Abstract

To improve the adsorption efficiency of layered double hydroxides (LDHs) for heavy metals, a novel sodium alginate (SA) intercalated MgAl-LDH (SA-LDH) was synthesized in this work. SA-LDH was characterized by XRD, FTIR, XPS and employed as adsorbent for Cd(II), Pb(II), Cu(II) elimination. Adsorbent dosage, initial pH and contact time, which are regarded as several key parameters, were optimized. The results showed that SA-LDH exhibited better adsorption performance compared with the pristine MgAl-LDH. The maximum adsorption capacities of SA-LDH for Cu(II), Pb(II) and Cd(II) reached 0.945, 1.176 and 0.850 mmol/g, respectively. The possible mechanisms were analyzed by XPS, XRD and FTIR. The results showed that Cd(II), Pb(II) and Cu(II) may be removed by SA-LDH via (i) bonding or complexation with Sur-OH or Sur-O- of SA-LDH, (ii) precipitation of metal hydroxides or carbonates, (iii) isomorphic substitution, and (iv) chelation with −COO in the interlayers. This work provides an effective method for the development of LDH-based adsorbent and the treatment of wastewater containing heavy metals.

HIGHLIGHTS

  • SA-LDH had – COO− groups beneficial for the removal of metal ions.

  • Mechanisms included precipitation, complexation, chelation and isomorphic substitution.

  • SA-LDH presented higher removal efficiencies for Cu(II), Pb(II) and Cd(II).

Graphical Abstract

Graphical Abstract
Graphical Abstract

INTRODUCTION

The discharge of heavy metals induces freshwater systems contamination and has attracted more attention in recent years. Heavy metals have been extensively detected in various water systems (i.e. rivers, oceans and lakes), resulting in adverse effects on ecosystem and human health because of their toxicity and persistence (Shang et al. 2019; Wang et al. 2019b). For example, lead is a long-lasting, highly accumulative pollutant, which can amass in organisms by virtue of the food coefficient cycle, usually concentrated in the cardiovascular, neurological, and renal systems (Yan et al. 2020). Various technologies have been explored for the treatment of heavy metals from contaminated water such as ion exchange, chemical precipitation, membrane filtration, electrolysis, adsorption, etc. Among them, adsorption is one of the most promising technologies due to its high removal efficiency and simplicity (Lee et al. 2019).

Various adsorbents (i.e. graphene oxide, chitosan, activated carbon, biopolymers and clays) were developed for heavy metals removal in water (Awad et al. 2017; Rahmanian et al. 2018; Bhatt & Padmaj 2019). Specifically, metal-based materials showed excellent adsorption for many heavy metals (i.e. lead, arsenic, nickel, cadmium, copper, mercury and cobalt (Wen et al. 2018; Jawad et al. 2019). For example, Zheng et al. (2019) found Corncob-supported aluminum-manganese binary oxide composite showed excellent adsorption ability for Cd(II). Recently, layered double hydroxides (LDHs) have emerged as effective adsorbents for heavy metals. As typical LDHs, MgAl-LDHs and MgFe-LDH loaded with magnetic carbon spheres could efficiently adsorb Cd(II), Cu(II) and Pb(II) (Shan et al. 2015; Xie et al. 2019). LDHs structural expression is generally accredited to , where M2+, M3+, and An− represent a divalent and trivalent cation and the interlayer anions (such as NO3, Cl, CO32−), respectively (Wang et al. 2018c), and x typically ranges from 0.17 to 0.33 (Jia et al. 2019). The exchanging interlayer anions provide a feasible scheme for improving the adsorption performance of LDHs. The MoS42− ion was intercalated into MgAl-LDH, and that has five times higher absorption quantities toward Cu(II) and Pb(II) than pristine LDH (Ma et al. 2016). Additionally, the removal efficiency of MgAl-LDH for Cu(II) could be significantly enhanced by introducing EDTA into the interlayer of MgAl-LDH (Shen et al. 2015).

Sodium alginate (SA) is derived from algae, which is a natural polysaccharide and a polymeric flocculant with many free carboxyl groups (−COO) (Ren et al. 2016). Previous studies showed that SA modified materials were superior to multifarious materials for heavy metal elimination (Wang et al. 2019a). For example, poly(acrylic acid)-sodium alginate nanofibrous hydrogels exhibited superior removal efficiency (qm = 591.7 mg/g) towards Cu(II) and outstanding regenerability (Wang et al. 2018a). Wang et al. (2019c) reported that sodium alginate functionalized materials showed a superior affinity and selectivity towards Pb(II). Moreover, Kong et al. reported sodium alginate fibroid hydrogel possessed very high saturated adsorption capacities and efficiently removed Cu(II), Pb(II) and Cd(II) (Kong et al. 2020). Hence, we hypothesize that SA intercalated LDH would show excellent adsorption abilities for heavy metals in water.

