Molybdate sorption by Zn–Al sulphate layered double hydroxides

Applied Clay Science 65-66 (2012) 128–133

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Molybdate sorption by Zn–Al sulphate layered double hydroxides Carla Ardau ⁎, Franco Frau, Elisabetta Dore, Pierfranco Lattanzi Department of Chemical and Geological Sciences, University of Cagliari, Via Trentino 51, 09127 Cagliari, Italy

a r t i c l e

i n f o

Article history: Received 17 February 2012 Received in revised form 17 May 2012 Accepted 18 May 2012 Available online 21 July 2012 Keywords: Zn–Al sulphate LDHs Molybdate–sulphate exchange Molybdate surface adsorption Polluted water treatment

a b s t r a c t The efficacy of Zn–Al sulphate layered double hydroxides (LDHs) in removing molybdenum from aqueous solution was tested through sorption of aqueous molybdate by synthetic samples in batch experiments. Up to ~ 54% Mo is removed; the most efficient mechanism is sulphate–molybdate exchange within the LDH interlayer, which preserves the layered structure, though with a partial loss of structural order. The Mo uptake is influenced primarily by the [Zn2+]/[Al 3+] ratio, which determines the ionic charge of the brucitelike layer; high ratios facilitate the release of sulphate from the interlayer and the entrance of molybdate, whilst for low ratios surface adsorption of molybdate becomes an important uptake mechanism. The Mo removing potential shown by Zn–Al sulphate LDHs may prove useful for the treatment of moderately contaminated waters. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Layered double hydroxides (LDHs), also known as hydrotalcitelike compounds or anionic clays (general formula M 2+1 − xM 3+x (OH)2(A n −)x/n · yH2O), are a group of compounds with a structure consisting of brucite-type metal-hydroxide layers, where bivalent cations are partially replaced by trivalent cations; these positively charged layers are balanced by interlayered inorganic or organic anions. This structure allows exchange with anions in solution (Rojas Delgado et al., 2008), making LDHs potentially suitable for the production of advanced catalytic materials, or for the removal of contaminants from waste waters (Smith et al., 2005; Vaccari, 1998, and references therein; Yang et al., 2005). Among the many possible LDHs compositions, we focused our interest on Zn–Al sulphate LDHs. They are much less studied and widespread in nature than the most common Mg–Al LDHs. However, Zn– Al–SO4 LDHs have been reported as secondary minerals in several mine areas, where they can exert a specific influence on the local geochemical cycle of As (Ardau et al., 2011a). Indeed, laboratory studies (Ardau et al., 2011b) documented an excellent capacity of Zn–Al– SO4 LDHs in removing arsenate from waters. Importantly, Zn–Al LDHs might be easily produced on a large scale by using waste materials from various industrial processes. Hence, their use in decontamination actions would have the additional benefit of reducing the need of waste disposal. The aim of this study is to investigate the capacity of Zn–Al–SO4 LDHs in removing molybdenum from aqueous solutions. Molybdenum is an essential micronutrient required for the growth of most ⁎ Corresponding author. Tel.: + 39 0706757715. E-mail addresses: [email protected] (C. Ardau), [email protected] (F. Frau), [email protected] (E. Dore), [email protected] (P. Lattanzi). 0169-1317/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2012.05.005

organisms. However, it has a narrow range between deficiency and toxicity (McGrath et al., 2010). Molybdenite (MoS2) is the most abundant Mo mineral, whilst in soils and natural waters the metal occurs predominantly in its anionic form MoO42 −. The WHO (World Health Organization) recommends a maximum level of 0.07 mg/L Mo in drinking water (WHO, 2006). Anomalously high concentrations of Mo may occur in association with metallurgical and mining activities (e.g., Leybourne and Cameron, 2008). There are only a few previous studies on the intercalation of molybdate in LDH structures, mostly related to technological applications (Mitchell and Wass, 2002; Smith et al., 2005; Yu et al., 2009; Zhang and Reardon, 2003). 2. Experimental 2.1. Synthesis of Zn–Al–SO4 LDHs Synthetic samples for this study were prepared by a coprecipitation method. An aqueous Zn–Al sulphate solution (0.2 mol/L), with established [Zn 2+]/[Al 3+] ratios, was added dropwise by a peristaltic pump into a reactor containing 200 mL of Milli-Q water (Millipore), under vigorous stirring (500 rpm). The precipitation was induced at pH ~8.5, by dropping a NaOH solution (0.5 mol/L) into the reactor. To avoid carbonate groups entering the interlayer, the experiments were conducted under controlled Ar atmosphere (Ardau et al., 2011b). Syntheses were performed for [Zn 2+]/[Al 3+] ratios equal to 5/3, 2/1 and 3/1 (from now on 1.7LDH, 2LDH, and 3LDH). Precipitation started instantly, and resulted in the formation of a white gel. After aging (≥7 days at 50 °C), the precipitates were filtered and washed with abundant deionised water. The physical appearance of the precipitate may be fairly different from one synthesis to another: in most cases a soft loose powder was obtained, whilst in

