Montmorillonite and amino acid composites for heavy metal ion adsorption

Summary

This study explores the interactions between montmorillonite (MMT) and selected amino acids (lysine, arginine, serine, and aspartate), as well as their complexes with heavy metal ions. Multi-technique characterization (e.g., XRD, FT-IR, XPS, SEM, TEM, and ICP-MS) revealed that MMT exhibits high affinity for basic amino acids, with the adsorption capacity following the order: amino > carboxyl > hydroxyl. The adsorption kinetics of lysine were well described by pseudo-second-order, Elovich, and Weber-Morris models, indicating a chemically controlled, two-stage process involving initial surface diffusion and subsequent internal diffusion. In contrast to the adsorption of amino acids, that of Cu²⁺ exhibited a higher initial rate and greater adsorption energy. Furthermore, amino acid-MMT composites effectively immobilized metal ions via coordination and electrostatic interactions. These findings enhance the understanding of mineral-organic interactions and suggest potential applications in biocatalysis, biopharmaceuticals, and environmental remediation.

Graphical abstract

Highlights

• •Montmorillonite exhibits a high adsorption capacity for basic amino acids
• •The adsorption follows a chemically controlled, two-stage diffusion process
• •The initial adsorption rate of Cu²⁺ or Pb²⁺ is faster than that of lysine
• •Synergistic coordination-electrostatic interplay immobilizes metal ions

Chemistry; Analytical chemistry; Chemical reaction kinetics; Thermal property

Introduction

Heavy metals are increasingly used in modern industries, and their effluents are frequently released into the environment. These metals can permeate soil and groundwater, leading to widespread environmental contamination. They accumulate in living organisms, posing serious risks to ecosystems. By binding to proteins and enzymes, heavy metal ions impair their function and disrupt normal physiological metabolic processes.¹ They also exhibit significant toxicity to microorganisms, inhibit plant growth, and indirectly affect animal development through food chains.² In humans and animals, exposure to heavy metals can cause acute, subacute, or chronic poisoning, as well as various functional impairments. These effects may contribute to the development of disorders such as cancer.³ Thus, heavy metal pollution has become a major global environmental issue. Ion exchange is a fundamental mechanism in many water treatment technologies. Natural montmorillonite (MMT) is considered an ideal material for this purpose due to its ion exchange properties. However, its cation exchange capacity (CEC) is relatively low. To enhance the ion exchange capacity and adsorption performance of MMT in heavy metal removal, quaternary ammonium salts are often incorporated.⁴ Nevertheless, long-chain quaternary ammonium salts are biologically toxic and may pose safety risks, particularly in environment-friendly or food-contact applications. Amino acids, as naturally occurring biological molecules, can serve as effective modifiers for MMT.⁵ For instance, proline can reduce the swelling of MMT by 22.5%.⁶ Amino acid-MMT composites have been extensively studied for the removal of heavy metal ions. Specific amino acids, such as lysine,⁷ histidine,⁸ cysteine,^9,10 and arginine monohydrochloride¹¹ are widely used in research targeting heavy metal ions including Cd²⁺, Pb²⁺, Cu²⁺, Hg²⁺, Co²⁺, and Zn²⁺. These composites can also effectively remove non-metallic elements such as As(V).¹² The binding mechanisms between amino acids and heavy metal ions include the formation of protonated amino groups (e.g., Lys⁺),⁷ which enhance adsorption through electrostatic interactions, as well as coordination between metal ions and oxygen atoms.¹⁰ The adsorption behaviors typically conform to the Langmuir isotherm and pseudo-second-order kinetic model,^7,8 and hybrid materials often exhibit high retention rates for heavy metal ions.⁹ Although numerous studies have explored the adsorption of metal ions by amino acid-modified MMT composites, research on the adsorption behavior, performance, and mechanisms remains insufficiently systematic and in-depth. For example, the influence of different types of amino acids—such as basic, acidic, and neutral amino acids—on adsorption performance has not been thoroughly investigated. Therefore, we conducted an in-depth study on the adsorption performance, behavior, and mechanism of MMT for various amino acids and different types of heavy metal ions. Our findings indicate that MMT exhibits high adsorption capacity for basic amino acids. The adsorption process follows a chemically controlled, two-stage diffusion mechanism, with metal ions immobilized through synergistic coordination and electrostatic interactions.

Results

Examining the structural properties of composite materials

The X-ray powder diffractometer (XRD) patterns of MMT and amino acid-modified MMT composites are depicted in Figure 1A. The graphic shows that 2θ = 7.0° corresponds to the d₀₀₁-value of MMT. After computation, the layer space of the original MMT is 1.26 nm, which increases to 1.35 nm after amino acid modification. This indicates that the amino acid could intercalate into the MMT layers. Adding amino acids to the composites reduced the reflection at 2θ = 6.5°, showing uneven layer space and decreased crystallinity. Reflections at 2θ = 6.5°, 26.6°, and 35° become smaller, indicating decreased crystallinity and smaller crystal particles. The peak at 14.2° is typically linked with the (002) crystal plane of MMT and is a secondary peak diffracting from the (001) crystal plane. As a result, the peak near 14.2° could imply that the sample is unaltered or slightly changed MMT. The peak near 28.4°, which could be a higher-order diffraction peak from the (004) crystal plane, may be significantly reduced or lost in intensity if the sample has been organically modified. When the (001) peak is dramatically shifted, the 14.2° and 28.4° peaks may disappear or weaken, indicating the presence of intercalation or exfoliation structures. The other reflections of MMT (2θ = 19.8°, 26.6°, 34.6°, and 36.0°) did not appreciably change. In addition, the composites reflected impurities, including cristobalite (2θ = 21.8°).

Figure 1.

Characterizing the structure and properties of composite materials

(A) XRD spectra for montmorillonite and its composites.

(B) FT-IR spectra for montmorillonite and its composites.

