Efficient Sequestration of Heavy Metal Cations by [Mo₂S₁₂]²⁻ Intercalated Cobalt Aluminum-Layered Double Hydroxide

Abstract

Heavy metal cations such as Ag⁺, Pb²⁺, and Hg²⁺ can accumulate in living organisms, posing severe risks to biological systems, including humans. Therefore, removing heavy metal cations from wastewater is crucial before discharging them to the environment. However, trace levels and high-capacity removal of the heavy metals remain a critical challenge. This work demonstrates the synthesis and characterization of [Mo₂S₁₂]²⁻ intercalated cobalt aluminum-layered double hydroxide, CoAl―Mo₂S₁₂―LDH (CoAl―Mo₂S₁₂), and its remarkable sorption properties for heavy metals. This material shows high efficiency for removing over 99.9% of Ag⁺, Cu²⁺, Hg²⁺, and Pb²⁺ from 10 ppm aqueous solutions with a distribution constant, K_d, as high as 10⁷ mL/g. The selectivity order for removing these ions, determined from the mixed ion state experiment, was Pb²⁺ < Cu²⁺ ≪ Hg²⁺ < Ag⁺. This study also suggests that CoAl―Mo₂S₁₂ is not selective for Ni²⁺, Cd²⁺, and Zn²⁺ cations. CoAl―Mo₂S₁₂ is an efficient sorbent for Ag⁺, Cu²⁺, Hg²⁺, and Pb²⁺ ions at pH~12, with the removal performance of both Ag⁺ and Hg²⁺ cations retaining > 99.7% across the pH range of ~2 to 12. Our study also shows that the CoAl―Mo₂S₁₂ is a highly competent silver cation adsorbent exhibiting removal capacity (q_m) as high as ~918 mg/g compared with the reported data. A detailed mechanistic analysis of the post-treated solid samples with Ag⁺, Hg²⁺, and Pb²⁺ reveals the formation of Ag₂S, HgS, and PbMoO₄, respectively, suggesting the precipitation reaction mechanism.

1. Introduction

Heavy metal contamination in water resources poses a significant threat to environmental and human health due to the toxicity, persistence, and bioaccumulation of these pollutants [1]. Contaminated water with heavy metals destroys the main metabolic process of humans by producing reactive oxygen species and free radicals, which cause high morbidity and mortality worldwide [2]. Not only humans but also the entire ecosystems are at serious risk for the contamination of water bodies by heavy metals [1]. As industrial activities continue to expand globally, the need for efficient and cost-effective methods to remove heavy metals from water has become increasingly urgent [3]. Among various remediation techniques, adsorption using engineered nanomaterials has emerged as a promising approach due to its high efficiency and simplicity [4].

Layered double hydroxides (LDHs) have garnered considerable attention as adsorbents for heavy metal removal owing to their unique layered structure, high anion exchange capacity, and tunable composition [5,6,7]. These materials comprise positively charged metal hydroxide layers with intercalated anions and water molecules in the interlayer spaces [8]. Modifying the interlayer anions offers opportunities to enhance the adsorption capacity and selectivity of LDHs for specific contaminants [9]. Recent studies have shown that the intercalation of sulfur-containing anions into LDHs can significantly improve their performance in heavy metal removal [6,8,10,11,12,13]. These findings highlight the importance of functionalizing chemically diverse LDHs with various metal sulfide anions. In particular, incorporating sulfur-rich anions provides abundant binding sites and enables potential redox interactions, significantly enhancing the overall removal efficiency.