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Table 1 Chemical analyses and empirical formulas of synthetic LDHs used for sorption experiments.

3LDH 2LDH 1.7LDH

Zn:Al starting solution

Zn

Molar ratio

wt.%

3.0 2.0 1.7

37 37 30

Al

Na

S

Zn:Al synthetic product

Δ Charge balancea

Empirical formula

Molar ratio 5.4 7.1 7.8

bdl 0.3 bdl

3.7 4.0 4.4

% [Zn0.74Al0.26(OH)2](SO4)0.15·mH2O Na0.01[Zn0.65Al0.35(OH)2](SO4)0.16 ·mH2O [Zn0.61Al0.39(OH)2](SO4)0.18 ·mH2O

2.9 1.9 1.6

− 1.75 + 1.71 − 1.26

dl = detection limit. a Calculated as ((Σ positive charges) − (Σ negative charges) / (0.5·(Σ positive charges) + (Σ negative charges)))·100.

others a hard, transparent, glass-like material was synthesised. The phenomenon was apparently random (i.e., it was not possible to positively establish any dependence on a specific condition). In this specific study, the sample 2LDH was glass-like, whilst 3LDH and 1.7LDH were soft. X-ray diffraction (XRD) patterns did not show differences between the two types of precipitate (see Section 3.2); indeed, when glass-like samples were re-hydrated and disaggregated by vigorous stirring during batch experiments, the final product acquired softness. Similar variations of the physical appearance of synthetic hydrotalcites as a function of [Me 2+]/[Me 3+] molar ratio were observed by Smith et al. (2005). The initial composition of LDHs used as sorbents in the sorption experiments and their empirical formulas are listed in Table 1. The [Zn 2+]/[Al 3+] ratio of synthetic phases corresponded approximately to that in the starting solutions. 2.2. Sorption of aqueous molybdate by synthetic LDHs in batch experiments

Al by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The analysis of dissolved Zn and Al allowed to determine whether a partial dissolution of LDH occurred during the sorption experiment, whilst the dissolved concentration of sulphate (adjusted for the possible contribution from dissolution) provided information on exchange process with molybdate. At the end of each experiment, the sorbent was washed with abundant deionised water and filtered. A portion of all solid materials was analysed by XRD before and after sorption experiments. XRD patterns were collected in the 5–70° 2θ angular range on an automated Panalytical X'pert Pro diffractometer with Ni-filter monochromatised Cu-Kα1 radiation (λ = 1.54060 Å), operating at 40 kV and 40 mA, using the X'Celerator detector. Because these phases are hydrophilic (Ardau et al., 2011b), all XRD patterns were collected after leaving samples in oven at 50 °C for 24 h. Another portion of the solids was dissolved in 10% v/v HNO3 (15 mol/L), and the solution analysed by ICP-AES to determine chemical composition. 3. Results

One gram of (X)LDH was suspended in a series of beakers, each containing 100 mL of aqueous molybdate solution, obtained by dissolving Na2MoO4 in Milli-Q water to achieve nominal Mo concentrations of 0.004, 0.008, 0.012, and 0.016 mol/L (from now on (a), (b), (c), and (d)). The suspension was left for ~72 h under vigorous stirring, and the pH was monitored, but not fixed; throughout all experiments, it varied slightly in the range ~ 7–8. At this pH, the dominant form of Mo in solution is the molybdate anion MoO42 −. Small volumes of solution were sampled at different time intervals, filtered at 0.4 μm, and acidified with HNO3 for the analyses of Mo, S, Zn, and

