Efficient Removal of Toxic Heavy Metals on Kaolinite-Based Clay: Adsorption Characteristics, Mechanism and Applicability Perspectives

Abstract

In this study, kaolinite-based clay (Ka-Clay) was used as an adsorbent for the efficient removal of Pb(II), Cd(II) and Hg(II) ions from aqueous media. The selection of Pb(II), Cd(II) and Hg(II) ions for experimental studies took into account their high toxicity, while the choice of Ka-Clay, the ease of preparation and high availability of this material were the most important arguments. Ka-Clay exhibits high adsorption performance, with removal percents over 98% for Pb(II) and 93% for Cd(II), even at high concentrations of metal ions (over 150 mg/L, pH = 6.5, 4 g adsorbent/L, 21 ± 1 °C). For Hg(II) ions, the adsorption percent does not exceed 55%, and this moderate value is mainly due to the significant change in pH. The adsorption behavior was in accordance with the Langmuir model (R² > 0.95) and the pseudo-second order kinetic model (R² > 0.99), indicating an adsorption process that occurs mainly through chemical interactions at the adsorbent surface between the metal ions and the functional groups. Adsorption processes are spontaneous (ΔG = −8.66 ÷ −15.76 kJ/mol) and endothermic (ΔH = 7.09 ÷ 21.81 kJ/mol), and the adsorption mechanism is the results of elementary processes of electrostatic attraction, ion exchange and superficial complexation. The insignificant effect of other ions (Ca(II), Mg(II), Na(I), K(I)) present in real wastewater samples as well as the desorption behavior of exhausted adsorbent highlight the practical utility of this adsorbent on a large scale. The experimental results included in this study suggest that Ka-Clay can be used as a promising adsorbent for the removal of high concentrations of toxic heavy metals with low cost and high efficiency, and this can contribute to the design of a sustainable wastewater treatment method.

1. Introduction

The continuous development of industrial fields generates significant volumes of wastewater with a high content of heavy metal ions, which, if not properly treated, will generate environmental pollution. In fact, improper wastewater discharge is currently the main source of environmental pollution, which significantly contributes to the degradation of ecosystem quality and threatens human health [1,2,3]. This is because some heavy metal ions, such as Pb(II), Cd(II) and Hg(II), are known to cause neurological damage, kidney disorders, respiratory and cardiovascular problems and are associated with increased risks of cancer [3,4]. In fact, the high toxic potential of Pb(II), Cd(II) and Hg(II) ions on all life forms, as well as their widespread use in many industrial activities (metallurgical industry, metal coating industry, varnish and paint industry, catalysts manufacturing, etc.) [4,5], were the bases for their selection for this study.

Therefore, finding efficient, economical and ecological methods for removing such toxic metal ions from aqueous environments is still an open research topic, and the results reported so far in the literature, although numerous, do not outline a clear solution to this problem.

There are many methods for treating wastewater containing heavy metal ions (such as chemical precipitation, coagulation, flocculation, phytoremediation, membrane-related processes, ion exchange, adsorption, etc.) [6,7,8,9], and some of them, due to their high efficiency, are applied on an industrial scale, even though they have some major disadvantages. Thus, although chemical precipitation, coagulation and flocculation are effective methods that can remove a wide variety of metal ions from wastewater, the high requirement of chemical reagents and energy, as well as the large amounts of sludge that are generated, limit their practical applicability [10,11]. On the other hand, phytoremediation methods are considered selective and environmentally friendly [12,13,14], but their moderate efficiency and the specific conditions that must be met to ensure the activity of microorganisms make then quite difficult to use on an industrial scale [12]. In the case of membrane-related processes, high operating costs and high energy consumption are the main disadvantages that make these methods used for wastewater treatment only in certain particular situations [15,16].

Compared with the above-mentioned methods, adsorption is cost-effective and has several important advantages that make it a promising alternative. Thus, high efficiency, ease of operation under different experimental conditions, applicability for a large number of metal ions, the potential for the quantitative recovery of adsorbed metal ions and the reuse of the adsorbent [17,18,19] are just few reasons why adsorption is, to date, one of the recommended methods for the removal of toxic metal ions from aqueous solutions. However, the success of using adsorption in industrial effluent decontamination processes depends largely on the choice of an efficient, inexpensive adsorbent that has high stability (mechanical and chemical). These conditions are, unfortunately, not always met by biomass-based adsorbents. Whether it is plant biomass or biomass waste (functionalized or not), these materials have low chemical and mechanical stability, being able to be used in a small number of cycles (adsorption/desorption) [20,21]. These limitations represent a major impediment to industrial scale applicability. Such difficulties do not arise in the case of clay materials, which in addition to excellent chemical stability have a large surface area and high ion exchange capacity [22,23]. Natural clays are versatile and abundant resources that have been frequently used as sustainable adsorbents in decontamination processes of effluents containing heavy metal ions [24,25,26]. Various clay minerals, such as bentonite, illite, kaolinite, halloysite, montmorillonite, etc. [27,28], have proven effective in retaining toxic metal ions, mainly due to their large surface area and high density of superficial negative charges [27]. Thus, numerous studies in the literature have reported the successful use of clay minerals for the removal of toxic heavy metal ions from aqueous media [23,25,27,28], even under extreme experimental conditions (in strongly acid/basic pH or at high temperature). In the NE part of Romania, the most widespread deposits are those containing kaolinite-based clay. These deposits are found on the surface, are spread over large areas and are easy to exploit on a large scale. Due to its high availability (at least regionally), ease of preparation (requiring only a few mechanical separation steps to remove soil, parental materials and other impurities) and low cost, kaolinite-based clay can be used as an adsorbent for removing pollutants from the environment. In addition, its layered chemical structure (aluminosilicate octahedral linked to Si-O tetrahedral through oxygen atoms) [29,30] makes kaolinite-based clay have a large number of functional groups, excellent stability, a large specific surface area and environmentally friendly qualities. All these considerations were the basis for the selection of this material (kaolinite-based clay) as an adsorbent for experimental studies, as it can provide a sustainable and efficient approach for the removal of toxic metal ions from aqueous media, which can be used in industrial effluent treatment processes.

