Dual-Electronic Nanomaterial (Synthetic Clay) for Effective Removal of Toxic Cationic and Oxyanionic Metal Ions from Water

Nguyen, Van Phuong; Nguyen, Khanh Thien Tran; Ton, Loc That; Nguyen, Dong Thanh; Nguyen, Khuong Quoc; Vu, Mai Thi; Tran, Hai Nguyen

doi:https://doi.org/10.1155/2020/1783749

Journal of Nanomaterials

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Metal Oxide Nanomaterials for Safe and Sustainable Water Treatment Technologies

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Research Article | Open Access

Volume 2020 | Article ID 1783749 | https://doi.org/10.1155/2020/1783749

Dual-Electronic Nanomaterial (Synthetic Clay) for Effective Removal of Toxic Cationic and Oxyanionic Metal Ions from Water

Van Phuong Nguyen,^1,2Khanh Thien Tran Nguyen,³Loc That Ton,⁴Dong Thanh Nguyen,⁵Khuong Quoc Nguyen,⁶Mai Thi Vu,⁷and Hai Nguyen Tran^2,8

Academic Editor: Laura Martinez Maestro

Received01 Feb 2020

Revised30 Mar 2020

Accepted21 May 2020

Published07 Sept 2020

Abstract

A synthetic clay (Mg/Al-layered double hydroxides; LDH) was directly synthesized through a simple coprecipitation method under a low-supersaturation condition. The clay was applied to remove metal cations (Cd²⁺, Cu²⁺, Pb²⁺, Ni²⁺, and Cr³⁺) and oxyanions (MnO₄^– and Cr₂O₇^2–) from a single aqueous solution. The result demonstrated that LDH exhibited a poor porosity (its specific surface area and total pore volume: 23.2 m²/g and 0.161 cm³/g, respectively) and positively charged surface within solution pH from 3.0 to 12. The X-ray powder diffraction (XRD) data suggested that the basal spacing of LDH was 0.773 nm. The presence of active CO₃^2– anions in the interlayer region of LDH that played an extremely important role in the adsorption process was identified by XRD and Fourier-transform infrared spectroscopy (FTIR). Energy-dispersive X-ray spectroscopy (SEM) analysis indicated that LDH possessed a surface morphology like a plate with a hexagonal shape. The adsorption isotherms of LDH towards various potentially toxic metals were conducted at 1.0 g/L, pH_Equilibrium 5.0, 30°C, and 24 h. The Langmuir maximum adsorption capacity of LDH towards the target metals exhibited the following order: 1.299 mmol/g (for Ni²⁺ adsorption) > 0.880 mmol/g (Cd²⁺) > 0.701 mmol/g (Cr³⁺) > 0.657 mmol/g (Pb²⁺) > 0.601 mmol/g (Cu²⁺) > 0.589 mmol/g (Cr₂O₇^2–) > . The synthetic clay can adsorb both cations and anions in the solution. Therefore, such LDH material can serve as a potential dual-electronic adsorbent for effectively eliminating various oxyanionic and cationic metal ions from water media.

1. Introduction

The appearance of potentially toxic metals in water bodies has caused more enormous concerns such as potential health risks for humans and environmental threats because of their high toxicity. In real wastewater, anionic and cationic contaminants always coexist in such environment. Therefore, it is necessary to explore and develop some advanced adsorbents with their excellent adsorption capacity towards various kinds of toxic contaminants, although commercial activated carbon (CAC) with its extremely high porosity has been acknowledged as a potential adsorbent for removing various contaminants from a water environment through pore-filling adsorption mechanism [1]. However, CAC, which is a high energy-consuming material, is expensive (~3 USD/kg) because it is often prepared through a pyrolysis process at high temperatures (>800°C) along with certain activation (i.e., physical or chemical) [2, 3]. Furthermore, an adsorption process related to pore-filling mechanism is usually irreversible for the adsorption of organic contaminants with a large molecule size (i.e., dye). Therefore, contaminant-laden AC cannot be effectively used for regeneration [2]. For example, Tran et al. [3] investigated the adsorption process of cation methylene green dye onto three kinds of AC material. They found that desorption efficiency was negligible when a chemical desorption method was used. In addition, Tomul et al. [4] concluded that the desorption of naproxen from three laden biochar samples by a chemical method was less efficient than that by a thermal method. However, the mass loss during the thermal method should be considered, such as approximately , , , and for each cycle of adsorption/desorption.