Figure 1B depicts the Fourier infrared spectrometer (FTIR) patterns of Lys/MMT composites with varying amino acid contents (0 ∼ 4CEC). Peaks at 3,200∼3,500 cm⁻¹ indicate the stretching vibration of intermolecular hydrogen bonds (OH), while peaks at 3,600–3,650 cm⁻¹ represent the stretching vibration of free hydroxyls (OH). After adding lysine, the distinctive peak of MMT at 3,430 cm⁻¹ gradually fades, showing that lysine has replaced the bound water in the MMT layers. The characteristic peak at 1,650 cm⁻¹ was similarly related to the presence of water, and the broadening of this peak indicated lysine adsorption. In addition, three additional characteristic peaks formed in MMT following the addition of lysine. Among them, the N–H stretching vibration peak of the amino group (–NH₂) in lysine at 3,300 cm⁻¹, which reflects the increase in lysine content in MMT; the asymmetric and symmetric stretching vibration peaks of the alkyl C–H bond were observed at 2,950 cm⁻¹, which is derived from the methyl (–CH₃) and methylene (–CH₂–) groups in the lysine molecule. The putative methyl (–CH₃) peak at 1,450 cm⁻¹ broadens as amino acid concentration increases in the sample. The peak at 1,350 cm⁻¹ could be caused by the reaction of the metal ions in MMT with the carboxyl group (–COOH) in lysine to generate carboxylates, or it could be the carbon-nitrogen single bond (C–N) stretching vibration peak in the lysine molecule. The zeta potential of MMT is −26.16 ± 0.33 mV, whereas Lys/MMT is −24.44 ± 1.92 mV. This is due to Lysine’s overall positive charge. The absolute value of the zeta potential is lower following lysine’s modification. This suggests that Lys/MMT has improved aggregation properties a little.

Figures 2A–2F depicts the X-ray photoelectron spectroscopy (XPS) spectra of the two composites following adsorption of the two metal ions. The C element exists mostly in the chemical forms of C–C, C–O, and C=O with binding energies of 284.8, 286.5, and 288.5 eV,¹³ respectively. O1s can be divided into two peaks at 531.5 and 532.5 eV, representing two oxygen forms in C–O and C=O, respectively. The peaks at 402.5 and 400.3 eV in the N1s XPS spectra correspond to the peptide bond (-NH-CO-) and amino group (NH₂). After adsorbing Pb²⁺, spectral peaks occurred at 139.3 and 144.2 eV, which correspond to Pb4f_7/2 and Pb4f_5/2,¹⁴ respectively. The binding energy spacing between Pb4f_7/2 and Pb4f_5/2 is 4.9 eV, indicating that the Pb–O bond exists on the surface. The distinctive peaks of Cu 2p_3/2 and Cu2p_1/2 occur at the binding energy of 933.5 and 953.5 eV, respectively, while the Auger peak appears at 943.3 eV, showing that Cu²⁺ is adsorbed on MMT in a positive bivalent state. Cu2p_1/2 can be regarded as free Cu²⁺, but Cu2p_3/2 belongs to the coordinated copper ion, implying that Cu²⁺ is coordinated with inorganic and organic ligands. This result supports the development of Cu-lysine complexes. Furthermore, the intensity and peak area of the second peak are greater than the first, indicating that Cu²⁺-ligands are the primary forms of the compounds.

Figure 2.

XPS examination of Lys/MMT composite

(A) XPS survey.

(B) C1s.

(C) O1s.

(D) N1s.

(E) Pb4f.

(F) Cu2p.

Figure 3 shows scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the MMT and Lys/MMT composites. SEM data demonstrate that the MMT surface is largely spherical and smooth, with an increase in microscopic burrs on the surface following amino acid adsorption by MMT (Figures 3A and 3B). This adds to an increase in Brunauer-Emmett-Teller (BET) surface area. The TEM data reveal that the MMT crystal particles are relatively complete, with broad and thin lamellae. After lysine attaches to MMT, the MMT lamellae tend to cluster, and the form of MMT crystal particles becomes more irregular (Figures 3C and 3D).

Figure 3.

SEM and TEM images of composites

(A and B) SEM images.

(C and D) TEM images.

(A and C) MMT.

(B and D) Lys/MMT.

The S_BET specific surface area, pore volume, and pore diameter of MMT and composite materials are depicted in Figures 4A and 4B. According to the findings, the adsorption-desorption curves correspond to the IV adsorption isotherm, indicating that the material is mesoporous. Moreover, the form of the hysteresis loop suggests that the pore structures of these two materials are predominantly lamellar. Table 1 summarizes the specific surface area, pore volume, and pore diameter of the composite materials. As can be observed, Lys/MMT has a bigger S_BET than MMT. However, amino acid modification greatly reduced pore volume and specific surface area.

Figure 4.

Composite isotherm curves for nitrogen adsorption and desorption

(A) MMT.

(B) Lys/MMT.

Table 1.

Micropore data for composite materials

Sample	BET-specific surface area (m2/g)	t-plot micropore area m2/g	t-plot micropore volume cm3/g	Adsorption average pore diameter nm (4V/A by BET)
MMT	14.1743	9.8768	5.289 × 10−3	8.7592
Lys/MMT	20.6275	5.0596	2.665 × 10−3	9.7218

The thermal analysis of these composites is depicted in Figure 5. At 135°C, mass loss rates reflect the presence of free water. The dehydration rates of lysine, arginine, and serine modified MMTs are lower than those of pure MMT. This means that the interlayer water molecules have been replaced by amino acid molecules. The mass loss peaks at 470°C indicate the dehydration of interlayer-bound and polycondensation water molecules (Figure 5A). The addition of lysine or arginine greatly improved mass loss at this temperature. However, serine and aspartate composites created fewer water molecules for lower amino acid content. We may conclude that the type of amino acid significantly influences the release of water molecules in composites. Derivative thermogravimetric analysis (DTG) data analysis shows that the elimination of free water peak for Arg/MMT, and Lys/MMT composites is at 94°C, aspartate is at 121°C, and serine is at 125°C (Figure 5B). The findings revealed that arginine and lysine molecules competed strongly with interlayer water molecules. However, the Ser/MMT’s interlayer water molecules can form more hydrogen bonds with the serine’s hydroxyl group, increasing the difficulty of removal. The 264°C peak implies amino acid polycondensation dehydration, which is obviously seen in arginine and lysine. The differential scanning calorimetry (DSC) figure shows that the heat absorption peak of evaporative random water molecules in Lys/MMT and Arg/MMT shifts to the lower temperature zone, whereas that of aspartate and serine composites shifts somewhat higher. Lys/MMT composites exhibit heat absorption peaks at 250°C for bound water elimination and polycondensation dehydration (Figure 5C).