In this study, Romanian kaolinite-based clay (Ka-Clay) sampled from Tomeşti deposit (NE region of Romania) was used for the adsorption of toxic Pb(II), Cd(II) and Hg(II) ions from an aqueous solution. The experimental results included in this study, which have not been reported in previous studies, provide a detailed perspective on the usefulness and limitations of this adsorbent for practical applications. To highlight the structural and morphological characteristics of kaolinite-based clay adsorbent, different analytical methods (FTIR, SEM and EDX) were used. The effect of the most important experimental parameters (pH, adsorbent dose, contact time, initial metal ions concentration and temperature) on the removal efficiency was also examined. The isotherm and kinetic data were analyzed using several isotherm, kinetic and thermodynamic models to outline the important elementary processes in the metal ion adsorption mechanism. The applicability of the Ka-Clay adsorbent was also assessed using real water samples, and the efficiency of the adsorption processes was evaluated through quality indicators, determined before and after adsorption.

2. Materials and Methods

2.1. Materials

The kaolinite-based clay adsorbent (Ka-Clay) used in this study was obtained from the Tomeşti deposit, NE Romania (geographical localization: Latitude: 47°08′01″ N, Longitude: 27°39′19″ E). After removal of foreign materials (stone pieces, plant roots, etc.), Ka-Clay samples were homogenized, dried (in over at 105 °C, 6 h), ground and sieved. For the experimental studies, only fractions with a particle size smaller than 2 microns were used. From a mineralogical point of view, Ka-Clay adsorbent contains 67.75% kaolinite, 8.25% illite, 5.93% smectite and 10.58% carbonates, while the other components (such as iron oxides and hydroxides (2.01%), sulphates (1.29%), silica (0.73%), heavy minerals (1.28%) and organic matter (0.24%)) are found below 2% [31]. Based on the mineralogical composition, the clay used as adsorbent in this study was named kaolinite-based clay adsorbent and was noted Ka-Clay.

From a chemical point of view, the clay sample contains in its composition (

Table 1. Chemical composition of kaolinite-based clay (Ka-Clay) used as adsorbent [31].

Major Components
Component	SiO2	Al2O3	Fe2O3	CaO	MgO	Na2O	K2O
(w/w), %	60.37	14.65	3.82	8.19	1.13	2.58	1.96
Minor Components
Component	Cu	Ni	Zn	Mn	Cr	Ti	Ba
mg/kg	62.44	17.08	103.71	379.37	30.55	94.73	39.17

) a series of mineral-specific oxides and small amounts of heavy metals, characteristics that make it suitable for use as an adsorbent for the removal of heavy metal ions.

The chemical reagents used in this study, such as lead nitrate (Pb(NO₃)₂ · 2 H₂O), cadmium nitrate (Cd(NO₃)₂ · 6 H₂O), mercury nitrate (Hg(NO₃)₂ · 2 H₂O), nitric acid (HNO₃) and sodium hydroxide (NaOH), were obtained from Chemical Company (Iaşi, Romania) and were of analytical grade (purity > 99.5%). All chemical reagents were used as received without further purification. The artificial wastewater sample was obtained from water taken from the Bahlui River (in the area of the Gheorghe Asachi Technical University of Iaşi), in which the concentration of Pb(II), Cd(II) and Hg(II) was adjusted to the desired value by adding laboratory solutions of the metal ions.

2.2. Adsorbent Characterization

To highlight the structural characteristics of the Ka-Clay adsorbent, X-ray diffraction (XRD), AAS spectrometry, FTIR spectrometry, EDX and SEM microscopy were used. XRD (XRD diffractometer with CuKa radiation (k = 1.5418 Å), scanning rate of 3°/min) [32] was used to obtain the mineral composition of Ka-Clay (Table 1), while atomic absorption spectrometry (AAS spectrometer NovAA 400P; acetylene/air flame; Analytik Jena, Jena, Germany) was used to measure the concentration of different elements (Cu, Ni, Zn, Mn, Cr, Ti and Ba) in the solutions obtained after the disaggregation of the clay sample (Table 1). FTIR spectra (Bio-Rad FTIR Spectrometer, Berlin, Germany, domain: 400–4000 cm⁻¹, 4 cm⁻¹ resolution, KBr pellet technique) were used to determine the nature of the functional groups on the adsorbent surface. EDX and SEM microscopy (SEM/EDX-Hitach S 3000N, Hitach, Düsseldorf, Germany) were employed to analyze the morphological structure and element distribution on the Ka-Clay surface.

2.3. Adsorption/Desorption Experiments

The adsorption experiments were performed in batch systems in order to examine the potential use of this adsorbent (Ka-Clay) for the removal of toxic heavy metal ions (Pb(II), Cd(II) and Hg(II)) from aqueous media. All studies were performed in batch system at different values of the experimental parameters (pH = 2.0–6.5, adsorbent dose = 4.0–20.0 g/L, initial metal ions concentration = 12.0–580.0 mg/L, contact time = 5–180 min and temperature = 10, 25 and 50 °C). In each case, an accurately weighed amount of adsorbent was mixed with 25 mL of aqueous solution containing the metal ions (in 100 mL Erlenmeyer flask). Each sample was stirred for a given period of time (at 150 rpm) and filtered (on quantitative filter paper). The concentration of each metal ion in the filtered solution was measured with an AAS Spectrometer (AAS NovAA 400P, air/acetylene flame, UV background, Analytik Jena, Jena, Germany) at the characteristic wavelength (217.0 nm for Pb, 228.8 nm for Cd and 253.7 nm for Hg).

The efficiency of heavy metal ion removal (R, %) and the adsorption capacity of Ka-Clay adsorbent (q, mg/g) were calculated using Equations (1) and (2).

$$R={\displaystyle \frac{c_0-c}{c_0}}\cdot 100$$

(1)

$$q={\displaystyle \frac{(c_0-c)\cdot V}{m}}$$

(2)

where c₀ and c are the initial and the equilibrium concentration of heavy metal ions in aqueous solution, (mg/L), V is the volume of aqueous solution (L) and m is the mass of Ka-Clay adsorbent (g).

