High uptake of Cu2+, Zn2+ or Ni2+ on calcined MgAl hydroxides from aqueous solutions: Changing adsorbent structures

Chemical Engineering Journal 272 (2015) 17–27

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High uptake of Cu2+, Zn2+ or Ni2+ on calcined MgAl hydroxides from aqueous solutions: Changing adsorbent structures Mingming Sun, Yuxin Xiao, Lin Zhang, Xue Gao, Wenbao Yan, Dongming Wang, Jixin Su ⇑ School of Environmental Science and Engineering, Shandong University, Jinan 250100, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 High affinity and adsorption capacity

of CHTs were in order of Zn2+ > Ni2+ > Cu2+.  Pseudo-second-order and Langmuir models well described kinetic and isotherm data. 2+ 2+  The uptake process for Cu , Zn or Ni2+ was spontaneous and endothermic.  The adsorbent could be used circularly 4 times efficiently.  Final removal of metal ions by reassembling hydrotalcites structures.

a r t i c l e

i n f o

Article history: Received 2 January 2015 Received in revised form 2 March 2015 Accepted 3 March 2015 Available online 11 March 2015 Keywords: Calcined MgAl hydroxides Potentially toxic metal ions Adsorption Regeneration Isomorphous substitution

a b s t r a c t Although previously a variety of studies have proposed the removal of anions and organic matters from contaminated water by calcined hydroxides (CHTs), their role in uptaking potentially toxic metal ions from effluents had rarely been investigated. In the present study, simulated wastewater containing Cu2+, Zn2+ or Ni2+ was used to investigate the adsorption performance of CHTs. Langmuir and Freundlich isotherm models were employed to fit the equilibrium experiments, and it was found that the Langmuir model was more appropriate to describe the adsorption isotherm. The maximum adsorption amount was higher than some other adsorbents, specifically, being 6.583, 7.535 and 6.152 mmol/g for Cu2+, Zn2+ or Ni2+ under the proposed conditions. For kinetic data, the pseudo-second-order kinetic model appeared to be the best-fitting model compared to the pseudo-first-order and Elovich models. Thermodynamic analysis revealed that Cu2+, Zn2+ or Ni2+ sorption on CHTs was spontaneous and endothermic. In the reusability study, the sorption capacity of the adsorbent did not vary remarkably in the initial four sorption/calcination cycles for Cu2+, Zn2+ or Ni2+ removal. By virtue of X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron dispersive X-ray analysis (EDX), it was speculated that the adsorption mechanism for Cu2+, Zn2+ or Ni2+ consisted of two steps: First, potentially toxic metal ions formed into hydroxide precipitations and adhered to the surfaces of adsorbents with high alkalinity. Second, the hydroxides participated in the process of adsorbents reconstructed the hydrotalcites structures through isomorphous substitution. Ó 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel./fax: +86 531 88362008. E-mail address: [email protected] (J. Su). http://dx.doi.org/10.1016/j.cej.2015.03.009 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.

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M. Sun et al. / Chemical Engineering Journal 272 (2015) 17–27

1. Introduction Environmental problems have been a major global concern for the last few decades [1]. Increasing worldwide water contamination has become one of significant environmental problems. Specifically, potentially toxic metals, such as copper, zinc, nickel, cadmium and lead, etc., which cannot be decomposed may gradually accumulate in environment and living organisms, and consequently result in severe diseases or even death. There are wide industrial sources of these potentially toxic metals, such as metal fabrication, mining, plating, batteries, paints, and so on. Therefore, wastewater regulations should be toughed to promote the development of efficient processes. The remediation of potentially toxic metals in wastewater urgently needs to be resolved [2,3]. So far, various treatment technologies have been employed for disposing potentially toxic metals in effluents, including chemical precipitation, adsorption, filtration, reverse osmosis, evaporation, etc [4,5]. Among them, adsorption is one of the most promising techniques due to its advantages of operation simplicity, high efficiency, low cost and potential for regeneration [6]. Moreover, adsorption reactions are prone to controlling the rate and transporting of metal ions. Due to the large surface area, high porosity and specific structure, layered double hydroxides (HTs) have been extensively utilized as efficient adsorption materials for adsorbing various anions [7–9] and metal cations [10–12]. Modified HTs with chelating ligands such as humate hybrid [13], diethylenetriaminepentaacetic acid (DTPA) [14] and ethylenediaminetetraacetate (EDTA) [15] were extensively studied recently in terms of metal cations adsorptions, limited promotion on the adsorption efficiency was found compared to HTs. Since calcined HTs (CHTs) possess a number of active sites, large surface area and the character of refactoring hydrotalcites structures in aqueous solution, they demonstrate higher adsorption capacity of anions than HTs, which had been confirmed in many previous studies [16,17]. It is worth noting that little attention has been paid to the removal of metal cations by CHTs [11], although CHTs may possess higher removal efficiency for metal ions comparing with HTs. The calcinations of HTs can greatly promote the adsorptions of potentially toxic metals ions, as reported by Yuan et al. [18]. In order to provide more relevant evidences and to remove metals ions more effectively, the studies on remediating wastewater containing potentially toxic metals ions by CHTs became important. This work committed to the adsorption of metal cations (Cu2+, Zn2+ or Ni2+) by CHTs, which are obtained by calcining HTs. The HTs is a class of two-dimensional nanostructure anionic clays. The crystals of HTs are composed of brucite-like layers (Mg(OH)2), where the divalent magnesium cations locate in the center of edge-sharing octahedral with hydroxyl anions at their vertices and planar, and neutral layers were formed [17,19]. When divalent metal ions are substituted by trivalent metal ions, the lamella shows positive residual charge. According to the charge neutrality principle, anions intercalate the interlayer regions together with water molecules, which promoted stacking of the sheets. Finally, a hydrotalcites type structure was formed. CHTs, as one class of mixed metal oxides, could utilize anions and metal cations in aqueous solution to reconstruct the lamellar structures of HTs, which was called ‘‘memory effect’’ [20]. When utilizing CHTs in purifying wastewater with anions or potentially toxic metals ions, metal cations could be removed by incorporating new sheets with isomorphous substitution, and anions could be removed by chelating or intercalating the interlayer regions with electrostatic attractions. The general formula of HTs and CHTs can be expressed as [M1x2+ Mx3+ (OH)2] (An) x/nyH2O and (M1x2+ Mx3+ O1+x/2), respectively, where, M2+ and M3+ are divalent