In general, layered double hydroxides (LDH) are normally known as hydrotalcite-like minerals/solids or synthetic anionic clays. The structure of LDH is composed of two oppositely charged parts: (1) the positively charged brucite-like sheets and (2) negatively charged anionic interlayers [5, 6]. The general formula of LDH is often expressed as [6]. In this formula, M²⁺ represents the divalent metal cations (i.e., Mg²⁺), M³⁺ the trivalent metal cations (i.e., Al³⁺), Aⁿ⁻ the interlayer charge-balancing anions of valence (i.e., CO₃^2–), and the molar ratio (ranging from 0.20 to 0.33) [6, 7]. According to the literature, LDH has been successfully synthesized by numerous methods (i.e., coprecipitation, ion exchange, hydrothermal method, and rehydration using a structural memory effect) [8]. Of those methods, coprecipitation prepared at a low supersaturation has been considered the most appropriate method for synthesizing LDH because of its simplicity in generating LDH with a high crystalline structure [6, 9]. The resultant LDH precipitates can be classified as a typical nanomaterial, with their size distribution being approximately 270 nm for Ni/Fe-LDH [10], 370 nm for Zn/Al-LDH [11], and 469 nm for glycerol-modified Ni/Fe-LDH [10].

The structural and morphological characteristics of LDH are strongly dependent on the synthesis process and the nature of used metal salts [10–12]. For example, LDH is synthesized from metal salts of nitrate (i.e., Al(NO₃)₃); the interlayer region of LDH often contains the NO₃^2– anions [6]. In addition, during the coprecipitation process of metal salts, CO₃^2– anions can be performed in the interlayer region of LDH because CO₂ gas is commonly dissolved in water. The presence of CO₃^2– anions in such region can be minimized if nitrogen gas flow is sufficiently used during coprecipitation to prevent the existence of CO₃^2– anions in solution. However, some previous studies demonstrated that the presence of host anions (i.e., CO₃^2–) in the interlayer region can serve as active exchangeable sites for removing various contaminant anions from water. Such anions comprise (1) inorganic contaminants: hexavalent chromium anion (i.e., CrO₄²⁻ and Cr₂O₇²⁻) [10, 12–14], arsenate anion (H₂AsO₄⁻ and HAsO₄²⁻) [12, 15, 16], and antimonate anion (Sb(OH)₆⁻) [15] as well as (2) organic contaminants: anionic reactive blue 21 dye [17], anionic methyl orange dye [18], three other anionic dyes (i.e., Congo red, indigo carmine, and sunset yellow FCF) [19], and anticancerous drug methotrexate [11]. Furthermore, the important role of CO₃^2– anions in the interlayer region in effectively removing potentially toxic metals from water through precipitation mechanism (i.e. CdCO₃) has been acknowledged in the literature. Such cationic metals include Cu²⁺ [9, 20–22], Zn²⁺ [20, 22, 23], Pb²⁺ [14, 22–24], Cd²⁺ [9, 23], Ni²⁺ [9], and Eu³⁺ [21]. To sum up, LDH can be classified as a “dual-electronic adsorbent” because of its unique structure characterization: positively charged sites on its external surface (high positively potential value) and abundantly exchangeable anions within its interlayer region (high anion exchange capacity).

Recently, some authors developed LDH modified with anionic surfactant sodium dodecyl sulfate (SDS) [18]. The resultant hierarchical flower-like LDH exhibited a hydrophobic surface with a high-water contact angle of 90.1°. Such LDH solid can be classified as an “amphiphilic adsorbent” to remove both polar and nonpolar organic compounds (i.e., anionic methyl orange dye and naphthalene) through adsorption and partition processes, respectively. However, the term of amphiphilic adsorbent is only for LDH modified with a certain anionic surfactant, not for pristine LDH (without any additional modification or treatment).

