Mineral Heterostructures for Simultaneous Removal of Lead and Arsenic Ions

Abstract

This study focuses on Pb²⁺ and As(V) adsorption on mineral heterostructures based on a mixture of Si, Fe, and Ti oxides (MOHs). Various techniques were performed to analyze the morphological and structural properties of the synthesized metal oxide samples. In addition to the experimental optimization of the parameters determined by the response surface method (RSM), the effects of pH, adsorbent dosage, temperature, and contact duration on the batch and column system adsorption efficiency of single-component and simultaneous lead and arsenate removal were tested. The pseudo-second-order kinetic model and Weber–Morris model were more relevant to the adsorption on the metal(loid)s. The adsorption of Pb²⁺ was related to the Langmuir isotherm model, while the adsorption of As(V) was fitted to the Freundlich isotherm model. The thermodynamic parameters indicate the spontaneity of the adsorption process with a low endothermic character. The MOHs were more effective in removing Pb²⁺ and As(V) in the multi-component system (87.7 and 46.1%, respectively) than in the single-component system (56.3 and 23.4%, respectively). This study demonstrates that mineral heterostructures can be effectively used to remove cations and anions from water systems, and due to their fast kinetics, they can be applied to the needs of rapid interventions after pollution.

1. Introduction

Long-lasting environmental problems arise from the presence of metals in different forms and matrices due to increased industrial, technological, and agricultural activity. A large amount of wastewater containing heavy metals, along with almost all (in)organic pollutants, is discharged into bodies of water. The distribution and concentration of arsenic and lead in water bodies vary widely depending on both natural geological processes and anthropogenic activities. In many regions, arsenic contamination arises from the weathering of arsenic-rich minerals, particularly in groundwater systems, while lead contamination is often linked to industrial discharge, legacy leaded gasoline, and aging infrastructure such as lead pipes [1,2,3]. The coexistence of arsenic and lead in the environment is a significant concern due to their widespread distribution and toxicity. These heavy metals often co-occur in areas affected by anthropogenic activities such as mining, smelting, and industrial processes, where metal ores containing both elements are extracted and processed. The historical use of lead-based paints and arsenic-based pesticides has further contributed to soil contamination, especially in agricultural and urban regions. Additionally, natural geological processes, such as the weathering of arsenic- and lead-rich rocks, can release both elements into groundwater and soil [4,5]. These heavy metal ions cannot be biodegraded in nature and accumulate in living organisms, and some of them can cause serious damage to the central nervous system, kidney, skin, liver, and lungs [6]. In particular, persistent exposure to arsenic can damage the central nervous system, cardiovascular system, endocrine system, liver, and skin, while exposure to lead can affect the kidneys, stomach, and nervous system [7,8]. Because their occurrence in water systems causes great threats to humans and biota, the concentrations of these chemicals in water are under regulation worldwide. According to the World Health Organization (WHO), the permissible limit of lead and arsenic ions in drinking water is up to 0.01 mg L⁻¹ [9].

The conventional methods of treatment of heavy metal contamination in aquatic systems include chemical precipitation, filtration, oxidation/reduction, ion exchange, membrane separation, reverse osmosis, electrochemical treatment, evaporation, solvent extraction, membrane technologies, and adsorption [10]. Among a variety of technologies, one of the environmentally friendly technologies is adsorption, which is now considered among the most effective, economic, and selective methods for water treatment and analysis purposes [11].

Nowadays, there are numerous natural, synthesized, and naturally developed synthesized materials used as adsorbents, such as zeolites, clay, fly ash, lignin microspheres/magnetite, biochar silica/Fe₃O₄, etc., for effective heavy metal and metalloid removal and wastewater treatment [12]. In order for an adsorbent to be used for various industrial purposes, it must have certain characteristics: highly efficient, multifunctional, economically profitable, environmentally friendly, and recyclable.

Metal(hydr)oxides are distinguished by their plentiful availability through natural resources, variable size and shape, ease of synthesis, high removal capacity, selectivity, unique catalytic activity, non-toxicity, and economic viability, all of which make them promising candidates for the efficient adsorptive removal of heavy metal and metalloid ions [13]. The adsorption parameters affecting the binding capacity of lead and arsenic to metal oxide-based materials are the adsorbent morphology, crystal structure, and active surface sites of the metal oxide nanoparticles and the pH of the medium. Heavy metal ions could bind to metal oxide nanomaterials, e.g, (i) to be adsorbed via the displacement of the protons, or (ii) to be attached to the negatively charged oxygen functionalities over the metal oxide nanoparticles, governed by the pH [14].

The most common mineral heterostructure adsorbents consist of metal(hydro)oxides such as iron oxides (γ-Fe₂O₃, α-Fe₂O₃, Fe₃O₄, α-FeOOH), manganese oxides (α-MnO₂), zinc oxides (ZnO), titanium oxides (TiO₂), aluminum oxides (γ-Al₂O₃), magnesium oxides (MgO), and cerium oxides (CeO₂), as well as their combinations [15,16,17]. One of the advantages of the three-component synthetic material is that the easy separation of Fe₃O₄-SiO₂-TiO₂ adsorbent by applying an external magnetic field facilitates the pre-concentration process and reuse of the adsorbent [18].

It is very important that the selected adsorbent can be used in several adsorption/desorption cycles. For practical and industrial applications, regeneration is one of the significant characteristics of a suitable adsorbent. In general, adsorbent regeneration has numerous advantages, such as the recovery of adsorbate molecules, the reuse of adsorbent in the adsorption process, the reduction of secondary waste, and the cost of the adsorption process [19]. Currently, there are more studies dealing with the study of adsorption compared to desorption and how to regenerate the used oxide-based materials for multiple uses.

The aim of this study was to (i) test the removal of Pb²⁺ and As(V) ions from single- and multi-component solutions onto mineral heterostructures, (ii) define the mechanisms and kinetics of the reactions in batch and column mode, and (iii) evaluate the regeneration of the used oxide-based materials for multiple practical applications. The novelty of this study is the simultaneous removal of cations (Pb²⁺) and oxyanions (As(V)) from aqueous solutions using silicate heterostructured materials.

2. Materials and Methods

2.1. Reagents, Standard Materials, and Equipment

For the purpose of this research work, the following chemicals were used: HNO₃ (Merck KgaA, Darmstadt, Germany); NaOH (Merck KgaA, Darmstadt, Germany); Arsenic—Plasma Emission Standard (Superlab, Belgrade, Serbia); Lead—Atomic Absorption Standard (Superlab, Belgrade, Serbia); and reference material—SLRS (River Water Reference Material for Trace Metals, National Research Council Canada, Ottawa, ON, Canada). Deionized water (Milli-Q, 18 MΩ·cm⁻¹) was used in the experiments (e.g., for dilutions). The concentrations of the Pb²⁺ and As(V) ions were measured using inductively coupled plasma mass spectrometry (ICP-MS) on Thermo Scientific^® iCAP Q (Waltham, MA, USA)

2.2. MOHs–Physico-Chemical Properties

For adsorption purposes in this work, MOHs were synthesized according to the previously described procedure [20]. The tested sample was a heterogeneous material that predominated in the content of Diopside (CaMgSi₂O₆—84.0 wt.%), Titanomagnetite (Fe_2.50 Ti_0.50O₄—9.1 wt.%), Hematite (Fe₂O₃—3.1 wt.%), Calcium Aluminium Silicate (Ca(Al₂Si₂O₈)—1.6 wt.%), Anatase (TiO₂—1.3 wt.%), and Rutile (TiO₂—0.9 wt.%).