3.1. Chemistry of batch solutions Results of sorption experiments are reported in Table 2, and graphically displayed in Fig. 1a and b. Sorption is quite fast, mostly occurring within the first 3 h. As a general rule, for a given [Zn 2+]/[Al 3+] ratio the uptake of Mo from the solution (Mo(removed)) increases from (a) to (d), that is for increasing values of Mo(start, aq) (where Mo(start, aq) is the starting aqueous Mo concentration) (Fig. 1a), whilst the percentage removed (Mo(removed)·100/Mo(start, aq)) decreases from (a) to (d) (Fig. 1b). The efficiency of removal is clearly

Table 2 Variation of solution chemical parameters during sorption experiments. Zinc and Al were determined only at 72 h. Sample Experiment label

3LDH Mo molarity

Sampling time

Al

mmol/L

(h)

mmol/L

a

4.0

b

8.0

c

12

d

16

0 3 24 48 72 0 3 24 48 72 0 3 24 48 72 0 3 24 48 72

nd nd nd nd bdl nd nd nd nd bdl nd nd nd nd bdl nd nd nd nd bdl

nd = not determined; dl = detection limit.

2LDH Mo

S

Zn

Al

1.7 LDH Mo

S

Zn

mmol/L 4.0 2.0 1.8 1.8 1.8 8.0 4.8 4.6 4.7 4.5 12.0 8.0 7.9 7.9 7.7 16.0 11.1 11.0 11.2 10.7

0.0 2.1 2.2 2.3 2.5 0.0 2.9 3.1 3.2 3.3 0.0 3.4 3.7 3.9 4.0 0.0 3.8 4.2 4.4 4.6

nd nd nd nd 0.44 nd nd nd nd 0.39 nd nd nd nd 0.36 nd nd nd nd 0.33

nd nd nd nd bdl nd nd nd nd bdl nd nd nd nd bdl nd nd nd nd bdl

Al

Mo

S

Zn

4.0 2.3 2.6 2.5 2.6 8.0 5.8 5.4 5.4 5.6 12.0 9.3 8.3 8.2 8.7 16.0 12.8 11.9 11.5 12.1

0.0 1.1 1.5 1.3 1.3 0.0 1.6 1.9 1.9 1.7 0.0 1.8 2.2 2.2 2.0 0.0 2.0 2.5 2.5 2.2

nd nd nd nd 0.033 nd nd nd nd 0.022 nd nd nd nd 0.018 nd nd nd nd 0.015

mmol/L 4.0 2.7 2.5 2.4 2.4 8.0 6.2 5.6 5.4 5.3 12 9.5 9.2 8.9 8.6 16.0 13.2 12.1 12.3 12.0

0.0 0.8 0.9 1.0 1.0 0.0 1.0 1.2 1.4 1.5 0.0 1.0 1.3 1.5 1.5 0.0 1.1 1.4 1.6 1.7

nd nd nd nd 0.078 nd nd nd nd 0.048 nd nd nd nd 0.032 nd nd nd nd 0.030

nd nd nd nd bdl nd nd nd nd bdl nd nd nd nd bdl nd nd nd nd bdl

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a

and 1.7LDH. Moreover, as observed for Mo, S released from 3LDH is more influenced by Mo(start, aq) values. Some Zn is released to the solution during the experiments, decreasing from (a) to (d) (Table 2), and according to the sequence: 3LDH >> 2LDH > 1.7LDH. This release indicates that at pH values in the range of 7–8 the LDHs are slightly soluble (3LDH more than 2LDH and 1.7LDH). Equilibrium calculations, carried out with the geochemical code PHREEQC (ver. 2.18.5570) coupled with the MINTEQ.v4 database (Parkhurst and Appelo, 1999), which includes Mo species, indicate oversaturation conditions with respect to ZnMoO4 in all solutions. Thus, it is possible that some Mo is subtracted from the solution by ZnMoO4 precipitation, but its amount is by far minor with respect to Mo sorbed by LDH and/or remaining in the solution.

10

S released to solution

8 6

(mmol/L)

4 2 0

a

b

c

d

-2 -4 -6

Mo removed from solution

-8 -10

Mo 1.7LDH S 1.7LDH

b

Mo 2LDH S 2LDH

Mo 3LDH S 3LDH

3.2. Sorbents

100

Mo removed from solution (%)

90 80 70 60 50 40 30 20 10

a

b

4

8

c

d

0 0

12

16

Mo starting concentration (mmol/L) 3LDH

2LDH

1.7LDH

Fig. 1. Variations in solution in sorption experiments after 72 h. a) Variations of S and Mo contents; b) percentage of Mo removed from solutions.