In the case of regeneration experiments, 5 g of Ka-Clay was loaded with each metal ion, under optimal experimental conditions (pH = 6.5 for Pb(II) and Cd(II) and 2.0 for Hg(II), adsorbent dose = 4.0 g/L, contact time = 180 min and temperature = 21 ± 1 °C), and the obtained values of the adsorption capacity were 20.86 mg/g for Pb(II) ions, 15.76 mg/g for Cd(II) ions and 4.61 mg/g for Hg(II) ions, respectively. Then, the loaded Ka-Clay adsorbent samples (0.1 g) were treated with 10 mL of HNO₃ solution of different concentration (10⁻³–1 mol/L), mixed for 3 h and filtered. The metal ion concentration in the filtered solution was analyzed as mentioned above (AAS Spectrometry), and the efficiency of desorption process (Desorption, %) was calculated using the equation:

$$Desorption,\%={\displaystyle \frac{c_{des}}{q_{ads}\cdot m}}\cdot 100$$

(3)

where c_des is the concentration of heavy metal ions in the filtered solution (after desorption), q_ads is the adsorption capacity calculated using Equation (2) in these experimental conditions and m is the amount of loaded Ka-Clay used in these experiments.

All experiments were conducted in triplicate, and the average values of the measurements were used for graphical representations and calculations.

2.4. Modeling of Adsorption Data

The kinetic data were analyzed using pseudo-first order (PFO), pseudo-second order (PSO) and intra-particle diffusion models (IPD) to establish the elementary processes that are essential in the adsorption of heavy metal ions on Ka-Clay. The mathematical equations of these models are summarized in

Table 2. The equations of the kinetic, isotherm and thermodynamic models used for the analysis of the experimental results [33,34,35,36,37].

Model		Equation	Notations
Kinetic models	PFO model	$q_t=q_e\cdot (1-e^{-k_1\cdot t})$	qt, qe—adsorption capacities at time t and at equilibrium, k1—rate constant of PFO model, k2—rate constant of PSO model, kdiff—rate constant of IPD model, c—equilibrium concentration of metal ions.
	PSO model	$q_t={\displaystyle \frac{k_2\cdot q_e^2\cdot t}{1+k_2\cdot q_e\cdot t}}$
	IPD model	$q_t=k_{diff}\cdot t^{1/2}+c$
Isotherm models	Langmuir model	$q={\displaystyle \frac{q_{\max }\cdot K_L\cdot c}{1+K_L\cdot c}}$	q—adsorption capacity, qmax—maximum adsorption capacity, KL—Langmuir constant, c—concentration of metal ions at equilibrium, n—heterogeneity factor, KF—Freundlich constant, KT—Temkin constant, BT—constant correlated with the adsorption energy.
	Freundlich model	$q=K_F\cdot c^{1/n}$
	Temkin model	$q=B_T\cdot \ln (K_T\cdot c)$
Thermodynamic modeling (van’t Hoff equations)		$\mathsf{\Delta }G=-RT\cdot \ln K_L$	ΔG—variation of free Gibbs energy, ΔH—variation of enthalpy, ΔS—variation of entropy, R—universal gas constant (8.314 J/K mol), T—absolute temperature.
		$\ln K_L={\displaystyle \frac{\mathsf{\Delta }H}{R\cdot T}}+{\displaystyle \frac{\mathsf{\Delta }S}{R}}$
		$\mathsf{\Delta }S={\displaystyle \frac{\mathsf{\Delta }H-\mathsf{\Delta }G}{T}}$

. For modeling the adsorption isotherms, the Langmuir, Freundlich and Temkin models were used (Table 2).

These isotherm models allow for the determination of how adsorption processes occur (mono- or multi-layer) and the calculation of quantitative parameters (ex. maximum adsorption capacity, adsorption energy, etc.), important in evaluation of their efficiency [33,34]. The choice of the most appropriate kinetic and isotherm model for the experimental data was based on the regression coefficients calculated from statistical analysis (ANOVA analysis). The parameters ΔG, ΔH and ΔS were calculated using van’t Hoff equations (Table 2). The values of these parameters allow for the evaluation of the spontaneity and the endo- or exo-thermal character of the examined adsorption processes.

2.5. Applicability Tests

River water samples (Bahlui River, Gheorghe Asachi Technical University of Iaşi area) were used to examine the effect of different interfering ions (Ca(II), Mg(II), Na(I) and K(I)) on the removal efficiency of Pb(II), Cd(II) and Hg(II) ions on Ka-Clay. In each experiment, the concentration of Pb(II), Cd(II) and Hg(II) ions was maintained at a constant value (20 ± 2 mg/L), while the concentration of Ca(II), Mg(II), Na(I) and K(I) ions was varied in the range of 10⁻³–1 mol/L. All experiments were performed under conditions established as optimal (pH of 6.5 for Pb(II) and Cd(II) and 2.0 for Hg(II), adsorbent dose of 4.0 g/L, contact time of 180 min and temperature = 21 ± 1 °C), according to the methodology described above (see Section 2.3), and the adsorption capacity values (calculated using Equation (2)) were compared with those obtained in the absence of interfering ions. In addition, for the river water samples, pH, TSS, turbidity and hardness before and after adsorption was measured using the specific methods [38].

3. Results and Discussion

The preliminary results reported earlier [31,32] showed that Ka-Clay has the potential to be used as an adsorbent for the removal of toxic metal ions from aqueous environments due to its (i) relatively high adsorption capacities, (ii) ease of preparation, and (iii) high thermal and chemical stability. Therefore, to demonstrate the usefulness of Ka-Clay in wastewater treatment processes, it is necessary to conduct a detailed study that highlights both the advantages and the practical limitations of this adsorbent.

3.1. Structural Features of Ka-Clay

The structural features of the Ka-Clay adsorbent were highlighted through FTIR and EDX spectrometry and SEM microscopy (Figure 1).

The presence of functional groups O–H (3628 cm⁻¹), O–M (3432 cm⁻¹—O–Si and 3407 cm⁻¹—O–Al), Si–O (1027 cm⁻¹) and O=M (1641 cm⁻¹—O=Si and 1389 cm⁻¹—O=Al) on the surface of Ka-Clay is highlighted in the FTIR spectrum (Figure 1a), and the relatively high intensity of these absorption bands suggests that such groups are abundant on the Ka-Clay surface and can participate in the adsorption processes of metal ions from the solution.