Furthermore, previous research indicated that Ca/Al-LDH can effectively remove chemical oxygen demand (COD; 73.9%), UV254 (85.8%), and total organic carbon (TOC; 74.7%) from water [25]. The authors [25] also found that COD removal by the LDH material (73.9%) was higher than that by the ion exchange resin (only 60%). Notably, Rahman and colleagues [22] compared sludge volume formed during the removal process of heavy metals using Mg/Al-LDH and Ca(OH)₂. They found that using Mg/Al-LDH for removing heavy metals only produced almost half as much sludge as using Ca(OH)₂. A similar trend of sludge reduction was obtained when comparing the application of Mg/Al-LDH and Ca(OH)₂ in treating the acidic mine wastewater [16]. Therefore, application of LDH for water treatment can minimize the major problems regarding sludge disposal.

In essence, each adsorbent material often exhibits an excellent affinity to each type of pollutant (i.e., cationic or anionic adsorbate). To tackle such problem, a dual-electronic material is developed by applying several further processes (i.e., modification, treatment, or grafting). After those processes, the dual-electronic material can simultaneously remove both cationic and anionic pollutants from the water environment. For example, Chao and Chen [26] prepared hexadecyltrimethylammonium bromide-(HDTMA-) modified NaY zeolite, concluding that this dual-electronic material can well adsorb both cationic (Cu²⁺, Zn²⁺, Ni²⁺, Pb²⁺, and Cd²⁺) and oxyanionic metal (Cr₂O₇^2– and MnO₄^–) ions in solution. An analogous conclusion was found for other materials—titanate nanotubes modified with hexadecyltrimethylammonium [27] and mesoporous silicas functionalized with amine and nitrilotriacetic acid anhydride [28].

Layered double hydroxides without further modification or treatment process are highly expected to remove both cationic and oxyanionic metal ions from solution through their unique properties. Therefore, in this study, a dual-electronic nanomaterial derived from layered double hydroxides was directly synthesized by a simple coprecipitation method. A hydrothermal treatment was subsequently used to further improve the crystalline structure of LDH. The synthesized nanomaterial was characterized by textual property, surface morphology and functionality, zeta potential, and crystalline structure. It was then applied as a potential adsorbent to adsorb five cationic metals (Cd²⁺, Cu²⁺, Pb²⁺, Ni²⁺, and Cr³⁺) and two oxyanion metals (MnO₄^- and Cr₂O₇^2-) in single aqueous solution. The relevant adsorption mechanism was also discussed herein.

2. Material and Method

2.1. Chemicals

Whole chemicals used in this investigation were of analytical reagent grade, so they were directly used without any further purification. Aluminum nitrate 9-hydrate and magnesium nitrate hexahydrate were bought from Merck. Sodium carbonate and sodium hydroxide puriss were purchased from BAKER. Four selective divalent cation metals include cadmium nitrate tetrahydrate (purchased from Alfa Aesar), nickel(II) nitrate hexahydrate (Merch), copper(II) nitrate trihydrate (Merch), and lead(II) nitrate (Sigma-Aldrich). One trivalent cation metal (chromium(III) nitrate nonahydrate) was delivered from Merch. Meanwhile, two oxyanionic metal ions—potassium permanganate and potassium dichromate—were selected from Sigma-Aldrich and Merch, respectively. Deionized distilled high-purity water was obtained from a Milli-Q water (Millipore) system.