and trivalent cations, An is the n-valent anion, and x is equal to the value of M3+/(M2++M3+) [21]. The uptake process of divalent cations (N2+) on CHTs can be expressed by Eq. (1) [22,23]: n M21x þ M3þ þ ðm þ 1 þ x=2ÞH2 O x O1þx=2 þ ðx=n þ 2y=nÞA  2þ 3þ þ yN2 ! M2þ 1x Ny Mx ðOHÞ2 Aðxþ2yÞ=n  mH2 O þ xOH

ð1Þ

The objective of this study was to evaluate the adsorption capacity of Cu2+, Zn2+ or Ni2+ on MgAl CHTs in single metal cation aqueous system. Adsorption equilibrium, kinetics and thermodynamics were analyzed to understand the adsorption mechanism and adsorption capacity for potentially toxic metals ions. The regeneration performance, one of the crucial characteristics for adsorbents in practical remediation applications, was also investigated. Moreover, the adsorption mechanisms for metal ion were further explored and described in detail. 2. Materials and methods 2.1. Materials All the chemicals used were of analytical pure. All water was deionized. The synthesized nitrate solutions of copper, zinc and nickel, with the concentration of 10,000 mg/L, were used to stock solutions. The HNO3 or NaOH solution (0.1 mol/L) was prepared to adjust the pH of solutions. 2.2. Synthesis of MgAl HTs and MgAl CHTs The Mg3Al–CO3 HTs were prepared by co-precipitation method [24]. In this method, the 400 mL solution containing Mg(NO3)6H2O (0.12 mol) and Al(NO3)39H2O (0.04 mol) was added drop wise to 400 mL of deionized water at room temperature. Simultaneously, the another mixture solution containing 1.6 mol of NaOH and 0.1 mol of Na2CO3 in 250 mL of water was added with vigorously stirring [19]. The pH of the reaction mixture was maintained at 10 ± 0.5. After stirring 2 h, the resulting slurries were separated by vacuum filtration, and then were aged at 80 °C for 24 h. The content of hydrotalcites was obtained by drying the above products. The calcined production (CHTs) was prepared by heating hydrotalcites (HTs) in a muffle furnace at 450 °C for 2 h. 2.3. Adsorption kinetics Kinetics experiments were carried out at specific reaction time intervals of 0, 10, 30, 50, 90, 120, 180, 240 and 300 min in a thermostat shaker under constant shaking (150 rpm) at 25, 35 and 50 °C. The initial concentration of Cu2+, Zn2+ or Ni2+was 200 mg/L, the volume of solution was 100 mL and the sorbent dosage was 0.5 g/L. Initial pH value of the solution was adjusted by 0.1 mol/L HNO3 or NaOH solution. Under a certain reaction time and temperature, 2 mL of reaction solution was taken and filtered by 0.22 lm membrane. The residual metal ion concentration in the filtrate was analyzed by flame atomic absorption spectrophotometer. The uptake quantity of Cu2+, Zn2+ or Ni2+ on CHTs at time t was determined by the following equation [25]:

qt ¼ ðC o  C t ÞV=m

ð2Þ 2+

2+

2+

where qt is the quantity of Cu , Zn or Ni adsorbed on CHTs at the time t (mg/g), qe = qt when adsorption reaches equilibrium, V is the volume of solution (L), Co and Ce are the initial and the time t concentration of Cu2+, Zn2+ or Ni2+ (mg/L), respectively, m is the mass of adsorbent (g).