Moreover, the high content of alkali and alkaline earth metals (Na, K, Ca, Mg) (Figure 1b) indicates that many of the surface functional groups of Ka-Clay can easily dissociate, and these metal ions represent the mobile ions in ion-exchange processes. All these easily ionizable superficial functional groups are found on a surface with a significant degree of heterogeneity (Figure 1c), which further facilitates the retention of transitional metal ions from aqueous media. All these structural features (although well-known in the case of clay materials) [29,30] recommend the use of Ka-Clay as an adsorbent for the retention of Pb(II), Cd(II) and Hg(II) ions from aqueous solutions and highlight the large-scale applicative potential of this inexpensive material.

3.2. Establishing Optimal Adsorption Conditions

The efficiency of metal ion adsorption processes is significantly influenced by the initial pH of the aqueous solution and the adsorbent dose [17,18]. Therefore, establishing the optimal pH and adsorbent dose, which ensure the high efficiency of the adsorption process, represents the first step in testing their practical applicability. In this study, the pH was varied in the range of 2.0–6.5, while the adsorbent (Ka-Clay) dose was increased from 4.0 to 20.0 g/L, and the values of the adsorption capacities obtained for each heavy metal ion (Pb(II), Cd(II) and Hg(II)) are presented in Figure 2.

Figure 2a shows that the variation in pH leads to a significant variation in adsorption capacities, which depends on the nature of the metal ion. Thus, in the case of Pb(II) and Cd(II) ions, the increase in pH (from 2.0 to 6.5) determines a significant increase in the adsorption efficiency (from 38.54% to 92.31% for Pb(II) and from 3.60% to 94.71% for Cd(II)), while in the case of Hg(II) ions, the removal percent decreases from 53.86% to 26.28%.

These variations in adsorption capacities can be explained by taking into account the previously determined pH_ZPC value of 4.72 [32]. At pH values lower than 4.72, most of the functional groups on the Ka-Clay surface are undissociated, meaning that metal ion retention occurs predominantly through physical interactions (hydrogen bonds or van der Waals interactions). Such interactions favor the retention of Hg(II) ions (due to their noble metal character), while the adsorption of Pb(II) and Cd(II) ions is less efficient. For example, at pH = 2.0, the adsorption of Pb(II) and Cd(II) ions is the lowest (7.73 mg/g and 0.37 mg/g, respectively), while in the case of Hg(II) ions, the adsorption capacity has the highest value (8.47 mg/g) (Figure 2a). However, in the event at pH = 2.0, the R % value for Hg(II) ions does not exceed 55%, which shows that the adsorption of these metal ions through physical interactions does not ensure efficient removal from aqueous media.

At pH values higher than 4.72, most of the functional groups of Ka-Clay are dissociated, and therefore, electrostatic interactions between these groups (negatively charged) and metal ions (positively charged) are favored. Such interactions are preferred by Pb(II) and Cd(II) ions (which have high electronegativity values) and less by Hg(II) ions (which at pH greater than 3.0 change their speciation form [39]). Thus, at pH = 6.5, the removal of Pb(II) and Cd(II) ions is quantitative (92.31% for Pb(II) and 94.71% for Cd(II)), while the removal percent of Hg(II) ions decrease to 27.28% (Figure 2a). Under these conditions, the optimal pH for Pb(II) and Cd(II) ion retention was selected as 6.5 and 2.0 for the Hg(II) ion retention, and these values were used in all other experiments.

However, it should be mentioned that after filtration, the final pH of the aqueous solution changes significantly (Figure 3) and reaches values greater than 7.0 regardless of the nature of the metal ion.

This significant increase in pH values at the end of the adsorption processes suggests that (i) there are numerous ionizable functional groups on the surface of Ka-Clay, which can interact with Pb(II), Cd(II) and Hg(II) ions during adsorption processes, and (ii) the ionization of these functional groups releases ions of alkali metals (Na(I) and K(I)) and alkaline earth metals (Ca(II) and Mg(II)), which in aqueous solutions lead to the formation of compounds with alkaline properties. This behavior is characteristic of adsorbents in the category of clay materials and is one of the factors responsible for their high efficiency in removing metal ions from aqueous media [23,40].

Another parameter that influences the efficiency of metal ion adsorption is the adsorbent dose. Increasing the amount of adsorbent from 4.0 to 20.0 g/L leads to a decrease in the adsorption capacity of Ka-Clay of more than 4.5 times for Pb(II) and Cd(II) and more than 3 times for Hg(II) under the mentioned experimental conditions (Figure 2b). The notable decrease in the adsorption capacity (q, mg/g) for the three metal ions with increasing Ka-Clay amount is probably determined by the agglomeration of the adsorbent particles. Thus, as the amount of adsorbent increases, the functional groups in different particles can interact each other, which significantly reduce their availability to bind metal ions in an aqueous solution [40]. However, within the same adsorbent dose range (4.0–20.0 g/L), the removal percent varies much less (from 96.13 to 98.39% for Pb(II), from 91.83 to 96.57% for Cd(II) and from 57.36 to 77.47% for Hg(II)). This less-significant increase in the values of the removal percent shows that even at the lowest adsorbent dose value, Ka-Clay has sufficient functional groups for the efficient adsorption of Pb(II), Cd(II) and Hg(II) ions from an aqueous solution. Therefore, the amount of adsorbent considered optimal was 4.0 g/L, and this value was considered in all subsequent experiments.

3.3. Effect of Contact Time and Kinetic Modeling

Figure 4 illustrates the effect of contact time on the adsorption performances of Ka-Clay for Pb(II), Cd(II) and Hg(II) ions under the experimental conditions established as optimal. The retention of Pb(II), Cd(II) and Hg(II) ions on Ka-Clay adsorbent occurs rapidly, within 30 min, suggesting that there are sufficient functional groups on the adsorbent surface to interact with metal ions in an aqueous solution. After 60 min, all adsorption processes reach the equilibrium, and the removal percents are 96.62% for Pb(II), 97.46% for Cd(II) and 60.93% for Hg(II), respectively (Figure 4). However, for Hg(II) ions, the R % values slightly increase by 23% in the range of 60–180 min (Figure 4c), and therefore, 180 min was established as optimal for the retention of Pb(II), Cd(II) and Hg(II) ions on Ka-Clay. This contact time value (180 min) was used in subsequent experiments.

