Inﬂuence of Salinity on the Removal of Ni and Zn by Phosphate-Intercalated Nano Montmorillonite (PINM)

: The salinity inﬂuence on the adsorptions of Ni and Zn onto phosphate-intercalated nano montmorillonite (PINM) were investigated. Single adsorption isotherm models ﬁtted the single adsorption data well. The adsorption capacity of Ni was higher than that of Zn onto PINM at di ﬀ erent salinities. The single adsorption parameters from Langmuir model ( Q mL and b L ) were compared with the binary adsorption ( Q ∗ mL and b ∗ L ). The Q ∗ mL of Zn was lower than that of Ni. The simultaneous presence of Ni and Zn decreased the adsorption capacities. The single and binary adsorptions onto PINM were a ﬀ ected by the salinity. The competitive Langmuir model (CLM), P-factor, Murali and Aylmore (M − A) models, and ideal adsorbed solution theory (IAST) were satisfactory in predicting the binary adsorption data; the CLM showed the best ﬁtting results. Our results showed that the PINM can be used as an active Ni and Zn adsorbent for a permeable reactive barrier (PRB) in the remediation of saline groundwater. cations in solution and solubility. Binary competitive adsorptions were analyzed by the Langmuir model, M − A model, CLM, P-Factor model and IAST predictions. Adsorption capacities of Ni for Langmuir, D − R, Sips, K − O and H − K models are higher than Zn. The competition between Ni and Zn decreased the adsorption retention on the speciﬁc sites in the adsorbents. The adsorption capacities of Ni and Zn in the Ni / Zn binary system were lower than those in the single system due to competition. The PINM could be used as a sustainable reactive medium in the PRB application for removing Ni and Zn in the presence of salinity.


Introduction
Removal of heavy metal-contaminated groundwater in coastal regions has become an urgent issue due to overuse of the limited amount of freshwater by industrial activities [1]. As industrial activities increase, demand for clean water is also expanding, causing a shortage of freshwater supplies and an increase in wastewater containing heavy metals into the surrounding environment [1,2]. Ni and Zn are commonly found heavy metals in wastewaters from industrial activities including mining, steel processing, electroplating and the production of batteries and paints [2]. Ni and Zn were found at alarming quantities around a metal refinery factory in Korea [3,4]. Furthermore, the Ni was detected up to 171 ppb in the Nakdong River, where refinery wastewater was discharged. Besides Korea, a health risk assessment has been conducted to the Ni-and Zn-contaminated groundwaters from the surrounding industrial areas, steel refineries, and mining sites in the other countries, in order to determine the Ni and Zn concentration limits according to the chronic daily intake (CDI) and the health risk index (HRI) [5][6][7]. Human body Ni and Zn exposures are strongly related to various health effects, ranging from common symptoms such as dermatitis, nausea, and diarrhea [8], to chronic symptoms such as cancer when the Ni and Zn concentration are above their threshold (>3000 µg·L −1 ) [9,10]. In order to meet the need for clean fresh water, several technologies for Ni-and Zn-contaminated groundwater remediation, such as chemical treatment (oxidation), biological processes using bacteria or plants, and physical treatment using permeable reactive barriers (PRBs), have been developed on full-scale [11]. Among these, the physical treatment using PRB is the most cost-effective technique (USD 60-245/ton) compared to the chemical treatment (USD 60-290/ton) and the biological processes

The Adsorbent Preparation and Characterization
The montmorillonite-KSF was purified using hydrogen peroxide (H 2 O 2 ; 30%, Duksan Chemical Co.) followed by washing using distilled and deionized (DDI) water (MilliporeSigma™ Synergy™ Ultrapure Water Purification System, Thermo Fisher Scientific, Waltham, MA, USA) at 60 • C, and drying at 60 • C for 24 h. PINM was synthesized from the purified montmorillonite mixed with 2000 mg/L of PO 4 3− (KH 2 PO 4 ; >98%, Yakuri Pure Chemicals Co.) using a rotary agitator in room temperature at 200 rpm for 24 h, followed by three times washing using 1 L of DDI water to remove excess H 2 PO 4 − ions, and air-dried for 3 days [21]. The physicochemical properties of the adsorbents were characterized. Determination of pH of point of zero charge (pH PZC ) of PINM was conducted by following the method by Ma et al. [21]. Cation exchange capacity (CEC) of PINM and montmorillonite were measured by the standard method [24]. Brunauer-Emmett-Teller (BET) surface area (A BET ) was determined from N 2 adsorption/desorption isotherm data (ASAP-2010 specific surface area analyzer, Micromeritics, Norcross, GA, USA) and fitted to the BET model. Pore size distribution was calculated using the Barrett Joyner Halenda (BJH) adsorption model (Quantachrome, Autosorb-iQ & Quadrasorb Si, Boynton Beach, FL, USA) and a specific surface area analyzer (Quantachrome, Nova, 2000, Boynton Beach, FL, USA). Scanning electron microscopy (SEM, Hitachi S−4200, Chiyoda City, Tokyo, Japan) was used to determine the morphology of montmorillonite before and after modification. The chemical composition of PINM and montmorillonite were characterized by EDS analysis (E−MAX EDS detector, Horiba, Kyoto, Japan). X−ray diffraction (XRD) patterns were measured using an X-ray diffractometer (PW2273 diffractometer, Philips, Guildford, UK) using Cu Kα radiation (λ = 1.54 Å) in the range from 5 • to 50 • of 2θ at a step size of 0.02 • and a step time of 1 s.