M. Sun et al. / Chemical Engineering Journal 272 (2015) 17–27

2.4. Adsorption equilibrium The experiments were conducted with initial Cu2+, Zn2+ or Ni2+ concentrations of 100, 200, 300, 400 and 500 mg/L in 250 mL plastics conical flasks at testing temperatures (25, 35 and 50 °C) with the contact time of 5 h. All batch experiments were conducted with 100 mL reaction solution and 0.5 g/L adsorbent in a thermostat shaker (stirring speed of 150 rpm). 2.5. Adsorption regeneration Using the same CHTs sample, adsorption cycles were evaluated by repeating more than three times cycles of adsorption/calcinations. A portion of 300 mg of the adsorbent was placed in 100 mL of Cu (NO3)2, Zn (NO3)2 or Ni (NO3)2 solution with the initial mental ion concentration 200 mg/L under 35 °C for 5 h, every time. The removal percentage of Cu2+, Zn2+ or Ni2+ was calculated according to Eq. (3) [26]:

Removal ð%Þ ¼ 100ðCo  Ce Þ=Co

ð3Þ 2+

2+

where Ce is the equilibrium concentration of Cu , Zn L).

2+

or Ni

(mg/

2.6. Characterizations of adsorbents and adsorption products X-ray diffraction (XRD) patterns were recorded using a Rigaku D/MAX-RA instrument at 40 kV and 50 mA with Cu-Ka (k = 0.154184 nm) radiation. Randomly oriented powder samples were scanned at the rate of 8°/min [24]. The specific surface area was measured by N2 adsorption/desorption at 77 K using a Quantachrome SI system. The pore-size distributions of samples were calculated from desorption branch according to the Barrett– Joyner–Halenda (BJH) method [27]. Scanning electron micrographs (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis of powder samples were obtained using a Hitachi S-4800 microscope and EDAX instruments, respectively. 3. Results and discussion 3.1. Characterization of HTs and CHTs Fig. 1 shows the XRD patterns of uncalcined and calcined HTs. Typical characteristic patterns of hydroxides could be observed, sharp and symmetric reflections for basal (0 0 3), (0 0 6), (1 1 0)

19

and (1 1 3) planes and asymmetric reflections for nonbasal (0 1 2), (0 1 5) and (0 1 8) planes, demonstrating that the synthetic material consisted of a single crystalline phase. The hexagonal lattice with rhombohedral 3R symmetry were indexed from the reflections of (0 1 2), (0 1 5), (0 1 8) and (1 1 0) [28]. Generally, most HTs containing CO2 3 have this structure, since the prismatic arrangement of hydroxyl groups allows those in both upper and lower layers to form hydrogen bonds with the oxygen atoms of CO2 3 . For the calcined sample, the characteristic patterns were similar to those of MgO, and the patterns of Al2O3 could not be observed. Moreover, the characteristic peaks (0 0 3) and (0 0 6) of hydrotalcites disappeared [29], suggesting that the calcination effectively destroyed the original hydrotalcites structures and formed mixed oxide Mg (Al) O. The typical N2 adsorption-desorption isotherms and the corresponding pore size distribution plots for the HTs and CHTs are illustrated in Fig. 1. The adsorption–desorption isotherms showed a type IV with H3 hysteresis loop according to the IUPAC classification, which was the characteristic of mesoporous materials with regular pore size distribution [30]. The pore size distribution curves of CHTs and HTs were broad in mesopores and narrow in microporous. CHTs possessed a larger unimodal pore size distribution centering at 35 nm than that of HTs. Additionally, the BET surface area of HTs was 72.07 m2/g, whereas CHTs was 212.8 m2/g. The above results demonstrated that CHTs had a larger surface area offering more active adsorption sites [28], which might induce a higher potentially toxic metal ion adsorption capacity for CHTs than HTs . As shown in Fig. 2, the SEM image of HTs presented abundant and variform flakiness, indicating thin crystals of HTs [17]. In terms of CHTs, there were no various morphologies of flakiness structures and the samples were compact and rough inversely, implying destroyed original hydroxides structures following calcinations, as the XRD results show.

3.2. Adsorption kinetics Adsorption kinetics was investigated to determine the time required for adsorption equilibrium and to further study the adsorption process of metal ions on CHTs. Adsorption kinetic data of Cu2+, Zn2+ or Ni2+ on CHTs from 0 to 300 min (5 h) at different temperatures was presented in Fig. 3. Obviously, the adsorptions of Cu2+, Zn2+ and Ni2+ have similar tendency, suggesting potentially homologous adsorption processes. The uptake of Cu2+, Zn2+ or Ni2+

Fig. 1. XRD patterns (left) and N2 adsorption–desorption isotherms and pore size distribution based on BJH analysis of adsorption data (inset) (light) for HTs and CHTs.