As per our information, it is rarely reported that SA was utilized to improve the adsorption performance of LDH for heavy metals. Then MgAl-LDH, which is one of the most used LDHs (Shan et al. 2014), was selected to synthesize the novel SA intercalated MgAl-LDH material (SA-LDH). The obtained LDH material was characterized and systematic batch experiments were carried out to investigate the adsorption properties for Cu(II), Pb(II) and Cd(II). The effects of initial pH, the amount of SA-LDH and contact time, adsorption isotherms, kinetics, and the possible adsorption mechanisms were conducted in this work.

MATERIALS AND METHODS

Chemicals

SA was provided by Macklin Biochemical Co., Ltd (Shanghai, China). Al(NO3)3·9H2O, Mg(NO3)2·6H2O, Cu(NO3)2·3H2O, Pb(NO3)2, Cd(NO3)2·4H2O, NaOH were obtained from Tianjin Damao Reagent Co., Ltd (Tianjin, China). All chemicals in the work were of analytical grade and used without further purification.

Preparation of SA-LDH

SA-LDH was fabricated through a low-saturation coprecipitation strategy according to a previous report (Shi et al. 2020). A salt solution with an Mg/Al molar ratio of 2:1 was prepared by Al(NO3)3·9H2O (0.025 mol) and Mg(NO3)2·6H2O (0.05 mol). 0.15 mol of NaOH and 0.025 mol of C5H7O4COONa were dissolved in distilled water to form an alkaline solution. Then above two mixed solutions were simultaneously dropped to distilled water under stirring, controlling the pH around at 10. The reaction process was in an oil bath at 60 °C and nitrogen atmosphere. After 12 h of dynamic reaction process, the precipitate was aged for 6 h. The resulting mixture was washed thoroughly with distilled water. The final mixture was oven-dried at 60 °C overnight and marked as SA-LDH.

Characterization

Fourier transformed infrared (FTIR) spectroscopy were characterized using a FTIR spectrometer (Perkin-Elmer, USA) in 4,000–400 cm−1 wavenumbers. The X-ray diffraction (XRD) patterns of materials were collected through CuKα radiation at 40 kV (λ = 0.154 nm) on an D8 X-ray diffractometer (Bruker, Germany). X-ray photoelectron spectroscopy (XPS) were measured using Al Kα X-ray on an ESCALAB 250Xi spectrometer (Thermo, USA).

Batch adsorption experiment

Adsorption experiments were performed in a series of 40 mL centrifuge tubes containing 0.05 g SA-LDH and 20 mL solution (200 rpm/min, 25 °C). The effect of dosage, contact time and pH were studied in the experiments. After a stated time, the suspension was centrifuged and filtered, 10 ml of the liquid supernatant were measured by an AA-7000 atomic absorption spectrophotometer (Shimadzu, Japan) to get the ion contents respectively. The adsorption capacity and removal ratio (Hu et al. 2019) were then calculated using Equations (S1) and (S2) in the Supporting Materials.

RESULTS AND DISCUSSION

Characterizations of SA-LDH

The XRD patterns of SA and synthetic SA-LDH were exhibited in Figure 1. It can be observed that the synthetic SA-LDH (Figure 1(b)) had the characteristic peaks of MgAl-LDH (peaks at 2θ = 13.24°, 22.88°, 35.09°, 39.19°, 47.23°, 60.97° and 62.34 corresponding to the (003), (006), (012), (015), (018), (110) and (113) planes) (Zhang et al. 2019) and SA (Figure 1(a)) (peaks at 2θ = 13.58°), indicating that the intercalation of SA into MgAl LDHs was successful. As shown in Figure 2, FTIR spectra of SA (Figure 2(a)) and SA-LDH (Figure 2(b)) are presented. The bands emerged at 1,042, 1,093, 1,411, 1,611 and 2,925 cm−1 in SA-LDH indicated the functional groups of SA were intercalated in the interlayer of MgAl-LDH. The bands located in 1,042 and 1,096 cm−1 resulted from C-O stretching vibrations of polysaccharides or polysaccharide-like substances of alginate (Sun et al. 2018; Yang et al. 2018). The symmetric stretching vibration and structural vibrations of antisymmetrical stretching of −COO were found at 1,411 and 1,611 cm−1 respectively (Li et al. 2019). Moreover, the stretch vibration peak of aliphatic C-H from glucose units in alginate chains was observed at 2,925 cm−1 (Guo et al. 2017).