Figure 5.

Heat treatment data analysis for MMT and amino acid montmorillonite composites

(A) TG curves.

(B) DTG curves.

(C) DSC curves.

The influence of amino acid types on adsorption performance

The adsorption capacity of MMT was dramatically influenced by amino acid species. Figure 6A shows that whereas MMT has a similar adsorption capacity for basic amino acids (lysine and arginine), it has a much lower adsorption capacity for aspartic acid and an even lower adsorption capacity for serine (Table S1). Principles governing the influence of amino acid concentration on MMT adsorption are identical. When the amino acid concentration is low, the adsorption capacity increases quickly. However, as the amino acid concentration increases, the rate of amino acid adsorption on MMT decreases.

Figure 6.

Adsorption statistics of amino acids on montmorillonite and heavy metal ions on composite materials

(A) Adsorption isotherms of four amino acids.

(B) Adsorption and slow-release rate of heavy metal ions on various composites.

Each experiment in Figure 6A was performed three times (n = 3), and the data are expressed as the mean ± standard error (see also Figure S1 and Table S1).

The effect of amino acid type, metal ion species, and ion concentrations

The adsorption characteristics of MMT and other amino acid MMT composites on various metal ions were investigated. The experimental results (Figure 6B) show that Lys/MMT and Arg/MMT can considerably increase metal ion adsorption at the doses of 200 and 400 mg/L. At 200 mg/L, Lys/MMT increased the adsorption rates of five metal cations by over 5%, reaching 9.4% to pure MMT. When the concentration was increased to 400 mg/L, the Pb²⁺ adsorption rate by Lys/MMT is 11.6% more than that of pure MMT. The adsorption effects of different metal ions vary remarkably as the concentration changes. However, in most cases, Ser/MMT lowers the adsorption effect of Zn²⁺ and Pb²⁺, with a maximum reduction of more than 40%. Asp/MMT also reduced the adsorption effect of Zn²⁺, Pb²⁺, and Cu²⁺, while dramatically increasing the adsorption rate of Cr³⁺.

The loading capacity of MMT and composites for various metal ions was noticeably different. When the ion concentration is 400 mg/L, the loading capacity of pure MMT on Cr³⁺, Ni²⁺, Cu²⁺, Zn²⁺, and Pb²⁺ is 46.28, 49.75, 56.02, 59.29, and 141.05 mg/g, respectively, while Lys/MMT can improve its adsorption capacity for the five ions listed above to 54.68, 65.45, 63.36, 75.16, and 159.65 mg/g, respectively. The adsorption rates of Cr³⁺ and Pb²⁺ are noticeably increased, while the adsorption rates of Zn²⁺ and Ni²⁺ fall significantly. This is due to the characteristics and binding energies of particular ions.

It differs significantly from the sustained release ratios of Cr³⁺, Ni²⁺, Zn²⁺, Cu²⁺, and Pb²⁺ ions absorbed by these composites. In Arg/MMT and Lys/MMT composites, Pb²⁺, Cu²⁺, and Ni²⁺ are released at a slower rate over time. The substance and metal ions form a more stable combination as the slow-release rate decreases. However, the slow-release rates of Ser/MMT for Ni²⁺, Cu²⁺, and Pb²⁺ rise to approximately 8%, 16% and 13%, respectively. Neutral amino acids (serine) with hydroxyl groups can form hydrogen bonds with water molecules existing in the MMT layer. Nevertheless, the hydroxyl group generates fewer coordination bonds with metal ions, resulting in a higher slow-release rate.

Analysis of the thermodynamic and kinetic mechanisms

Analysis of the adsorption isotherm model

MMT’s adsorption isotherms for lysine, arginine, serine, and aspartate fit the Langmuir and Freundlich models, as illustrated in Table 2. However, the Langmuir model fits better than the Freundlich model, indicating that MMT adsorption sites have homogeneous surfaces. This agrees with the single-layer saturated adsorption concept. Simultaneously, the decrease in adsorption rate as concentration increased highlighted the hierarchy of the adsorption process, which was somewhat congruent with the Freundlich model’s description of heterogeneous surfaces. As a result, it is possible to conclude that the limited effective adsorption site of MMT may undergo a transition from single to multi-layer. Therefore, the energy distribution of the adsorption site as well as the concentration of the solution, influences amino acid adsorption.

Table 2.

Fitting parameters to amino acid adsorption isotherms

Adsorbate	Langmuir isotherm			Freundlich isotherm
	Qmax (mg/g)	KL (L/mg)	R2	n	KF (mg/g)	R2
Arg	4,482.59	4.41 × 10−4	0.9955	0.5972	20.33	0.9682
Lys	2,829.39	3.29 × 10−4	0.9992	0.5668	27.55	0.9672
Ser	1,264.12	1.67 × 10−3	0.9968	0.4042	49.52	0.9335
Asp	1,625.61	9.89 × 10−4	0.9985	0.4157	52.81	0.9333

Kinetic analysis of lysine adsorption on MMT

Figure 7 depicts an adsorption kinetics curve fitting. The experimental findings show that the adsorption capacity rose drastically from 0 to 153.41 mg/g within the first 10 min of the adsorption procedure, after which the adsorption rate steadily dropped until equilibrium. This is due to the large specific surface area with an abundance of active sites on the surface of MMT lamella, which allows for fast interaction with lysine molecules. As adsorption progresses, the number of accessible sites diminishes resulting in a lower adsorption rate. The adsorption rate was also impacted by lysine concentration. The pseudo-first-order and second-order kinetic fitting curves are presented in Figures 7A and 7B, and the kinetic parameters may be determined using Equations 1 and 2.

log(q_e-q_t) = log(q_e) - k₁·t/2.303. (Equation 1)

t/q_t= 1/ (k₂q_e²) + t/q_e. (Equation 2)

Figure 7.

Kinetic curves of amino acid adsorption on montmorillonite

(A) Pseudo first-order fitting curve.

(B) Pseudo second-order fitting curve.

(C) Weber-Morris equation fitting curve.

(D) Elovich equation fitting curve.

q_e represents the equilibrium adsorption capacity (mg/g), q_t represents the adsorption capacity at t time (mg/g), k₁ is the pseudo-first-order rate constant (min⁻¹), and k₂ is the pseudo-second-order rate constant (g·mg⁻¹·min⁻¹).