The kinetic curves resulting from modeling of the experimental data using PFO, PSO and IPD models (Table 2) are also shown in Figure 4, and the kinetic parameters calculated for the adsorption of Pb(II), Cd(II) and Hg(II) are presented in

Table 3. Kinetic parameters for Pb(II), Cd(II) and Hg(II) ion adsorption on Ka-Clay.

Model	Parameters	Pb(II)	Cd(II)	Hg(II)
PFO	R2	0.9799	0.9613	0.9011
	qeexp, mg/g	23.59	16.41	12.73
	qecalc, mg/g	7.62	5.15	1.85
	k1, 1/min	0.0051	0.0214	0.0035
PSO	R2	0.9949	0.9995	0.9981
	qeexp, mg/g	23.59	16.41	12.73
	qecalc, mg/g	23.86	16.50	12.67
	k2, g/mg mim	0.0057	0.0243	0.0282
IPD	R2	0.9547	0.9942	0.9165
	c1, mg/L	13.53	11.74	9.63
	kdiff1, mg/g min1/2	0.6167	0.5602	0.3497
	R2	0.9998	0.9051	0.9697
	c2, mg/L	14.72	15.78	10.58
	kdiff2, mg/g min1/2	0.7509	0.5325	0.1477

.

The results of kinetic modeling (Figure 4) indicate that the adsorption of Pb(II), Cd(II) and Hg(II) ions on Ka-Clay is very well described by the PSO model (Table 3). Therefore, the retention of these metal ions on the adsorbent surface involves chemical interactions (most likely electrostatic) involving functional groups of Ka-Clay.

However, two clarifications should be made: (i) the regression coefficients calculated for the PFO model are quite high (R² > 0.9) (Table 3), and (ii) the rate constants of the PFO and PSO models are comparable except for Hg(II) ions. These observations suggest that the retention of metal ions to the Ka-Clay surface occurs in two successive steps. Thus, the metal ions near the adsorbent surface first interact with a functional group (according to PFO model), and then, once fixed on the surface, they interact with a second functional group (according to PSO model). In the case of Pb(II) and Cd(II) ions, these two interactions occur rapidly (practically simultaneously), which results in the rate constants obtained for the PFO and PSO models having similar values (Table 3). In the case of Hg(II) ions, their pronounced tendency to hydrolyze causes the interaction with the first functional group to occur more easily than the interaction with the second functional group (the ratio of the rate constants is greater by an order of magnitude) (Table 3).

However, in the realization of these interactions, the elementary diffusion processes play an important contribution. The modeling of experimental data using the IPD model shows that although this model fits the experimental data quite well (Figure 4, Table 3), the diffusion stages do not limit the adsorption rate of Pb(II), Cd(II) and Hg(II) ions on Ka-Clay. This is due to the fact that in the linear dependencies of the IPD model (Figure 5), two distinct regions can be observed, corresponding to the diffusion of metal ions from the solution layer (external diffusion) (region 1) and the diffusion of metal ions into the outer surface of the adsorbent into the internal pores (region 2). Moreover, regardless of the nature of the metal ion, the parameters of this model (k_diff and c) have similar values for both regions (Table 3), which suggests that once the metal ions reach the Ka-Clay surface, they interact with the functional groups of the adsorbent.

All these observations highlight two important aspects in the characterization of the adsorption of Pb(II), Cd(II) and Hg(II) ions on Ka-Clay, namely that (i) the binding of metal ions on the adsorbent surface is predominantly achieved through chemical interactions (most likely electrostatic), which indicates the potential use of this adsorbent in the decontamination processes of industrial effluents, and (ii) the adsorbent has a large number of superficial functional groups, suggesting its potential use for the removal of high concentrations of metal ions. These aspects will be considered in the following sections.

3.4. Effect of Initial Metal Ions Concentration and Temperature

To test the adsorptive performances of Ka-Clay, experimental studies were conducted at different temperatures (10, 20 and 50 °C) and varying concentrations of metal ions (0–575 mg/L for Pb(II), 0–160 mg/L for Cd(II) and 0–560 mg/L for Hg(II)). The removal percent values (R, %) obtained for each metal ion are illustrated in Figure 6.

Regardless of the nature of the metal ion, increasing temperature (from 10 to 50 °C) causes an increase in the efficiency of adsorption processes (Figure 6) over the entire initial concentration range. This improvement in adsorption efficiency is much more noticeable at high concentrations of metal ions, where, with an increase in temperature, the removal percentages increase by 40% for Pb(II), 29% for Cd(II) and 23% for Hg(II) (Figure 6). However, a quantitative removal of metal ions (over 99%) is achieved only in the case of Pb(II) and Cd(II) ions at 50 °C (when their initial concentration is below 240 mg Pb(II)/L and 30 mg Cd(II)/L, respectively), while for Hg(II) ions, the removal percentages do not exceed 55% regardless of temperature or initial concentration. These adsorptive performances highlight the practical applicability of these adsorption processes and justify the potential use of the Ka-Clay adsorbent for treating industrial effluents.

3.5. Isotherm and Thermodynamic Modeling

The modeling of adsorption isotherms is essential for obtaining a quantitative assessment of the efficiency of using a given adsorbent for the removal of metal ions from aqueous media [33,34].

In this study, the modeling of adsorption isotherms obtained in the case of Pb(II), Cd(II) and Hg(II) ion adsorption on Ka-Clay was performed using three models, Langmuir, Freundlich and Temkin (see Section 2.4), and the values of the characteristic parameters are presented in

Table 4. Isotherm parameters of Pb(II), Cd(II) and Hg(II) ion adsorption on Ka-Clay.