Building upon these findings, our study focuses on the development of a novel adsorbent material: [Mo₂S₁₂]²⁻ intercalated cobalt aluminum-layered double hydroxide (CoAl―Mo₂S₁₂). The [Mo₂S₁₂]²⁻ anion, a sulfur-rich molybdenum complex, is expected to provide numerous binding sites for heavy metal cations through strong metal–sulfur interactions [14] and particularly for soft heavy metal cations through hard and soft acid-base principles where sulfide ions act as a soft base [15]. It is important to note that MogAl-Mo₃S₁₃, a previously reported material, offers impressive sorption properties for heavy metals. This work reports the synthesis and characterization of CoAl-Mo₂S₁₂-LDH [6]. Importantly, the host–guest interactions depend on numerous factors, including the unique chemical properties of the layered cations and the guest anions. The layered composition varies from Mg²⁺ to Co²⁺, resulting in deviations in interlayer spacing and M-O bonding strength within the layered MoO₆ octahedra. Furthermore, Mo₃S₁₃ and Mo₂S₁₂ ions exhibit distinct differences, including but not limited to the following: (i) Mo₃S₁₃ ions contain a trinuclear cluster with Mo-Mo bonds in a quasi-spherical structure, whereas [Mo₂S₁₂] ions comprise a dinuclear Mo-Mo cluster with a linear structure, (ii) molybdenum atoms in Mo₂S₁₂ anions have a pentavalent oxidation state, while those in Mo₃S₁₃ anions are tetravalent, and (iii) variations in charge density and ionic sizes further differentiate their chemical and physical behaviors. These features create remarkable distinctions in the host-guest interactions, significantly influencing the physicochemical properties of the resulting materials. Therefore, further investigation is necessary to understand these interactions fully.

In this manuscript, we report the synthesis, characterization, and efficiency of [Mo₂S₁₂]²⁻ intercalated CoAl―LDH for the sequestration of various heavy metal cations from aqueous solutions. We explore the adsorption mechanisms, kinetics, and sorption capacity to elucidate the underlying processes governing the removal of these contaminants. Additionally, we examine the material’s selectivity under different environmental conditions to assess its practical applicability in water treatment scenarios. By combining the advantageous properties of CoAl―LDH with the metal-binding capabilities of [Mo₂S₁₂]²⁻, we developed a highly efficient and versatile adsorbent for heavy metal removal. This study contributes to the growing body of knowledge on functionalized LDHs for environmental remediation and provides insights into the design principles for next-generation adsorbent materials targeting specific water pollutants.

2. Results and Discussions

2.1. Synthesis and Characterization

[Mo₂S₁₂]²⁻ intercalated CoAl-layered double hydroxide (CoAl―Mo₂S₁₂―LDH) was synthesized at ambient conditions using the anion exchange method via the substitution of NO₃⁻ with [Mo₂S₁₂]²⁻ anions into the positively charged gallery of the host CoAl─NO₃─LDH. The XRD patterns of CoAl―CO₃―LDH, CoAl―NO₃―LDH, and CoAl―Mo₂S₁₂―LDH show the characteristics of Brag’s diffraction peaks of 003 and 006, belonging to the hydrotalcite structure type layered double hydroxide (Figure 1A). The corresponding d₀₀₃ values for CoAl―CO₃, CoAl―NO₃, and CoAl―Mo₂S₁₂ are obtained at 0.75 nm, 0.89 nm, and 1.08 nm, respectively (Figure 1A). The gradual increase in the interplanar d₀₀₃ distances from CoAl―CO₃ → CoAl―NO₃ → CoAl―Mo₂S₁₂ are attributed to the larger ionic sizes of intercalated anions having a trend of CO₃²⁻ < NO₃⁻ < Mo₂S₁₂²⁻.

Infrared spectroscopy revealed the C-O stretching and bending modes of CO₃²⁻ were confirmed by the sharp band at 1355 cm⁻¹ and a strong band at 787 cm^−1, demonstrating the formation of CoAl―CO₃―LDH (Figure 1B) [16]. Furthermore, a doublet of N-O stretching modes of micro-hydrated NO₃⁻ observed at 1355 and 1407 cm⁻¹ confirm the exchange of CO₃²⁻ by NO₃⁻ (Figure 1B) in the gallery of CoAl―LDH [16,17]. The appearance of a very low intensity of NO₃⁻ peak at the FTIR spectrum of CoAl―Mo₂S₁₂ reveals the almost disappearance of the NO₃⁻ group (Figure 1B). The steric hindrance of the disulfide-rich large [Mo₂S₁₂]²⁻ might inhibit its complete exchange with NO₃⁻ ions [18]. A similar finding was also reported for other bulky metal–sulfide interaction LDH [19]. Therefore, the FTIR spectrum indicates the formation of CoAl―CO₃ → CoAl―NO₃ → CoAl―Mo₂S₁₂ through the ion-exchange method, which is also corroborated by XRD analysis.

