surfaces of QC-MC and QC-MT are heterogeneous and the adsorption of Cd (II) ions is made of the multilayer. ese results show that the maximum amount of retention of Cd (II) ions was 4.23 mg/g for QC-MC and 5.85 mg/g for QC-MT.

Kinetics of Adsorption.
e kinetics of adsorption are essential for evaluating and discussing the mechanism of fixation of the solute on the adsorbent surface and for describing the adsorption rate; with an initial concentration of 10 mg/L at 25°C temperature, the adsorbent dose is 1.5 g, and the pH � 5. In this context, two models have been applied to describe the mechanism of kinetic adsorption of Cd 2+ onto QC-MC and QC-MT, pseudo-first-order, and pseudo-second-order (Figure 8). e kinetic constants for both adsorbents are given in Table 3. According to the comparison of correlation coefficient (R 2 ), the pseudo-second-order values are higher than pseudo-first-order for QC-MC and QC-MT. erefore, the adsorption of Cd (II) onto QC-MC and QC-MT followed the pseudo-second-order model, which signified that the adsorption mechanism is of a chemical nature (chemisorption) involving valence through the sharing or exchange of electrons between QC-MC and QC-MT adsorbents and Cd (II) ions, and the interaction and the retention of Cd (II) ions would be strong.

Intraparticle Diffusion.
e intraparticle diffusion model was also used to describe the adsorption mechanism of several solid-liquid systems.
is model assumes that the process of intraparticle diffusion is the limiting step that controls the rate of transfer of a solute from an aqueous phase to a solid phase. e intraparticle diffusion model plot is shown in Figure 9 for QC-MC and QC-MT. From the results, the multilinearity of Cd (II) ions adsorption on QC-MC and QC-MT adsorbents suggests that the entire adsorption process can be separated into three steps. e first step characterizes the transfer of Cd (II) ions from the aqueous solutions to the external surface of the QC-MC and QC-MT (film diffusion). e second step describes the transfer of Cd (II) ions from the external surface to the pores of the clay (intraparticle diffusion). e third step represents the fixation of Cd (II) ions to the surface of QC-MC and QC-MT (Adsorption). From Figure 9, the rate constant values of the intraparticle diffusion model of the second step (K 2 ) are higher than those of the first (K 1 ) and third (K 3 ) steps for QC-MC and QC-MT adsorbents. e kinetic data indicate that the Cd (II) ions adsorption on both natural clays is rapid during the second step, revealing that the adsorption rate was controlled by intraparticle diffusion.

3.2.8.
ermodynamic Study. Temperature is one of the important and effective parameters in the adsorption process. Figure 10 represents the variation in the maximum amount adsorbed from Cd (II) by QC-MC and QC-MT. According to the results, QC-MC and QC-MT exhibit similar behavior. It is observed that the adsorbed amount of QC-MC and QC-MT decreases slightly with temperature increase reflecting the exothermic nature of adsorption.
ermodynamic parameters obtained are summarized in Table 4. According to the results, the values of the standard enthalpy ΔH°of QC-MC and QC-MT are negative which confirms that the process of adsorption of Cd (II) by QC-MC and QC-MT is exothermic. A negative ΔS°entropy value was found for QC-MC and QC-MT which means that Cd (II) molecules are more organized at the solid/liquid interface than in the liquid phase for these systems. e negative values of ΔG°obtained reveal that the adsorption process is favorable and that it is spontaneous and it was observed that ΔG°values increase slightly with temperature for QC-MC and QC-MT which reflects the negative effect of temperature on adsorption of Cd (II).

Regeneration of QC-MC and QC-MT.
Adsorbent regeneration is considered an important economic aspect in reducing treatment costs. e regeneration of QC-MC and QC-MT polluted by Cd (II) was affected using the acid method, and the results are presented in Figure 11. After the regeneration cycles of QC-MC and QC-MT, the percentage desorption of Cd (II) by nitric acid was 18% for QC-MC and 15% for QC-MT, whereas hydrochloric acid was desorbed 21% for QC-MC and 25% for QC-MT, and sulfuric acid was reduced to 11% for QC-MC and 10% for QC-MT. However, the results of the regeneration of QC-MC and QC-MT show that hydrochloric acid is the most effective for the desorption of Cd (II) ions.   it was noticed that there is no change in the bands of mica for QC-MC and the montmorillonite for QC-MT; therefore, it can suggest that the Cd (II) ions were adsorbed on the surface of QC-MC and QC-MT.