Model	Parameters	Pb(II)			Cd(II)			Hg(II)
		10 °C	20 °C	50 °C	10 °C	20 °C	50 °C	10 °C	20 °C	50 °C
Langmuir	R2	0.9756	0.9838	0.9742	0.9853	0.9863	0.9761	0.9522	0.9761	0.9763
	qmax, mg/g	48.08	69.93	116.08	22.67	72.41	107.41	25.64	31.03	45.66
	KL, L/g	0.036	0.158	0.661	0.603	1.313	2.140	0.004	0.005	0.008
Freundlich	R2	0.9237	0.9195	0.9024	0.8399	0.8688	0.9044	0.8390	0.8854	0.8906
	1/n	0.15	0.23	0.23	0.16	0.23	0.57	0.65	0.71	0.78
	KF, L/g	18.02	22.11	22.17	4.52	5.69	6.07	0.33	0.38	0.78
Temkin	R2	0.9316	0.9492	0.9061	0.8824	0.9452	0.9044	0.8851	0.8470	0.8247
	KT, L/g	26.86	17.36	11.09	2.33	1.93	1.19	0.03	0.03	0.04
	B, kJ/mol	4.81	9.71	12.77	6.21	11.91	21.27	3.71	9.33	10.49

.

As observed in Table 4, the highest regression coefficients (R² > 0.95) are obtained for the Langmuir model, regardless of the nature of the metal ion (Pb(II), Cd(II) or Hg(II)) or the temperature (10, 20 or 50 °C). This means that the adsorption of Pb(II), Cd(II) and Hg(II) ions on Ka-Clay is best described by this model and occurs through specific interactions between the metal ions and the functional groups of the adsorbent, leading to the formation of a complete monolayer on its surface. The agreement between the Langmuir model and the experimental data can also be observed in Figure 7, which shows the experimental isotherms and those calculated using this model.

As can be seen from Table 4, the maximum adsorption capacity (q_max, mg/g) increases in the order Pb(II) > Cd(II) > Hg(II), which suggests that Pb(II) ions are the most efficiently retained on the Ka-Clay surface under the given experimental conditions. However, the highest values of the Langmuir constants (K_L, g/L) are obtained for Cd(II) ions (Table 4), indicating that these ions can interact most easily with functional groups of the adsorbent compared with Pb(II) and Hg(II) ions. These observations are also supported by experimental data (Figure 6), which show that over a fairly wide range of initial concentrations (lower than 240 mg Pb(II)/L and 30 mg Cd(II)/L), Pb(II) and Cd(II) ions are quantitatively removed from aqueous solutions (>99% at 50 °C) through adsorption on Ka-Clay.

The values of the Freundlich model parameters (Table 4) indicate that the adsorption processes are favorable for all metal ions (n ranges between 1 and 10) and occur on a heterogeneous surface (1/n < 1). Moreover, the adsorption energy values (calculated using the Temkin model) are less than 22 kJ/mol (Table 4), which suggests that the adsorption of metallic ions (Pb(II), Cd(II) and Hg(II)) on the Ka-Clay surface takes place predominantly through electrostatic interactions (as ion exchange) [41]. The contribution of electrostatic interactions in adsorption processes depends on the type of the metallic ion and decreases in the order Cd(II) > Pb(II) > Hg(II), supporting the hypothesis that the interactions between Cd(II) ions and the functional groups occur with the highest intensity (see K_L values, Table 4).

To evaluate the efficiency of Ka-Clay in retaining Pb(II), Cd(II) and Hg(II) ions, the maximum adsorption capacity values (calculated from the Langmuir model) were compared with those reported in the literature for other adsorbents (

Table 5. The maximum adsorption capacities for Pb(II), Cd(II) and Hg(II) ions obtained for various adsorbents.

Adsorbent	Metal Ion	pH	Adsorbent Dose, g/L	Adsorption Capacity, mg/g	Reference
Bentonite clay	Pb(II)	5.0	2.0	0.60	[42]
Natural illitic clay	Cd(II)	5.0	1.0	5.25	[43]
	Pb(II)	5.0	1.0	15.20
Activated carbon	Cd(II)	2.0	1.0	0.63	[44]
	Hg(II)	2.0	1.0	7.97
Acid processed montmorillonite clay	Pb(II)	2.0	1.0	5.98
Nanoscale zero-valent iron composite	Pb(II)	6.0	1.0	59.35	[45]
Bentonite-crown-5 composite	Pb(II)	5.0	-	101.11	[46]
Montmorillonite-DDTC composite	Cd(II)	6.1–6.9	2.0	21.53	[47]
Montmorillonite-dimercapro composite	Hg(II)	4.0–5.0	0.2	3.21	[48]
MoS2/clay mineral composites	Pb(II)	5.0	1.5	89.45	[49]
	Cd(II)	5.5	1.6	280.39
	Hg(II)	3.0	-	1836.00
Ka-Clay	Pb(II)	6.5	4.0	69.93	This study
	Cd(II)	6.5	4.0	72.41
	Hg(II)	2.0	4.0	31.03

).

Also, the increase in the values of q_max, K_L, K_F, 1/n and B (Table 4) with increasing temperature, observed for all studied metal ions, is characteristic of endothermic adsorption processes. This behavior is also confirmed by the values of the thermodynamic parameters (

Table 6. Thermodynamic parameters for Pb(II), Cd(II) and Hg(II) ion adsorption on Ka-Clay.

Metal Ion	Temperature, °C	ΔG, kJ/mol	ΔH, kJ/mol	ΔS, J/mol·K
Pb(II)	10	−15.67	21.81	77.71
	20	−13.23		85.04
	50	−7.19		85.42
Cd(II)	10	−12.73	7.09	17.74
	20	−11.79		26.46
	50	−8.66		31.38
Hg(II)	10	−14.83	13.01	86.09
	20	−12.92		86.17
	50	−11.37		88.44

), which support the hypothesis of retention of heavy metal ions on the Ka-Clay surface predominantly through electrostatic interactions. Negative Gibbs energy values (∆G) indicate that the adsorption process of Pb(II), Cd(II) and Hg(II) ions on Ka-Clay is thermodynamically favorable and has the potential to occur spontaneously regardless of the experimental conditions.

As seen in Table 5, Ka-Clay has maximum adsorption capacities comparable to those reported in the literature, even when functionalized composite materials are used. However, it should be emphasized that adsorption capacity values significantly depend on experimental conditions (pH, adsorbent dosage, contact time), and this is why a rigorous study is necessary when using Ka-Clay as an adsorbent for retaining Pb(II), Cd(II) and Hg(II) ions.