We also analyzed the pristine CoAl―Mo₂S₁₂ by X-ray photoelectron spectroscopy (XPS) to determine the oxidation states of cobalt, molybdenum, and sulfur ions. The high-resolution XPS spectrum of CoAl―Mo₂S₁₂ shows 3 doublets for Mo 3d with binding energies (BEs) of 229.17 and 232.37 eV, ∆BE = 3.20 eV; 231.15 and 234.35 eV, ∆BE = 3.20 eV; 232.32 and 235.52 eV, ∆BE = 3.20 eV corresponding to the 3d_5/2 and 3d_3/2 of Mo⁴⁺, Mo⁵⁺, and Mo⁶⁺, respectively [8,20,21]. Similarly, 3 doublets for S 2p orbitals (Figure 1D) at 161.76 and 162.94; 163.06 and 164.24; 168.02 and 169.20 eV with ∆ = 1.18 eV identified its three different oxidation states as −2, −1, and +6, respectively [8,21,22]. The deviation of the Mo and S oxidation states may suggest a change in the structure of the redox-active Mo₂S₁₂ anions during the intercalation process, which is yet to be understood. This kind of phenomenon was also observed for MgAl-MoO₂S₂ [23]. Additionally, XPS also revealed peaks at 781.25 and 797.56 eV, corresponding to Co 2p_3/2 and 2p_1/2 energy, respectively [8]. Hence, the distance between the main and satellite peaks of Co 2p_3/2 orbitals is 5.5 eV, which indicates the bivalency of Co ions in the CoAl―Mo₂S₁₂ (Figure S1) [24].

The thermogravimetric analysis (TGA) shows an initial loss of about 8 and 6.7 weight percentages up to 160 °C for CoAl―NO₃ and CoAl―Mo₂S₁₂, respectively (Figure S2A). This weight loss is due to the removal of surface and interlayer water molecules. The thermal decomposition of the CoAl―NO₃ structure is observed around 250 °C by the loss of OH⁻ and NO₃⁻ groups and, subsequently, led to the formation of CoAl₂O₄ (Figure S2A) [25]. However, the TGA of CoAl―Mo₂S₁₂ showed a continuous loss of weight that reached ~34% before it became stable at about 550 °C. Such a gradual loss is related to losing water, nitrate, and sulfide species. After TGA, the X-ray powder diffraction pattern of the CoAl―Mo₂S₁₂ shows low-intensity hump-like features of few diffraction peaks at 2Ɵ = 14.7, 26.2, 30.5, 46.9, and 53.9° which are related to CoMoS_2.17O_1.12 (PDF-2 Entry # 00-016-0438) (Figure S2B) [26].

The SEM image shows a random orientation of the aggregated plate-like crystallites of CoAl―Mo₂S₁₂―LDH (Figure 2A). The energy-dispersive X-ray spectrum of CoAl―Mo₂S₁₂―LDH confirms the presence of Mo and S besides Co and Al (Figure 2B). The atomic abundance of Co-Al is 0.61:0.39. Considering the amount of trivalent Al³⁺ ion, the maximum 0.195 moles of [Mo₂S₁₂]²⁻ can be inserted into the CoAl―LDH layers, and, therefore, the molecular formula of CoAl―Mo₂S₁₂―LDH would be Co_0.61Al_0.39(OH)₂(Mo₂S₁₂)_0.195·H₂O. However, bearing in mind the molar ratios of (Mo + S)/Co and (Mo + S)/Al which are 3.08 and 4.78, respectively, the molecular formula of the synthesized CoAl―Mo₂S₁₂ would be Co_0.61Al_0.39(OH)₂(Mo₂S₁₂)_0.134(NO₃)_x(CO₃)_0.06−x/2·H₂O (where x = 0.0 to 0.12) instead of Co_0.58Al_0.42(OH)₂(Mo₂S₁₂)_0.21·H₂O. Such a mix of anionic intercalation can agree with the IR spectrum (Figure 1B). The TEM image of the LDHs reveals the transparent hexagonal morphology, which demonstrates the ultrathin nature of the platelike crystallites (Figure 2C).