Figure 1

XRD patterns of (a) SA, (b) SA-LDH before adsorption, SA after adsorption of (c) Cu(II), (d) Pb(II), (e) Cd(II), (□) CdCO3, and (*) Pb3(CO3)2(OH)2.

Figure 1

XRD patterns of (a) SA, (b) SA-LDH before adsorption, SA after adsorption of (c) Cu(II), (d) Pb(II), (e) Cd(II), (□) CdCO3, and (*) Pb3(CO3)2(OH)2.

Figure 2

FTIR spectra of (a) SA, (b) SA-LDH before adsorption and after adsorption of (c) Cu(II), (d) Pb(II), and (e) Cd(II).

Figure 2

FTIR spectra of (a) SA, (b) SA-LDH before adsorption and after adsorption of (c) Cu(II), (d) Pb(II), and (e) Cd(II).

Adsorption behavior

In order to investigate the broader utility and adsorption performance of the synthetic SA-LDH, a series of systematic experiments were performed. Influence of adsorbent dosage and initial solution pH on removal ability of SA-LDH, adsorption kinetics and isotherms were studied systematically.

Effect of dosage

As shown in Figure 3, compared with the pristine MgAl-LDH, SA-LDH exhibited superior sorption efficiencies for Cd(II) (Figure 3(c)), Pb(II) (Figure 3(b)) and Cu(II) (Figure 3(a)) at the same dosage. It was shown that the removal ratios of Cd(II), PbII) and Cu(II) increased significantly as the dosage of adsorbent increased, and then reached the equilibrium values. For example, the adsorption efficiency of Cu(II) increased from 27.12% to 96.83% as the amount of SA-LDH rose from 0.01 g to 0.05 g, then did not increase when the dosage of SA-LDH further increased. A reasonable explanation was that as the dosage of adsorbent increased, more adsorption sites for Pb(II), Cd(II) and Cu(II) are available (Du et al. 2019). Accordingly, the increase of dosage enhanced the agglomeration of the adsorbent, and prevented the increase of adsorption efficiency (Mallakpour & Hatami 2019). Considering the low cost and effectiveness, optimal SA-LDH the dosage was selected as 0.05 g to remove heavy metals in the following tests.

Figure 3

Effect of adsorbent dosage on (a) Cu(II), (b) Pb(II) and (c) Cd(II) adsorption by SA-LDH. Experimental conditions: C0 = 1.5 mmol/L (Cu(II)), 2.5 mmol/L (Pb(II)) and 1.0 mmol/L Cd(II)); t= 240 min; m = 0.01–0.15 g; pH = 5.48 (Cu(II)), 5.54 (Pb(II)), and 6.15 (Cd(II)).

Figure 3

Effect of adsorbent dosage on (a) Cu(II), (b) Pb(II) and (c) Cd(II) adsorption by SA-LDH. Experimental conditions: C0 = 1.5 mmol/L (Cu(II)), 2.5 mmol/L (Pb(II)) and 1.0 mmol/L Cd(II)); t= 240 min; m = 0.01–0.15 g; pH = 5.48 (Cu(II)), 5.54 (Pb(II)), and 6.15 (Cd(II)).

Effect of solution pH

As is well-known, pH is crucial for affecting the removal efficiency of adsorbent for heavy metal ions. It affects both the speciation of the heavy metal ions and the electrostatic interaction between the adsorbate-adsorbent particles, thus affect the adsorption efficiencies of adsorbents (Jawad et al. 2019). As illustrated in Figure 4, the adsorption capacities of SA-LDH for Cu(II) (Figure 3(a)), Pb(II) (Figure 3(b)) and Cd(II) (Figure 3(c)) ions increased with the increase of initial pH values, and the trends of removal efficiency were consistent with equilibrium pH. The relatively low adsorption efficiencies of SA-LDH might due to the protonation of −OH and −COO in SA-LDH at low pH values, resulting in electronic repulsion between heavy metal and adsorptive sites on SA-LDH. Besides, the competitive adsorption of positively charged H3O+ and heavy metal ions also decrease the adsorption ability of SA-LDH at low pH (Wang et al. 2018b). With the increase in pH, the negative charges on SA-LDH surface increased and the H3O+ decreased. Thus, the adsorption capacities of SA-LDH increased because of electrostatic attraction (Xiong et al. 2019). Considering that the precipitation of heavy metal ions can form at high pH values, the original pH values of the prepared Cd(II), Pb(II) and Cu(II) solutions which are 6.15, 5.54 and 5.48, respectively, were not adjusted for the following tests.