Adsorption Isotherm Experiments
For single adsorption, the experiments were performed in 50 mL conical centrifuge tube (polyethylene, SPL Labware, Pocheon-si, Gyeonggi-do, Korea) at 25 • C. Firstly, 1.0 g of PINM was prepared in 50 mL tubes. The solution of Ni 2+ and Zn 2+ dissolved in artificial seawater (30% ) and DDI water (0% ), respectively, were added into the tubes.  [25]. To prevent the formation of metal hydroxides and carbonates, the solution pH was controlled. The program of MINEQL + version 4.6 for Windows (Environmental Research Software, Hallowell, ME, USA) was used to predict the molar distributions of nickel and zinc species at pH 5.0. To eliminate the carbon dioxide effect on adsorption, the headspace in the vials were minimized. The sample was mixed using a tumbler at 10 rpm for 24 h. Preliminary kinetic experiments showed that adsorption equilibrium was reached within 3 h. However, adsorption experiments were conducted for 24 h throughout this study to ensure adsorption equilibrium. The vials were collected and followed by the centrifugation at 3000 rpm (=1977 g) for 20 min. Then, the supernatant was filtered using 0.2 µm syringe filter (Whatman, cellulose nitrate membrane filter, φ = 25 mm). The preliminary study showed that the cellulose nitrate membrane filter had no effect on sorption of the Ni and Zn. The Ni and Zn concentration in the aqueous phase was analyzed using an inductively coupled plasma optical emission spectrometry (ICP-OES, Optima 2100 DV, Perkin-Elmer Co., Industry Drive Pittsburgh, PA, USA). Duplicate experiments were performed.
Metal solution at the same molar concentration (0.0170, 0.085, 0.170, 0.340, 0.681, 1.022, 1.363 and 1.704 mM) in a 1:1 volume ratio for each solute were prepared for binary adsorption experiments (Ni/Zn). The adsorption experiment in the binary system were performed in the same manner as those in the single system.

Isotherm Equations and Fitting Method
Single and binary adsorption models are summarized in Table 2. Table 2. Adsorption isotherm models.

Model Equation Reference
Single Adsorption

Adsorbent Characteristics
The raw montmorillonite and PINM physicochemical properties were compared. The A BET and pore volume remarkably increased from 2.6 m 2 /g to 115.9 m 2 /g and from 0.011 cm 3 /g to 0.1 cm 3 /g, respectively, due to the strongly adsorbed phosphate ions through the formation of Al-O-P-OH surface precipitates and an inner-sphere complex [32]. The pore size also slightly increased from 38.05 Å to 40.73 Å. As shown in Figure 1, SEM images showed that the raw montmorillonite particle size was bigger than that of PINM. However, the CEC of PINM (58.1 meq/100 g) was higher than that of raw montmorillonite (52.7 meq/100 g). Due to the H + displacement, the pH PZC values of raw montmorillonite decreased from pH 5.5 to 4 after phosphate-intercalation. The results of chemical analysis were obtained by EDS data, presented in Figure 1. In the intercalation process, Ca in the montmorillonite (Figure 1c) disappeared and was replaced by P in the PINM (Figure 1d). The O in the montmorillonite was slightly reduced in the PINM. However, the Si spectra did not change. This result indicated a successful modification of montmorillonite to become PINM, which was in good agreement with the literature [21]. The X−ray diffraction (XRD) patterns of the PINM and montmorillonite are illustrated in Figure 2. As the phosphate-intercalation expanded the interlayer spaces [21], the basal spacing of PINM (15.33 Å) was larger than that of raw montmorillonite (12.07 Å). These results successfully confirm the phosphate-intercalated nano montmorillonite. Crystal size of montmorillonite and PINM were calculated at optimum peak intensity of montmorillonite (19.8 • ) and PINM (19.8 • ) using the Scherrer equation, where the crystal size of montmorillonite (322.7 nm) was bigger than that of PINM (34.2 nm). This result suggests that the modification using phosphate decreased the montmorillonite crystal size.