Moreover, increasing the temperature within the range of 10–50 °C (a range relevant for practical applications) leads to a significant rise in the ΔG values characteristic of the adsorption process: 8.48 kJ/mol for Pb(II), 4.07 kJ/mol for Cd(II) and 3.46 kJ/mol for Hg(II) (Table 6). These differences show that as the temperature increases, the energy required for the adsorption processes decreases, which is characteristic of endothermic processes [50]. Consequently, the nature of the interactions involved in the adsorption processes of Pb(II), Cd(II) and Hg(II) ions on Ka-Clay does not change with increasing temperature, and they remain predominantly electrostatic in nature.

The endothermic nature of the retention of Pb(II), Cd(II) and Hg(II) ions on Ka-Clay is supported by the positive values of ΔH (Table 6, Figure 8). These positive values of adsorption enthalpy show that the binding of Pb(II), Cd(II) and Hg(II) ions on the Ka-Clay surface involves the formation of bonds that require greater energy consumption than that needed to break the initial physical bonds (hydrogen bonds or van der Waals forces) on the surface of the adsorbent and the metal ion [18]. This allows us to state that (i) in solution, the adsorbent is most likely stabilized through the formation of physical bonds between water molecules and functional groups (which is an advantage in terms of practical applicability), and (ii) the adsorption of metal ions on the Ka-Clay surface involves the existence of an affinity between the functional groups and the metal ions, which is characteristic of chemical interactions (electrostatic or covalent).

As shown in Table 6, the variation in adsorption enthalpy depends on the nature of the metal ion and increases in the order Cd(II) < Hg(II) < Pb(II), which suggests that Pb(II) ions have the highest affinity for the functional groups of Ka-Clay. This order of metal ion affinity for the functional groups of the adsorbent is primarily dictated by the electronegativity values (Pb(II) (2.33) > Hg(II) (2.00) > Cd(II) (1.69) [51]), which further emphasizes the importance of chemical interactions in the adsorption processes. However, for all the studied metal ions, the ΔH values are around 20 kJ/mol, which indicates a physico-chemical adsorption process [37]. Therefore, the interactions between Pb(II), Cd(II) and Hg(II) ions and the functional groups of Ka-Clay are most likely electrostatic (ion exchange).

The retention of Pb(II), Cd(II) and Hg(II) ions on Ka-Clay predominantly through electrostatic interactions is also supported by the positive values of ΔS (Table 6). The positive ΔS values obtained in the case of the three studied metal ions indicate the high compatibility between them and the functional groups of the adsorbent. Moreover, the relatively high values of entropy variation (Table 6) suggest that during the adsorption, at the liquid/solid interface, the randomness varies significantly [52] and is most likely due to the release of mobile ions (Ca(II) or Na(I)) from the Ka-Clay surface and the water molecules that hydrate both the adsorbent and the metal ions.

The spontaneous and endothermic nature (proved by the thermodynamic parameters—Table 6) of the adsorption processes shows that Ka-Clay enables efficient retention of Pb(II), Cd(II) and Hg(II) ions from aqueous environments. Furthermore, this adsorbent can be used in large-scale applications with low operating costs and reduced environmental impact.

3.6. Desorption Study

The recovery of the adsorbed metal ions is crucial when selecting an adsorbent for industrial applications and commercial purposes, as it provides valuable information about the efficiency of reuse and the stability of the adsorbent, leading to potential cost reductions [53]. In this study, Pb(II), Cd(II) and Hg(II) ions retained on Ka-Clay were desorbed using HNO₃ solutions with varying concentrations (0.001–1 mol/L) as the desorption agent, and the results obtained are shown in Figure 9. The selection of HNO₃ as the desorption agent was made considering the following: (i) Pb(II) and Cd(II) ions are retained at pH = 6.5, so their desorption most likely occurs in an acid media, and (ii) in the case of Hg(II) ions, although their adsorption occurs at pH = 2, after filtration, the final pH of the solution becomes greater than 7.0 (see Figure 3), so their desorption is also expected to take place in an acid media.

As shown in Figure 9, desorption efficiency increases with the concentration of HNO₃ for all the studied metal ions. The highest desorption percentages are obtained when using the 1 mol/L HNO₃ solution, where more than 49% of the metal ions retained on Ka-Clay (64.39% for Pb(II), 49.73% for Cd(II) and 83.13% for Hg(II)) are released into the solution and can be recovered. However, only in the case of Hg(II), the desorption of these ions is quantitative (over 83%), and the adsorbent can be reused in at least another three adsorption/desorption cycles until its adsorption capacity for Hg(II) ions decreases (at least theoretically) below 50%. In the case of Pb(II) and Cd(II) ions, even when treating the loaded adsorbent with a 1 mol/L HNO₃ solution, the desorption percentages are much lower (64.39% for Pb(II) and 49.73% for Cd(II)), which suggests that a significant portion of these ions is immobilized on the surface of Ka-Clay and cannot be released through desorption. Therefore, the recovery of Pb(II) and Cd(II) ions as well as the reuse of Ka-Clay in multiple adsorption/desorption cycles (for such situations) are less efficient, and consequently, an alternative method for utilizing the exhausted adsorbent needs to be found.

However, these experimental results also highlight another important aspect, namely that the type of interactions between the functional groups of Ka-Clay and the Pb(II), Cd(II) and Hg(II) ions during the adsorption process depends on the nature of the metal ion. Thus, if in the case of Hg(II) ions the interactions with the functional groups of Ka-Clay are predominantly ionic exchange (the only possible ones under the optimal experimental conditions), then in the case of Pb(II) and Cd(II) ions, in addition to ionic exchange interactions, surface complexation interactions also occur. Consequently, the adsorption efficiency of Hg(II) ions is modest (below 55%), specific to an ionic exchange equilibrium process, whereas the adsorption of Pb(II) and Cd(II) ions is quantitative (over 98%), characteristic of irreversible surface complexation processes.

3.7. Real Water Sample Tests

As mentioned in Section 2, for the tests on real water samples, water samples were collected from the Bahlui River (Gheorghe Asachi Technical University of Iaşi area). These tests had two objectives: (i) studying the influence of the concentration of common cations (Na(I), K(I), Ca(II) and Mg(II)) on the efficiency of retaining Pb(II), Hg(II) and Cd(II) ions on Ka-Clay and (ii) determining quality indicators of the water samples before and after using Ka-Clay as an adsorbent.