2.2. Preparation of Layered Double Hydroxides

The Mg/Al-LDH nanoparticles were synthesized through a two-step process: coprecipitation and thermal crystallization [29]. Briefly, the mixture solution of Mg(NO₃)₂·6H₂O and Al(NO₃)₃·9H₂O was added dropwise into another (NaOH and Na₂CO₃) under stirring. The pH of the solution was controlled at approximately for 3 h at 45°C to obtain white precipitates. Notably, nitrogen gas was not used during the precipitation process to expect the spontaneous formation of abundant CO₃^2– anions in the interlayer region of LDH. Subsequently, the solution containing the precipitates was transferred into a Teflon-lined autoclave. The autoclave was then heated at 190°C for 24 h (Figure 1). The resultant nanoparticles were collected by centrifugation, washed repeatedly with pure water, dried at 60°C for 48 h, and stored in a desiccator until further use. A primary test was conducted to explore the effect of Mg/Al molar ratios (i.e., 1 : 1, 2 : 1, 3 : 1, 4 : 1, and 5 : 1) on the adsorption capacity of LDH to the selective metals. The adsorption results in Figure 2 indicated that an increase in the Mg/Al ratio from 1 : 1 to 3 : 1 resulted in increasing the adsorption capacity of LDH to Ni²⁺ and Cr₂O₇^2– ions in a single solution. However, a further increase in the ratio to 5 : 1 caused a significant decrease in the adsorption capacity of the LDH towards Ni(II) cation or an insignificant change of the adsorption capacity of the LDH towards Cr(VI) anion. Therefore, the molar ratio of M²⁺ to M³⁺ of 3 : 1 was selected for further studies of the characterization of LDH and the adsorption of potentially toxic metals in solution.

2.3. Characterization of Layered Double Hydroxides

X-ray diffraction (XRD) data were obtained from a PANalytical PW3040/60 X’Pert Pro. Fourier-transform infrared (FT-IR) spectrum was detected by a PerkinElmer 1600 FT-IR spectrophotometer. A scanning electron microscope (SEM, S-3000N, Hitachi) was used to measure the morphological surface property of LDH. The external surface charge of LDH was analyzed by a zeta potential analyzer (Colloidal Dynamics; ZED-3600). The porosity of LDH was calculated from the nitrogen adsorption/desorption isotherm (Micromeritics ASAP 2020 sorptometer) at 77 K. The LDH sample had been dried in a vacuum oven at 105°C for 24 h before it was used for analyzing its aforementioned properties.

2.4. Study of Adsorption Isotherm

The study of adsorption isotherm was concluded in a single metal solution to avoid the phenomenon of competitive adsorption. Adsorption conditions were described as follows: a solid/liquid ratio of 1.0 g/L (), temperature of 30°C, contact time of 48 h, and controlled solution pH of 5.0. The concentration of toxic metal ions before and after adsorption was determined by an Inductively coupled plasma mass spectrometry (ICPMS-NexION 2000, US). The blank sample (the initial concentration of adsorbate without the presence of LDH denoted as the value) was conducted simultaneously. The amount of metal adsorbed by LDH (; mmol/g) was calculated from the mass balance equation (Equation (1)). Each study of adsorption was conducted in triplicate, and the average values were reported.

In this study, the Langmuir model (Equation (2)) [30] was applied to estimate the maximum adsorption capacity of LDH towards each target metal. Meanwhile, the Freundlich model [31] (Equation (3)) is commonly used for the real design of water system because it is an empirical equation. Two adsorption isotherm models have been widely applied in the literature to model the experimental data of adsorption equilibrium because of the help of their parameters [3, 5, 7, 32, 33]. The important role of the parameters of two selective models in the adsorption process was discussed in Section 3.2. where and (mmol/L) are the concentrations of metal in solution before and after equilibrium adsorption, respectively; (g) is the mass of used LDH; (L) is the volume of the metal solution; (mmol/g) is the maximum saturated adsorption capacity of LDH; (L/mmol) is the Langmuir constant related to the affinity between metal and LDH; [] is the Freundlich constant, which characterizes the strength of adsorption; and (dimensionless; ) is a Freundlich intensity parameter, which indicates the magnitude of the adsorption driving force or surface heterogeneity; the adsorption isotherm becomes linear with , favorable with , and unfavorable with .

To minimize error functions during modeling, a nonlinear optimization technique was applied for computing the relevant parameters of the selective models [32]. The adjusted coefficient of determination (Adj-) and chi-square test () were automatically calculated from the Origin software; meanwhile, standard deviation of residues (SD), Marquardt’s percent standard deviation (MPSD) [34], and Bayesian information criterion (BIC) [35] are expressed in Equation (4), (5), and (6), respectively. The best fitting model exhibits the highest Adj- value, but the lowest value of the others (i.e., , SD, MPSD, and BIC). where is the amount of adsorbate in solution adsorbed by adsorbent from the experiment (calculated from Equation (1)); is the amount of adsorbate in solution adsorbed by adsorbent estimated from the selective model; is the number of experimental points used for modelling; and is the number of parameters in the selective model.

