Insights into phosphate adsorption behavior on structurally modified ZnAl layered double hydroxides

Applied Clay Science 165 (2018) 234–246

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Research paper

Insights into phosphate adsorption behavior on structurally modified ZnAl layered double hydroxides

T

⁎

E.M. Seftela,b, , R.G. Ciocarlanb, B. Michielsena, V. Meynena,b, S. Mullensa, P. Coolb a b

VITO Flemish Institute for Technological Research, Boeretang 200, B-2400, Belgium Laboratory of Adsorption and Catalysis, Department of Chemistry, University of Antwerp (CDE), Universiteitsplein 1, 2610 Wilrijk, Antwerp, Belgium

A R T I C LE I N FO

A B S T R A C T

Keywords: Layered double hydroxides Structural modification Adsorption Phosphate Competitive sorption

The present study focuses on the phosphate uptake by synthetic ZnAl-layered double hydroxides having different charge density due to the modification of the cationic ratio (Zn2+/Al3+) within the brucite-like sheets. The structure of the as-synthesized ZnAl-LDH was confirmed by X-ray diffraction and 27Al NMR and micro-Raman spectroscopy characterization techniques. The materials were applied for sorption of phosphate anions in aqueous media under relevant conditions. Parameters affecting the sorption process were thoroughly investigated, such as the layer cationic ratio, the exchangeable interlayer anion, thermal treatment and competitive sorption in the presence of co-existing anions. The sorption data provided information regarding the relationship between the phosphate uptake and the physical-chemical properties of these materials. The phosphate adsorption onto the non-calcined LDH occurred via the anionic exchange mechanism while both structural reconstruction and precipitation mechanisms were observed for the calcined materials. The obtained results suggest that the Zn containing LDH are suitable candidates for the phosphate recovery or removal from aqueous media in wastewater treatment processes.

1. Introduction Phosphorous is an essential nutrient necessary for cell growth in plants, animals or bacteria and is essential ingredient in fertilizers, together with nitrogen and potassium, to sustain and enhance the crop yield. Over the past decades, the continuous progress and industrialization concomitant with an exponential increase in population and civilization expansion has intensified the nutrients use but also their uncontrolled release at different ecosystem levels. Most of the released phosphorous enters into rivers, lakes or oceans causing eutrophication, which may be translated as “dead zones” in aquatic systems where nothing can live due to the lack of oxygen (Cheng et al., 2009; Halajnia et al., 2013; Lalley et al., 2016). Different conventional techniques may be applied for the removal of phosphorous in aqueous media which, depending on their nature may be classified as physical (e.g. sedimentation, flotation or filtration), chemical (e.g. precipitation, ion exchange or adsorption) and biological (e.g. bacteria, algae or plants) with various levels of success. Among these processes, adsorption has gained an increased interest due to its simplicity, economic viability and technical feasibility. Moreover, it has the major advantage that it can achieve high efficiency in areas where the aqueous effluents contain low concentrations of pollutant

⁎

molecules, but still high for the maximum accepted levels for surface and drinking waters, where chemical precipitation or other similar techniques cannot be applied (Morse et al., 1998, He et al., 2010. Adsorption has been intensively studied for the uptake/removal of phosphorous/nutrients from aqueous effluents by using a wide variety of sorbent materials, such as polymeric hydrogels (Kofinas and Kioussis, 2003), modified ion exchange resins (Blaney et al., 2007), waste materials such as ZnCl2-activated coir pith carbon (Namasivayam and Sangeetha, 2004), Fe and Al oxides and hydroxides (Tanada et al., 2003, Li et al., 2016, filtralite (Ádám et al., 2007), MnO2 (Ouvrard et al., 2002, La2O3 (Ning et al., 2008), layered double hydroxides (LDH) and calcined products (He et al., 2010), etc. Among these, the layered double hydroxide (LDH) materials attract major attention due to their high capacity which may be compared to the organic anion exchange resins. These materials have been applied as sorbent materials for a wide range of anionic contaminants, such as oxyanions (nitrates, phosphates, arsenates, chromates, selenates…) (Goh et al., 2008; Grover et al., 2010; Lu et al., 2015) or monoatomic anions (chloride, fluoride..) (Bhatnagar et al., 2011 in aqueous media. LDH, also called hydrotalcites, are a class of clay minerals having positive layers and negative exchangeable interlayer species that occur naturally and may be easily synthesized in laboratory conditions. LDH-