Figure 4

Effect of solution pH on the adsorption of (a) Cu(II), (b) Pb(II) and (c) Cd(II) by SA-LDH. Experimental conditions: C0 = 1.5 mmol/L (Cu(II)), 2.5 mmol/L (Pb(II)) and 1.0 mmol/L Cd(II)); t = 240 min; m = 0.05 g; pH = 2.0–6.0 (Cu(II)), 2.0–5.5 (Pb(II)), and 2.0–8.0 (Cd(II)).

Figure 4

Effect of solution pH on the adsorption of (a) Cu(II), (b) Pb(II) and (c) Cd(II) by SA-LDH. Experimental conditions: C0 = 1.5 mmol/L (Cu(II)), 2.5 mmol/L (Pb(II)) and 1.0 mmol/L Cd(II)); t = 240 min; m = 0.05 g; pH = 2.0–6.0 (Cu(II)), 2.0–5.5 (Pb(II)), and 2.0–8.0 (Cd(II)).

Adsorption kinetics

The adsorption capacity qt of Pb(II), Cd(II) and Cu(II) is depicted in Figure 5 as a function of time t. The adsorption reactions of SA-LDH toward Cd(II), Pb(II) and Cu(II) increased rapidly before 90, 240 and 120 min, respectively, and then increased in small increments with the gradually decreased adsorption sites on the SA-LDH.

Figure 5

Effect of contact time on Cu(II), Pb(II) and Cd(II) adsorption by SA-LDH. Experimental conditions: C0 = 1.5 mmol/L (Cu(II)), 2.5 mmol/L (Pb(II)) and 1.0 mmol/L Cd(II)); m = 0.05 g; pH = 5.48 (Cu(II)), 5.54 (Pb(II)), 6.15 (Cd(II)); t = 0–360 min.

Figure 5

Effect of contact time on Cu(II), Pb(II) and Cd(II) adsorption by SA-LDH. Experimental conditions: C0 = 1.5 mmol/L (Cu(II)), 2.5 mmol/L (Pb(II)) and 1.0 mmol/L Cd(II)); m = 0.05 g; pH = 5.48 (Cu(II)), 5.54 (Pb(II)), 6.15 (Cd(II)); t = 0–360 min.

The adsorption behavior of SA-LDH toward Cd(II), Pb(II) and Cu(II) were analyzed using the pseudo-first-order and pseudo-second-order models (Equations (S3) and (S4)). By comparison with the R2 obtained by fitting the kinetic data, the pseudo-second-order model (0.999, 0.996 and 0.999) were all higher than that of the pseudo-first-order model (0.911, 0.954 and 0.895) (Table 1), indicating that the adsorption process all fit well to the pseudo-second-order model. The rate constant k2 arrayed according to the Pb(II) < Cu(II) < Cd(II) order, indicating that the adsorption rates of three heavy metals onto SA-LDH followed the order of Pb(II) < Cu(II) < Cd(II). This result was in accord with the equilibrium time of 90, 120 and 240 min for Cd(II), Cu(II) and Pb(II), respectively. According to the pseudo-second-order model, calculated qe values were in agreement with experimental ones, which were the equilibrium concentrations of heavy metals in the adsorbed SA-LDH (Shi et al. 2020). Therefore, the fitting results fitted well with the pseudo-second-order model, which is an indication that chemisorption occurred in the adsorption process according to the hypothesis of the pseudo-second-order model (Luo et al. 2019; Xie et al. 2019).

Table 1

Calculated parameters of the pseudo-first-order and pseudo-second-order kinetics models for adsorption of Cu2+, Pb2+ and Cd2+ onto SA intercalated Mg-Al-LDH

AdsorbatePseudo-first-order
Pseudo-second-order
qe, exp (mmol/g)qe, cal (mmol/g)k1 (1/min)R2qe, cal (mmol/g)k2 (g/(mmol·min))R2
Cu(II) 0.572 0.131 0.00928 0.911 0.592 0.314 0.999 
Pb(II) 0.925 0.437 0.00917 0.954 0.943 0.0725 0.996 
Cd(II) 0.348 0.0681 0.0111 0.895 0.359 0.702 0.999 
AdsorbatePseudo-first-order
Pseudo-second-order
qe, exp (mmol/g)qe, cal (mmol/g)k1 (1/min)R2qe, cal (mmol/g)k2 (g/(mmol·min))R2
Cu(II) 0.572 0.131 0.00928 0.911 0.592 0.314 0.999 
Pb(II) 0.925 0.437 0.00917 0.954 0.943 0.0725 0.996 
Cd(II) 0.348 0.0681 0.0111 0.895 0.359 0.702 0.999 