2.3. Adsorption Experiments

2.3.1. Optimization Methods

Optimizing and predicting the optimal conditions for the adsorption of heavy metals is quite challenging, due to the numerous interactions between the various independent variables involved in the adsorption reactions. For process optimization, certain statistical programs are used to establish a design of experiment (DOE) using response surface methodology (RSM) to develop a mathematical function that relates the response to different predictors [21].

In this work, the adsorption process was optimized by numerical and graphical optimization methods using a Box–Behnken Design (BBD) with response surface methodology (RSM). A four-factor BBD with RSM (adsorption time (A), adsorbent dosage (B), temperature (C), and pH (D)) was used to maximize the Pb²⁺ ion removal, and the experimental and predicted results are shown in

Table 1. Four-factor BBD for RSM with observed Pb²⁺ removal.

Run	A t (min)	B m (mg)	C T (°C)	D pH	Y qe (mol g−1) × 10−5
1.	16	11	45	12	0.359
2.	16	2	45	7	4.36
3.	2	11	25	7	0.957
4.	16	20	25	7	1.13
5.	30	11	25	7	2.02
6.	2	11	45	7	0.998
7.	30	11	45	7	1.94
8.	30	11	35	12	0.396
9.	2	11	35	2	0.367
10.	16	2	35	12	0.89
11.	16	11	35	7	1.83
12.	16	11	35	7	1.83
13.	16	11	35	7	1.83
14.	2	2	35	7	2.20
15.	16	11	35	7	1.83
16.	2	11	35	12	0.204
17.	30	11	35	2	0.713
18.	30	2	35	7	4.40
19.	16	20	35	12	0.221
20.	16	20	45	7	1.08
21.	2	20	35	7	0.61
22.	16	2	25	7	4.54
23.	16	11	35	7	1.83
24.	16	11	25	12	0.374
25.	16	20	35	2	0.399
26.	16	2	35	2	1.6
27.	16	11	25	2	0.673
28.	16	11	45	2	0.646
29.	30	20	35	7	1.2

. The ANOVA of Pb²⁺ removal modeled by quadratic modeling in the optimization of Pb²⁺ adsorption on the MOHs is given in Table S1, and the expected responses and associated confidence intervals were calculated based on Equation (S1) in the Supplementary Materials.

2.3.2. Adsorption Behaviors

The adsorption behaviors of lead and arsenic onto the MOHs adsorbent were investigated by isotherms and kinetic and thermodynamic experiments. The results were used to estimate the maximum uptake capacity of the adsorbent, to assess the impacts of factors such as temperature and pH, and to evaluate whether the adsorption behavior was spontaneous. The adsorption capacity q_t (mg g⁻¹) of the tested MOHs adsorbent for the removal of the selected metals at time t was calculated by the following equation:

$$q_t={\displaystyle \frac{(C_i-C_f)V}{m}}$$

(1)

where Ci and Cf are the initial and final concentrations (mg dm⁻³) of lead and arsenic in the solution, V is the volume of the solution (mL), and m is the mass of the investigated adsorbent (mg).

2.3.3. Single Metal Adsorption

Stock solutions of arsenic and lead with an initial concentration of 5 mg dm⁻¹ for both ions were prepared in deionized water. The impact of pH in the adsorption of lead and arsenic was investigated in the pH range of 2–12 using a multiparameter ion meter (model PC 5, XS Instruments, Carpi MO, Italy). The chemicals HNO₃ (0.1 M) and NaOH (0.1 M) were used to adjust the pH of the solution. The adsorption experiments were conducted in a batch system by the addition of 2, 3, 4, 5, 10, 15, and 20 mg of the adsorbent in a vial of 10 mL containing 5.312 and 4.336 mg dm⁻³ of the standard solutions of Pb²⁺ and As(V), respectively. The optimal pH was defined at 5.0 for Pb²⁺ and 6.0 for As(V). The kinetic studies were carried out by adding 5 mg of MOHs material to a 10 mL solution (C_i = 5.312 and 4.336 mg dm⁻³ for Pb²⁺ and As(V), respectively) at the predetermined times of 2, 4, 6, 8, 10, 30, 60, and 180 min. The rate of adsorption was determined by pseudo-first-order, pseudo-second-order, and intraparticle diffusion. The adsorption equilibrium and thermodynamic parameters were evaluated at three temperatures (25, 35, and 45 °C) at the optimal pH values of 5.0 for Pb²⁺ and 6.0 for As(V). The kinetic, isotherm, and thermodynamic models/studies used for the fitting adsorption data are given in the Supplementary Materials. After the adsorption process, the sample solutions were filtered through a membrane syringe filter (pore size 0.45 µm).

2.3.4. Competitive Adsorption Studies

Competitive adsorption experiments were conducted when both Pb²⁺ and As(V) ions were present together in solution. In the simultaneous adsorption studies, the initial concentrations of the Pb²⁺ and As(V) ions were 2.419 and 2.265 mg dm⁻³, respectively. The above tests were carried out in flasks containing 10 mL of a solution with 5 mg of the selected material at 200 rpm at 25 °C at a predetermined time. The optimal pH for competitive adsorption is defined as 5.5.

2.3.5. Bed Column Experiments

In order to achieve optimal packing of the system, the adsorbent MOHs were carefully packed into the glass column. For the flow-through column experiment with a down-flow design, 1.2 × 5 cm (d × H) glass tubes with sintered filters and PTFE valves packed with MOHs were used. Sand was placed to the top of the column bed after pretreatment to prevent uniform distribution of the flow, while a column adapter was attached to the top of the column to allow the placement of the PTFE tubing into the column. Feeding solution (DV) was pumped through the tube to remove any impurities from the rig, and after its addition, a vacuum was applied to remove the air bubbles entrapped during the feeding solution flow. The mass of the adsorbent in the columns was m_ads = 0.9699 mg. The solution of pollutants (C_i = 0.5312 and 0.4336 mg dm⁻³ for Pb²⁺ and As(V), respectively) of known concentration was adjusted to pH 6 and passed through the column at 5.0, 10.0, and 15.0 cm³ min⁻¹ flow rates. The effluent samples were collected at a predetermined period and the concentration of Pb²⁺ and As(V) in the effluent was determined using the ICP-MS technique. The schematic diagram of the adsorption column experiment device is shown in Figure 1.

2.4. Regeneration

The regeneration of metal oxide heterostructures is an important feature to evaluate their repeatability in use. The regeneration efficiency mainly depends upon the nature of the metal adsorption, i.e., how the metals interact with the MOHs, as well as the components of the eluting reagents [15].

To evaluate the regenerability of the adsorbents, different solutions such as deionized water, NaOH (0.1; 0.5; 1 M), HCl (0.1; 0.5; 1 M), and HNO₃ (0.1; 0.5; 1 M) were used in the desorption experiments. The desorption experiments were performed for a mixture of lead and arsenic by adding a metal-free solution to the already used MOH materials after the adsorption process. The mixtures were rotated again for 30 min at 25 °C, after which the samples were filtered and acidified.