3. Results and Discussion

3.1. Property of Layered Double Hydroxides

The structure feature of LDH was confirmed by the XRD spectrum (Figure 3). As expected, the synthesized LDH was a typically well-crystallized material. Two shaped diffraction peaks observed at two-theta degree of 11.44° (a typical 003 characteristic) and 22.82° (006) were consistent with the standard JCPDS file of the hydrotalcite structure (JCPDS No. 89–0460) and in the literature [12, 36]. According to the Bragg’s law, the basal spacing () of LDH was calculated to be 0.773 nm. An identical result was reported for similar Mg/Al-LDH materials (i.e., 0.80 nm [22], 0.780 nm [15], 0.769 nm [19], 0.762 nm [37], 0.760 nm [14], and 0.758 nm [12]) and for the other LDH (Zn/Al-LDH (0.887 nm) [11], Ni/Fe-LDH (0.782 nm) [10], Mn/Mg/Fe-LDH (0.777 nm) [24], and Zn/Al-LDH (0.755 nm) [17]). In addition, because the host CO₃^2– anions were abundantly presented in the interlayer region of LDH (confirming by the XRD spectrum at 29.39°), the synthesized nanoparticles can be classified as LDH containing the interlayer carbonate anions (CO₃-LDH). The carbonate anions can (1) effectively interact with selective cationic metals (i.e., cadmium) in solution to form the metal-carbonate precipitation [5] or (2) provide the exchangeable anion sites towards oxyanionic metals (i.e., dichromate) in solution [6, 7].

The isotherm of nitrogen gas adsorption/desorption onto LDH is illustrated in Figure 4. According to the report of physisorption data for gas/solid systems published by IUPAC [38], the physisorption exhibited a typical characteristic feature of Type IV isotherm, with a type H3 hysteresis loop being observed at a relative pressure () higher than 0.8 [19]. The IV-type isotherm combined with H3-type loop was an indication of mesoporosity with slit-shaped pores [6, 10, 18]. An analogous observation has been reported elsewhere [12, 36]. Furthermore, the textural property of LDH (Table 1) obtained from the physisorption isotherm indicated that LDH was a nonporous material. This is because it exhibited a low Brunauer-Emmett-Teller (BET) surface area () and total pore volume (). Similarly, some scholars reported that LDH solids exhibited a low specific surface area and total pore volume, such as Mg/Al-LDH ( and ) [19], Ni/Fe-LDH (34.2 m²/g and 0.06 cm³/g) [10], Mg/Al-LDH (15.7 m²/g and 0.078 cm³/g) [37], and hierarchical flower-like Mg/Al-LDH (3.58 m²/g and 0.076 cm³/g) [18]. This means that the adsorption of selective metals through well-known pore-filing mechanism seems negligible [6]. In this case, LDH can remove potentially toxic metals from water media through other adsorption mechanisms (e.g., surface precipitation, complexation, and ion exchange).

The main functional groups on the LDH’s surface were qualitatively detected by FTIR (Figure 5). Several corresponding key bands were highlighted and provided in the FTIR spectrum. Firstly, a broad band at approximately 3500 cm^-1 is designated to the –OH group of the hydroxide layers, interlayer water molecules, and even moisture. Secondly, a profound band at nearly 1640 cm^-1 is assigned to the C=O overlapped N=O functional groups in the interlayer region that might mainly correspond to the interlayer CO₃^2– and NO₃^– anions, respectively. Thirdly, the overlapping of C–O (relative to host CO₃^2– anion) and N–O (host NO₃^– anion) vibration is well-identified at around 1370 cm^-1. Lastly, two bands corresponding to the metals (M=Mg and Al) vibrations are observed at approximately 676 cm^-1 (O–M–O) and 420 cm^-1 (M–OH) [6, 12, 36, 37, 39]. The result suggested that (1) LDH might contain the abundant host anions (CO₃^2– and NO₃^–) in the interlayer region and (2) the external surface of LDH might be positively charged because of the abundant presence of –OH groups (primarily derived from Al–OH and Mg–OH) on its surface.