Adsorption isotherms

The isotherm data in this study were tested using isotherm equations (Equations (S5) and (S6)) and the results of nonlinear fitting are shown in Figure 6. The fitting parameters are listed in Table 2. The Freundlich and Langmuir isotherm are empirical equations employed to describe heterogeneous and homogeneous adsorption processes (Zhang et al. 2020). Results revealed that Langmuir model of Pb(II) and Cu(II) better described the adsorption experiments than the Freundlich model (Figure 6). It is suggested that the uptake of Pb(II) and Cu(II) were homogeneous adsorption processes but Cd(II) fitted the Langmuir and Freundlich models, indicating the uptake process included heterogeneous and homogeneous adsorption. From fitting data of the Langmuir isotherm, the maximum adsorption capacities of SA-LDH for Cu(II), Pb(II) and Cd(II) were 0.945, 1.176 and 0.850 mmol/g, respectively. Furthermore, a dimensionless invariable RL was a crucial specialty of the Langmuir model as a separation factor (Xia et al. 2019). When the condition is favorable, the value of RL is between 0 and 1. In Table 2, all the RL values ranged between 0 and 1. It can be deduced that the adsorption process of SA-LDH toward three heavy metals were favorable.

Table 2

Parameters of Langmuir and Freundlich equations for adsorption of Cu2+, Pb2+ and Cd2+ onto SA intercalated Mg-Al-LDH

AdsorbateLangmuir equation
Freundlich equation
qm (mmol/g)b (L/mmol)RLR2kf1/nR2
Cu(II) 0.945 8.313 0.0169–0.546 0.994 0.687 0.275 0.463 
Pb(II) 1.176 50.961 0.00163–0.164 0.999 1.0185 0.227 0.493 
Cd(II) 0.850 4.605 0.0416–0.685 0.949 0.587 0.264 0.974 
AdsorbateLangmuir equation
Freundlich equation
qm (mmol/g)b (L/mmol)RLR2kf1/nR2
Cu(II) 0.945 8.313 0.0169–0.546 0.994 0.687 0.275 0.463 
Pb(II) 1.176 50.961 0.00163–0.164 0.999 1.0185 0.227 0.493 
Cd(II) 0.850 4.605 0.0416–0.685 0.949 0.587 0.264 0.974 
Figure 6

Adsorption isotherms of Cu(II), Pb(II) and Cd(II) onto SA-LDH. Experimental conditions: m = 0.05 g; pH = 5.48 (Cu(II)), 5.54 (Pb(II)), and 6.15 (Cd(II)); t = 240 min; C0=0.1–7 mmol/L (Cu(II)), 0.1–12 mmol/L (Pb(II)) and 0.1–5 mmol/L (Cd(II)).

Figure 6

Adsorption isotherms of Cu(II), Pb(II) and Cd(II) onto SA-LDH. Experimental conditions: m = 0.05 g; pH = 5.48 (Cu(II)), 5.54 (Pb(II)), and 6.15 (Cd(II)); t = 240 min; C0=0.1–7 mmol/L (Cu(II)), 0.1–12 mmol/L (Pb(II)) and 0.1–5 mmol/L (Cd(II)).

Moreover, for comparison, data of various adsorbents towards Cu(II), Pb(II) and Cd(II) are presented in Table S1. The adsorption capabilities of the three heavy metals on SA-LDH were equal to or better than the other adsorbents (Kenawy et al. 2018; Li et al. 2018; Sun et al. 2018; Wang et al. 2018c; Kostenko et al. 2019; Shang et al. 2019; Xia et al. 2019; Xie et al. 2019; Zhang et al. 2019).

Adsorption mechanisms

To ascertain the mechanisms involved in Cd(II), Pb(II) and Cu(II) adsorption on SA-LDH, several technologies (XRD, XPS and FTIR) were used to reveal the characteristics of SA-LDH before and after adsorption.