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M. Sun et al. / Chemical Engineering Journal 272 (2015) 17–27

Fig. 2. Scanning electron microscopy images of calcined (a) and uncalcined (b) MgAl HTs.

Fig. 3. Kinetics of Cu2+, Zn2+ or Ni2+ adsorption on CHTs from aqueous solution at different temperatures. Solid lines represent predicted data by pseudo-second- order, and the symbols are the experimental data ([Cu2+] = [Zn2+] = [Ni2+] = 200 mg/L).

was rapid for the first 20, 50 and 50 min, respectively. Then it slowed down and gradually got closed to saturation. The adsorption rate of CHTs for Cu2+ was the highest, whereas Zn2+ had the maximum adsorption ability. Furthermore, the adsorption capacity of CHTs increased with the rising temperature, which demonstrated that a higher temperature was beneficial to enhance the adsorption efficiency [31]. The adsorption processes of metal ions on the active sites of CHTs might be endothermic and could be elucidated by more available active sites of adsorbents at higher temperatures. It could also be explained that rising temperature facilitated metal ions transferring from the bulk solution toward the adsorbent surface and enhanced the accessibility to the adsorbent active sites [32].

In order to investigate the adsorption mechanism of potentially toxic metal ions on calcined hydrotalcites, the experimental results were fitting with the pseudo-first- order [33], pseudo-secondorder [34] and Elovich models [25] as expressed by Eqs. (4–6).

ln ðqe  qt Þ ¼ ln qe  K 1 t

ð4Þ

t=qt ¼ 1=ðK 2 q2e Þ þ t=qe

ð5Þ

qt ¼ K i t 0:5

ð6Þ

where qe and qt are the amount of the adsorbate adsorbed (mg/g) at equilibrium and time t (min); k1 (min1), k2 (g/(mg min)) and ki

21

M. Sun et al. / Chemical Engineering Journal 272 (2015) 17–27 Table 1 Kinetic constants of Cu2+, Zn2+ or Ni2+ sorption on CHTs for initial concentrations of 200 mg/L at different temperatures and analyzed by different models. T (K)

Cu2+

Zn2+

Ni2+

298 308 323 298 308 323 298 308 323

qe,exp (mg/g)

122.3 175.2 228.0 302.8 362.2 408.4 120.9 140.3 177.6

Pseudo-first-order

Pseudo-second-order 2

qe,cal (mg/g)

K1 (/min)

R

qe,cal (mg/g)

K2(10

53.50 51.72 63.83 215.0 228.8 269.5 128.9 152.0 167.5

0.0122 0.0141 0.0026 0.0097 0.0096 0.0156 0.0134 0.0186 0.0190

0.8672 0.7771 0.9193 0.9959 0.9844 0.9937 0.9910 0.9899 0.9943

124.8 177.0 230.4 297.6 320.5 350.9 157.5 167.2 198.4

7.27 10.91 15.85 0.2700 0.3500 0.5200 0.7200 1.170 1.560

Table 2 Relevant parameters of the activation energy for Cu2+, Zn2+ or Ni2+ adsorption on CHTs at different temperatures.

Cu2+ Zn2+ Ni2+

Particle diffusion 4

R2

Ea (kJ/mol)

ln A

ln K2 298 K

308 K

323 K

24.68 21.69 24.22

2.765 1.792 0.2980

7.226 10.54 9.537

6.821 10.27 9.058

6.447 9.862 8.764

0.9733 0.9978 0.8957

(mg/(g min0.5)) are the rate constants of pseudo-first-order, pseudosecond-order and particle diffusion models. The kinetic equation parameters which are determined based on the slopes and intercepts of the linear plots, are listed in Table 1. It could be obtained that the computed adsorption capacities of metal ions at equilibrium of the pseudo-second-order kinetic model were close to experimental results at different temperatures. Furthermore, the highest and most stable correlation coefficients (R2) of the pseudo-second-order kinetics model suggested better description of the Cu2+, Zn2+ or Ni2+ adsorptive process on CHTs, it could be concluded that the potentially toxic metal ion adsorption was a chemisorption process [35]. Moreover, both the rate constant K2 and the equilibrium capacity for Cu2+, Zn2+ or Ni2+ increased as temperature rose from 298 to 323 K, which also indicate that the increasing temperatures can promote adsorption processes. To well describe the rate constant K2 and the above phenomenon, the Arrhenius equation, defined as the free energy variation transferring when one mole of metal ion from infinity in solution to the surface of the solid, was referenced [36,37].