2.5. Material Characterization

For the analysis of morphological and structuralproperties, the Brunauer–Emmett–Teller (BET) method, Fourier transform IR spectroscopy (ATR-FTIR), X-ray diffraction analysis (XRD), and scanning electron microscopy (SEM) were utilized. The prior and post-adsorption structural analysis of the surface functional groups of the samples was performed by Fourier transform infrared (ATR-FTIR) spectroscopy. The Fourier transform infrared spectra were recorded using Nicolet iS10 FT-IR spectrometer (Thermo Scientific, Waltham, MA, USA) spectroscopy, and the spectra were recorded in transmittance mode from 400 to 4000 cm⁻¹ using eight scans at 4 cm⁻¹ resolution. The X-ray diffraction (XRD) method was used to analyze the morphology of the synthesized samples using an ENRAF NONIUS FR590 XRD (Bruker AKSS, Manning Park Billerica, MA, USA) diffractometer. The XRD patterns of the MOHs were compared with the diffraction powder files (PDF2) for Diopside, CaMgSi₂O₆ (reference pattern: 75-1092), Anatase, syn, Fe₂O₃ (73-1764), Hematite, syn, Fe₂O₃ (89-8103), and Titanomagnetite, syn, Fe_2,50 Ti_0,50O₄ (75-7376). The specific surface of a selected sample was determined by the BET method (Brunauer, Emmett, Teller) analyzing the nitrogen adsorption isotherms, measured by a Micromeritics ASAP 2020 V 1.05 H surface area analyzer. The mineral morphology was examined by scanning electron microscopy (SEM) with TESCAN MIRA 3 XMU equipment (TESCAN, Brno, Czech Republic). Mössbauer spectroscopy (MB) was used to provide information on the positions, valence states, and spins of the iron ions in the MOHs. The measurement of the ⁵⁷Fe-Mössbauer spectra was performed in transmission geometry using a ⁵⁷Co(Rh) source at room temperature, and this spectrum was calibrated by the spectrum of the normal iron foil. The isomer shift values (δ) were in reference to metallic alpha iron (δ = 0). The Mössbauer spectrum was determined using the WinNormos-Site software package (IgorPro 5.05A) based on the least squares method [22].

3. Results

3.1. Optimization of Process Parameters

In order to maximize the removal of Pb²⁺ ions (q_e) under optimized experimental conditions, four predictor parameters, adsorption time (A), adsorbent dose (B), temperature (C), and pH (D), were used with the Response Surface Method (RSM). The parameter optimization was performed using Equation (S1) as the objective function for Pb²⁺ removal, which belongs to the BBD numerical optimization methods. The independent variables were used as limits in the defined critical ranges. Graphs of the optimization are presented in Figure 2. The critical properties in the graphics area are represented in intense red. More precise optimization conditions were obtained by point prediction through the software based on the factors/components included in the model. The red dots in the figure represent critical conditions of maximum effectiveness for Pb²⁺ ion removal (adsorption) under various experimental parameters. In response surface plots, intense colors (in this case, red) typically represent high values, suggesting that these red dots are positioned where Pb²⁺; removal efficiency is at its peak. Finally, the obtained optimum experimental conditions were validated by an extra experiment, which presented the Pb²⁺ removal (q_e = 4.64 × 10⁻⁵ mol g⁻¹). The observed, experimental Pb²⁺ removal under optimum conditions was within the 95% confidence interval of the Pb²⁺ ion removal and the 2% relative standard deviation (RSD) of the predicted value, confirming the accuracy and reliability of the developed model.

3.2. The Influence of pH Value

The influence of pH on the removal of Pb²⁺ and As(V) ions depends on the nature of the adsorbent through the surface tension, surface properties, and degree of ionization of the groups present on the surface of the adsorbent, as well as the ion speciation in the aqueous solution at a certain pH value.

Figure 3 shows the influence of pH on the removal of Pb²⁺ and As(V) ions. The optimal pH values for the sorption of Pb²⁺ and As(V) ions on the MOHs adsorbent are 5.0 and 6.0, respectively. At these pH values, the adsorption can be explained by the electrostatic interaction between the structure of the adsorbent and the analyzed ions.

A surface charge property, i.e., zero net electrical charge on the material surface (pH_pzc), depends on the influence of the functional groups present in each sample. When the pH < pH_PZC (6.6), the particle surface is positively charged, and due to the electrostatic repulsion causing attraction with negatively charged ions, it is difficult to adsorb the positively charged ion. Conversely, when pH > pH_PZC, the surface has a net-negative charge, which is the result of the deprotonation of hydroxyl groups present on the surface of the adsorbent, causing low adsorption efficiency for the oxyanion and high efficiency for the cation [23,24].

The adsorption efficiency of the Pb²⁺ and As(V) ions on the MOHs was monitored in the range of pH values from 2 to 12 (Figure 3). The pH value affects the charge of the adsorbent surface, the degree of ionization, the amount of other metals in the aqueous solution, and the properties of the active surface of the adsorbent. The behavior of the Pb²⁺ and As(V) ions at various pH levels can be explained based on their speciation in the solution. At pH values higher than 7, the precipitation of lead occurs, forming a stable hydroxide ionic species (Pb(OH)₂) [25], while at a lower pH (i.e., 2, 3, 4), more H⁺ and H₃O⁺ cations are available to be adsorbed, resulting in a lower adsorption effectiveness [26]. For the Pb²⁺ ions, at pH = 5, the ion exchange and the complex formation process are the main procedures to remove the metal ions from the solution. The optimal pH for the lead is 5, because at pH > 6, the lead is hydrolyzed into Pb(OH)⁺ and Pb(OH)₂, thereby reducing the amount of free Pb²⁺ available for complexation [27,28].

At pH > pH_PZC, the electrostatic attraction between the positively charged surface of the adsorbent and the negatively charged species of arsenic compounds leads to more favorable adsorption of that ion. At pH values higher than 7.5, additional trivalent ionic species of arsenic have a negative effect on the adsorption of this ion on the adsorbent. At pH < pH_PZC, the negatively charged H₂AsO⁴⁻/HAsO₄²⁻ species take part in an electrostatic attraction with the positively charged adsorbent surface. The pH_PZC is not a crucial factor determining the effectiveness of heavy metal removal due to the complexity of the adsorption mechanism and the extent of the electrochemical interactions [29].

3.3. Batch Adsorption Experiments

3.3.1. Single Metal Adsorption—Isotherm Studies

The experimental adsorption data related to the adsorption isotherms was analyzed by fitting to the Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherm models (Equations (S2)–(S5)). The data values of the non-linear parameters of the adsorption isotherms of the Pb²⁺ and the As(V) adsorption (25, 35, and 45 °C) are presented in

Table 2. Non-linear parameters of adsorption isotherms of adsorption of Pb²⁺ and As(V) ions on MOHs (Ci[Pb²⁺] = 5.312 mg dm⁻³; Ci[As(V)] = 4.336 mg dm⁻³; m/V = 200–2000 mg dm⁻³; pH 5 for Pb²⁺ and 6 for As(V)).