The data in Table 3 show that the equilibrium adsorption quantity q_e calculated by the pseudo-first-order and pseudo-second-order kinetic equations are 348.342 and 375.94 mg/g, respectively. The former is slightly lower than the experimental value, while the latter is close to the experimental value of 367.26 mg/g. As a result, the adsorption process is more consistent with the pseudo-second-order kinetic model (Table 3), indicating that it is most likely regulated by chemical adsorption. This causes the creation of hydrogen or ionic bonds between lysine molecules and MMT lamellae.

Table 3.

Kinetic parameters for the adsorption of Lys, Cu²⁺, and Pb²⁺ on MMT or Lys/MMT

Adsorbent	Ions' species	Model	K/min−1	qe /mg·g−1	T/Qt	R2
MMT	Lys	pseudo first-order	4.13 × 10−2	348.342	–	0.9457
MMT	Lys	pseudo second-order	2.66 × 10−3	–	3.56 × 10−2	0.9982
MMT	Cu2+	pseudo first-order	1.29 × 10−1	49.007	–	0.9792
MMT	Cu2+	pseudo second-order	1.96 × 10−2	–	5.62 × 10−2	0.9998
Lys/MMT	Cu2+	pseudo first-order	3.42 × 10−1	52.181	–	0.9202
Lys/MMT	Cu2+	pseudo second-order	1.69 × 10−2	–	1.06 × 10−1	0.9957
MMT	Pb2+	pseudo first-order	1.96 × 10−1	124.003	–	0.8033
MMT	Pb2+	pseudo second-order	7.63 × 10−3	–	1.36 × 10−2	0.9994
Lys/MMT	Pb2+	pseudo first-order	3.35 × 10−1	144.377	–	0.9636
Lys/MMT	Pb2+	pseudo second-order	6.64 × 10−3	–	1.31 × 10−2	0.9996

The Weber-Morris equation, an internal diffusion model, is frequently used to describe the adsorption kinetics of solid/liquid systems. The model expression is provided in Equation 3. Figure 7C exhibits the fitting curve of the Weber-Morris equation for MMT adsorption of lysine. The fitting parameters reveal that the adsorption process may be separated into two stages: surface diffusion (k₁) and interior diffusion (k₂). The rate constants for the two stages are 27.196 and 1.803, respectively. The two stages have a good linear fit, demonstrating that the Weber-Morris equation accurately describes lysine adsorption by MMT.

Q_t = K_s·t^0.5 + C. (Equation 3)

Q_t (mg/g) indicates the adsorption capacity at time t, while K_s represents the adsorption rate coefficient (mg·g⁻¹·min^−0.5). C represents the boundary layer effect constant (mg/g), and a value of zero means that the adsorption process is solely controlled by the internal diffusion process.

The first stage of adsorption has a quicker adsorption rate, which could be owing to faster surface adsorption or an increase in external diffusion rate (lysine from solution to MMT’s surface). As the adsorption process advances, the number of accessible sites diminishes, resulting in a lower adsorption rate. The second stage is primarily internal diffusion or pore diffusion (lysine molecules enter the MMT lamellar structure), the adsorption rate slows, and the adsorption curve is nearly parallel to the x axis, indicating that the diffusion rate in the particles decreases significantly, and the adsorption process tends to equilibrium. The turning point between the two stages is around t^0.5 = 10, and the equivalent adsorption period is about 100 min. This suggests that in the first 100 min, lysine adsorption by MMT is mostly regulated by diffusion on the particle’s exterior surface. Whereas, in the later stages, it is primarily controlled by diffusion within the composite. Furthermore, the curve’s division into two portions suggests that, in addition to surface and internal diffusion, chemical reactions may occur on the adsorbent’s surface.

The Elovich equation (Equation 4) is commonly used to describe processes with a variety of reaction mechanisms, including solute diffusion in the bulk phase of a solution or at an interface, as well as activation and deactivation actions at surfaces. It is particularly well suited to processes with large changes in activation energies during reactions, such as those at the soil-sediment interface.¹⁵

Q_t = 1/B·(ln(A·B·t+1)). (Equation 4)

Q_t (mg/g) represents the adsorption capacity at time t (min). The initial adsorption rate is denoted by parameter A, and the constant associated with surface activation energy during the adsorption process is represented by parameter B.¹⁶

Figure 7D depicts the fitting curve and fitting parameters of Elovich’s equation. The good fitting degree implies that this model well describes the kinetic mechanism of lysine adsorption on MMT (Table 4). It can be shown that there were highly significant variances in the initial rates, coefficient A, which were in descending order of Pb²⁺, Cu²⁺, and Lys (Table 4). If the B value is low, it implies that the adsorbent’s surface requires little activation energy and that the adsorption process occurs easily. The fitting finding also suggests that the adsorption of lysine by MMT is mostly carried out by chemical bonding rather than physical adsorption.

Table 4.

Kinetic parameters for the Elovich equation fitting curves

Type	A	B	R2
MMT_Lys	8.77 × 101	0.0164	0.9638
MMT_Cu	3.26 × 103	0.235	0.9522
Lys/MMT_Cu	1.45 × 104	0.233	0.9627
MMT_Pb	1.75 × 1027	0.524	0.9980
Lys/MMT_Pb	5.32 × 1022	0.384	0.9986

Adsorption kinetics of Cu²⁺ and Pb²⁺ by Lys/MMT

Figure 8A shows the adsorption curve fitting results of MMT and Lys/MMT for Cu²⁺. This figure shows that their initial adsorption rates were very fast, with the adsorption amount rapidly increasing to 47.44 mg/g (Lys/MMT) during the first 30 min. At 300 min, the adsorption amount reached 59.76 mg/g (Lys/MMT), approaching the equilibrium adsorption condition. Figure 8B shows the results of the adsorption curve fitting of MMT and Lys/MMT on Pb²⁺. The adsorption capacity increased rapidly to 141.784 mg/g (Lys/MMT) in the first 30 min and reached 149.272 mg/g (Lys/MMT) at 300 min, approaching the equilibrium adsorption condition. Because the adsorbent surface has a large number of active adsorption sites for metal ions to occupy, the mechanism of rapid adsorption in the initial stage is primarily surface adsorption and/or external diffusion. As the adsorption period increases, internal diffusion (the process of transferring ions in the pores of the composite material) may limit the adsorption rate until it reaches the adsorption equilibrium point.