K 2 ¼ A expðEa=RTÞ

ð7Þ

ln K 2 ¼ ln A  Ea=RT

ð8Þ

where A is the pre-exponential factor and Ea is the activation energy (kJ/mol). When the value of activation energy (Ea) is less than 20 kJ/mol, the adsorption process is controlled by diffusion [38,39]. The adsorption energy (Ea) for Cu2+, Zn2+ and Ni2+ calculated via plotting ln K2 against 1/T were all above 20 kJ/mol (Table 2), indicating that the removal process of Cu2+, Zn2+ or Ni2+ was similar and dominated by the rate of reaction of metal ions with CHTs rather than by diffusion. Hence, it was concluded that the adsorptive process of Cu2+, Zn2+ or Ni2+ on CHTs was probable chemisorption. 3.3. Adsorption equilibrium Adsorption equilibrium was investigated to obtain the quantitative information about the adsorption capacity of Cu2+, Zn2+ or Ni2+ on CHTs and the distribution of adsorbates between liquid phase and solid phase at the equilibrium condition [27]. The experimental data was usually fitted by Langmuir and Freundlich

) (g/mg min))

2

R

Ki (mg/(g min0.5)

R2

0.9983 0.9998 0.9999 0.9911 0.9950 0.9927 0.9886 0.9901 0.9961

5.320 7.230 8.970 13.58 15.16 17.93 7.560 8.610 10.16

0.6172 0.4990 0.4177 0.9914 0.9723 0.9159 0.9792 0.9745 0.9584

models, which were represented mathematically as Eqs. (7) and (8), respectively [25,40]:

C e =qe ¼ 1=qm K L þ C e =qm

ð9Þ

ln qe ¼ ln K F þ 1=n ln C e

ð10Þ

where Ce (mg/L) and qe (mg/g) are the equilibrium adsorbates concentrations in the aqueous and solid phases, respectively; qm (mg/g) is the maximum adsorption capacity; KL (1/mg) is the constant of the Langmuir isotherm; KF (mg/g)(L/mg)n and n are parameters of the Freundlich model. The fitted Langmuir and Freundlich models at 25, 35 and 50 °C are shown in Fig. 4. Obviously, the fitting curve of Langmuir model was more consistent with the experimental data compared to the Freundlich model. Besides, the correlation coefficients (R2 > 0.99) of Cu2+, Zn2+ or Ni2+ in Langmuir model were all higher than that in Freundlich model. Results demonstrated that the adsorption isotherm of Cu2+, Zn2+ or Ni2+ on CHTs could be expressed as Langmuir model, and the surfaces of CHTs were homogeneous with the adsorption mechanism of monolayer uptake. Besides, seen from Fig. 4, the amounts of adsorbed Cu2+, Zn2+ or Ni2+ increased with ascending equilibrium concentrations of Cu2+, Zn2+ or Ni2+, and the equilibrium concentrations of potentially toxic metal ions increased with increasing initial concentrations. At the same equilibrium concentration, the increased temperature promoted the adsorption of Cu2+, Zn2+ or Ni2+, which was in line with the kinetic results. Therefore, the maximum adsorption capacity of Cu2+, Zn2+ or Ni2+ based on Langmuir model was about 418.3 mg/g (6.583 mmol/g), 492.7 mg/g (7.535 mmol/g) or 361.1 mg/g (6.152 mmol/g), where the initial concentrations of Cu2+, Zn2+ or Ni2+ was 500 mg/L under 50 °C. Obviously, the affinities and adsorption capacities of CHTs were in the following order: Zn2+ > Cu2+ > Ni2+. The uptake of Cu2+, Zn2+ or Ni2+ on different adsorbents had been studied and reported in several literatures [5,11,41–47]. Although the reported data was obtained under different experimental conditions, they can be adopted as a criterion for comparison. It can be obtained from Table 3 that CHTs possessed higher adsorption capacity and affinity for Cu2+, Zn2+ or Ni2+ than the other listed adsorbents. It is notable that CHTs have higher superiority for Cu2+, Zn2+ or Ni2+ removal than HTs, which could also verify that CHTs could improve the adsorption capacity of Cu2+, Zn2+ or Ni2+. Therefore, it could be concluded that CHTs indeed showed better performance in removing Cu2+, Zn2+ or Ni2+ from wastewater [31,48]. 3.4. Adsorption thermodynamics In order to evaluate the orientation and feasibility of the adsorptive reaction and to provide more information concerning the inherent energy and structural changes, the typical

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M. Sun et al. / Chemical Engineering Journal 272 (2015) 17–27

Fig. 4. The adsorption isotherms of Cu2+, Zn2+ or Ni2+ on CHTs at different temperatures. Solid lines represent predicted data by Langmuir model, dash lines represent predicted data by Freundlich model and the symbols are the experimental data.