In essence, the value of the hydroxyl group is often higher than 11 [40]. Therefore, the –OH group is protonated into the –OH₂⁺ group when solution . This is well consistent with the analysis result of zeta potential of LDH (Figure 6). The zeta () potential values of LDH were positive, suggesting that the external surface of LDH was highly positively charged within the solution pH range from 3.0 to 12. Similarly, Lee and coworkers [15] reported that the pH values of isoelectric point (IEP) of Mg/Al-LDH, LDH calcinated at 400°C, and LDH calcinated at 600°C were 12.1 (pH_IEP), 12.7, and 12.5, respectively. An identical observation was summarized in a recent review article [6]. Furthermore, LDH had a high positive value at pH 5.0 () and 6.0 () which is consistent with the report of Abo El-Reesh and coworkers [10] for Ni/Fe-LDH (43.3 mV at pH 6.0) and glycerol-modified Ni/Fe-LDH (32.8 mV at pH 6). The result suggested that LDH exhibited a high affinity to anions in solution through electrostatic attraction.

Finally, as portrayed by SEM image (Figure 7), LDH exhibited a plate-like morphology on the surface. This is a typical morphology of synthesized LDH materials. As aforementioned in Section 1, the morphology of LDH strongly depended on the synthesis process and the nature of used metal salts. Different morphologies of LDH were observed in the literature such as the morphology like sheet [22], nanofoil [11], 3D hierarchically flower [12, 18], and interconnecting flower [20]. To some extent, the morphological property of LDH has a less impact on its adsorption capacity compared to the others (i.e., its surface area and charge).

3.2. Adsorption Isotherm

The adsorption isotherms of target potentially toxic metals onto LDH are presented in Figure 8. The pH solutions were maintained at around to avoid the precipitation in the form of metal hydroxide (also known as precipitation by pH). For example, copper metal ions can be spontaneously precipitated in solution in the form of Cu(OH)₂ without being adsorbed by LDH when solution pH values are higher than 6.0. Furthermore, in a low solution pH value (i.e., 2.0), the structure of the synthesis clay might not be stable. This is because the LDH particles were synthesized through the coprecipitation process at a high pH value of 12 (Section 2.1). In other words, the synthesized particles had a low chemical stability under acidic solution.

(a)

(b)

According to the shape classification of adsorption isotherm [41], the shape of the adsorption isotherms of metal cations and oxyanions was categorized as L-type or F-type (not H-type) without a strict plateau. The relationship between isotherm parameters and isotherm shapes has been analyzed by Tran and coworkers [32]. The value of the parameter of the Freundlich model in Table 2 ranged from 0.203 to 0.446 (lower 1.0), suggesting that the adsorption isotherm was favorable. The conclusion was confirmed by the concave downward curve of adsorption isotherm in Figure 8. The outcome suggested that LDH tended to exhibit a higher affinity to the pollutants under high concentrations in solution. Therefore, it should be given a considerable concern when LDH is applied to remove the pollutants under their low concentrations (or trace levels) from the water environment.

The corresponding parameters of the Langmuir and Freundlich models (Table 2) were calculated from the nonlinear optimization method to minimize the error function during modeling. According to a higher adjusted coefficient of determination, the Langmuir model (Adj-–0.994) was more appropriate to describe the experimental data of equilibrium adsorption of the target metals by LDH than the Freundlich model (Adj-–0.962) did. This conclusion is well consistent with the result of other statistics; for example, the value of , SD, MPSD, and BIC of the Langmuir model was lower than that of the Freundlich model (Table 2). As expected, the prepared LDH can effectively adsorb cationic and oxyanionic metal ions in aqueous solutions. This means that it can serve as a promising dual-electronic adsorbent for removing both cationic and anionic pollutants from water media.