Figure 8.

Kinetic curves of heavy metal adsorption by MMT and Lys/MMT composites

(A) Cu²⁺.

(B) Pb²⁺.

The kinetic model accurately describes the adsorption behavior of Cu²⁺ and Pb²⁺ on MMT and Lys/MMT. It is clear that the pseudo-secondary reaction kinetic equation applies (Table 3).¹¹ Because the pseudo-secondary model is typically linked with chemisorption processes, the adsorption process could include valence bond formation, electron sharing, or transfer, and other interactions between adsorbed molecules and the adsorbent surface.

The Weber-Morris adsorption model for Cu²⁺ and Pb²⁺ adsorption by MMT follows a similar pattern to that for lysine adsorption. They can be separated into two phases: the surface diffusion phase (k₁) and the internal diffusion phase (k₂), with the major characteristics indicated in Table 5. The fitted straight line for the first phase of Pb²⁺ goes through the coordinate origin, showing that this system’s internal diffusion is governed by a single rate. The other adsorption processes do not reach the origin, indicating that internal diffusion is not a unique factor influencing the adsorption process. The t^0.5 values of the turning points for the two adsorption phases are around 10 min for lysine adsorption on MMT, 4.4 min for Cu²⁺ adsorption on MMT, and 2.2 min for others (Figures 9A and 9B). When converted to a precise time, lysine takes 100 min, Cu²⁺ takes roughly 20 min, and the rest take about 5 min to reach stage 2. This suggests that Cu²⁺ and Pb²⁺ adsorb faster than lysine. But the lysine modification increased the rate of Cu²⁺ adsorption. Table 4 shows the primary parameters of the Elovich equation’s fitting curve (Figures 9C and 9D). The curve’s parameter A reflects the initial adsorption rate, and the higher the value, the faster the starting rate in the early stage. The parameter A of the adsorption rate of Cu²⁺ and Pb²⁺ increased dramatically, showing that the initial adsorption rate is quite large, and the lysine modification also did. This is mostly due to the creation of coordination interactions between Cu²⁺ and amino acid molecules, which accelerates the Cu²⁺ adsorption rate. This is also proved by the result of XPS research.

Table 5.

Kinetic parameters for the Weber-Morris equation fitting curves

Type	K1	C	R2	K2	C	R2
MMT_Lys	27.1961	72.46	0.952	1.8032	326.717	0.9080
MMT_Cu	10.5037	2.22	0.958	0.3485	44.996	0.8906
Lys/MMT_Cu	20.0888	3.55 × 10−15	1	1.1465	41.285	0.8990
MMT_Pb	55.2076	0	1	0.5112	121.383	0.8577
Lys/MMT_Pb	62.5491	0	1	0.6897	137.739	0.9586

Figure 9.

Kinetic curves of the adsorption of metal ions on composites

(A) Weber-Morris equation fitting curve of Cu (II).

(B) Weber-Morris equation fitting curve of Pb (II).

(C) Elovich equation fitting curve of Cu (II).

(D) Elovich equation fitting curve of Pb (II).

Thermodynamic study of lysine, Cu²⁺, and Pb²⁺ adsorption

To further understand the mechanism of amino acid adsorption by MMT, thermodynamic investigations were conducted. Equation 5 determines the standard enthalpy change (ΔH^θ) and standard entropy change (ΔS^θ) for the Van’t Hoff equation. The change in Gibbs free energy (ΔG^θ) can be estimated by Equation 6.

lnK_d= -ΔH^θ/R·1/T + ΔS^θ/R. (Equation 5)

K_d is the partition coefficient, R is the ideal gas constant (8.314 J/mol·K), T is the absolute temperature, and its unit is K.

ΔG^θ= ΔH^θ− TΔS^θ. (Equation 6)

Figures 10A–10C and Table 6 depict the thermodynamic fitting curve and parameters for lysine, Cu²⁺, and Pb²⁺. ΔH^θ > 0 shows that the adsorption process is endothermic. Therefore, increasing the ambient temperature may improve amino acid adsorption capabilities by providing more energy to overcome the barrier between the adsorbent and the adsorbate. The presence of guest molecules increases system chaos, as indicated by ΔS^θ > 0. At 273.15–363.15 K, these adsorptions are spontaneous, with ΔG^θ < 0. The free energies of lysine adsorption by MMT and metal ion adsorption by composites were negative, and the absolute values rose with increasing temperature, showing that the adsorption process was spontaneous and that warming facilitated the adsorption reaction. The adsorptions of amino acids by MMT and Pb²⁺ by composites have negative slopes with substantial absolute value (ΔH^θ > 0). This implies that the reactions are highly adsorptive, and increasing the temperature greatly raises the equilibrium constants. MMTs adsorption of Pb²⁺ shows a positive and substantial intercept (ΔS^θ > 0), indicating an increase in product disorder. Changes in entropy contribute more to free energy. The absolute value of Gibbs free energy for Pb²⁺ adsorption exceeds that of Cu²⁺ adsorption, showing that Pb²⁺ has a superior spontaneous property than Cu²⁺.

Figure 10.

The Vant’ Hoof equations for determining the thermodynamic parameters of adsorption

(A) Lysine on MMT.

(B) Cu²⁺, Pb²⁺ on MMT.

(C) Cu²⁺, Pb²⁺ on Lys/MMT.

Table 6.

Thermodynamic characteristics for Cu²⁺ and Pb²⁺ adsorption by various materials

Ions' type	Materials' type	ΔHθ (J·mol−1)	ΔSθ (J−1·mol−1)	ΔGθ (J·mol−1)
				273.15K	303.15K	333.15K	363.15K
Lys	MMT	118.20	0.48	−13.82	−28.32	−42.81	−57.31
Cu2+	MMT	20.28	0.31	−64.40	−73.70	−83.00	−92.30
	Lys/MMT	2.12	0.23	−60.70	−67.60	−74.50	−81.40
Pb2+	MMT	323.94	1.52	−91.52	−136.85	−182.45	−228.05
	Lys/MMT	275.11	1.04	−8.97	−40.17	−71.37	−102.57

Calculate and analyze the adsorption energy

The calculated adsorption energy (see Scheme 1) is shown in Table 7. When the adsorption energy is negative, it means that the adsorption process is exothermic, making the adsorbates more likely to be adsorbed on adsorbents’ surface. Furthermore, a higher negative number for adsorption energy suggests that more energy is liberated, making the adsorption process easier to carry out. The adsorption energy of Cu²⁺ is greater than that of the amino acids. The absolute value of the adsorption energy of MMT with serine, aspartate, and lysine is Lys›Ser›Asp. This partly suggests that the adsorption energy decreases in the sequence: NH₂, OH, and COOH groups. However, the adsorption energy of amino acids with MMT has no positive association with the adsorbed amount of Ser. This means that, when compared to charged amino and carboxyl groups, hydrogen bonding cannot significantly boost amino acid adsorption capacity. MMT adsorption is more dependent on ionic bonding.