influenced by [Zn2+]/[Al3+] ratios: higher ratios favour molybdate uptake from solution, according to the sequence: 3LDH >>2LDH ≥ 1.7LDH, but the decrease of percentage Mo removed with increasing Mo(start, aq) is more pronounced for 3LDH (Fig. 1b). Molybdenum uptake is accompanied by a concomitant release of S to solution, increasing from (a) to (d) according to the sequence: 3LDH >> 1.7LDH > 2LDH (Fig. 1a). Sulphur released from the 3LDH sample is about twice the amount released from the 1.7LDH sample. This difference increases if the percentage of S released is considered, since the 3LDH sample contains less S than the 1.7LDH sample (Table 1). As expected the difference between 2LDH and 1.7LDH is less pronounced. The molar amount of Mo removed almost always exceeds that of S released; the difference is very low for 3LDH but significant for 2LDH

The Mo and S contents of solid sorbents before and after the experiments are shown in Table 3. The inequality S(sol) > (S + Mo)a/b/c/d generally occurs, where S(sol) is the starting S concentration in the solid sorbent, whilst (S + Mo)a/b/c/d is the sum of S and Mo at the end of each experiment. This inequality is probably due to the rinsing of LDHs with abundant deionised water after the experiments (see Section 2.2), with consequent desorption/release phenomena, which could lead to an underestimation of real concentrations in the solids. However, the overall trend (loss of S and gain of Mo) in the course of all experiments matches well with the opposite trend of elements in solution. For a given [Zn 2+]/[Al 3+] ratio, Mo enrichment increases from (a) to (d). Molybdenum enrichment and S loss are proportional in the four aliquots of the 3LDH sample, whilst in 2LDH and 1.7LDH samples the behaviour of these two elements appears less coupled, as already observed for solution chemistry. In Figs. 2 and 3 the XRD patterns of solid sorbents before and after the experiments are shown. Several samples (i.e. 1.7LDH from (a) to (d), and 2LDH (c) and (d)) show a shoulder in the left edge of the (003) reflection (2θ ~ 10°). This effect can be ascribed to a residual (003) reflection of highly hydrated Zn–Al–SO4 LDHs (2θ ~ 8°). The same origin has a very weak reflection (2θ ~ 16°), ascribable to the residual (006) reflection, present in a smaller number of samples (e.g. 2LDH(d)). The transition from highly hydrated to less hydrated Zn–Al–SO4 LDHs was described by Ardau et al. (2011b). In correspondence of the above chemical changes, there is a certain degree of structural rearrangement of the samples, more evident for increasing [Zn 2+]/[Al 3+] ratios. For the 3LDH samples, the analysis of the portion in the 5–30° 2θ angular range (Fig. 3), where basal reflections fall, indicates a shift of basal reflections toward higher 2θ values (which implies a shortening of d-spacings), and the splitting of the (006) reflection (2θ ~ 20°) into two broad reflections (2θ = 21.5° and 23°), suggesting a loss of structural order, probably due to inhomogeneities of interlayers (e.g., systematic variations of anion distribution). For 1.7LDH and 2LDH samples, the shift of the basal X-ray reflections is not evident, whilst in 2LDH samples the (006) reflections (2θ ~ 20°) present a gradual broadening from (a) to (d) suggesting a loss of structural order, and an asymmetry, which could be a hint of a reflection splitting.

Table 3 Variations of solid sorbents chemistry (Mo and S contents) before and after sorption experiments. 3LDH

mmol/g S Mo

2LDH

1.7LDH

Sorbent

a

b

c

d

Sorbent

a

b

c

d

Sorbent

a

b

c

d

1.15 –

0.84 0.15

0.81 0.23

0.72 0.28

0.66 0.35

1.24 –

1.16 0.06

1.03 0.13

1.03 0.15

1.01 0.19

1.37 –

1.32 0.09

1.16 0.14

1.07 0.19

1.11 0.22

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a

131

a A B C D

b b A

B C D

c c A B C D Fig. 3. A detail of the XRD patterns of LDHs with different [Zn2+]/[Al3+] ratios (portion in the 5–30° 2θ angular range), before and after the d experiment.

Fig. 2. Comparison of XRD patterns (λ = 1.5405 Å) of LDHs with different [Zn2+]/[Al3+] ratios, before and after sorption experiments.