Ni and Zn Adsorption
The effects of saline water on the adsorption capacities of single adsorption, Ni and Zn onto PINM at pH 5.0 were expressed in Figure 3. The single adsorption data were fitted by 2-parameter isotherm models (the Freundlich, Langmuir, and D−R models) and the model parameters for PINM are listed in Table 3. All models were fitted well to the experimental data (Freundlich: 0.95 < R 2 < 0.99, Langmuir: 0.98 < R 2 < 0.99, and DR: 0.97 < R 2 < 0.98). The Freundlich isotherm has been extensively used to define adsorption of heavy metal ions onto clay [38] and considers the adsorption affinity and nonlinearity. This shows that the adsorption of Ni and Zn mainly occurred onto the PINM surface active sites. The K F value for 0% was higher than K F value for 30% at pH 5.0 due to less solubility in saline water. The K F values of Zn were consistently higher than those of Ni. The Freundlich exponent, N F , is the heterogeneity factor indicate as 0.1 < N F < 1; favorable adsorption process [39,40]. The N F values for adsorptions of Ni and Zn onto adsorbents were in the range of 0.39−0.46, indicating that both Ni and Zn adsorption was favorable [41]. In the previous studies, the adsorption capacities of montmorillonite and various modified montmorillonite were compared in Table 4. The maximum adsorption capacity (Q mL ) of Ni and Zn at 0% in this study was higher than that of Ni and Zn at 30% . The Q mL values of Ni were higher than those of Zn. The Langmuir parameter, named separation factor (S f ) describes that adsorption isotherm can be favorable (0 < S f < 1), unfavorable (S f > 1), linear (S f = 1), or irreversible (S f = 0) [42,43]: The calculated values of S f ranged 0.90 and 0.94 (Table 3), indicating that adsorptions of Ni and Zn onto PINM are favorable.
The D−R model parameter also fitted well to the single adsorption data (0.97 < R 2 < 0.98). The Q mD value of the D−R model increased in the order of 0% > 30% for both Ni and Zn at pH 5.0. The Q mD values of D−R model were slightly less than the Q mL values of the Langmuir model ( Table 3). The value of E in D−R model can be used to differentiate the adsorption mechanisms (physical or chemical). When the E value is in the range of 8 to 16 kJ/mol, the adsorption occurs by ion-exchange. The E value less than 8 kJ/mol indicates the physical adsorption process, whereas E value greater than 16 kJ/mol, the adsorption is chemical [44]. The calculated E values in this study were less than 4.6 kJ/mol indicating that adsorption of Ni and Zn onto PINM occurs via physical adsorption in nature [27].
The comparison of adsorption capacity by Freundlich adsorption coefficient (K F ), Langmuir adsorption capacity (Q mL ) and D−R adsorption capacity (Q mD ) in Table 3, Ni was higher than Zn for both 0% and 30% containing PINM at pH 5.0. The maximum adsorption capacity (Q mS of the Sips model, Q mK of K−O model and Q mK of H−K model) at 0% was higher than those of 30% for both metals. The maximum capacity of values of Ni were higher than Zn at the same salinity. Several literatures have reported similar results (adsorption capacities of Ni > that of Zn) for adsorbents such as Na-montmorillonite [18]. The adsorption affinity of Ni and Zn decreased with the increase salinity (30% ) owing to competition between the added metals and cations in background solution by the limited cation exchange sites [45][46][47]. Table 3 summarizes the comparison of Ni and Zn sorption capacities of Langmuir model for various adsorbents found in literature. The Q mL values of Ni onto PINM in this study were slightly higher than those of Ni onto montmorillonite in the literature [48,49].