Scheme 1.

A schematic diagram of the adsorption energy calculation procedure

(A) MMT and lysine aqueous solution.

(B) MMT and aspartate aqueous solution.

(C) MMT and serine aqueous solution.

(D) MMT and Cu²⁺ aqueous solution.

Table 7.

Adsorption energy of montmorillonite with lysine, aspartate, serine, and CuCl₂ solution

Adsorbate	Lys	CuCl2	Ser	Asp
Adsorption energy (kcal/mol)	−2862.61	−5560.3	−2798.34	−2678.61
Standard error	149.9688	220.8005	123.794	132.4427

Discussion

MMT is more capable of adsorbing arginine and lysine, mainly because these amino acids carry more positive charges amino group than the other two amino acids, and can undergo strong electrostatic attraction and cation exchange with the negatively charged MMT.¹⁷ As aspartic acid carries a charged R group (containing a carboxyl group), this has a certain effect on increasing the adsorption capacity.¹⁸ Serine is electrically neutral and is only held together by weak hydrogen bonds, so the amount of these amino acids adsorbed by MMT is much less. The order of groups that are beneficial for enhancing the adsorption capacity of amino acids is as follows: amino group, carboxyl group, and hydroxyl group.

Amino and carboxyl groups of amino acids can be protonated or deprotonated in solution, and their groups can coordinate or interact electrostatically with metal ions to increase adsorption capacity.¹⁹ Positively charged guanidine groups (such as arginine) and amino groups (such as lysine) can generate more stable electrostatic attraction and coordination interactions with metal cations,^20,21 resulting in greatly improved composite adsorption capability. Negatively charged carboxyl groups are less likely to coordinate with metal ions.²² It can be said that the ionic and coordination bonds of amino acid side chains are two major determining variables.

The adsorption and slow-release rates of heavy metal ions on different composites varied significantly due to noticeable variances in their characteristics. Ions’ significant properties are indicated as follows: hydration heat, valence, ionic radius, and effective ionic radius (Table 8). Metal ions’ hydration energy greatly influences their affinity. Ions with low hydration energy can easily remove water molecules from the interlayer and become stuck in the interlayer. The Pb²⁺ with the lowest hydration energy is most likely to shed the coordinated water molecules and become a bare ion in the MMT interlayer.²³ It also has a large ionic radius and excellent polarization ability, making it easier to establish stable coordination bonds with a wide range of chemical groups. These variables make the compound more stable, and the adsorption amount is the highest. According to the results of the above analysis, MMT’s adsorption ability on these five heavy metal ions is in the following order: Pb²⁺ > Zn²⁺ > Ni²⁺ > Cu²⁺ > Cr³⁺. The adsorption capacity of Cr³⁺ is also affected by its ionic valence and isomorphism.²⁴ However, the amino-acid-modified MMT composites all contribute to a significant increase in the sustained-release rate of Zn²⁺, which ranges between 20% and 40%. This is owing to the too high initial adsorption capacity along with higher hydration energy.

Table 8.

The influencing properties of five metal ions

Ion Species	Pb2+	Zn2+	Ni2+	Cu2+	Cr3+
Ionic radius/nm	0.119	0.083	0.069	0.073	0.0615
Heat of hydration/kJ	1502.1	2058.5	2121.3	2121.3	4623.3
Ionic potential	435.3	760.2	863.3	884.9	2047.2

MMT adsorbs amino acids and metal ions primarily via chemical, physical, and ion-exchange adsorption. Physical adsorption is based on the interaction of the adsorbent and the adsorbate molecules (Gerstner orientation effect, Debye induction effect, and London dispersion effect)²⁵ and thus has very little adsorption selectivity. Chemical adsorption occurs when electrons are redistributed at the solid-liquid interface, leading to the formation of chemical bonds. The hydroxyl, amino, and carboxyl groups on the surface of clay mineral crystals can react with heavy metal ions or their hydrolyzed species, leading to surface complexation. Among these, amino groups exhibit stronger electrostatic attraction. Outer-layer complexes are primarily formed through electrostatic force, whereas inner-layer complexes involve chemical bonding. Chemical adsorption is highly selective and typically characterized by monolayer adsorption. When ions have lower hydration energy (such as Pb²⁺), the stability constant of the formed complexes increases, resulting in a greater adsorption capacity. However, if the concentration of ions at the mineral-water interface exceeds the threshold number for precipitation nucleation, precipitation occurs rapidly. The valence state of an ion also influences its adsorption during ion exchange. High-valent metal ions usually possess a smaller hydration radius, facilitating their approach to the MMT surface. Their higher charge densities lead to stronger electrostatic interaction with the negatively charged layer of MMT, thereby enhancing adsorption. The strong dependence of metal ions’ adsorption on pH further indicates that the process is predominantly governed by surface complexation and ion exchange mechanisms.

In summary, the types of amino acid groups affect the adsorption capacity of MMT for amino acids. The order of groups that are beneficial for enhancing the adsorption capacity of amino acids is: amino group, carboxyl group, and hydroxyl group. Except for serine, arginine, and lysine competed heavily with interlayer water molecules, facilitating adsorption. The Langmuir model, pseudo-second-order kinetics, Elovich, and Weber-Morris models all agree that MMT adsorbs lysine. This shows that the adsorption mechanism is predominantly chemical in nature. The adsorption process can be divided into two parts. The absolute value of adsorption energy decreases in the following order: Lys, Ser, and Asp. Amino acid/MMT composites exhibit a stronger adsorption capacity for metal ions due to their ability to coordinate with or electrostatically interact with the ions. Cu²⁺ and Pb²⁺ exhibit similar adsorption tendencies to amino acid molecules, but inorganic ions commence adsorption more quickly. A number of factors influence adsorption efficacy, including hydration heat, valence, ionic radius, effective ionic radius, and ion coordination impact. The adsorption amount of Pb²⁺ with the lowest hydration energy, a large ionic radius, and excellent polarization is the highest. The absolute value of Gibbs free energy for the adsorption of Pb²⁺ is greater than that for the adsorption of Cu²⁺. This study gives strong evidence for the use of amino acid-modified MMT in industrial and agricultural productivity, as well as environmental pollution prevention.