Isotherm Model	Ion	Pb2+			As(V)
	Temperature	25 °C	35 °C	45 °C	25 °C	35 °C	45 °C
Langmuir isotherm	qm (mg g−1)	9.504	8.3122	7.2543	3.378	3.613	3.813
	KL (dm3 mg−1)	8.312	9.5707	11.304	1.913	2.049	2.217
	R2	0.816	0.838	0.840	0.991	0.991	0.981
Freundlich isotherm	KF (mg g−1)(dm3 mg−1)1/n	6.643	7.637	8.776	2.107	2.307	2.494
	n	3.668	3.4439	3.2886	2.232	2.289	2.360
	R2	0.991	0.997	0.992	0.967	0.940	0.910
Temkin isotherm	AT (dm3 g−1)	286.17	240.04	240.02	18.20	18.82	19.63
	bT	1.51	1.38	1.21	0.75	0.81	0.87
	B (J mol−1)	1643	1860	2185	3287	3146	3039
	R2	0.931	0.952	0.958	0.990	0.985	0.973
Dubinin–Radushkevich isotherm	qm (mg g−1)	7.95	7.24	6.51	2.52	2.77	3.01
	Kad (mol kJ−2)	10.17	10.26	10.37	10.30	10.21	10.12
	Ea (kJ mol−1)	7.012	6.980	6.944	6.968	6.999	7.028
	R2	0.859	0.878	0.888	0.961	0.969	0.974

and Figure 4.

The best fitting parameter adsorption isotherms for Pb²⁺ and As(V) adsorption were found to follow the Freundlich and Langmuir isotherms, respectively. These isotherm models for Pb²⁺ indicated the following decrease in the fitting order: Freundlich > Dubinin–Radushkevich > Temkin > Langmuir, compared to the following for As(V): Langmuir > Dubinin–Radushkevich > Temkin > Freundlich, as shown by the R² correlation coefficient (Table 2).

In the Langmuir model, the adsorption is assumed to be monolayered on a homogeneous surface where all sorption sites are found to be identical and energetically equivalent, while the Freundlich model assumes the adsorption to be multilayered on a heterogeneous adsorbent surface with a non-uniform distribution [30]. In Table 2, the high monolayer coverage capacities (q_m) for the As(V) ions, obtained using the Langmuir isotherm model, increase with the temperature increase. The distribution coefficients (K_L; dm³ mg⁻¹) were higher for the Pb²⁺ ions, indicating that they are the most retained cations; on the other hand, As(V) can be easily exchanged and substituted by other metals [31]. The Freundlich isotherm is the best fitting one for the Pb²⁺ adsorption. A value of the Freundlich parameter (Table 2) n close to zero indicates a very heterogeneous surface; values n < 1 imply a chemisorption process, while values n > 1 indicate combined adsorption, e.g., physisorption and chemisorption with different contributions to the process in different steps of the system equilibrium. The results from Table 2 indicate that the adsorption is combined in all cases. Some information about the studied systems can be obtained by calculating the parameters of the Temkin and Dubinin–Radushkevich models. In the Temkin isotherm, A_T is the bond constant that represents the maximum binding energy (dm³ mg⁻¹), and it can be seen from the table that the values are much higher for the removal of the Pb²⁺ ions [32]. In the Dubinin–Radushkevich isotherm model, Ea represents the mean free energy, and for the selected ions, Ea < 8 kJ mol⁻¹ indicates whether the physical process dominates [33].

3.3.2. Thermodynamic Studies

The thermodynamic parameters of the Gibbs free energy change, ΔG^Θ, enthalpy change, ΔH^Θ, and entropy change, ΔS^Θ, are given in

Table 3. Calculated Gibbs free adsorption energy, enthalpy, and entropy for adsorption of Pb²⁺ and As(V) on MOHs at 25, 35, and 45 °C.

Ion	ΔGΘ (kJ mol−1)			ΔHΘ (kJ mol−1)	ΔSΘ (J mol−1 K−1)	R2
	25 °C	35 °C	45 °C
Pb2+	−45.55	−47.44	−49.42	12.11	193.33	0.996
As(V)	−39.39	−40.88	−42.42	5.80	151.54	0.996

.

The thermodynamic parameters are important to check the spontaneity (ΔG^Θ) and feasibility (ΔH^Θ and ΔS^Θ) of the process. The negative values of ΔG^Θ indicate the spontaneity and feasibility of the adsorption process, which increase with increasing temperature. According to the literature [34] for ΔG^Θ in the range of −20 to −80 kJ mol⁻¹, both physisorption and chemisorption participate, in whose scope are the results of this research. Information about the nature of the process, whether it is exothermic or endothermic, is given by the parameter ΔH^Θ. In this work, the positive values of ΔH^Θ confirm the endothermic nature of the adsorption process. A positive value (ΔS^Θ) indicates an increase in the degree of freedom of the adsorbed species; that is, it reflects the affinity of the adsorbent for the adsorbate molecules [35].

3.3.3. Kinetic Study

The reaction rate and adsorption mechanisms for the appropriate system are obtained by calculating the kinetic parameters. The physico-chemical characteristics of the adsorbent control the removal rate of the selected ions in the solution. The plots of the adsorption kinetics of Pb²⁺ and As(V) are given in Figure 5a, while the plots of the intraparticle diffusion model are given in Figure 5b. The kinetic data obtained from the Pb²⁺ and As(V) sorption studies were fitted to the kinetic models of pseudo-first-order (PFO), pseudo-second-order (PSO), and second-order (

Table 4. Pseudo-first-, pseudo-second- and second-order model parameters for adsorption of Pb2+ and As(V) on MOH adsorbent (Ci[Pb²⁺] = 5.312 mg dm⁻³; Ci[As(V)] = 4.336 mg dm⁻³; m/V = 200 mg dm⁻³; pH 5 for Pb²⁺ and 6 for As(V); T = 25 °C).

Pollutant	Eq. Parameter	PFO	PSO	Second-Order
Pb2+	qe	1.941	6.493	6.493
	k (k1, k2)	0.189	0.141	0.00464
	R2	0.891	0.999	0.572
As(V)	qe	3.484	6.888	6.888
	k (k1, k2)	0.164	0.060	0.0009
	R2	0.944	0.998	0.659

).

Based on Figure 5a, it can be concluded that the equilibrium in the system is reached after 30 min. The coefficient of determination (R²) and parameters of the kinetic models were calculated to determine the conformity of the model with the obtained experimental data. According to the R² coefficient (Table 4), the pseudo-first-order kinetic model is far from unity, and the pseudo-second-order kinetic model is near unity (R² = 1). The process described by PSO is chemisorption between the analyte ions and adsorbent, indicating the existence of more than one rate-controlling step [36]. In this research, a very fast establishment of equilibrium was observed, which indicates the very fast kinetics of the process of removing lead and arsenic from the system. These results indicate physical sorption and not chemisorption, which is characterized by a very slow establishment of equilibrium.

In general, on the surfaces of the metal oxides, physisorption occurred both on the sites of the metal cations and on the sites of the oxygen anions in the surface layer. Between these two places, the strength of the connection can be drastically different. The bond is stronger at the metal site than at the large O²⁻ anion because metal cations have a significantly higher charge density, and therefore can induce a stronger polarization of the adsorbate [37].

Due to the successful application of the PSO model, the results of the parameters of the PSO model for the adsorption of Pb²⁺ and As(V) on the MOHs adsorbent are shown in

Table 5. The parameters of the pseudo-second-order model for the adsorption of Pb²⁺ and As(V) ions on the MOHs.