4. Discussion The results of batch experiments document a moderate ability of Zn–Al–SO4 LDHs to remove molybdate from solution through sorption processes. In the conditions of our experiments, removal of Mo ranges between 24% and 54% of the total in solution, i.e. much less than reported for arsenate in similar experiments (Ardau et al., 2011b). In agreement with other studies on LDH (e.g., Goh et al., 2008) the efficiency of removal is influenced by [Me 2+]/[Me 3+] ratios, with the maximum removal obtained for the highest [Zn 2+]/ [Al 3+] ratio. For a given [Zn 2+]/[Al 3+] ratio, the absolute amount of Mo removed increases with Mo(start, aq), but at the same time the process becomes less efficient (i.e., the percentage of total Mo removed decreases). Two different mechanisms control the removal of anions by LDHs from aqueous media: interlayer anionic exchange and surface adsorption (Mohan and Pittman, 2007). The former is favoured by high [Me 2+]/[Me 3+] ratios, due to a reduced positive charge of brucite-like layers, which facilitate the release of interlayer anions (in this case, sulphate) from the solid to the aqueous medium

(SO4 2 −(sol) → SO4 2 −(aq)). Ionic radii of MoO42 − and SO42 − are fairly close, being respectively 0.246 and 0.230 nm; considering also the same charge, MoO42 − and SO42 − should have a similar affinity for the LDH interlayer. On the other hand, for an exchange process involving exclusively anions in the interlayer of the LDH structure, the amount of MoO42 −(removed) should be equal to SO42 −(released). On the contrary, in all experiments MoO42 −(removed) exceeds SO42 −(released); the difference increases toward lower [Zn 2+]/[Al 3+] ratios of the sorbent, where SO42 −(sol) in the interlayer is more strongly bound to the structure due to a higher positive charge of brucite-like layers. Accordingly, we suggest that Mo uptake partially takes place through adsorption onto the external surface of the LDH phase, which does not involve the passage SO42 −(sol) → SO42 −(aq). The contribution from adsorption is higher for low [Zn 2+]/[Al 3+] ratios. An effective visualisation of the two processes is shown in Fig. 4, where chemical changes in solids phases are plotted. To sort out the underestimation of real concentrations in solid phases (see Section 3.2), S and Mo values were calculated as (S(sol) − S(released)) and (Mo(start, aq) − Mo(removed)), respectively. As a first approximation, we will assume that the Mo sorption by Zn–Al–SO4 LDHs can totally be explained by the quantification of the anions removed from/released to solution. Under this assumption, Mo concentration under the dashed line in

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1.6 1.4

highest. In this case a shift of the X-ray reflections toward higher angles (lower d-spacing values) is also observed, giving indirect evidence, in agreement with solution data, of the sulphate-molybdate exchange in the interlayer of the structure. Splitting of basal reflections is also observed, suggesting inhomogeneous characteristics of interlayers (i.e., preferential distribution of sulphate/molybdate in specific interlayers).

1.7LDH

mmol/g

1.2 1 0.8 0.6 0.4

5. Conclusions

0.2 0

sorbent

a

b

c

d

b

c

d

According to sorption experiments performed in this study, Zn–Al–SO4 LDHs can act as removers of molybdate from water, although less efficient than for arsenate. The value of [Zn 2+]/[Al 3+] ratio in LDH is critical for the efficacy of the process. In fact, it determines the extent of the two main mechanisms of MoO42 − removal: interlayer anionic exchange, and surface adsorption. The former clearly prevails for [Zn 2+]/[Al 3+] = 3, whilst the latter plays an important role for [Zn 2+]/[Al 3+] ≤ 2. The sulphate–molybdate exchange in the LDH interlayer is the most efficient mechanism, although the most affected by the MoO4 2 −(start, aq) value. The highest efficiency can thus be obtained using [Zn 2+]/[Al 3+] ratio = 3 or more, providing a well balanced MoO42 −(start, aq)/SO42 −(sol) ratio. The layered structure is preserved during the sorption process, though with a partial loss of the structural order. An undesirable consequence of the use of 3LDHs could be a release of some Zn into solution; however, this solubility becomes negligible at pH ≥ 8 (Ardau et al., 2011b). In conclusion, Zn–Al–SO4 LDHs can be effective in the treatment of waters moderately contaminated by molybdate with pH in the range of 7–8.

1.6 1.4

2LDH

mmol/g

1.2 1 0.8 0.6 0.4 0.2 0 1.6 1.4

sorbent

a

3LDH

mmol/g

1.2 1

Acknowledgements

0.8 0.6 0.4 0.2 0

sorbent

a

b S

c

d

Mo

Fig. 4. Calculated variations of sorbent chemistry (Mo and S contents) before and after sorption experiments at 48 h. Dashed line marks the amount of Mo entered the interlayer needed to balance the S released; the excess Mo above the dashed line is assumed to be adsorbed at the external surface.