Limitations of the study

The recycling procedures and reusability performance of composite materials have only been tested through modest studies. Future in-depth study can benefit from additional relevant experiments. In practical applications, concerns such as the dosage of Lys/MMT composites, the effect of ionic strength in the solution, the appropriate pH range, and operational stability will all be thoroughly investigated in subsequent tests. Due to the uniqueness and complexity of the MMT structure, this article only performed analysis and calculation on this supercell and repetitive verification. In the future, we plan to add other similar approaches for comparing the unit cell to experimental measurements.

Resource availability

Lead contact

Further information and requests for resources and reagents should be directed to and will be fulfilled by the lead contact, Yinghai Lyu (yhlv@sdust.edu.cn).

Materials availability

The materials used in this investigation were amino acid/MMT composites synthesized in our laboratory in accordance with the experimental section.

Data and code availability

• •The datasets used in this investigation are available from the lead contact on reasonable request.
• •This paper does not report original code.
• •Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.

Acknowledgments

The authors would like to express their gratitude to Kang Jiabao and Sui Xinyi for their hard work in the data supplementation and verification experiments. Special thanks to the editors of the publishing house for their valuable feedback.

Author contributions

Conceptualization, Y. L. and H.L.; methodology, H.L. and J.L.; investigation, H.L., X.L., and X.Y.; writing – original draft, J.L. and X.L; writing – review and editing, Y.L. and J.L.; funding acquisition, Y.L. and G.L.; resources, Y.Y. and X.S.; supervision, G.L. and Y.L.

Declaration of interests

The authors declare no competing interests.

STAR★Methods

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Chemicals, peptides, and recombinant proteins
Chromium (Ⅲ) chloride (CrCl3)	Shanghai Shanpu Chemical Co., Ltd	CAS:10025-73-7
Copper(Ⅱ)chloride dihydrate (CuCl2·2H2O)	Tianjin BASF Chemical Co., Ltd	CAS:10125-13-0
Copper(Ⅱ)nitrate trihydrate (Cu(NO3)2·3H2O)	Xilong Scientific Co., Ltd	CAS:10031-43-3
Hydrochloric acid(HCl)	Sinopharm Chemical Reagent Co., Ltd	CAS:7647-01-0
L-Arginine (L-Arg)	Shanghai Macklin Biochemical Co., Ltd	CAS:74-79-3
L-Aspartic acid (L-Asp)	Shanghai Macklin Biochemical Co., Ltd	CAS:56-84-8
Lead nitrate (Pb(NO3)2)	Tianjin Guangfu Fine Chemical Research Institute	CAS:10099-74-8
L-Lysine (L-Lys)	Shanghai Macklin Biochemical Co., Ltd	CAS:56-87-1
L-Serine (L-Ser)	Shanghai Macklin Biochemical Co., Ltd	CAS:56-45-1
Montmorillonite	Zhejiang Fenghong New Materials Co., Ltd	N/A
Nickel chloride hexahyrate (NiCl2·6H2O)	Xilong Scientific Co., Ltd	CAS:7791-20-0
Ninhydrin hydrate	Shanghai Macklin Biochemical Co., Ltd	CAS:485-47-2
Nitric acid (HNO3)	Laiyang Kangde Chemical Co., Ltd	CAS:7697-37-2
Zinc Chloride (ZnCl2)	Tianjin Dingshengxin Chemical industry Co.,Ltd	CAS:7646-85-7
Software and algorithms
Adobe photoshop	Adobe photoshop 2025	https://www.adobe.com
Digital Micrograph	Digital Micrograph 3.5	https://www.gatan.com
Materials Studio	Materials Studio 2020	https://www.3ds.com
MDI Jade	MDI Jade 6.5	https://www.materialsdata.com
Origin	Originlab 2024b	https://www.originlab.com
Thermo Avantage	Thermo Avantage v5.9	https://www.thermofisher.cn

Method details

Preparing composite materials

The montmorillonite, purchased from Zhejiang Feng Hong New Materials Co., LTD., has a cation exchange capacity (CEC) of 73 mmol/100 g. Based on preliminary experiments, lysine was selected as the modifier to prepare the lysine/montmorillonite composite (Lys/MMT). The composite was synthesized by adding lysine (equivalent to 2 CEC) to a suspension of 4.0 g montmorillonite in 160 mL of distilled water. The mixture was left to stand for 0.5 hour, followed by 0.5 hour of sonication. It was then magnetically stirred for 4 hours and aged at room temperature for 12 hours. Subsequently, the suspension was centrifuged at 4200 r/min for 15 minutes (or longer if necessary). The resulting precipitate was washed and dried at 57 °C to obtain the final product, designated as the 2CEC Lys/MMT composite (2LM).

How amino acid species affect montmorillonite's adsorption capabilities

Arginine, lysine, serine, and aspartate were chosen as organic modifiers, and their relevant information is shown in the key resources table. The modified ninhydrin colorimetric method was used to determine the amino acid content.²⁶ Their standard curves are shown in Figure S1. Four 100 mL beakers were filled with four 0.1g pieces of montmorillonite. 40 mL of distilled water was added to separate beakers containing 2CEC of arginine, lysine, serine, and aspartate. After mixing and standing for half an hour, they were sonicated for another half an hour and then allowed to stand for an additional twelve hours. To separate the supernatants from the precipitates, centrifugation was performed for 15 minutes (or longer if necessary) at 4200 r/min. The amino acid contents in the supernatants were then determined using the modified ninhydrin colorimetric method. Before being used, the precipitates were cleaned, dried, crushed, and sealed.