Corresponding author at: VITO Flemish Institute for Technological Research, Boeretang 200, B-2400, Belgium. E-mail address: [email protected] (E.M. Seftel).

https://doi.org/10.1016/j.clay.2018.08.018 Received 15 December 2017; Received in revised form 13 August 2018; Accepted 21 August 2018 Available online 31 August 2018 0169-1317/ © 2018 Elsevier B.V. All rights reserved.

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E.M. Seftel et al.

type materials are generally described by the chemical formula [M2+ 13+ x+ 2+ (Anare divalent cations (Mg2+, xMx (OH)2] x/n) · mH2O, where: M Zn2+, Ca2+, etc.), M3+ are trivalent cations (Al3+, Fe3+, Mn3+, etc.) and An- are interlayer anions (CO32−, NO3−, etc.) (Cavani et al., 1991; Scott et al., 2004). These materials may be used as suitable sorbents for phosphate uptake due to their high anion-exchange capacity, surface areas, and flexible interlayer region accommodating various anionic species. Moreover, their chemical composition can be easily tuned allowing not only a high ion exchange capacity but also high selectivity towards the phosphate ions. There are four mechanisms that can be involved in the removal of anions with the LDH-type materials, namely surface complexation, surface precipitation, interlayer anion exchange and structural reconstruction via the unique memory effect property. This unique memory effect property allows to a calcined material, which consist of a solid solution of mixed metal oxides also called layered double oxides (LDOs), to rebuild its original layered structure when placed in contact with an aqueous solution containing anions. This property also induces an increased sorption capacity when compared with the non-calcined LDH material. Many factors may affect the sorption capacity as well as the selectivity towards phosphate anions when dealing with LDH-type materials, such as the composition of the brucite-like layers, the interlayer exchangeable anions or the calcination temperature and competing anions in the polluted aqueous solutions. For example, many studies deal with the use of the parent of the LDH family, namely the MgAlcontaining mineral called hydrotalcite, reporting different sorption capacities as a function of the Mg/Al cationic ratio or the interlayer anion (Morimoto et al., 2012; Halajnia et al., 2013). Furthermore, studies involving different cation combinations between divalent Mg2+, Zn2+, Ca2+, Ni2+, Co2+, Cu2+ and trivalent Al3+, Fe3+, Mn3+, even monovalent Li+ or tetravalent Zr4+ cations, are also reported (Das et al., 2006; Chitrakar et al., 2010). These all indicate different sorption capacities, mechanisms or kinetics as a function of the layer composition. Selectivity may be a problem for anion exchangers as competing anions (e.g. sulphate, chloride, carbonate or bicarbonate) may co-exist in wastewaters and in some cases are taken up in favour of phosphate because of their greater concentrations and/or enhanced electrostatic interaction (Blaney, Cinar et al. 2007). Zn-containing LDH and their calcined products are reported as good candidates with high adsorption selectivity towards phosphate anions via an anionic exchange or memory effect mechanism, respectively (He, Kang et al. 2010). He et al. concluded that the calcined LDOs and the NO3−-type ZnAl-LDH exhibit high selectivity towards phosphate ions only. The Cl−-type LDH shows selectivity towards both sulphate and phosphate ions while the CO32−LDH did not show remarkable selectivity. On the other hand, this study as well as many reports that are dealing with the investigation of the selectivity towards phosphate sorption by LDH describe the competitive adsorption behavior in the presence of sulphates, nitrates and chlorides but rarely carbonates. The latter ones may actually have the most interfering effect among all the competitive anions due to their wellknown high affinity for the LDH structures. Secondly, very important are the sorption capacity and rate which, for feasibility in large scale applications, have to be high. For the Zn-containing LDH, the reported sorption capacities are rather low, e. g. average around 20 mg-P/g (He, Kang et al. 2010). Very limited reports are describing sorption capacity of the ZnAl-LDH system up to 50 mg-P/g (Cheng et al., 2009). Therefore, Zn-containing LDH represents a good candidate for selective sorption of phosphates if their sorption capacity and sorption rate can be increased. This study aims at the development of highly selective ZnAl-LDH with increased sorption capacity for efficient phosphate sorption in aqueous media. The increase of their sorption capacity is achieved by the structural modification of the brucite-like sheets towards increased 235