Ni/Zn Adsorption
Binary adsorption of Ni and Zn onto PINM at different salinities were presented in Figure 4. The binary adsorption data of Ni and Zn onto PINM and the predictions of M−A, CLM, P-factor and IAST models are shown in Figure 4 (Table 5).  In Table 5, the M−A model predicted the binary adsorption well (0.75 < R 2 < 0.98). In the M−A model fitting results, the competition factor (a ij ) of Zn (a 21 = 0.30) was consistently higher than that of Ni (a 12 = 0.05) at 0% , which explains that Zn was more affected than Ni in binary competitive adsorption. In contrast to the results at 0% , the a 12 (0.58) was higher than the a 21 (0.33) at 30% . The values of a 12 and a 21 increased from 0.05 to 0.58 and from 0.30 to 0.33 as the salinity increased from 0 to 30% . This indicates that the competition effect between the two metals are more affected by the presence of co-solutes (Na + , K + , Ca 2+ , and Mg 2+ ) in the seawater [51]. The CLM prediction was well fitted with the binary adsorption in terms of R 2 values (0.86 < R 2 < 0.93). The same is true for the P-factor model prediction (R 2 > 0.89), except Zn at 30% . The prediction using IAST varied with single adsorption model and metal solution. For the most binary adsorptions, the IAST predictions were in good agreement with data (Table 5).
Compared to the single adsorptions (Table 3), the Q mL values of binary adsorptions were reduced due to competition (Table 6). In both single and binary adsorptions, the Langmuir parameters, Q mL and b L , were not correlated. In Table 7, the estimated maximum adsorption capacity values of binary adsorption (Q * mL ) were compared with those of single adsorption (Q mL ). The Q mL,Ni /Q mL,Zn and Q * mL,Ni /Q * mL,Zn ratios were higher than unity at both 0% and 30% . This suggests the higher adsorption affinity of Ni than Zn, regardless of salinity. The Q mL,i /Q * mL,i ratios were mostly less than unity, indicating the simultaneous presence of both Ni and Zn reduced adsorption due to competition for adsorption sites in the adsorbent. In addition, Q mL,Ni /Q * mL,Ni > Q mL,Zn /Q * mL,Zn at 0% but vice versa at 30% indicates that Ni adsorption was more affected than Zn adsorption at 0% but vice versa at 30% in binary adsorption process in the simultaneous presence of a co-solute.   The affinity constant (b L ) of Langmuir model was calculated from adsorption isotherm data to estimate the free energy change of adsorption [38,52]. Higher b L values are related to specifically adsorbed metals at high energy surfaces with low dissociation constants. Meanwhile, lower b L values were related to adsorption at low energy surfaces with high dissociation constants [53]. The binding energy coefficient (b L,Ni and b L,Zn for single adsorption and b * L,Ni and b * L,Zn for binary adsorption, respectively) varied with salinity and metal solution. In the single adsorption, the adsorption affinity of Zn was higher than Ni (b L,Zn > b L,Ni ) at both 0% and 30% . On the other hand, b * L,Zn > b * L,Ni at 0% and b * L,Zn < b * L,Ni at 30% were observed for binary competitive adsorption. It was also found that b i > b * i at 0% , whereas b i < b * i at 30% . This indicates that co-solute present in the seawater may affect the adsorption affinity of the metals onto PINM.

Conclusions
Effect of salinity on the adsorptions of Ni and Zn onto PINM have been investigated using single and bimary systems at pH 5. In single adsorption, Freundlich, Langmuir, D−R, Sips, K−O and H−K models were fitted well. The adsorption affinity (K F ) and capacities (Q mL , Q mD , Q mS , Q mKO , and Q mHK ) of Ni were consistently higher than Zn at different salinities. The adsorption capacities of Ni and Zn at 0% were slightly higher than at 30% , mainly owing to the competition between the metals and cations in solution and solubility. Binary competitive adsorptions were analyzed by the Langmuir model, M−A model, CLM, P-Factor model and IAST predictions. Adsorption capacities of Ni for Langmuir, D−R, Sips, K−O and H−K models are higher than Zn. The competition between Ni and Zn decreased the adsorption retention on the specific sites in the adsorbents. The adsorption capacities of Ni and Zn in the Ni/Zn binary system were lower than those in the single system due to competition. The PINM could be used as a sustainable reactive medium in the PRB application for removing Ni and Zn in the presence of salinity.  absolute temperature (K) Q mD theoretical saturation capacity (mg/kg) Q mG maximum adsorption capacity of the adsorbent (mmol/g) Q mHK constant in the H−K model [(L N KC mmol 1−N KC /g)] Q mL maximum adsorption capacity (mmol/kg) Q mL,I maximum adsorption capacity for component i in a single system (mmol/g) Q * mL,i maximum adsorption capacity for component i in a single system (mmol/g) Q mS maximum adsorption capacity (mmol/g) π spreading pressure.