Sorbate	Temperature	qe (mg g−1)	k2 (g (mg min)−1)	R2	Ea (kJ mol−1)
Pb2+	25 °C	6.493	0.14064	0.999	2.151
	35 °C	6.552	0.14454	0.999
	45 °C	6.611	0.14853	0.999
As(V)	25 °C	6.475	0.05771	0.997	2.216
	35 °C	6.888	0.05973	0.998
	45 °C	7.310	0.06095	0.997

. The activation energy can be determined using the Arrhenius equation, based on the results of the kinetics performed at the temperatures of 25, 35, and 45 °C. The linear form of the Arrhenius Equation (2) is as follows:

$$\ln K^{\prime }=-{\displaystyle \frac{E_a}{RT}}+\ln A$$

(2)

where K′ is the reaction rate constant at a given temperature, E_a shows the activation energy (kJ mol⁻¹), R is the universal gas constant (8.314 JK⁻¹mol⁻¹), T is the temperature (K), and A is the Arrhenius factor (the frequency for a given reaction). The increase in the activation energy occurs as a consequence of the adsorption mechanism on the used adsorbent.

Physisorption, or physical adsorption, generally has energy up to 40 kJ mol⁻¹, while chemisorption requires higher energy and activation energy over 40 kJ mol⁻¹. Based on the obtained results, where Ea for the Pb²⁺ ion adsorption is 2.151 kJ mol⁻¹ and for the As(V) ion adsorption is 2.216 kJ mol⁻¹, we can conclude that the main mechanism of the adsorption is physical adsorption.

Due to the complexity of the adsorption process, a mass transfer process that varies the steps could significantly contribute to the overall adsorption process. To describe sequential/competitive adsorption processes as transfer models, the intraparticle diffusion models were used: Weber–Morris (W-M), Dunwald–Wagner (D-W), and Homogeneous Solid Diffusion (HSDM) [22]. The rate constants as determined using the W-M, D-V, and HSDM models for the adsorption of the selected ions are given in

Table 6. Kinetic parameters of W-M, D-W, and HSDM models for adsorption of Pb²⁺ and As(V) ions on MOHs adsorbent (Ci[Pb²⁺] = 5.312 mg dm⁻³; Ci[As(V)] = 4.336 mg dm⁻³; m/V = 200 mg dm⁻³; pH 5 for Pb²⁺ and 6 for As(V); T = 25 °C).

Model	Eq. Parameters	Pb2+	As(V)
Weber–Morris (Step 1)	kp1 (mg g−1 min−0.5)	1.623	1.609
	C (mg g−1)	1.563	0.782
	R2	0.998	0.997
Weber–Morris (Step 2)	kp2 (mg g−1 min−0.5)	0.102	0.135
	C (mg g−1)	5.736	5.571
	R2	0.999	0.994
Weber–Morris (Step 3)	kp3 (mg g−1 min−0.5)	0.00597	-
	C (mg g−1)	6.177	-
	R2	0.999	-
Dunwald–Wagner	K	0.698	0.0567
	R2	0.693	0.820
Homogenous Solid Diffusion Model	Ds	7.44 × 10−11	6.58 × 10−11
	R2	0.680	0.799

.

Based on the results (Table 6 and Figure 5b), the W–M fitting result shows that the adsorption process can be described through several steps. The values of the constants of the W-M model (k_p1 > k_p2 and k_p1> k_p2> k_p3) indicate that the kinetics of the adsorption are significantly faster in the first period due to the large number of available sites on the adsorbent [38]. The values of intercept C (mg g⁻¹) provide data about the thickness of the boundary layer, and the higher the value of C, the higher the resistance originating from the diffusion within the particles. On the other hand, the low values of the intersection in the first linear range (W–M1) indicate the possibility that this phase takes place quickly and partially inhibits mass transfer, while the higher values in the second phase (W–M2) indicate the dominance of the intra-diffusion of the particles [39].

Comparison of the MOHs material in this study with heterostructured materials collected from the literature offers valuable information on their adsorption performances.

Table 7. The review of the literature data on the adsorption characteristics and the experimental parameters for sorbents applied for Pb²⁺ and As(V) removal.

Sorbent	pHpzc	Experimental Conditions	qmax (mg g−1)	Adosrption Isotherm	Kinetic Model	Thermodynamic Parameters	Reference
Iron oxide-modified sericite alginate beads	4.9	m = 0.8 g; V = 50 mL; pH = 5; T = 25 °C; t = 15 h.	qPb = 133.73 qAs = 21.61	Freundlich (Pb2+ and As(V))	PFO (Pb2+) PSO (As(V))- chemisorptions	-	[40]
CoFe2O4@SiO2-NH2	-	m/V = 0.4 g L−1; Co(Pb2+) = 80 mg L−1; pH = 7; T = 35 °C, t = 0–780 min.	qPb = 181.6	Langmuir	PSO	ΔG° < 0 spontaneous ΔH° > 0 endothermic ΔS° > 0	[41]
Illite–smectite clay	-	m/V = 0.625–12.5 g L−1; Co(Pb2+) = 1 mg L−1; T = 298 K; t = 60 min.	qPb = 0.256	Langmuir	PSO	ΔG° < 0 spontaneous ΔH° > 0 endothermic ΔS° > 0	[42]
Shanghai silty clay	-	m/V = 15 g L−1 (Pb2+); m/V = 40 g L−1 (As(V)); Co(Pb2+) = 100 mg L−1; Co(As(V)) = 50 mg L−1; pHAs = 7; pHPb = 6; T = 298 K; t = 24 h.	qAs = 2.8 qPb = 26.46	Langmuir (Pb2+) Freundlich (As(V))	PSO- chemisorption	ΔG° < 0 spontaneous ΔH° > 0 (Pb2+) endothermic ΔH° < 0 As(V) exothermic ΔS° > 0	[43]
Clay	-	m = 1 g; V = 100 mL; Co(Pb2+) = 100 mg L−1; pH = 7; T = 25 °C t = 120 min.	qPb = 36.23	Langmuir	PSO	-	[44]
Bentonite clay (BC) calcined at 500 °C	5.89	m = 100 mg; V = 200 mL; pH = 5; T = 20 °C; t = 0– 500 min.	qPb = 94.01	Langmuir	PFO	ΔG° < 0 spontaneous ΔH° > 0 endothermic ΔS° > 0	[45]
TiO2/kaolinit	-	m = 0.5 g; V = 20 mL; Co(Pb2+) = 5–80 mg L−1; pH = 6; T = 30 °C; t = 80 min.	qPb = 333.33	Langmuir	-	ΔG° < 0 spontaneous ΔH° > 0 endothermic ΔS° > 0	[46]
Nanocrystalline kaolinite	-	m/V = 0.5 g L−1; Co(As(V)) = 30 mg L−1; pH = 8; T = 303 K; t = 120 min.	qAs = 43.67	Langmuir	-	ΔG° < 0 spontaneous ΔH° > 0 endothermic ΔS° > 0	[47]
α -Fe2O3 nanoclusters	-	m = 3 mg; Co(As(V)) = 2–150 mg L−1; pH = 3; T = 25 °C, t = 12 h.	qAs = 181.82	Langmuir	PSO (intraparticle) diffusion rather	-	[48]
Zeolite-supported nanoscale zero-valent iron	5.6	m/V = 0.5 g L−1; Co(Pb2+) = 100 mg L−1; Co(As(III)) = 5 mg L−1; pHPb = 5.5; pHAs = 7; T = 25 °C; t = 10 h.	qPb = 85.90 qAs = 12.84	Langmuir As(III)- complexation, Pb2+ -reduction	PSO	-	[49]
MOHs	6.6	m/V = 200–2000 mg L−1;Co(Pb2+) = 5 mg L−1; Co(As(V)) = 5 mg L−1; pHPb = 5; pHAs =6, T = 25 °C; t = 30 min.	qPb = 9.50 qAs =3.81	Freundlich (Pb2+) Langmuir (As(V))	PSO	ΔG° < 0 spontaneous ΔH° > 0 endothermic ΔS° > 0	This study

summarizes the experimental conditions, isotherms, kinetic and thermodynamic models, and maximum absorption of Pb²⁺ and As(V) in the MOHs analyzed in this study, as well as other mineral materials reported in the literature.