sorption sites which may enhance their sorption features. The sorption behavior of phosphate anions on the newly developed ZnAl-LDH is investigated in depth as a function of the cationic ratio in the LDH sheets as well as the different intercalated anions (CO32−, NO3−, Cl−) and further, on their derived mixed metal oxides (LDOs) obtained by controlled heat treatment. The selectivity towards phosphates is also investigated in the presence of competitive anions, such as carbonates and bicarbonates, sulphates, nitrates and chlorides. 2. Materials and methods 2.1. Synthesis of ZnAl-LDH materials The ZnrAl-CO3 or ZnrAl-NO3, where r stands for the Zn/Al cationic ratio in the synthesis solutions having hydrotalcite structures were prepared by the conventional co-precipitation method at constant pH (detailed on synthesis procedure in SI file, S2.1.) [1]. The chlorine containing LDH, Zn1.25Al-Cl, was obtained following the acid–salt method applied on the Zn1.25Al-NO3 LDH sample (detailed on synthesis procedure in SI file, S2.1.) (Liu et al., 2006. The derived mixed metal oxides (ZnrAl-LDOs) were prepared by heating the powders at different temperatures, with a heating rate of 2 °C/min and an isothermal period of 4 h in air. The corresponding calcined products were denoted as ZnrAlcT, where c stands for calcined product and T is the corresponding calcination temperature. 2.2. Characterization Energy Dispersive X-ray Spectra (EDX) were recorded in order to determine the chemical composition of the LDH samples. The reported compositions are calculated as an average of 5 measurements. The structure and the crystal phases of the prepared materials were investigated by X-ray diffraction (XRD) recorded on a PANalytical X'Pert PRO MPD diffractometer with filtered CuKα radiation; measurements were done in the 2θ mode using a bracket sample holder with a scanning speed of 0.04°/4 s in continuous mode. Raman measurements were done using a HORIBA XploRA PLUS V1.2 MULTILINE Cofocal Raman microscope under the same setting conditions. A 532 nm diode-pumped solid-state (DPSS) laser with a power of 25 mW was used for excitation and all the spectra were collected with an accumulation time of 20 s. 27 Aluminum solid-state MAS NMR spectra were acquired on an Agilent VNMRS DirectDrive 400 MHz spectrometer (9.4 T wide bore magnet) equipped with a T3HX 3.2 mm probe. Magic angle spinning (MAS) was performed at 20 kHz with ceramic rotors of 3.2 mm in diameter (34 μL rotors). The signal of AlCl3 was used to calibrate the chemical shift scale (0 ppm). Acquisition parameters used were the following: a spectral width of 192 kHz, a 90° pulse length of 2.8 μs, an acquisition time of 15 ms, a recycle delay time of 3 s and 2000 scans. Porosity and surface area studies were performed on a Quantachrome Quadrasorb-SI automated gas sorption system using nitrogen as the adsorbate at liquid nitrogen temperature (−196 °C). Prior outgassing step was performed under vacuum for 24 h at room temperature for the LDH samples and at 200 °C for the LDO samples, respectively. The surface area was calculated using the BET method in the range of relative pressure 0.05–0.35. The thermoanalytical measurements were performed on a Mettler Toledo TGA/SDTA851e thermobalance. Samples were heated at a heating rate of 5 °C/min under O2 flow. 2.3. Batch adsorption experiments Batch adsorption experiments were carried out to evaluate the performances of the ZnAl-LDH materials and the derived calcined mixed oxides as phosphate adsorbents. The adsorption kinetics and adsorption isotherms together with the role of co-existing anions (Cl−,

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E.M. Seftel et al.