2.2. Sorption of Individual Heavy Metal Cations

We investigated the interactions of CoAl―Mo₂S₁₂ with various heavy metal cations: Cu²⁺, Hg²⁺, Ag⁺, Pb²⁺, Cd²⁺, Ni²⁺, and Zn²⁺ (Figure 3, Table S2). Among them, CoAl―Mo₂S₁₂ was remarkably effective in capturing Ag⁺, Cu²⁺, Hg²⁺, and Pb²⁺ from aqueous solutions. This finding revealed that CoAl―Mo₂S₁₂ could remove over 99.9% of Ag⁺, Cu²⁺, Hg²⁺, and Pb²⁺ from 10 ppm (mg/L) spiked solutions of each cation from the individual cationic states. Such a highly efficient removal of Ag⁺, Cu²⁺, Hg²⁺, and Pb²⁺ led to the final concentrations of ~0.5, 22.0, 26.0, and 3.0 ppb, respectively, with their corresponding distribution constants of 2.0 × 10⁷, 4.5 × 10⁵, 3.8 × 10⁵, and 9.9 × 10⁶ mL/g. The distribution constant, K_d, represents the affinity of sorbent to a sorbate, and any value of ≥10⁴ mL/g is deemed exceptional [27,28]. Hence, CoAl―Mo₂S₁₂, with such an excellent removal performance and unprecedented selectivity toward a large number of toxic heavy metal cations, including Ag⁺, Cu²⁺, Hg²⁺, and Pb²⁺, and an outstanding K_d value, places this material among the top candidates for the sorption of heavy metals ions from aqueous solutions.

2.3. Sorption of Mixed Heavy Metal Cations

To investigate the selective affinity and competitive sorption of different heavy metal cations, CoAl―Mo₂S₁₂ sorbents were suspended in a solution containing Mⁿ⁺ = Ni²⁺, Cu²⁺, Zn²⁺, Ag⁺, Cd²⁺, Pb²⁺, and Hg²⁺, which we refer to as mixed-ion states (Figure 4A, Table S3). The sorption experiment was carried out in aqueous solutions with 10 ppm concentrations of each cation at pH~7 for contact times of 6, 24, and 48 h. Importantly, a solution with 10 ppm of each of the seven cations yields a total concentration of 70 ppm in the mixed-ion state. This experiment showed that even in the presence of competing cations, the removal performance for the Ag⁺ and Hg²⁺ cations remains over 99.5%. However, our results show in the mixed-ion state, the removal efficiency of Cu²⁺ ions dropped from ~70% (K_d~2.3 × 10³) to ~20% (2.4 × 10² mL/g), whereas it increased from 7 (K_d~0.8 ×10²) to 33% (K_d~4.9 × 10²) for Pb²⁺ ions with a duration of 6 h to 48 h. Additionally, other harder cations, such as Ni²⁺, Cd²⁺, and Zn²⁺, are not selective for the CoAl―Mo₂S₁₂. The selectivity order for these ions was Pb²⁺ < Cu²⁺ ≪ Hg²⁺ < Ag⁺ (Table S3). Hence, such a high selectivity for Ag⁺ and Hg²⁺ can be attributed to their higher chemical softness and polarizability, which drive high affinity to bind with chemically soft and polarizable Lewis basic sulfide anions of the CoAl―Mo₂S₁₂―LDH, following Pearson’s hard-soft Lewis acid-base principle [15].

The sorption rates and mechanisms can be analyzed by pseudo-first and pseudo-second-order rate equations, as described in Equations (1) and (2), respectively [29].

$$\mathrm{Pseudo}\text{-}\mathrm{first}\text{-}\mathrm{order}: \ln \left(q_{\mathrm{e}}-q_{\mathrm{t}}\right)=\mathrm{l}\mathrm{n}{q}_{\mathrm{e}}-k_1\mathrm{t}$$

(1)

$$\mathrm{Pseudo}\text{-}\mathrm{second}\text{-}\mathrm{order}: {\displaystyle \frac{\mathrm{t}}{q_{\mathrm{t}}}}={\displaystyle \frac{1}{k_2{q}_{\mathrm{e}}^2}}+{\displaystyle \frac{\mathrm{t}}{q_{\mathrm{e}}}}$$

(2)

where q_e (mg/g) is the amount of adsorbed element per unit mass of adsorbent at equilibrium and q_t (mg/g) is the adsorbed amount at time t, while k₁ (min⁻¹) and k₂ (g/mg·min⁻¹) are equilibrium rate constants of pseudo-first-order and pseudo-second-order adsorption interactions, respectively [30]. Fitting of the experimental data led to a decent linear relationship for the ‘t/q_t’ versus ‘t’ plot (Figure 4B), demonstrating the sorption of Ag⁺ and Hg²⁺ follow the pseudo-second-order rate equations where the rate constant, k₂, for Ag⁺ and Hg²⁺ is ~4.86×10⁻² and 1.00×10⁻¹ g/mg·min⁻¹, respectively, and the correlation coefficients (R²) were nearly equal to unity. Overall, a combination of such remarkable sorption efficiencies establishes CoAl―Mo₂S₁₂―LDH as a highly promising adsorbent for the removal of heavy metals from complex samples and wastewater treatment.