Table 3 Comparison of Cu2+, Zn2+ or Ni2+ adsorption capacity (qm) of tested material with some literature values. Contaminant 2+

Cu

Zn2+

Ni2+

Adsorbents

qm (mmol/g)

References

Grape stalks wastes DTC-S ASA–PGMA/SiO2 Layered double hydroxides (HTs) Calcined hydrotalcites (CHTs) GH-T-P SG-PS-azo-IM Modified coir fibers Layered double hydroxides (HTs) Calcined hydrotalcites (CHTs) Cork bark Grape stalks wastes CHA/MFC Layered double hydroxides (HTs) Calcined hydrotalcites (CHTs)

0.16 0.27 0.42 0.03–2.50 6.58 0.05 0.05 0.12 0.67–4.76 7.54 0.07 0.18 0.70 0.07–0.73 6.15

[39] [40] [41] [12] This work [42] [43] [44] [12] This work [45] [39] [5] [12] This work

thermodynamic parameters were analyzed [49]. The temperature ranged from 298 to 323 K in the work. Gibbs free energy (DG°), enthalpy change (DH°) and entropy change (DS°) of the Cu2+, Zn2+ or Ni2+ adsorption were computed at various temperatures by the following thermodynamic relations (Eqs. (9–11)) [32,37,49].

DG ¼ DH  T DS

ð11Þ

DG ¼ RT ln K

ð12Þ

ln K ¼ DS =R  DH =RT

ð13Þ

where DG° is the standard free energy change (J/mol); R is the molar gas constant (8.314 J/mol K);T is the absolute temperature (K); K is the thermodynamic equilibrium constant (dimensionless). The value of K is determined by Eq. (12) [31]

K ¼ qb

ð14Þ

where b is the adsorption constant in Langmuir equation (L/mg); and q is density of water (106 mg/L). On the basis of Eq. (11), DH° (KJ/mol) and DS° (KJ/mol) can be calculated from the slope and intercept of the straight line of Van’t Hoff plots of ln KD versus 1/T [50], which were shown in Fig. 5. The thermodynamic parameters for adsorption of Cu2+, Zn2+ or Ni2+ on CHTs were tabulated in Table 4. The positive DH° verified that the adsorption natures were endothermic and a strong binding might exist between metal cations and adsorbent [11]. The positive values of DS° elucidated the randomness increased at the solid/solution interface during the adsorption process [51]. As seen in Table 4, the DG° is more negative at higher temperature, showing the spontaneity of the adsorption process increasing with the rising temperature. As well, the results were agreed with the adsorption rate and uptake capacity of Cu2+, Zn2+ or Ni2+ increasing with rising temperature, which was in well consistent with above studies. 3.5. Adsorption regeneration On the basis of the cost-effectiveness, it is important to evaluate the reutilization ability of adsorbents in sequential adsorption processes [52]. The regeneration of MgAl CHTs could be achieved due to the structure reconstruction, namely the ‘‘memory effect’’ [53]. The adsorption of Cu2+, Zn2+ or Ni2+ using regenerated CHTs after

M. Sun et al. / Chemical Engineering Journal 272 (2015) 17–27

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Fig. 5. Van’t Hoff plot for adsorption of Cu2+, Zn2+ or Ni2+ onto CHTs.

Table 4 Thermodynamic data for adsorption of Cu2+, Zn2+ or Ni2+ onto CHTs at temperatures ranging from 25 to 50 °C (298–323 K).

Cu2+ Zn2+ Ni2+

DH (kJ/mol)

DS (kJ/mol)

DG (kJ/mol) 298 K

308 K

323 K

5.334 11.88 5.360

0.02600 0.05800 0.02560

2.404 5.345 2.295

2.699 6.088 2.523

3.058 6.816 2.933

five times of thermal treatment is shown in Fig. 6. It could be obtained that the majority of metal ions in solutions were removed by CHTs in the previous four times. Nevertheless, the removal efficiency of Cu2+, Zn2+ or Ni2+ after the fifth cycle dramatically reduced with the order of Ni2+>Cu2+>Zn2+. These results implied that the thermal regeneration of CHTs was feasible only within the previous four cycles under the same experimental conditions. Afterwards the regenerated materials would show large loss in terms of adsorption capacities, which might be resulted from the reduced crystallinity of HTs as the cycle number of adsorption increased [50,54]. In Section 3.3, the adsorption equilibrium research revealed the irreversible adsorption, for which adsorbed ions were accumulated in adsorbents occupying adsorption sites and participated in the next cycle adsorption process. With the increasing cycle time, the accumulations increased with the decreased adsorption sites. When the adsorption reaches equilibrium, there is no available adsorption site bringing about the reduced adsorption efficiency. In addition, after five cycles, the total removal amount of Cu2+, Zn2+ and Ni2+ was 4.768 mmol/g (303.0 mg/g), 4.871 mmol/g (318.5 mg/g) and 4.740 mmol/g (278.2 mg/g), respectively, which suggested that the adsorption capacity of CHTs decreased with the order Zn2+ > Cu2+ > Ni2+ as shown in kinetic studies.

Fig. 6. Removal efficiency of Cu2+, Zn2+ or Ni2+ in different cycles by using CHTs ([Cu2+] = [Zn2+] = [Ni2+] = 200 mg/L, T = 35 °C; CHTs = 0.3 g/100 mL).