Based on the results from Table 7, it can be concluded that the Langmuir model prevails for isotherm models, while the PSO model stands out for kinetic models. The predominant sorption mechanisms proposed for Pb²⁺ and As(V) in the existing literature include chemisorption. Although the results showed that the PSO model may suggest some level of chemisorption, the lower R² values for Langmuir indicate that physisorption is also involved in lead removal. The parameter results in this study suggest that the mechanism for lead removal could be physisorption, but chemisorption may be present under certain conditions. On the other hand, chemisorption is demonstrably present as a mechanism for removing arsenic from aqueous solutions. During adsorption on silicate materials, arsenate ions (H₂AsO₄⁻) and lead ions (Pb²⁺) are adsorbed through the formation of complexes with functional groups on the silicate surface or ion exchange [40,50]. The adsorption of Pb²⁺ and As(V) on MOHs is a thermodynamically feasible, spontaneous, and endothermic process. In Table 7, numerous literature sources are presented that provide the maximum sorption capacities of Pb²⁺ and As (V) sorption on various materials from single-component solutions. The differences in q_max are due to the nature and properties of each adsorbent, the adsorbent particle size, the surface area, and the main functional groups in the structure of the adsorbent. The given results are in a wide range of sorption capacity values, and it is noticeable that no district correlation is observed between these values and the experimental conditions. The results of this research show a low sorption capacity of the material, but the fast kinetics of the adsorption process demonstrate that the synthesized MOHs material is suitable for accidental and emergency situations, where it is necessary to react quickly and efficiently.

3.4. Simultaneous Adsorption of Pb²⁺ and As(V) Ions

Examining simultaneous adsorption is very important due to the fact that aqueous systems are mixtures of different substances. Therefore, there is a great interest in efficient adsorbents for the simultaneous removal of both anions and cations from the aquatic system. In order to obtain a realistic picture of the possible uses of the synthesized adsorbent for the purification or treatment of industrial wastewater or model water containing a mixture of different pollutants, it is necessary to investigate the simultaneous adsorption in multi-component systems. A graphic representation of adsorption efficiency as a function of time in single- and multi-component systems is shown in Figure 6. Also, the removal of Pb²⁺ and As(V) ions in the binary system depending on the initial concentration and time are shown in the Figure 7.

Based on the results, the adsorption selectivity of the synthetic material can be evaluated in relation to the pollutant mixture. The obtained results showed a higher affinity of the MOHs adsorbent in relation to Pb²⁺ ions, while a lower affinity was found for As(V) ions. The adsorption efficiency of anion/cation removal depends on the pollutant nature (ionic radius, electronegativity, etc.), solution pH, temperature, and surface properties of the adsorbent. In both cases, it was observed that the Pb²⁺ and As(V) were adsorbed very rapidly, and most of the uptake occurred within 30 min. Based on the obtained results, the analyzed adsorbent is more efficient for the simultaneous removal of the selected ions. It can be explained by the fact that in the case of the single metal adsorption, the initial concentrations were high, and the adsorbent mass was small. The removal efficiency from the multi-component system compared to the single-component system increased from 56.4 to 87.7% for Pb²⁺, while for As(V) it increased from 23.4 to 46.1% for the same experimental conditions. Increasing the adsorption time beyond 30 min did not result in a significantly further removal of ions. The removal rate of lead and arsenic shows a decreasing trend in 40–60 min due to possible competition between the positive and negative ions after reaching a maximum in the multi-component solution. In the binary system, the As(V) adsorption on TiO₂-based materials increased, which can be explained by the fact that in the presence of cations, As(V) adsorption is facilitated due to electrostatic attraction and the presence of the co-adsorption of more metals [13]. Also, the presence of Fe oxide in the adsorbent structure allows the simultaneous removal of multiple metal(loid) contaminants. The adsorption of As(V) mainly depends on the incorporated Fe content, where the arsenic was mainly adsorbed on the Fe oxides due to electrostatic and specific adsorption, while lead was adsorbed on the Fe oxides via electrostatic and physical adsorption, respectively [51,52]. For the simultaneous removal of heavy metals and oxy anions, it is very important to choose an adsorbent suitable for removing both positive and negative ions. SiO₂ usually has a lower adsorption capacity for the removal of As(V), but their modification with metal oxides (Al, Fe, Mn, Ti, Sn, Zn, Mg) greatly improved the adsorption capacity due to the removal of impurities and increase in the surface area of the adsorbent. Silica and metal oxides have rather different surface properties: (i) silica contains both hydrophilic and hydrophobic sites (siloxanes), while the normal metal oxides are hydrophilic; (ii) silica has an asymptotic point of zero net proton charge and a highly non-Nernstian surface potential, while the metal oxides’ charging curves are linear to the point of zero net proton charge and their surface potentials are close to Nernstian [53]. Various oxide complexes can be obtained by synthesis and modification, which lead to an increase in the efficiency in removing the heavy metals and oxy anions.

3.5. Bed Column Experiments

For the possible application of the MOHs adsorbent for real application in water purification processes, a fixed-bed column study was conducted. In order to obtain the optimal empty-bed contact time (EBCT), the values of several parameters (the flow rate Q, empty bed volume (EBV), and pH value) were adjusted. All experiments were performed at 25 °C while the flow rate of the feed solution was varied at Q = 5.0, 10.0, and 15.0 cm³ min⁻¹. Using an Ismatec peristaltic pump, the feed water was passed through the hybrid adsorbent bed. The residence time EBCT was calculated according to the following: EBCT = H/γ, where H is the bed depth (cm) and γ is the linear flow rate (cm³ cm⁻² s⁻¹). The breakthrough point is marked when the concentration in the eluent reaches the value of the maximum allowed concentration for the given ion in drinking water, where on the graph C_mac/C_i = 0.0188 for lead and 0.023 for arsenic (C_mac = 0.01 mg dm⁻³).

The operational performance in a dynamic mode requires the determination of the breakthrough curve (Figure 8), and thus the calculation of the capacity was performed (

Table 8. Bohart–Adams, Yoon–Nelson and Thomas model fitting for Pb²⁺ and As(V) ion adsorption by MOHs (Ci[Pb²⁺] = 5.312 mg dm⁻³; Ci[As(V)] =4.336 mg dm⁻³; m_ads = 0.9699 mg; T = 25 °C; pH = 6, BV= 2.826 cm⁻³).