Fig. 1. XRD patterns of the ZnrAl-LDH samples with different cationic ratio and interlayer anions.

NO3−, SO42−, HCO3– and CO32−) were investigated. Batch experiments were realized at a solid/liquid ration of 1 g-adsorbent/L of KH2PO4. The kinetic study was realized using an initial concentration of 200 mg-P/L over 360 min of magnetic stirring. Adsorption isotherms were determined at initial concentrations ranging from 5 mg-P/L to 200 mg-P/L and 24 h stirring. Detailed description on the experimental procedures for study of the effect of contact time, and the role of co-existing anions, as well as the calculation of the adsorption capacity and evaluation of the adsorption kinetics and isotherms are included in SI file, S2.3.

material with a cationic ratio of 2 (Scott et al., 2004; Costa et al., 2012) (Table 1). Taking into account the high Al degree, especially in the samples with low cationic ratio up to 1.25, Raman and 27Al solid state NMR were recorded to assess the purity of the as-synthesized materials as well as to detect any degree of disorder within the brucite-like layers. The obtained Raman spectra are included in Fig. 2 A together with the 27 Al solid state NMR spectra for the Zn1.25Al-NO3 sample (Fig. 2B) with the highest Al content (44 mol%Al). All the LDH samples present the same adsorption characteristics, e. g. the doublet centered at 490 cm−1 and 550 cm−1 corresponding to the translational vibrations modes EgT and EuT of the M-OH planes as well as the band at 710 cm−1 corresponding to the EuR(OH) vibration mode (Pushparaj et al., 2015). Close inspection of the Raman spectroscopic profiles in the low-energy domain (below 300 cm−1) reveals the presence of a weak shoulder centered at ~260 cm−1 for the samples with high Al content, e. g. higher than 33 mol%Al as in the Zn2Al-LDH type material. This shoulder may be related with the presence of sites composed of amorphous aluminium hydroxide phase (AlOH) which could not be detected by XRD (Pushparaj et al., 2015). This weak shoulder at 260 cm−1 only starts to appear with the increase of the Al content as in the Zn1.25Al-NO3 sample. Therefore, we recorded the 27Al solid state NMR spectra for further investigate the phase purity of the Zn1.25Al-NO3 sample (Fig. 2B). Thermal stability and porosity evolution during calcination were investigated further. Fig. 3A shows the TG-DTG curves obtained for the

3. Results and discussions 3.1. Structural characterization In this study, Zn-containing anionic exchangers with characteristic layered double hydroxide structure were prepared. The XRD patterns of the as-synthetized layered double hydroxides are presented in Fig. 1. In all cases, the XRD patterns indicate the formation of highly crystalline LDH structures, with sharp and symmetric reflections assigned to the basal (003) and (006) planes together with the asymmetric and less sharp reflections assigned to the (101), (015), (018), (110) and (113) planes (Cavani et al., 1991; Seftel et al., 2008). The calculated basal spacing of the nitrate containing LDH is ~0.899 nm, while for the carbonate and the chlorine correspondents these are 0.762(1) nm and 0.778(1) nm, respectively, corresponding well with the values reported in the literature for a ZnAl-LDH type

Table 1 Chemical and structural characteristics of the prepared LDH materials and their corresponding calcined products. Sample

Zn/Al initial

Zn/Al final

x

Chemical composition of the brucite-like layers

d003, nm

c, nm

a, nm

Cd, e/nm2

SBET, m2/g

Zn1.25Al-NO3 Zn1.5Al-NO3 Zn2Al-NO3 Zn1.25Al-CO3 Zn1.25Al-Cl Zn1.25Alc325 Zn1.25Alc500