3.6. Adsorption mechanism The variation of solution pH was essential for understanding the adsorption mechanism of potentially toxic metal ions on adsorbents, since it was relative to the chemical speciation of metal ions [41]. The solution pH variation was studied at the same adsorption kinetics condition under 35 °C. The variation of pH value versus t (min) is presented in Fig. S1(Supplementary Fig. 1). Initial solutions were all adjusted with 0.1 mol/L HNO3 or NaOH solution to achieve pH at about 2.1. CHTs (Mg(Al)O) could take place rehydration in aqueous solution bringing about the release of OH- with the

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M. Sun et al. / Chemical Engineering Journal 272 (2015) 17–27

Fig. 7. X-ray diffraction patterns of CHTs-Cu, CHTs-Zn or CHTs-Ni (after Cu2+, Zn2+ or Ni2+ adsorption on CHTs).

increase of pH value [55]. Therefore, the pH value constantly increased at varying rates in solutions (Fig. S1). Seen from Fig. S1, pH values remained in the range of 2.1–5 during the whole adsorption process, being much lower than the pHPZC (which was 12.5, as reported by Xiao et al. [55]), revealing the positive charge of the surfaces of CHTs [29]. Hence, hydroxyls released and attached to the surfaces of adsorbents, and partial hydroxyls shifted from the solid-liquid interfaces to liquid phases, inducing decreased alkalinity gradient. Metal ions easily formed chemical precipitates with abundant hydroxyls. Meanwhile, part of magnesium or aluminum hydroxides may be generated. Hence, some precipitates were generated, with most of them concentrating on the surfaces of adsorbents owing to the high alkalinity. Besides, more hydroxyls were released in salt solutions of Cu2+, Zn2+ and Ni2+ than in the deionized water (Fig. S1), which demonstrated that metal ions contributed to releasing hydroxyls and promoted the reconstruction of hydrotalcites. In the view of chemical reactions, the consumption of hydroxyls utilized to generate metal precipitates could facilitate the process of the forward reaction in Eq. (1). Though

Fig. 8. The scanning electron microscopic (SEM) image and corresponding EDX spectrum of CHTs-Cu (a), CHTs-Zn (b) or CHTs-Ni (c).

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precipitates could not be observed in solutions during the whole adsorption process, metal ions were largely removed according to the kinetics experiment. These also implied that metal precipitates mainly attached to the surfaces of adsorbents. The pH value and adsorption quantity were nearly constant with increasing time, which might be resulted from decreased metal ion concentrations. As a conclusion, potentially toxic metal ions firstly removed from solutions by forming hydroxide precipitations with adhering to the surfaces of adsorbents. Apart from pH value, ionic property was also one important factor for analyzing adsorption mechanism. Ni, Cu and Zn are adjacent transition metals in the periodic table of chemical element, thus they may have the same adsorption mechanism on CHTs. The same total ionic charge and approximately the same size as replaced Mg2+, can lead to the isomorphic substitution [11]. Furthermore, Kim et al. [56] successfully incorporated Co2+ into MgAl HTs lattices with isomorphous substitution, and demonstrated that the incorporations were highly dependent on the dissolution of Mg2+. According to the study of Liang et al. [11], the sorption mechanisms of Cu2+, Zn2+ and Ni2+ on HTs might be all isomorphic substitution. These studies implied that Cu2+, Zn2+ and Ni2+ could be incorporated into HTs lattices via isomorphous substitution. The ionic radius of Cu2+, Zn2+ and Ni2+ were 0.72 Å, 0.74 Å and 0.72 Å, nevertheless the affinity was in the order of Zn2+ > Cu2+ > Ni2+ according to Sections 3.3 and 3.5. It indicated that cations with small ionic radius might have less chance to be incorporated into hydrotalcites structures, because small ionic radius caused a steric hindrance for isomorphic substitution. In order to further explore the Cu2+, Zn2+ or Ni2+ adsorption mechanism on MgAl CHTs, the samples after adsorption (CHTs-R) were analyzed by XRD (Fig. 7), SEM and EDX (Fig. 8). It was obvious that the characteristic patterns of CHTs-R were similar to HTs (Fig. 1) with decreasing relative intensities, and the XRD patterns of metal hydroxides could not be detected, revealing that CHTs (Mg(Al)O) changed to the hydrotalcites-like compounds. Along

with the adsorption process, previously generated precipitates gradually disappeared through certain reactions, for example, electrolysis. Meanwhile, CHTs (mixed metal oxides) might form hydroxides on the surface of adsorbents with abundant OH– and occur to electrolysis releasing Mg2+ and Al3+. The substitution of Mg2+ by Cu2+, Zn2+ or Ni2+ might occur. These reactions may be expresses as follows:

MgðAlÞO þ H2 O ! MgðAlÞ—O . . . H þ OH— 2þ

Ni

þ 2OH— ! NiðOHÞ2 #

ð15Þ ð16Þ

MgðAlÞ—O . . . H þ OH— ! MgðOHÞ . . . Sur1 . . . AlðOHÞ2

ð17Þ

MgðOHÞ . . . Sur1 . . . AlðOHÞ2 þ NiðOHÞ2 3þ

! Al 3þ

Al

2þ

. . . Sur1 . . . M2þ ðMg2þ =Ni Þ þ H2 O 2þ

 . . . Sur1 . . . M2þ ðMg2þ =Ni Þ þ H2 O þ CO2 3 =NO3 ! Sur2

ð18Þ ð19Þ

where Sur1 or Sur2 respects the surface structures of the calcined or uncalcined hydrotalcites. Cell parameters and interlayer spacing of HTs before and after adsorption were calculated and listed in Table S1 (Supplementary Table 1) [29,57]. Parameter ‘‘a’’ showed slight decrease, speculating substitution of partial Mg2+ with small ionic radius (0.65 Å) by Cu2+, Zn2+ or Ni2+ with relatively large ionic radius (0.72 Å, 0.74 Å and 0.72 Å, respectively). Cell parameter ‘‘c’’ (the layer thickness) and interlayer spacing increased, suggesting that the hydrotalcites structures slightly changed after the uptake of heavy mental ions comparing with original hydrotalcites structures. SEM images of CHTs-R showed plentiful flakiness with alveolate-like morphology and resembled the images of HTs, which also proved CHTs reconstructing hydrotalcites structures after adsorption, as previously shown by XRD. The chemical compositions of

Scheme 1. Schematic representation of probable adsorptive mechanism between the heavy metal ion (with Cu2+, Zn2+ or Ni2+) and calcined MgAl HTs.

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CHTs-R could be observed in the corresponding EDX spectrums (Fig. 8). In all EDX spectrums, the elements of Mg, Al, C and O contained in MgAl-CO3 HTs were present, which indicated that CHTs might be reformed to HTs. Moreover, Cu, Zn or Ni element could also be observed, indicating the existence of Cu, Zn or Ni in the structures of adsorption products. Ultimately, the above results suggested that Cu2+, Zn2+ or Ni2+ was finally removed on CHTs by the isomorphous substitution in the course of HTs refactoring. The previous analysis demonstrated that the adsorption mechanisms of Cu2+, Zn2+ and Ni2+ might be similar, which included two steps: (i) the potentially toxic metal ions in aqueous solution formed hydroxide precipitates on the surfaces of adsorbents with high alkalinity; (ii) CHTs experienced rehydration, together with hydroxide precipitates forming new hydrotalcites compounds. Simultaneously, part of Mg was substituted by Cu, Zn or Ni. These were depicted in Scheme 1. 4. Conclusions Sorption performance of CHTs was investigated for the removal of Cu2+, Zn2+ or Ni2+ from aqueous solutions. Pseudo-second-order model gave the best fitting to the adsorption kinetic results and the activation energies were over 20 kJ/mol, implying the possibility of chemisorptions. Adsorption equilibrium could be well depicted by Langmuir model, illustrating the homogeneous adsorption. The adsorbents had high adsorption capacity and affinity in the order of Zn2+ > Cu2+ > Ni2+. Thermodynamics parameters demonstrated that the adsorption of Cu2+, Zn2+ or Ni2+ was endothermic and spontaneous with increasing randomness of the system. The regeneration study revealed that the adsorbents could be efficiently recycled for four times under the experimental conditions. Besides, Cu2+, Zn2+ or Ni2+ firstly isolated from aqueous solution and adhered to the surfaces of absorbents as hydroxides. Then hydroxides took place to reconstruct the hydrotalcites structures together with absorbents, and the layered sites of Mg2+ was substituted by Cu2+, Zn2+ or Ni2+ through isomorphism substitution. As a consequence, calcined hydroxides possessed vast application potential in treatment effluents containing Cu2+, Zn2+ or Ni2+ owing to the high affinity and efficiency, ease of operation, low-cost and potential for regeneration. Acknowledgments Financial supports from Science and Technology Development Plan of Shandong Province of China (No. 2012GGE27098) are gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2015.03.009. References [1] C.G., G. Zhao, X. Wang, S. Yu, Synthesis of polyacrylic acid stabilized amorphous calcium carbonate nanoparticles and their application for removal of toxic heavy metal ions in water, J. Phys. Chem. C 114 (2010) 12948–12954. [2] T. Madrakian, A. Afkhami, B. Zadpour, M. Ahmadi, New synthetic mercaptoethylamino homopolymer-modified maghemite nanoparticles for effective removal of some heavy metal ions from aqueous solution, J. Ind. Eng. Chem. (2014). [3] J. Gómez-Pastora, E. Bringas, I. Ortiz, Recent progress and future challenges on the use of high performance magnetic nano-adsorbents in environmental applications, Chem. Eng. J. 256 (2014) 187–204. [4] M. González, I. Pavlovic, R. Rojas-Delgado, C. Barriga, Removal of Cu2+, Pb2+ and Cd2+ by layered double hydroxide–humate hybrid. Sorbate and sorbent comparative studies, Chem. Eng. J. 254 (2014) 605–611.

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