Model			Pb2+			As(V)
	Q	cm3 min−1	5.0	10.0	15.0	5.0	10.0	15.0
Bohart–Adams	kBA	dm3 mg−1 min−1	6.277	10.08	15.53	29.86	37.89	33.90
	qo	mg g−1	8.204	7.001	6.132	2.841	2.616	2.335
	R2		0.988	0.996	0.998	0.992	0.990	0.987
Yoon–Nelson	kYN	min−1	0.667	0.541	0.550	1.295	1.095	0.979
	θ	min	14.98	12.67	11.19	6.355	5.853	5.224
	R2		0.988	0.995	0.997	0.991	0.989	0.986
Thomas	kTh	L min−1 mg−1	1.255	1.018	1.035	2.986	2.526	2.529
	qo	mg g−1	22.97	19.42	17.17	8.01	7.38	6.60
	R2		0.947	0.957	0.951	0.991	0.989	0.985

). The performance of the column was described by the determination of the breakthrough curve, whose characteristics are crucial for the adsorption dynamic response and the process design of the fixed-bed adsorption column because they directly influence the possible applications and economics of the adsorption process. The adsorption efficiency, performance, and applicability of the adsorbents for operations at a laboratory and industrial level were studied using the Bohart–Adams, Yoon–Nelson and Thomas models.

Figure 8 shows the results for the experiments on the column, which represents the pollutant concentration in the effluent normalized to the influent, C_i/C_o, in relation to the volume of solution through which it passed column.

The results in Table 8 show good agreement with the experimental results for all process parameters studied, indicating that the models were suitable for the MOHs adsorbent and for the fixed-bed column design. In all cases for Pb²⁺ and As(V), the best fit was obtained using the Bohart–Adams model, but in any case, all presented results showed significant adsorption behavior. The throughput volume for the breakthrough of the heavy metal(oid)s for different flow rates (5; 10 and 15 cm³ min⁻¹) was in the order of Pb²⁺ (5.4; 3.2; 1.2 dm⁻³) and As(V) (2.8; 1.6; 0.7 dm⁻³). As the flow rate increases, the breakthrough time and adsorption capacity decrease. Higher flow rates influence the decrease in external diffusional resistance, and thus low internal adsorbent surface saturation, which is reflected in the lower adsorption capacity. The correlation between the predicted and experimental results indicates the high potential of using MOHs in a real water treatment system.

3.6. Regeneration and Desorption

After adsorption, two methods were applied: the desorption of the ions after simultaneous adsorption and the regeneration of the MOHs in a flow system.

3.6.1. Desorption of Ions After Simultaneous Adsorption

The effect of desorption was tested by calculating the percentage of desorbed Pb²⁺ and As(V) (Equation (S15)) in the mixture on the spent adsorbents by using different strengths of eluents. Their performance depended on the adsorbate/surface functionality of the binding type/strength. The results in Figure 9 show that the desorption of lead was favorable using an acidic eluent (Figure 9a), while for arsenic, it was an alkaline eluent (Figure 9b). The alkaline solutions achieved high regeneration performance in oxyanion desorption, in which NaOH plays the role of a source of competing OH⁻ ions. Also, the adsorption of As(V) on materials based on metal oxides is through the ion exchange between the surface hydroxyl groups (M-OH) and arsenic species, without the occurrence of a redox reaction. Heavy metal cations (Pb²⁺) can be desorbed using acidic solutions (such as HCl, HNO₃, H₂SO₄, etc.) due to the electrostatic repulsion between the metal cations and the protonated surface of the adsorbent [54]. These are the possible mechanisms and conditions of desorption by acids: (1) the low pH favors the desorption and/or dissolution of the metal cations, (2) strong competition between the cations and H⁺ ions for the adsorption sites causes the displacement of cations into the acid solution, (3) the acidic conditions favor the dissolution of the Fe and Al adsorption surfaces of oxides/silicates, and thus the release of the adsorbed/surface-precipitated metals, and (4) the acid reacts with the residual alkalinity and reduces the adsorption capacity [55].

The arsenic and lead desorption increased upon increasing the concentration of desorption agents from 0.05 to 0.1 M. The highest percentage of the lead desorption was when 0.1 M HNO₃ (60.9%) was used, while for arsenic it was when 0.1 M NaOH (36.0%) was used. It is very important to properly choose the desorption agent and the design of the overall technology in order to achieve a minimal negative impact on the properties of the adsorbent and the environment.

3.6.2. Regeneration of MOHs in Flow System

The adsorbent’s applicability was evaluated according to the following performance parameters: adsorption uptake, kinetics, selectivity, period of usability, and stability. The number of adsorption/desorption cycles present indicates the period of use of the adsorbent, after which the exploitation of the adsorbent is not sustainable. The desorption study was carried out in order to achieve a satisfactory ecotoxicological effect of the entire process and to check the degree of fulfillment of the established criteria for obtaining a low degree of deterioration of the adsorbent in successive cycles of adsorption/desorption. The most important parameters for the selection of a suitable desorption agent are the functionality of the surface, the mechanism of adsorption, and the electronic and geometric characteristics. The decision of a realistically applicable process depends on the choice of operating factors: regenerator, concentration, flow rate, pH value, and desorption time. An upward-flow fixed-bed column desorption study was selected as a more convenient method (simpler handling and maintenance).

Acidic or alkaline agents are usually applied in the regeneration process. Among many desorption agents, 0.2 mol dm⁻³ HNO₃ for Pb²⁺ and 0.5 mol dm⁻³ NaOH for As(V) (100 cm³) were selected as the most effective. The adsorption/desorption results are shown in

Table 9. The five adsorption/desorption cycles of the Pb²⁺ and As(V) using MOHs at the following desorption conditions: (Q = 5.0 cm³ min⁻¹; Ci[Pb²⁺] = 0.5312 mg dm⁻³; Ci[As(V)] = 0.4336 mg dm⁻³; m_ads = 0.9699 mg), using 0.2 mol dm⁻³ HNO₃ for Pb²⁺ and 0.5 mol dm⁻³ NaOH for As(V) (V = 100 cm³).

Adsorbate	Pb2+						As(V)
	I	II	III	IV	V	Ʃ3	I	II	III	IV	V	Ʃ3
Adsorption (mg g−1) 1	8.20	8.04	7.85	7.53	7.13		2.84	2.64	2.44	2.20	1.99
Desorption (mg g−1) 1	8.10	7.88	7.53	7.15	6.69		2.68	2.48	2.24	2.00	1.75
C (ppm) 2	81.0	78.8	75.3	71.5	66.9		26.8	24.8	22.48	20.03	17.51
Δq (mg g−1) 3	0.10	0.16	0.31	0.38	0.44	1.39	0.16	0.16	0.20	0.20	0.24	0.95

¹ adsorption capacity and quantity of desorbed pollutants; ² concentrations of pollutants in effluent water; ³ quantity of bonded pollutants after nth cycle and overall amount after five cycles.

.

A low decreasing trend in desorption efficiency was obtained (Table 9): from 98.7% to 93.8% for the Pb²⁺ ions (overall 81.5%), and from 94.5% to 88.0% for the As(V) ions (overall 61.65%), from the first to the fifth desorption cycle, respectively. Due to the low strength of the adsorbate/adsorbent surface interactions, regeneration is feasible process. Loosely bonded As(V) ions, due to pronounced anionic properties, are displaced by anionic regenerators due to electrostatic repulsion. A similar mechanism is operative for Pb²⁺ for electrostatic repulsion with a positive adsorbent surface. It indicates that a labile bond is the main prerequisite to obtain good desorption ability. The desorption efficiency decrease follows a similar trend after the tenth cycle: 92.3% for Pb²⁺ and 85.2% for As(V). Also, the influence of a higher flow rate on the deterioration in desorption efficiency was noticed (1st cycles): 98.7% and 88.6% for Pb²⁺ (Q = 5.0 and 10.0 cm³ min⁻¹, respectively). For the desorption process to be ideal, the adsorbent must return to its initial performance, allowing for efficient reuse for the maximum number of cycles.