The effect of amino acid dose on MMT adsorption characteristics

Several pieces of 0.1g montmorillonite were weighed and placed in separate beakers containing 0.5, 1, 2, 4, 8, 12, 16, 24, and 32 CEC of lysine. After adding 40 mL of distilled water, the same procedures as those used in the amino acid species experiments were carried out. Arginine, serine, and aspartate dosages were treated in the same way as lysine.

The adsorption performance of amino acid/montmorillonite for metal ions

The impact of amino acid type, metal ion species, and ion concentrations

After the Arg/MMT, Ser/MMT, and Asp/MMT composites were created by following the Lys/MMT preparation procedure, 0.1g of pure MMT and four types of composites were placed in 100 mL beakers with 40 mL of 200 mg/L NiCl₂, CuCl₂, CrCl₃·6H₂O, ZnCl₂, and Pb(NO₃)₂ solution. They were stood for 0.5 hours, then sonicated for 0.5 hours before standing for another 12 hours. The adsorption capabilities of five types of heavy metal cations by five types of materials were estimated using supernatant concentrations. After the suspensions were centrifuged at 4200 r/min for 15 min, a small amount of supernatant was diluted to a certain ratio and measured by ICP-MS. If the metal ion concentration was 400 mg/L, the adsorption and determination tests were carried out using the 200 mg/L protocol.

Comparison of ion release performance

After the 200 mg/L metal cations' adsorption trials were finished, the sustained release tests were performed according to the steps below. The composite precipitations were placed in a beaker, mixed with 40 mL of distilled water, and left for 12 hours. The supernatants were collected after centrifugation at 4200 r/min for 15 min. The concentration of metal ions in the supernatants was determined by ICP-MS, and the slow-release quantity of heavy metal cations was calculated.

Kinetic and thermodynamic study of lysine adsorption on montmorillonite

The kinetic analyses were performed using the following methodologies. Samples of the solutions were collected at 5, 10, 20, 30, 40, 60, 80, 120, 240, 360, 480, and 960 minutes after the 0.1g montmorillonite and 4CEC lysine were mixed in beakers. The amino acid content was calculated using the modified ninhydrin colorimetric method. The thermodynamic analyses of lysine adsorption by montmorillonite were performed later. Three beakers containing 0.1g montmorillonite and 16CEC lysine suspension were placed in three water bath kettles and kept at 0 °C, 30 °C, and 60 °C for 2 hours, respectively. The subsequent operations, such as centrifugation and determination, are identical to those in the amino acid adsorption experiments.

Kinetic and thermodynamic study of metal ions adsorption on Lys/MMT

A kinetic study of metal ions adsorption on Lys/MMT was conducted in the following steps. 0.1 g of MMT and Lys/MMT were weighed into a beaker, and 40 mL of Cu(NO₃)₂ and Pb(NO₃)₂ solution with a metal cation concentration of 400 mg/L were added. The beaker was placed in an intelligent thermostatic water bath at 30 °C for 2 hours. The metal ion concentrations in the diluted supernatants were measured using a flame atomic absorption spectrophotometer at 5, 10, 20, 30, 60, 120, 180, 240, 300, and 320 minutes.

A thermodynamic study of metal ions adsorption on Lys/MMT was conducted in the following steps. 0.1 g of MMT and Lys/MMT were weighed into a beaker, and 40 mL of Cu(NO₃)₂ and Pb(NO₃)₂ solution with a metal cation concentration of 400 mg/L were added to the beaker. The beaker was placed in an intelligent thermostatic water bath with temperatures of 0 °C, 30 °C, 60 °C, and 90 °C to adsorb the metal ion concentration in the diluted supernatant for 2 hours. The metal ion content in the supernatant was measured using a flame atomic absorption spectrophotometer.

Calculate and analyze adsorption energy

The solution models of lysine, aspartate, serine, and Cu²⁺ were created using MS2020 software's Amorphous cell module building block, with a density of 1.005, a molar ratio of water molecules to solute of 600:1, and a COMPASSII forcefield. The space group of montmorillonite crystals is monoclinic C₂/m. The crystal constants are a = 5.23Å, b = 9.06Å, and c = 12.50Å, with a layer of water molecules existing in interlayer spaces.²⁷ The three angles employed in this structure are α=90°, β=99.00°, and γ=90°. 6a×6b×1c montmorillonite supercells with space group P1 were grown on a layer of 001 crystal surface. Size of the MMT supercell is 3.138nm × 3.138nm × 3.999nm, with angles α = β = 90°; γ = 60°. Following that, four solid-liquid two-phase models were developed for lysine, aspartate, serine, and Cu²⁺ solutions using MMT (Scheme 1A–1D). Finally, the adsorption energies of montmorillonite with lysine, aspartate, serine, and Cu²⁺ solutions were estimated using the Forcite module. The parameters are listed below. Ensemble: NVT. Initial velocities are random. Temperature: 298K. Time step: 1.0 fs. Total simulation time: 100ps. Thermostat: Nose. Equation 7 was used to compute the adsorption energies in the frame.

E_int = E_a-b-E_a-E_b. (Equation 7)

Characterization

The D8 Advance X-ray powder diffractometer (XRD) and Nicolet 510P Fourier infrared spectrometer (FTIR) were used to analyze the structure and groups. The XRD specifications include a Cu target, a tube voltage of 40 kV, a tube current of 40 mA, a scanning speed of 5 °/min, and a range of 5–80°. The topographical study was performed using the FEI APREO scanning electron microscope (SEM) and the Talos F200s field emission transmission electron microscope (TEM). The ion contents were determined by the Thermo Fisher's iCAP PQ inductively coupled plasma mass spectrometer (ICP-MS) or the Beijing Puxi TAS-986F atomic absorption spectrophotometer (AAS). BET analysis is carried out with the completely automated specific surface area and porosity analyzer, Michael ASAP2460, fully. The Sensys Evo DSC differential scanning calorimeter and Setline STA synchronous thermal analyzer were used to perform the TG-DSC analysis. The working gas is Ar, and the carrier gas flow rate is 20 mL/min. The heating rate is 20 °C /min, with a temperature range of 50-600°C.

Quantification and statistical analysis

Statistical analysis of data was performed using Excel (Microsoft) and Origin (OriginLab). Each experiment in Figure 6A was performed three times, and the data are expressed as the mean ± standard error.

Published: December 12, 2025