1.25 1.5 2 1.25 1.25 1.25 1.25

1.25 1.44 2.01 1.22 1.2 1.25 1.25

0.443 0.410 0.333 0.451 0.454 – –

[Zn0.557Al0.443(OH)2] [Zn0.590Al0.410(OH)2] [Zn0.667Al0.332(OH)2] [Zn0.549Al0.451(OH)2] [Zn0.545Al0.454(OH)2] – –

0.899(1) 0.899(1) 0.896(2) 0.762(1) 0.778(1) – –

2.697(5) 2.697(3) 2.688(8) 2.286(3) 2.334(5) – –

0.307(7) 0.308(3) 0.309(3) 0.307(6) 0.307(7) – –

nd 4.980(8) 4.019(3) nd nd – –

40 23 10 51 62 90 181

x – MIII/MII + MIII (molar ratio). Cd – charge density calculated as Cd = xe/(a2sin60°), e/nm2; nd – not determined as the sample may contain Al-enriched sites or AlOH amorphous phase. a and c – unit cell parameters a = 2d110 and c = 3d003, nm. SBET – specific surface area calculated by the BET method, m2/g. 236

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Fig. 2. A: micro-Raman spectra of the (a) Zn1.25Al-NO3, (b) Zn1.5Al-NO3, (c) Zn2Al-NO3 samples; B:

Zn1.25Al-NO3 sample indicating three main weight loss stages. The first weight loss stage up to 160 °C corresponds with the loss of the water molecules from the surface and interlayer gallery. The second stage up to 325 °C corresponds with the decomposition of the nitrate anions and dehydroxylation of the LDH sheets (overlapping steps), followed by a third broad weight loss up to 500 °C corresponding to the completion of transformation into a mixed metal oxide lattice (Seftel et al., 2008). Accordingly, for the investigation of the phosphate sorption via the reconstruction mechanism, the Zn1.25Al-NO3 sample was thermally treated at 325 °C and 500 °C, respectively. Further, porosity features were investigated by using the N2 adsorption/desorption technique. Fig. 2B shows the N2 sorption isotherms of the Zn1.25Al-NO3 and calcined products at different temperatures. The Zn1.25Al-NO3 and its correspondent calcined at 325 °C show Type II isotherms which are obtained in case of aggregates of plate-like particles. The non-reversible character of the isotherm forming a hysteresis loop similar to a H3 type may be associated with the development of a pore network generated by the interconnection of the plates. These types of isotherms are now termed Type IIb and indicate the presence of external interparticle porosity characteristic for clay powders. The Zn1.25Alc500°C shows a Type IV isotherm with a H3 hysteresis loop indicating the presence of slit-shaped pores (Rouquerol et al., 1999). A decrease in porosity may be observed as the Zn/Al cationic ratio decreases which may be associated with the increase of the Zn content, while an increase in the mesoporous features may be observed for the

27

Al NMR spectra of the Zn1.25Al-NO3 sample.

calcined products. The release of anions from the interlayer gallery increases the porosity up to 90 m2/g. The porous features are further enhanced by complete transformation into a mixed metal oxide lattice (Table 1) (Rouquerol et al., 1999; Seftel et al., 2008; Li et al., 2011). 3.2. Phosphate adsorption study In order to successfully apply LDH for phosphate adsorption processes, it is important to investigate the phosphate adsorption behavior as a function of time. Fig. 4 includes the sorption data obtained on the studied ZnAl-LDH materials and calcined materials. The data generated show that, when working with the non-calcined LDH, the adsorption performances are strongly dependent on the material properties in terms of both layer and interlayer composition, which is expected to affect not only the anionic exchange capacity but also the selectivity/affinity towards the phosphate anions (Fig. 4A and B). Further, the sorption data obtained on the thermally treated LDH showed a slower (up to 360 min) uptake of the phosphate anions due to the different sorption mechanism involved in the reconstruction of the hydroxylated layers, restacking into a LDH lattice and the re-population of the interlayer gallery. Although slower, the sorption capacity enhances with approximately 22 mg-P/g when the sample is thermally treated at 325 °C (Fig. 4C). The adsorption data obtained on the LDH materials having different molar cationic ratio in the brucite-like sheets (Fig. 4A), indicate that the

Fig. 3. (A) TG-DTG curves of Zn1.25Al-NO3 and (B)N2 adsorption/desorption isotherms of the Zn1.25Al-NO3 and calcined products.