3.7. Structural and Morphological Characteristics

3.7.1. X-Ray Diffraction

The crystallinity of the analyzed materials and their phase purity were determined by powder X-ray diffraction (XRD), and are presented in Figure 10.

The silicate structure is represented by diopside, which belongs to the group of calcium pyroxenes. The main constituent is diopside (84.0 wt.%), with characteristic peaks at 2θ° = 29.9, 30.4, 35.8, 39.3, 42.5, 52.3, and 65.6 [56]. The peak of intense diffraction (2θ° = 29.9) determines the size of the dioxide crystallite in the MOHs material. In addition, many low-intensity diopside peaks were also noticed in the range of 30 to 75° [57]. Meanwhile, the XRD spectrum of the MOHs displays two similar broad peaks at 33° and 63°, indicating the presence of the rhombohedral α-phase Fe₂O₃ (hematite) [58,59]. The structure of titanomagnetite (Fe_2,50 Ti_0,50O₄) indicates that Ti and Fe are the principal elements of this compound. Titanomagnetite was observed at 2θ° = 34.9, 43.5, 56.5, and 60.45 [60]. Also, within the prominent crystal features present for the sample MOHs in Figure 9 are the anatase and rutile that occur at 25.4° and 27.5°, respectively [61,62]. To quantify the percentages of these minerals obtained by the XRD analysis, EDS (see Section 3.7.3) was used in this study.

3.7.2. FT-IR Analysis

The FT-IR spectra obtained for the MOHs samples, before and after the adsorption of the lead and arsenic, are shown in Figure 11. The intensity of the band indicates the quantity of the functional groups and the physico-chemical composition of the tested mineral materials responsible for IR absorption.

The structural hydroxyl groups and the water molecules in the interlayer occur at 3750–3400 cm⁻¹ (Figure 11) [63]. The peak of very low intensity at 1634 cm⁻¹ was the evidence of the water molecules’ bending vibrations, or the O-H groups [46]. The intense band located at 1054 cm⁻¹ is assignable to the Si–O–Si symmetric stretching vibrations [57,64,65]. The Si–O–Ti vibrations are observed only at 950 cm cm⁻¹, which confirms the semi-quantitative analysis performed where the MOHs contained Ti components [66]. The bands near 730 cm⁻¹ correspond to the vibrations of the Ti-O bonds in the TiO₆ polyhedrons [67]. It can be seen (Figure 11) that the band at 440–400 cm⁻¹ shows a complicated vibration mode, so in this absorption band, it is difficult to determine the various vibrations of the Si–O–Si, Al–O–Al, and Si–O–Al linkages [68]. In the same range, the functional groups associated with diopside can be found; the band at 415 cm⁻¹ represents the non-bridging bending vibrational modes of O–Ca–O, and the peak at 465 cm⁻¹ shows the non-bridging bending modes of the O–Mg–O bonds [69,70].

3.7.3. SEM and EDS Analysis

Figure 12 displays the scanning electronic microscopy (SEM) and energy dispersive spectroscopy (EDS) images of the sample MOHs, which reveal their spherical and uniform spherical-shaped structures.

The SEM image of the surface of the diopside pellets (Figure 12a) shows a porous surface with agglomerated spherical particles of about 5–10 µm in diameter. The particles appear as aggregates, with the micro and macro structures scattered irregularly over the surface and randomly organized with a rough morphology. The particles were irregular in shape, wherein some particles had a sharp shape, while some had a plate morphology [71]. On the surface of the diopside pellets, the pores that occurred might be due to the release of volatile materials through the calcination and sintering procedures [69].

Also, the EDS analysis is shown in Figure 12b. The results reveal the presence of Fe, O, C, Si, and Ti, which confirms the existence of iron oxide, silica, and titania within our MOH material. The presence of the main adsorbent components (%) is given in

Table 10. The presence of the main adsorbent components after the simultaneous removal of lead and arsenic ions.

Spectrum Label	O	Mg	Al	Si	Ca	Ti	Fe	As	Pb	Total
Spectrum 11	55.38	-	-	16.64	0.47	24.53	1.96	0.47	0.56	100.00

. The presence of Ca and Mg determined by the EDS analysis originates from the diopside.

The textural properties of the analyzed MOHs material result in a specific surface area of 271.7 m² g⁻¹. The analysis revealed that the total surface area of the mineral materials is high for the MOHs material. The MOHs material can be classified as microporous and used for industrial applications (range of 100–1500 m² g⁻¹) [72].

3.7.4. Mossbauer Spectroscopy

The method was used to examination of local, structural, microdynamic characteristics, magnetic properties, and oxidation states of iron of the observed adsorbent. The ⁵⁷Fe-Mössbauer spectra of MOHs are presented in Figure 13.

The nine Mössbauer subspectra that could be distinguished in the MOHs spectrum correspond to the different Fe sites in the MOHs sample; these Fe sites correspond to the different local atomic structures and/or valence states of Fe in the sample. In Figure 13, the solid circles represent the experimental data, while the corresponding fits are represented in red solid lines. The fitted lines of the Mössbauer subspectra are seven magnetic sextets (blue, black, dark cyan, orange, red, magenta, and black color lines) and three paramagnetic doublets (dark cyan, pink, and blue lines). The vertical arrow (relative transmission = 1.9%) on the right side of the spectrum shows the relative position of the lowest peak in relation to the baseline. The Mossbauer subspectra’s fitted lines are plotted above the main spectrum fit as follows: diopside (3 doublet), hematite/titanohematite phase (1 sextet), and titanomagnetite phase (5 sextets). The red solid line in the lower part represents the error calculated as the difference (Th-Ekp), where the highest value of the absolute difference is less than 0.2%.

4. Conclusions

Globally, the contamination of water with various pollutants, especially multiple heavy metals, has caused increasing concerns due to its harm to the environment. It is a huge challenge to find a suitable method and material for simultaneously efficient and economical removal of heavy metals. In this study, the successfully prepared synthesized Si/Ti/Fe oxide heterostructures were shown in terms of their removal properties of arsenic and lead ions. Metal oxides are an integral part of soil, which have been extensively exploited and modified as highly effective adsorbents for the removal of inorganic pollutants from water systems. The silicate structure of the material was upgraded with Ti and Fe particles to increase the efficiency of cation and oxyanion removal. The results of the kinetic parameters demonstrated that the adsorption speed is very fast, with the maximum adsorbed percentage up to 30 min after the process starts. The maximum removal efficiency of Pb²⁺ and As(V) ions in the multi-component solution was 87.7 and 46.1%, while in the single-component solution it was 64.3 and 50.7%, respectively. The experimental results in this paper pointed out that alkalis are efficient desorbing agents for the desorption of oxyanions (As(V)), while acids are efficient for desorbing metal cations (Pb²⁺). The low values of adsorbent capacity and fast kinetics of the process during simultaneous adsorption demonstrated that the synthesized MOHs material is suitable for accidental and emergency situations, where it is necessary to react quickly and efficiently. For the further steps, it would be useful to investigate the cost benefit analysis of this material and the possibility of its commercial use for environmentally friendly pollution removal.