2.4. pH-Dependent Sorption Studies

A pH-dependent sorption experiment of CoAl―Mo₂S₁₂ for heavy metal cations was conducted in an aqueous solution at neutral, acidic, and alkaline conditions. A detailed sorption analysis of mixed heavy metal cations shows that CoAl―Mo₂S₁₂ is the most efficient sorbent for Ag⁺, Cu²⁺, Hg²⁺, and Pb²⁺ ions at pH~12 (Figure 5 and Table S5). At this pH, it achieves over ≥ 99.9% removal of Ag⁺ and Hg²⁺ cations with K_d values of >10⁸ and 10⁶ mL/g, respectively. Remarkably, the removal rate for both Ag⁺ and Hg²⁺ cations remains > 99.7 and 98.9%, respectively, over the pH range of ~2 to 12. For Cu²⁺ and Pb²⁺, the removal efficiency increases as the pH of the solution increases. Hence, the higher removal of Cu²⁺ and Pb²⁺ in alkaline conditions could be attributed to the cooperative effect of adsorption and metal hydroxide precipitation. This high removal performance, along with such high distribution constants, highlights CoAl―Mo₂S₁₂ as a unique sorbent for heavy metal cations.

2.5. The Sorption Capacity of CoAl―Mo₂S₁₂―LDH

Sorption experiments were conducted with 20.0 mg of the CoAl―Mo₂S₁₂―LDH for Ag⁺ and Pb²⁺ using their individual solution prepared by spiking a range of concentrations up to 3000 ppm. To understand the sorbent and sorbate interactions, the experimental data were fitted by the Langmuir isotherms model for heterogeneous surfaces (Equation (3)). This model predicts that the sorbate undergoes single-layer type exposure on the surface of the sorbent materials. It also predicts that once an adsorption site is covered, no further adsorption can take place at the same site [30].

$$\mathrm{Langmuir} \mathrm{isotherm}: q=q_m{\displaystyle \frac{b{C}_{\mathrm{e}}}{1+b{C}_{\mathrm{e}}}}$$

(3)

where C_e (mg/L) is the concentration at equilibrium, q (mg/g) is the equilibrium sorption capacity of the sorbed Ag⁺, q_m (mg/g) is the theoretical maximum sorption capacity, b (L·mg⁻¹) is the Langmuir constant that is related to the interaction energy of CoAl―Mo₂S₁₂―LDH and Ag⁺, and C_e (µg/mL) is the equilibrium concentration.

The sorption performance of the CoAl―Mo₂S₁₂―LDH towards Ag⁺ and Pb²⁺ increases with the initial concentrations up to 3.0 ×10³ ppm. Our study shows that the CoAl―Mo₂S₁₂―LDH is a highly competent silver cation sorbent exhibiting removal capacity (q_m) as high as ~918 mg/g (Figure 6A, Tables S6 and S7), placing this material among the topmost sulfide-based sorbent known to date (

Table 1. Comparison of sorption capacities for Ag⁺ ions with the known high-performing sorbents.

Cation	Adsorbent	qm (mg/g)	Ref.
	CoAl―Mo2S12―LDH	918	This work
	KCMS chalcogel	1378	[35]
	KMS―1	156	[36]
	LDH―Sn2S6	978	[13]
	Mo3S13―ppy	408	[19]
Ag+	Ni/Fe/Ti―MoS4―LDH	856	[37]
	Mn―MoS4	564	[38]
	MoS4―ppy	480	[39]
	MoS4―LDH	450	[7]
	Sx―LDH	383	[40]
	KMS―2	408	[41]
	Fe―MoS4	565	[42]
	MgAl-Mo3S13	1073	[6]