For the XRD spectra of SA-LDH before and after Pb(II) adsorption (Figure 1(d)), a series of new peaks can be detected as Pb3(CO3)2(OH)2 (Jia et al. 2019). Compared to the fresh adsorbent, a new peak presented on the SA-LDH with Cd(II) (Figure 1(e)) was attributed to CdCO3 and the intensity of peaks decreased (Tran et al. 2018; Lyu et al. 2019). However, the XRD patterns after Cu(II) (Figure 1(c)) adsorption was not found in any new phases. Nonetheless, the peaks of Cu 2p, Pb 4f and Cd 3d were obviously presented on the surface of SA-LDH in Figure S1.

The surface chemical compositions of SA-LDH before and after the uptake of Cd(II), Pb(II) and Cu(II) were studied by XPS (Figure S1 and Figure 7). The peaks of Mg 1s, Al 2p, C 1s and O 1s can be observed before and after adsorption in Figure S1, which indicated that the structure of the composite was not significantly changed. As displayed in Figure 7(a), three peaks located at 943.1, 953.7 and 963.3 eV implied the formation of complexations (Sur-OH-Cu(II), Sur-O-Cu(II) or Sur-COO-Cu(II)) (Xie et al. 2019). In addition, eliminating Cu(II) can be devoted to formation of Cu2O and CuO with peaks at 933.4 eV (Yue et al. 2017) and the formation of Cu hydroxides with the peak at 935.3 eV in SA-LDH after adsorption (Sun et al. 2018). In Figure 7(b), we can see the characteristic Pb 4f peaks located at 138.4 and 143.3 eV, indicating the formation of complexations, such as Sur-OH-Pb(II), Sur-O-Pb(II) or Sur-COO-Pb(II). Moreover, the peaks around 138.4 eV were located between the position of Pb carbonates (peak around 138.7 eV) and Pb hydroxides (peak around 137.3 eV), suggesting the presence of both Pb carbonates and Pb hydroxides (Zhou et al. 2018). The results were consistent with the XRD results, suggesting the formation of Pb3(CO3)2(OH)2 (Figure 1(d), PDF-#13-0131). The characteristic Cd 3d peaks located at 405.5 and 412.2 eV in Figure 7(c) imply that Cd carbonate and Cd hydroxide existed in the process of adsorption, which was consistent with the XRD results (Figure 1(e)).

Figure 7

HR-XPS spectra of (a) Cu 2p, (b) Pb 4f and (c) Cd 3d in SA-LDH after heavy metal ions adsorption.

Figure 7

HR-XPS spectra of (a) Cu 2p, (b) Pb 4f and (c) Cd 3d in SA-LDH after heavy metal ions adsorption.

The FTIR spectra of SA-LDH before and after the adsorption reaction were also studied, but no significant differences were found after adsorption (Figure 2(c)–2(e)). Besides, the XPS results demonstrated that atomic ratios of Mg/Al on SA-LDH obviously decreased from 1.44 to 0.63, 1.14 and 0.89 after Cu(II), Pb(II) and Cd(II) uptake. This indicated the release of Mg(II) in SA-LDH into solution in the whole removal processes. Presumably, the isomorphic substitution also made a contribution to adsorption mechanisms (Zhou et al. 2020).

Obviously, Cu(II), Pb(II) and Cd(II) may be removed using SA-LDH via complexation with Sur-OH or Sur-O-, isomorphic substitution, the precipitation of metal hydroxides or carbonates, and chelation with the functional ligand (such as −COO) in the interlayers.

Conclusion

In summary, a novel SA-LDH material was prepared, characterized and applied to Cd(II), Pb(II) and Cu(II) eliminations. The synthetic SA-LDH exhibited good adsorption capacity and the maximum adsorption capacities were 0.945, 1.176 and 0.850 mmol/g for Cu(II), Pb(II) and Cd(II). The Langmuir model can well describe the uptake process of SA-LDH toward Cu(II) and Pb(II), while that for Cd(II) was expressed by the Langmuir and Freundlich models. Kinetic studies indicated that the adsorption process towards heavy metals on SA-LDH followed a pseudo-second-order model. From the detailed XRD and XPS analysis, surface complexation, the precipitation of metal hydroxides or carbonates, isomorphic substitution, and chelation were involved in the adsorption process. The results suggested SA-LDH possessed great potential to eliminate heavy metal in aqueous solutions.

ACKNOWLEDGEMENTS

This work was funded by the Natural Science Foundation of China (21577048) and Shandong Province Water Supply and Drainage Monitoring Center.

DATA AVAILABILITY STATEMENT

All relevant data are included in the paper or its Supplementary Information.

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Supplementary data