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

Institute of Research and Development, Duy Tan University, Danang 550000, Vietnam Faculty of Environmental and Chemical Engineering, Duy Tan University, Da Nang City, Vietnam Department of Environment Sustainable Development, An Giang University, An Giang, VNU-HCM, Vietnam Department of Management Science, Thu Dau Mot University, Binh Duong, Vietnam Institute of Environmental Technology, Vietnam Academy of Science and Technology, Hanoi, Vietnam Department of Crop Science, College of Agriculture, Can Tho University, Can Tho, Vietnam Hanoi University of Natural Resources and Environment, Ministry of Natural Resources and Environment, Hanoi, Vietnam Institute of Fundamental and Applied Sciences, Duy Tan University, Ho Chi Minh City, Vietnam


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 13% ± 2:5, 17% ± 7:4, 34% ± 8:4, and 41% ± 0:95 for each cycle of adsorption/desorption.
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 (Cu 2+ , Zn 2+ , Ni 2+ , Pb 2+ , and Cd 2+ ) and oxyanionic metal (Cr 2 O 7 2and MnO 4 -) 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 2 Journal of Nanomaterials 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 (Cd 2+ , Cu 2+ , Pb 2+ , Ni 2+ , and Cr 3+ ) and two oxyanion metals (MnO 4 and Cr 2 O 7

2-
) in single aqueous solution. The relevant adsorption mechanism was also discussed herein.

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 9hydrate 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.

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(NO 3 ) 2 ·6H 2 O and Al(NO 3 ) 3 ·9H 2 O was added dropwise into another (NaOH and Na 2 CO 3 ) under stirring. The pH of the solution was controlled at approximately 12 ± 0:3 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 CO 3 2anions 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 Ni 2+ and Cr 2 O 7 2ions 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 M 2+ to M 3+ of 3 : 1 was selected for further studies of the characterization of LDH and the adsorption of potentially toxic metals in solution.

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  Step 1: preparation of LDH by the co-precipitation method 3 Journal of Nanomaterials dried in a vacuum oven at 105°C for 24 h before it was used for analyzing its aforementioned properties.

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 (m/V), 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 C o value) was conducted simultaneously. The amount of metal adsorbed by LDH (q e ; 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 C o and C e (mmol/L) are the concentrations of metal in solution before and after equilibrium adsorption, respectively; m (g) is the mass of used LDH; V (L) is the volume of the metal solution; Q o max (mmol/g) is the maximum saturated adsorption capacity of LDH; K L (L/mmol) is the Langmuir constant related to the affinity between metal and LDH; K F [ðmmol/gÞ/ðL/mmolÞ n ] is the Freundlich constant, which characterizes the strength of adsorption; and n (dimensionless; 0 < n < 1) is a Freundlich intensity parameter, which indicates the magnitude of the adsorption driving force or surface heterogeneity; the adsorption isotherm becomes linear with n = 1, favorable with n < 1, and unfavorable with n > 1.
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-R 2 ) and chisquare test (χ 2 ) 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-R 2 value, but the lowest value of the others (i.e., χ 2 , SD, MPSD, and BIC).
BIC = n ln ∑ q e,exp − q e,model 2 n 2 6 4 where q e,exp is the amount of adsorbate in solution adsorbed by adsorbent from the experiment (calculated from Equation (1)); q e,model is the amount of adsorbate in solution adsorbed by adsorbent estimated from the selective model; n is the number of experimental points used for modelling; and p is the number of parameters in the selective model.

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 (d 003 ) of LDH was calculated to be 0.773 nm. An identical result was reported for similar
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 (p/p o ) 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 (S BET = 23:2 m 2 /g) and total pore volume (V Total = 0:161 cm 3 ). Similarly, some scholars reported that LDH solids exhibited a low specific surface area and total pore volume, such as Mg/Al-LDH (S BET = 51:0 m 2 /g and V Total = 0:236 cm 3 /g) [19], Ni/Fe-LDH (34.2 m 2 /g and 0.06 cm 3 /g) [10], Mg/Al-LDH (15.7 m 2 /g and 0.078 cm 3 /g) [37], and hierarchical flower-like Mg/Al-LDH (3.58 m 2 /g and 0.076 cm 3 /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 CO In essence, the pKa value of the hydroxyl group is often higher than 11 [40]. Therefore, the -OH group is protonated into the -OH 2 + group when solution pH < its pKa value. This is well consistent with the analysis result of zeta potential of      Figure 6). The zeta (ζ) 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 ζ value at pH 5.0 (42:5 ± 3:5 mV) and 6.0 (44:2 ± 1:5 mV) 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).

Adsorption
Isotherm. The adsorption isotherms of target potentially toxic metals onto LDH are presented in Figure 8. The pH solutions were maintained at around 5:0 ± 0:2 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.
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 n 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-R 2 = 0:985-0.994) was more appropriate to describe the experimental data of equilibrium adsorption of the target metals by LDH than the Freundlich model (Adj-R 2 = 0:853-0.962) did. This conclusion is well consistent with the result of other statistics; for example, the value of χ 2 , 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 dualelectronic 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 n parameter ≈ 1:0). The linear isotherm shape is commonly known as the partition phenomenon [26]. In this study, the n parameter was lower than 1.0; therefore, the   ). The result suggested that CO 3 -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 CO 3 2anion in the interlayer region of LDH (i.e., CdCO 3 ) was more dominant than anion exchange mechanism between anionic metal ion in solution and CO 3 2anion 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).

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 ζ 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 CO 3 2anions in the interlayer region of LDH can spontaneously simulate the precipitation process occurring between cationic metals (i.e., Pb 2+ and Cu 2+ ) and carbonate ions to form carbonate hydroxides (i.e., Pb 3 (OH) 2 (CO 3 ) 2 and Cu 2 CO 3 (OH) 2 , respectively) [9,24]. In addition, Rahman and coworkers [22] analyzed the precipitates (characterized by XRD) after the adsorption process of heavy metals (Cu 2+ , Pb 2+ , and Zn 2+ ) onto LDH. They found the important role of host NO 3 2anions in the interlayer region of Mg/Al-LDH in removing potentially toxic metals. The formed precipitates after the adsorption process included copper nitrate hydroxide [Cu 2 (OH) 3   7 Journal of Nanomaterials 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 Xray 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 Cu 2+ or Zn 2+ ions. An identical conclusion was obtained for the adsorption of Pb 2+ ions onto Mn/Mg/Fe-LDH [24], Cu 2+ ions onto sulfonated calix [4]arene intercalated Mg/Al-LDH [21], and Pb 2+ , Cu 2+ , Cd 2+ , and Zn 2+ 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 Pb 2+ in solution and Mg 2+ 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 Cr 2 O 7 2and MnO 4 anions onto LDH might involve anion exchange between the oxyanions in the solution and the anions (CO 3 2-and NO 3 -) 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 threedimensional hierarchical flower-like Mg/Al-LDH [12]. Some detail information on the adsorption mechanism  Journal of Nanomaterials related to anionic exchange has been reported by Goh and coworkers [7]. Lastly, the mechanism of adsorptioncoupled 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].

Conclusions
The layered double hydroxide-based nanoadsorbent was successfully and rapidly synthesized from two low-cost metal salts of Mg(NO 3 ) 2 and Al(NO 3 ) 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 m 2 /g and 0.161 cm 3 /g, respectively. The surface charge of LDH was positive within solution pH from 3.0 to 12. The CO 3 2-and NO 3 anions existed abundantly in the interlayer region of LDH.

Data Availability
No data were used to support this study.

Conflicts of Interest
The authors declare that they have no conflicts of interest.