The selection of Na(I), K(I), Ca(II) and Mg(II) as coexisting cations for this study was made considering the following: (1) all these cations are present in significant concentrations in water samples, and (2) they can significantly influence ion exchange equilibria. The results obtained for the retention of Pb(II), Cd(II) and Hg(II) ions onto Ka-Clay in the presence of different concentrations of these ions are shown in Figure 10.

As can be observed in Figure 10, Na(I), K(I), Ca(II) and Mg(II) ions have a rather limited influence on the efficiency of Pb(II), Cd(II) and Hg(II) ion adsorption onto Ka-Clay regardless of their concentration. However, at high concentrations (1 mol/L), particularly in the case of Ca(II) ions, a decrease in the adsorption capacity of Ka-Clay for Pb(II) ions (1.79%), Cd(II) ions (0.98%) and Hg(II) ions (3.01%) can be observed. This decrease, although insignificant, is most likely due to the regression of ion exchange equilibria responsible for the retention of Pb(II), Cd(II) and Hg(II) ions onto Ka-Clay. Therefore, the use of Ka-Clay allows for the efficient removal of Pb(II) (97.81%), Cd(II) (95.49%) and Hg(II) (51.15%) ions, even from effluents with high concentrations of common ions, further highlighting the practical potential of this adsorbent.

To test the behavior of Ka-Clay in adsorption processes on real water samples, water samples were collected from the Bahlui River (Gheorghe Asachi Technical University of Iasi area). In each water sample, the concentration of Pb(II), Cd(II) and Hg(II) ions was adjusted to 20 ± 2 mg/L (single-component systems), while the other experimental parameters were set to the values established as optimal (see Section 3.2). After adding the adsorbent and intermittent stirring (3 h, at a temperature of 21 ± 1 °C), the concentrations of Pb(II), Cd(II) and Hg(II) ions as well as certain quality indicators (pH, TSS, turbidity and hardness) were determined.

Table 7. The values of the analyzed indicators before and after the adsorption of Pb(II), Cd(II) and Hg(II) ions on Ka-Clay.

Indicator	Pb(II)		Cd(II)		Hg(II)		Permissible Limit [54]
	Before	After	Before	After	Before	After
cM(II), mg/L	20.68	0.23	21.72	0.14	20.02	1.71	<(0.01–0.5)
pH	6.50	7.54	6.50	7.47	2.00	7.54	6.5–8.5
TSS, mg/L	219.15	278.45	219.15	285.01	219.15	279.23	-
Turbidity, NTU	10.70	14.20	10.70	15.10	10.70	13.90	-
Hardness, °Ge	14.58	15.98	14.58	16.13	14.58	15.79	-

presents the values of these indicators (before and after adsorption) compared to the maximum permissible values imposed by Romanian legislation (NTPA 002/2005) [54].

Analyzing the data presented in Table 7, it can be seen that the removal of Pb(II) and Cd(II) ions onto Ka-Clay is efficient, as the concentration values of these ions after adsorption are lower than the maximum permissible values (0.5 mg Pb(II)/L and 0.3 mg Cd(II)/L, respectively) [54]. This is not valid for Hg(II) ions, where after adsorption their concentration in solution remains high (over 1.7 mg/L). Moreover, regardless of the initial pH value (6.5 for Pb(II) and Cd(II) or 2.0 for Hg(II)), after adsorption, it increases to 7.4–7.5, a value that falls within the limits imposed by NTPA (Table 7). Additionally, a slight increase in the values of the TSS and hardness parameters can also be noted after adsorption (Table 7), but these variations are less significant as long as the legislation does not impose maximum permissible values for them. Most likely, the increase in the values of these parameters is a consequence of the ionization of the functional groups of Ka-Clay and the occurrence of ion exchange processes.

Much more important from an applicative point of view is the increase in turbidity of the solutions obtained after filtration. This increase in turbidity (approximately 5% after the removal of the adsorbent) is most likely due to very small Ka-Clay particles passing through usual filtration systems and end up in the solution. To support this observation, a control test was performed under the same experimental conditions but using distilled water. The variation of the solution turbidity from approximately 0.01 to 3.69 NTU clearly shows that Ka-Clay particles are responsible for the increase in the value of this parameter. Although on a laboratory scale the variation in turbidity before and after adsorption is not significant, it can have particularly important consequences when such a process is applied on a large scale. This drawback still needs to be minimized, and the proposed solution must take into account both efficiency and the costs.

4. Conclusions

Kaolinite-based clay (Ka-Clay) has been used to remove heavy metal ions with high toxic potential from aqueous environments. The selection of this adsorbent for adsorption studies took into account its low cost and high regional availability. For the experimental studies, the ions Pb(II), Cd(II) and Hg(II) were chosen, ions recognized for their negative effects on the environment and human health. The results presented in this study show that Ka-Clay exhibits high adsorption performance under optimal conditions that are easily achievable. Thus, at pH = 6.5, an adsorbent dose of 4.0 g/L and ambient temperature (21 ± 1 °C), Pb(II) and Cd(II) ions are quantitatively removed (over 95% in the case of Pb(II) and over 93% in the case of Cd(II)), even when their concentration is high (around 150 mg/L). Unfortunately, in the case of Hg(II) ions, the performance of Ka-Clay is more modest, with the removal percentage not exceeding 55% (even at low concentrations and at optimum pH of 2.0). The modeling of isotherms and kinetic curves shows that regardless of the nature of the metal ions, the adsorption process is spontaneous and endothermic and occurs in accordance with the Langmuir model and the pseudo-second-order kinetic model. These results, correlated with the desorption behavior characteristic of each metal ions, suggest that the retention of Pb(II) and Cd(II) ions involves a combined adsorption mechanism (ion exchange and surface complexation), while in the case of Hg(II) ions, the predominant interactions responsible for their retention are ion exchange. Therefore, Ka-Clay can be used as a promising adsorbent for the removal of high concentrations of toxic heavy metal ions with low cost and high efficiency and can contribute to the design of a sustainable wastewater treatment method. However, before its widespread use, the disadvantages (e.g., particle agglomeration, increased solution turbidity, etc.) must be minimized, and the methods by which this can be achieved will be presented in future studies.