). The high sorption of Ag⁺ can be attributed to the strong affinity of Ag⁺ ions toward the Lewis-basic sulfide anions. However, the contribution of the topotactic ion exchange between the layered cations and Ag⁺ cannot be entirely ruled out. In contrast, CoAl―Mo₂S₁₂―LDH offers a moderate sorption capacity of ~202 mg/g for Pb²⁺ cations (Figure 6B). This value of sorption capacity for Pb²⁺ ions is comparable with Mg₂Al-LS-LDH (123 mg/g) [31], CTS/PAM gel (138 mg/g) [32], EDTA-LDH (180 mg/g) [33], cellulose-based chalcogel (240 mg/g) [34], and MoS₄-LDH (290 mg/g) [7].

2.6. Application Potential of CoAl―Mo₂S₁₂―LDH

To assess the potential in the application of CoAl―Mo₂S₁₂―LDH towards heavy metal removal from wastewater, we performed the sorption experiments in the presence of highly competitive ions in naturally contaminated water, e.g., Mississippi River water (MRW water, from Louisiana) and Tap water (TAP), which were contaminated with various types of inorganic ions, e.g., Na⁺, K⁺, Ca²⁺, Mg²⁺, CO₃²⁻, HCO₃⁻, SO₄²⁻, PO₄³⁻, and Cl⁻ and other organic pollutants. A total of 20.0 mL water from different sources was spiked with a mixed solution of Ag⁺, Hg²⁺, Pb²⁺, Ni²⁺, Cu²⁺, Zn²⁺, and Cd²⁺, containing 10 ppm of each metal cation resulting in a total concentration of 70 ppm for seven metal ions. CoAl―Mo₂S₁₂ can efficiently remove Ag⁺ and Hg²⁺ from those water samples even if they contain a variety of chemically diverse species, such as large quantities of cations, anions, and organic species. In particular, Ag⁺ and Hg²⁺ are removed from MRW and TAP with a removal performance of above 99% (

Table 2. Sorption results of CoAl―Mo₂S₁₂―LDH in potable Tap water and Mississippi River water containing seven metal ions of 10 ppm for each (70 ppm total), C_i = initial (pre-sorption) concentration, C_f = final (post-adsorption) concentration.

Mixed-Ions	Ci (ppm)	Cf (ppm)	SD	Removal (%)	Kd (mL/g)	qm (mg/g)	Cf (ppm)	SD	Removal (%)	Kd (mL/g)	qm (mg/g)
	MRW						Tap Water
Cu2+	10.0	5.6637	0.052	43.36	765.6	4.336	7.498	0.019	25.02	333.6	2.501
Hg2+	10.0	0.0420	0.004	99.78	2.37 × 105	9.958	0.041	0.004	99.59	2.41 × 105	9.958
Ag+	10.0	0.0223	0.002	99.79	4.47 × 105	9.977	0.031	0.001	99.68	3.14 × 105	9.968
Pb2+	10.0	9.3199	0.063	6.80	73.0	0.680	9.456	0.101	5.43	57.5	0.543
Cd2+	10.0	10.0	-	0.0	0.0	0.0	10.0	-	0.0	0.0	0.0
Ni2+	10.0	10.0	-	0.0	0.0	0.0	10.0	-	0.0	0.0	0.0
Zn2+	10.0	10.0	-	0.0	0.0	0.0	10.0	-	0.0	0.0	0.0

). This finding implies that CoAl―Mo₂S₁₂ has exceptional potential for the selective extraction of Ag⁺ and Hg²⁺ from wastewater and for the selective separation of these elements from contaminated waterways.

2.7. Post-Sorbed Analysis of Solid Sorbent

After the sorption experiments, the solid adsorbent was collected, air-dried at ambient temperature, and analyzed using XRD, TEM, and XPS. The XRD pattern of the 100 ppm Ag⁺ interacted samples showed disruption of the LDH structure, which was significant in 3000 ppm Ag⁺ interacted sorbent and showed humps with very weak peaks at 2Θ~25.5, 31.4, and 43.0°, identified as Ag₂S (Figure S3A) [43]. Furthermore, TEM analysis revealed the secondary phase (Figure S3B,C), suggesting a precipitation mechanism. Likewise, XRD patterns of 100 ppm Hg²⁺ and Pb²⁺ treated samples showed retention of the LDH structure with the formation of insoluble HgS and PbMoO₄, respectively, indicative of precipitation mechanisms (Figure 7A). TEM images of post-treated samples clearly show the formation of secondary phases (HgS and PbMoO₄) along with the hexagonal nanosheet of pristine sorbent (Figure 7B,C).