237

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Fig. 4. Adsorbed phosphate amounts as a function of time on the ZnAl-LDH materials with (A) different Zn/Al molar ratio, (B) different interlayer anion and (C) Zn1.25Al-NO3 calcined at different temperatures.

first stages of the process. Thus, samples with low Zn/Al molar ratios (Zn1.25Al-NO3 and Zn1.5Al-NO3) adsorb about 70% of phosphate in 1 min and about 90% after 3 min, while the Zn2Al-NO3 sample adsorbs only ~70% of phosphate in the first 3 min. This is due to the decrease in Zn/Al ratios which is in agreement with literature reports describing higher adsorption rates of LDH materials at lower M2+/M3+ molar ratios (Das et al., 2006; Islam and Patel, 2009; Halajnia et al., 2013). It is related with the increased charge densities on the LDH surfaces by lowering the M2+/ M3+ ratios. The calculated charge densities as a function of cationic ratio are also observed to increase with the same trend. The adsorption data showed that 56 mg-P is adsorbed per gram of Zn2Al-NO3 powder having a charge density of ~4.019 e/nm2. This amount increases up to ~88 mg-P per gram of LDH powder when decreasing the cationic ratio up to 1.5 due to the increase of the charge densities to 4.980 e/nm2 (see Table 1). After this value, no increase in sorption ability may be observed, which may be assigned on the phase inhomogeneity due to the high Al content. The anionic exchange abilities are also influenced by the anionic species located in the interlayer gallery of the LDH materials. This is highlighted by the adsorption data included in Fig. 4B where the LDH with different interlayer anions were used for phosphate adsorption studies. It is well known that the LDH-type materials have high affinity for carbonate anions which limits their anionic exchange abilities. The reported series for the anion affinity for the LDH, solids which also

phosphate adsorption is strongly influenced by the Al content in the LDH sheets which is correlated with the phase purity as demonstrated by spectroscopic investigation by micro-Raman and 27Al SS NMR. The phosphate uptake ability results from the positively charged hydroxylated sheets generated by the isomorphic substitution of the Al3+ within the Zn-OH lattice. The decrease in the Zn/Al molar ratio, thus the substitution of more Al3+ cations within the LDH lattice, enhances the charge density on the brucite-like sheets with subsequent increase of their anionic exchange capacities. On the other hand, for the sample with high Al content (up to 44 mol%Al), the additional AlOH type phase brings different adsorption sites at the surface of the LDHtype sheets. The increase of sorption capacity is observed when the cationic ratio decreases up to 1.5 which can be correlated with the increase of the charge density on the surface of the LDH sheets. Further decrease of the cationic ratio to 1.25 does not result in an increase of sorption capacity (Fig. 4A). Taking into account the relative intensity of the two types δiso (27Al) in the 27Al SS NMR deconvoluted spectra, it can be assumed that an LDH phase with the Zn/Al ration of ~2.3 similar to the Zn2Al-NO3 sample mixed with a AlOH amorphous phase is formed. As such, it is difficult to propose a sorption mechanism in this case, but we may conclude that a combined anionic exchange with surface sorption at the Al-enriched sites may be responsible for the high sorption capacity observed for the Zn1.25Al-NO3 sample. The data show not only enhanced adsorption capacity at lower Zn/ Al molar ratios but also a slightly faster rate of anionic exchange in the 238

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E.M. Seftel et al.