Fig. 4 (marking out the S(sol) value) represents the exchanged molybdate, whilst Mo concentration above the dashed line represents molybdate adsorbed onto the surface. When [Zn 2+]/[Al 3+] = 3, the adsorbed fraction is negligible or within the analytical error, whilst for 2 and 1.7 ratios the estimated amount of Mo adsorbed increases significantly. This double mechanism of Mo removal, although is a simplification of the exchange process, can explain the more pronounced decrease of Mo percentage removed from (a) toward (d), observed for increasing [Zn 2+]/[Al 3+] molar ratios (Fig. 1b). As said above, molybdate uptake in the interlayer implies an expulsion of sulphate from the structure. This expulsion slows down approaching the maximum exchange capability of the phase (a → d). On the contrary, when Mo uptake takes place on the external surface of the phase, the number of available adsorption sites is independent on the release SO4 2 −(sol) → SO4 2 −(aq). The loss of crystallinity after MoO42 − sorption suggested by XRD patterns could reflect a distortion of the brucite-like layer from planarity, as a consequence of a disordered spatial distribution of anions on each side of the layer (Tumiati et al., 2008). The maximum distortion is observed in the 3LDH samples, where the molybdate loading is

Financially supported by the Italian Ministry of Education, University and Research (MIUR; PRIN 2008 grant to Pierfranco Lattanzi) and by the Consorzio AUSI (Consorzio per la Promozione delle Attività Universitarie del Sulcis-Iglesiente, grant to Franco Frau). The authors wish to thank the associate editor and two anonymous reviewers for their useful comments and suggestions that greatly improved the manuscript. References Ardau, C., Cannas, C., Fantauzzi, M., Rossi, A., Fanfani, L., 2011a. Arsenic removal from surface waters by hydrotalcite-like sulphate minerals: field evidences from an old mine in Sardinia, Italy. Neues Jahrbuch für Mineralogie, Abhandlungen 188/1, 49–63. Ardau, C., Frau, F., Ricci, P.C., Lattanzi, P., 2011b. Sulphate-arsenate exchange properties of Zn–Al layered double hydroxides: preliminary data. Periodico di Mineralogia 80 (2), 339–349. Goh, K.-H., Lim, T.-T., Dong, Z., 2008. Application of layered double hydroxides for removal of oxyanions: a review. Water Research 42, 1343–1368. Leybourne, M.I., Cameron, E.M., 2008. Source, transport, and fate of rhenium, selenium, molybdenum, arsenic, and copper in groundwater associated with porphyry‐Cu deposits. Atacama Desert, Chile. Chemical Geology 247, 208–228. McGrath, S.P., Micó, C., Curdy, R., Zhao, F.J., 2010. Predicting molybdenum toxicity to higher plants: influence of soil properties. Environmental Pollution 158, 3095–3102. Mitchell, P.C.H., Wass, S.A., 2002. Propane dehydrogenation over molybdenum hydrotalcite catalysts. Applied Catalysis A: General 225, 153–165. Mohan, D., Pittman Jr., C.U., 2007. Arsenic removal from water/wastewater using adsorbents. A critical review. Journal of Hazardous Materials 142, 1–53. Parkhurst, D.L., Appelo, C.A.J., 1999. User's guide to PHREEQC (Version 2)-A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations: U.S. Geological Survey Water-Resources Investigations Report, 99–4259. 310 pp. Rojas Delgado, R., De Pauli, C.P., Barriga Carrasco, C., Avena, M.J., 2008. Influence of MII/ MIII ratio in surface-charging behavior of Zn–Al layered double hydroxides. Applied Clay Science 40, 27–37. Smith, H.D., Parkinson, G.M., Hart, R.D., 2005. In situ absorption of molybdate and vanadate during precipitation of hydrotalcite from sodium aluminate solutions. Journal of Crystal Growth 275, 1665–1671. Tumiati, S., Godard, G., Masciocchi, N., Martini, S., Ponticelli, D., 2008. Environmental factors controlling the precipitation of Cu-bearing hydrotalcite-like compounds from mine waters. The case of the “Eve verda” spring (Aosta Valley, Italy). European Journal of Mineralogy 20, 73–94.

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