We also investigated the post-interacted solids after treating 100 ppm of Ag⁺, Hg²⁺, Pb²⁺_, and Cu²⁺ using X-ray photoelectron spectroscopy. The bands observed at 374.41 and 368.40 eV (∆ = 6.01eV, Figure 8A), 105.18 and 101.10 eV (∆ = 4.08 eV, Figure 8B), 139.51 and 144.33 eV (∆ = 4.08 eV, Figure 8C), and 933.29 and 953.04 eV (∆ = 19.75 eV, Figure 8D) correspond to binding energy for Ag (3d_3/2; 3d_5/2) [21], Hg (4f_5/2; 4f_7/2) [8,21], Pb (4f_5/2; 4f_7/2) [21], and Cu (2p_1/2; 2p_3/2) [44], respectively. These findings further suggest the precipitations of the lead, silver, and mercury cations occur through the interactions of the CoAl―Mo₂S₁₂―LDH.

3. Materials and Methods

3.1. Material Synthesis

Cobalt aluminum nitrate-layered double hydroxides (CoAl―NO₃―LDH) were synthesized using the ion-exchange method from cobalt aluminum carbonate-layered double hydroxides (CoAl―CO₃―LDH), which were synthesized by hydrothermal method from their corresponding nitrate precursor using hexamethyl-tetramine at 140 °C according to literature reported by the author [11].

The sulfur-rich ammonium thiomolybdate (NH₄)₂Mo₂(S₂)₆·2H₂O was prepared according to the method summarized by Draganjac et al. [45]. In detail, freshly prepared 30 mL of dark red (NH₄)₂S_x solution was added to the reddish-brown aqueous solution (30 mL) containing 2.0 g of ammonium paramolybdate, (NH₄)₆Mo₇O₂₄.4H₂O, and 1.5 g of hydroxylamine hydrochloride (NH₂OH·HCl). The resulting reddish-brown solution was warmed at 50 °C and 90 °C for 1 and 4 h, respectively, with cooling and filtering in each step. A total of 10 mL of (NH₄)₂S_x solution was also added to the resulting reddish-brown solution before being kept in the Argon atmosphere overnight. Finally, dark-black crystals were filtered, washed with ice-cold water, 2-propanol, carbon disulfide, and diethyl ether a couple of times, and dried at room temperature (Figure 9A).

CoAl―Mo₂S₁₂―LDH layered double hydroxides (LDH―Mo₂S₁₂) were synthesized at room temperature by intercalating [Mo₂S₁₂]²⁻ anions into the positively charged gallery of the host CoAl―NO₃―LDH using the anion exchange method [11,12]. In detail, 600 mg of CoAl―NO₃―LDH and 600 mg of (NH₄)₂(Mo₂S₁₂) were stirred in 15.0 mL of formamide separately for 1h, and then dispersed solution of (NH₄)₂(Mo₂S₁₂) was added dropwise to the CoAl―NO₃ solution followed by stirring for 24 h at RT. The black solid was then centrifuged, washed with water, and dried under a vacuum at 70 °C (Figure 9B).

3.2. Characterization

X-ray powder diffraction: A Rigaku MiniFlex 600 diffractometer equipped with a D/teX Ultra detector was used to collect the high-resolution powder X-ray diffraction (PXRD) of CoAl―CO₃, CoAl―NO₃, and CoAl―Mo₂S₁₂ using Cu K_α1 (λ = 1.540593 Å) radiation generated from a sealed-tube X-ray source at 40 kV and 15 mA. The diffraction intensity data were collected from 5 to 80° 2θ at a scan rate of 3.5° min⁻¹ (0.02° resolution).

Energy dispersive spectroscopy and electron microscopy: Elemental analysis was conducted using the ESED―II environmental secondary electron detector. The powdered samples were subsequently placed on a carbon-taped metal stub for imaging of the surface morphology of the samples. The samples were then analyzed using energy dispersive spectroscopy (EDS), and the experiment was performed in at least four different spots, with the average measured composition reported throughout. High-resolution Lyra3―Tescan scanning electron microscopy (SEM) was used for imaging and elemental analysis of the samples. The theoretical resolution of the instrument is 1.2 nm at 30 keV. An accelerating voltage of 20 kV and 120 s of accumulation time were maintained during data collection. Transmission electron microscopy (TEM) imaging was conducted using JEOL 1011 at 100 KV.