Fourier Transform Infrared Spectroscopy.
e FTIR spectra before and after adsorption of Cd (II) by QC-MC and QC-MT are presented in Figure 13. A large broad absorption around 3380-1639 cm −1 represents asymmetrical stretching and bending vibrations of the OH group, respectively. e FTIR spectra of QC-MC confirm the existence of Quartz in the peaks 1038 cm −1 , 983 cm −1 , and 779 cm −1 which are due to Si-O symmetric stretching and bending [23][24][25]. Calcite was also detected in QC-MC at 1425 cm −1 and 1246 cm −1 , which are signified to doubly degenerate asymmetric stretching and C�O stretching, respectively [25,26]; the frequency of 876 cm −1 is due to Al-O-H stretching mode vibration signified to the presence of mica [27,28]. e infrared spectrum of QC-MT is characterized by several bands, the frequencies of Si-O symmetric stretching and bending at 1058 cm −1 , 986 cm −1 , and 778 cm −1 are due Table 2: Parameters of adsorption isotherm model.

Adsorbent
Langmuir constants Freundlich constants Q max (mg/g)   to the presence of Quartz (Ko, T.H. and Chu, H., 2005), montmorillonite was also present in QC-MT at 843 cm −1 which is due to Al-Mg-OH bending [26,29], the FTIR spectroscopy detects the presence of calcite at 1451 cm −1 due to doubly degenerate asymmetric stretching, and 1247 cm −1 is due to C�O stretching in QC-MT [30,31].
After adsorption of Cd (II) ions, the FTIR spectra remained largely unchanged, indicating that QC-MC and QC-MT have good stability and hence the potential for high reusability. e absorption peak was the same in QC-MC and QC-MT; it was observed at 549 cm −1 , which was caused by Cd (II) ions Cd-O stretching vibrations.

Scanning Electron Microscopy.
e scanning electron microscopy was affected to analyze the texture and characterize mineralogical assemblies of the samples. Figure 14 presents the images of SEM with different enlargements of               [32]. Clays have permanent charges (isomorphic substitutions) and load variables of silanol groups and aluminol groups also of iron oxide group and aluminum oxide group, which give them the ability to fix an inclusive variety of cadmium. However, this fixation of Cd (II) ions to the surface of the adsorbents under the effect of the negative electrical charges present on the surface formed an internal sphere complex between the adsorbent and cadmium ions. Recently, researchers have used clay minerals to remove cadmium ions from an aqueous solution like kaolinite, montmorillonite, bentonite, and other clays raw. Table 5 gathers some adsorbents and their adsorption capacity reported in the literature. ese results show that these adsorbents have a remarkable adsorption capacity compared to other absorbents used for the treatment of Cd (II) ions, which makes them alternatives and good adsorbents for the Cd (II) ions.

Conclusion
In summary, both natural clays from the Middle Atlas region in Morocco were successfully analyzed and applied for the treatment of aqueous medium contaminated by Cd (II) ions heavy metal. e quantity of Cd (II) ions adsorbed on both natural clays is found to increase with the increase in the adsorbent dose, pH of the solution, initial heavy metal concentration, contact time, and temperature. e kinetic study revealed that the adsorption mechanism can be described by a pseudo-second-order model. e isotherm data indicated that the nature of Cd (II) ions adsorption onto both natural clays is described by the Freundlich model, thus indicating multilayer adsorption of Cd (II) ions on heterogeneous active sites of both natural clay adsorbents. e maximum Cd (II) ions adsorption capacity is 5.85 mg/g for QC-MT and 4.23 mg/g for QC-MC at 25°C. e thermodynamic study allowed concluding that the Cd (II) adsorption onto QC-MC and QC-MT was exothermic, favorable, and spontaneous. e regeneration tests suggested that hydrochloric acid was most effective in desorbing Cd (II) ions from the surface of QC-MC and QC-MT. e results of the adsorption process indicated that both Moroccan natural clays are effective and low-cost natural adsorbents for the purification of aqueous environments polluted by Cd (II) ions.
Data Availability e datasets used and analyzed during the current study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article.