In general, the Freundlich model can be applied for estimating the maximum adsorption capacity of an adsorbent when adsorption shape is nearly linear (the ). The linear isotherm shape is commonly known as the partition phenomenon [26]. In this study, the parameter was lower than 1.0; therefore, the maximum adsorption capacity of LDH was estimated through the Langmuir model. Under the same operational conditions of adsorption study, the maximum adsorption capacity () of LDH towards the potentially toxic metals exhibited the following order: 1.299 mmol/g (for Ni²⁺ adsorption) > 0.880 mmol/g (Cd²⁺) > 0.701 mmol/g (Cr³⁺) > 0.657 mmol/g (Pb²⁺) > 0.601 mmol/g (Cu²⁺) > 0.589 mmol/g (Cr₂O₇^2–) > 0.522 mmol/g (MnO₄^–). The result suggested that CO₃-LDH possessed a higher adsorption capacity of cationic metals than anionic ones in solution. This is because the precipitation mechanism between cationic metal ion in solution and CO₃^2– anion in the interlayer region of LDH (i.e., CdCO₃) was more dominant than anion exchange mechanism between anionic metal ion in solution and CO₃^2- anion in LDH. In addition, the cationic metal ions highly tended to react with the –OH groups on the external surface of LDH through nonelectrostatic attraction (inner-sphere complexation).

Because most LDH materials have applied for the removal of hexavalent chromium from water, such adsorbate was selected as a target pollutant for comparison. The Mg/Al-LDH adsorbent ( mg/g) exhibited a higher adsorption capacity towards Cr(VI) that the other LDH materials reported in the literature, such as Mg/Al-LDH (16.3 mg/g) [42], Ni/Fe LDH (50.4 mg/g) [10], Mg/Al-LDH (58.8 mg/g) [37], and Mg/Al-LDH calcinated at 500°C (65.2 mg/g) [12]. Notably, a higher zeta potential value of Mg/Al-LDH (42.7 mV) in this study compared to that of Mg/Al-LDH (~18 mV) reported by other researchers [37] might lead to a higher affinity to Cr(VI) in solution through electrostatic attraction. As a result, the prepared Mg/Al-LDH adsorbent (127.3 mg/g) exhibited a remarkably higher maximum adsorption capacity than the Mg/Al-LDH one (58.8 mg/g) reported by Zhu and colleagues [37]. Interestingly, the synthesized Mg/Al-LDH nanomaterial ( mg/g) possesses an outstanding adsorption capacity compared to activated carbon (103 mg/g) and biochar (83.5 mg/g) [43], HDTMA-modified zeolite (39.7 mg/g) [26], and HDTMA-modified titanate nanotubes (18.6 mg/g) [27].

3.3. Feasible Adsorption Mechanism

In essence, the mechanism of pollutant adsorption is often strongly dependent on the solution pH value. This is because solution pH greatly affects both the species of adsorbate and the surface of adsorbent. In this study, potential adsorption mechanisms were discussed at the solution of pH_Equilibrium 5.0.

As shown in Figure 6, the prepared LDH adsorbent exhibited a positively charged surface because of its positive value of 42.7 mV. Therefore, electrostatic attraction was ruled out for the adsorption of cationic metal ions, but the integral contribution to the adsorption of oxyanionic metal ions onto LDH. Furthermore, a previous study indicated that the abundant presence of CO₃^2– anions in the interlayer region of LDH can spontaneously simulate the precipitation process occurring between cationic metals (i.e., Pb²⁺ and Cu²⁺) and carbonate ions to form carbonate hydroxides (i.e., Pb₃(OH)₂(CO₃)₂ and Cu₂CO₃(OH)₂, respectively) [9, 24]. In addition, Rahman and coworkers [22] analyzed the precipitates (characterized by XRD) after the adsorption process of heavy metals (Cu²⁺, Pb²⁺, and Zn²⁺) onto LDH. They found the important role of host NO₃^2– anions in the interlayer region of Mg/Al-LDH in removing potentially toxic metals. The formed precipitates after the adsorption process included copper nitrate hydroxide [Cu₂(OH)₃NO₃], lead nitrate hydroxide [Pb₄(OH)₄(NO₃)₄], zinc nitrate hydroxide hydrate [Zn(NO₃)(OH)H₂O], and zinc nitrate hydroxide [Zn₅(NO₃)₂(OH)₈]. In this study, the interlayer region of Mg/Al-LDH contained both the host CO₃^2– and NO₃^– anions. Therefore, it is expected that the precipitates in the form of carbonate and nitrate were formed after the adsorption process. This mechanism might be dominant than the others (i.e., isomorphic substitution and complexation).