Fourier transform infrared spectroscopy: Infrared spectra of the grounded polycrystalline powders of CoAl―CO₃, CoAl―NO₃, and CoAl―Mo₂S₁₂ and post-sorbed solid were collected using the PerkinElmer FTIR Spectrometer in the range of 400 to 4000 cm⁻¹.

X-ray photoelectron spectroscopy: Finely ground powder samples were pressed onto indium metal foil for data acquisition. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Kratos Supra+ spectrometer with an Al-Kα X-ray source running at 150 W. The analysis spot size was 300 × 700 µm, and an electron flood gun was applied to minimize sample charging. Survey scans were recorded at pass energy of 80 eV, with high-resolution scans taken at 20 eV. All spectra were referenced to the C 1s peak at 284.6 eV for calibration.

Thermogravimetric analysis: The stability of the powdered samples of the CoAl―NO₃―LDH, CoAl―MoS₄―LDH was studied by Shimadzu DTG-60 from RT to 600 °C using Al crucible under N₂ atmosphere (30 mL/min).

Sorption experiments: The batch approach was used to evaluate the uptake of heavy metal ions, Mⁿ⁺ = Ni²⁺, Cu²⁺, Zn²⁺, Ag⁺, Cd²⁺, Pb²⁺, and Hg²⁺, under ambient conditions. To assess the affinity of CoAl―Mo₂S₁₂ for Mⁿ⁺ cations, 20.0 mg of sorbent material was suspended in Mⁿ⁺ solutions of various concentrations and pHs and interacted for varying amounts of time. Subsequently, the solutions were centrifuged at 13,000 RMP for 30 min to precipitate the adsorbents from the heavy metal-contaminated aqueous solutions. The remaining metal ion concentrations in the supernatant solutions were determined using inductively coupled plasma-optical emission spectrometry (ICP-OES). The post-interacted solid dried samples were also analyzed by XRD, TEM, and XPS.

The sorption efficiency was calculated from the difference in pre- and post-sorbed Mⁿ⁺ ion concentrations. The affinity of CoAl―Mo₂S₁₂ to the heavy metal cations was determined by the distribution coefficient, K_d, which is defined by the equation, K_d = (V[(C₀ − C_f)/C_f])/m; where V is the volume of the solution, C₀ and C_f are the initial and final concentration of Mⁿ⁺ ions in ppm, m is the mass of the functionalized sorbent. The removal rate of Mⁿ⁺ was determined using the equation [C₀ − C_f)/C₀] × 100. The removal capacity, q_m (mg/g), was obtained by the expression of q_m = {(C₀ − C_f)V/m} × 10⁻³.

4. Conclusions

The hexagonal ultrathin nanosheet of [Mo₂S₁₂]²⁻ intercalated cobalt aluminum-layered double hydroxide, CoAl―Mo₂S₁₂―LDH, was synthesized by subsequent exchange of CO₃²⁻ and NO₃⁻ ions. The formation of CoAl―Mo₂S₁₂―LDH was characterized using XRD, FTIR, SEM/EDX, XPS, and TEM analysis. The synthesized material exhibits exceptional sorption performance for Ag⁺, Cu²⁺, Hg²⁺, and Pb²⁺ from 10 ppm aqueous solutions in individual tests. The CoAl―Mo₂S₁₂ demonstrated awe-inspiring performance as a silver cation adsorbent, with a maximum adsorption capacity of 918 mg/g, placing it among the topmost Ag sorbent known to date. XRD, XPS, and TEM analysis of Ag⁺, Hg²⁺, and Pb²⁺ sorbed solid samples revealed the formation of Ag₂S, HgS, and PbMoO₄, respectively, which reveal the precipitation sorption mechanism. Competitive sorption experiments revealed a selectivity order of Pb²⁺ ≪ Cu²⁺ ≪ Hg²⁺ < Ag⁺; however, the limited affinity for harder cations such as Ni²⁺, Cd²⁺, and Zn²⁺ following the HSAB rule as sulfur is a soft base.