Notably, LDH abundantly contained the –OH group on its surface; such group can act as an active site to binding cationic metal ions in solution through surface complexation [5]. For example, Zhao and coworkers [20] applied the X-ray photoelectron spectroscopy (XPS) spectrum of Cu 2p and Zn 2p and concluded that the surface complexation occurred between the OH groups on the external surface of Li/Al-LDH and Cu²⁺ or Zn²⁺ ions. An identical conclusion was obtained for the adsorption of Pb²⁺ ions onto Mn/Mg/Fe-LDH [24], Cu²⁺ ions onto sulfonated calix[4]arene intercalated Mg/Al-LDH [21], and Pb²⁺, Cu²⁺, Cd²⁺, and Zn²⁺ onto sulfide-selector intercalated Ni/Fe-LDH [23]. Another feasible mechanism was isomorphic substitution. Zhou and colleagues [24] found that the isomorphic substitution occurring between Pb²⁺in solution and Mg²⁺ in the structure of Mn/Mg/Fe-LDH played an integral role in adsorption mechanism. Isomorphic substitution has been recognized by some other scholars for the adsorption of potentially toxic metals onto LDH-based materials [20].

In contrast, the adsorption process of Cr₂O₇^2– and MnO₄^– anions onto LDH might involve anion exchange between the oxyanions in the solution and the anions (CO₃^2– and NO₃^–) in the interlayer region of LDH. A similar conclusion was reported by other scholars for adsorption of Cr(VI) onto Ni/Fe-LDH [10], polyaniline-modified Mg/Al-LDH [37], in situ synthesized Mg/Al-LDH [13], anisotropic Mg/Al-LDH nanosheets [14], and three-dimensional hierarchical flower-like Mg/Al-LDH [12]. Some detail information on the adsorption mechanism related to anionic exchange has been reported by Goh and coworkers [7]. Lastly, the mechanism of adsorption-coupled reduction has been identified during the process of Cr(VI) adsorption onto LDH-based materials. More detail information on such mechanism has been recently reviewed by Tran and colleagues [6].

4. Conclusions

The layered double hydroxide-based nanoadsorbent was successfully and rapidly synthesized from two low-cost metal salts of Mg(NO₃)₂ and Al(NO₃)₃ through the simple coprecipitation method. The basal spacing of LDH was 0.773 nm. LDH was nonporous material, with S_BET and V_Total being 23.2 m²/g and 0.161 cm³/g, respectively. The surface charge of LDH was positive within solution pH from 3.0 to 12. The CO₃^2– and NO₃^– anions existed abundantly in the interlayer region of LDH.

The Langmuir maximum adsorption capacity () of LDH towards various target toxic metals followed the decreasing order: Ni²⁺ (1.299 ± 0.029 mmol/g) > Cd²⁺ (0.880 ± 0.028 mmol/g) > Cr³⁺ (0.701 ± 0.023 mmol/g) > Pb²⁺ (0.657 ± 0.014mmol/g) > Cu²⁺ (0.601 ± 0.038 mmol/g) > Cr₂O₇^2– (0.589 ± 0.038 mmol/g) > MnO₄^– (0.522 ± 0.015 mmol/g). It can be experimentally concluded that LDH was a promising dual-electronic adsorbent for effectively removing both toxic metal oxyanions and cations from the water environment. However, some further studies should be continued to feasibly apply this material for commercial purposes and real water treatment. They include the studies of adsorption column, regeneration, cost estimation, and toxicity.

Data Availability

No data were used to support this study.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2019.54.

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Copyright © 2020 Van Phuong Nguyen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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