Table 2 Determined kinetic parameters for phosphate adsorption on the different LDH materials, modelled for an initial concentration of 200 mg-P/L. Kinetic model Experiment Pseudo-first order

ln(qe − qt ) = ln(qe ) − Pseudo-second order t qt

1

=

k 2 qe 2

+

t qe

Elovich

qt =

1 b

ln(ab) +

1 b

ln(t )

k1 t 2.303

qe (mg/g) qe (mg/g) k1 (min−1) R2 qe (mg/g) k2 (g/mg.min) R2 a (mg/g.min) 1/b (mg/g) R2

Zn1.25Al-NO3

Zn1.5Al-NO3

Zn2Al-NO3

Zn1.25Al-Cl

Zn1.25AlCO3

Zn1.25Alc325

Zn1.25Alc500

84 34.328(6) 0.605(9) 0.584(7) 84.317(1) 0.035(9) 0.999(9) 3.67*107 4.251(9) 0.631(2)

88 32.414(3) 0.315(5) 0.680(1) 81.967(2) 0.020(1) 0.999(9) 1.09*105 6.655(7) 0.827(8)

56 47.874(8) 0.752(1) 0.978(1) 58.823(5) 0.016(7) 0.999(8) 6.00*102 7.219(6) 0.828(7)

57 45.672(2) 0.095(2) 0.836(8) 50.581(7) 0.022(9) 0.999(7) 3.39*105 3.245(8) 0.905(6)

14 9.647(8) 0.239(4) 0.849(5) 14.836(8) 0.023(3) 0.998(8) 22.0 2.340(7) 0.963(6)

104 86.383(7) 0.276(4) 0.985(4) 111.111(1) 0.365*10−3 0.995(2) 16.3 17.424(0) 0.994(7)

94 85.614(1) 0.253(6) 0.987(5) 106.383(1) 0.183*10−3 0.979(0) 8.43 17.040(7) 0.972(1)

qt and qe are the amount of phosphate sorbed (mg-P/g) at time t and at equilibrium, respectively; k1 and k2 are the apparent adsorption rate constants, a and b are constants of the Elovich equation; constants in pseudo-second order and Elovich equations are calculated without using q0 (t = 0, q0 = 0).

phosphate sorption on the studied ZnAl-LDH type materials. It is observed that, depending on the material composition and thermal treatment, the phosphate sorption reached equilibrium differently. The non-calcined materials reach equilibrium in very short times via anionic exchange. The fast anionic exchange process is confirmed by X-ray diffraction acquired on the recovered materials after the sorption tests (as it will be demonstrated in a later section). The situation is, however, different for the thermally treated materials which are attaining equilibrium within longer times. This is due to the reconstruction mechanism that occurs through a dissolution-recrystallization mechanism with concomitant re-hydroxylation into brucitic lattice and gradual restacking of the layers on top of each-other in longer periods of time. In this case, the phosphate sorption profiles are better fitted by more than one kinetic model as compared with the non-calcined LDH, indicating a possible combination of uptake mechanisms. The data generated from the effect of time experiments were fitted into four kinetic models: Lagergren pseudo-first and pseudo-second order, Elovich and intra-particle diffusion kinetic models (S., 1898; Weber and Morris, 1963; Fierro et al., 2008; Olu-Owolabi et al., 2014; Suresh and S, 2014). These common models are used in order to determine the rate constants of adsorption and to find out information on the sorption mechanisms. Table 2 includes the kinetic models parameters calculated by the four kinetic models for the ZnrAl-LDH and the calcined materials. According to the adsorption data included in Fig. 4, at low Zn/Al cationic ratios (e. g. 1.25 and 1.5) the anion uptake is sharp at initial times (up to 3 min.) followed by a slower uptake in the next stage reaching equilibrium in 20 to maximum 60 min. For the Zn2Al-NO3 sample, the phosphate uptake occurs slower from the start, and this is also reflected in the calculated rate constants (see Table 2). This may be correlated well with the previous observations related with the phase purity when the Al content is higher than 33 mol% indicating that the Al-rich sites and/or the presence of AlOH amorphous sites may be responsible for the different sorption behavior of the as-prepared materials, as it is well known that the aluminium hydroxides manifest also high affinity for phosphates (Tanada et al., 2003). The equilibrium sorption capacity (qe) for each material was also estimated from the kinetic models. These values together with the correlation coefficients (R2) evaluated by pseudo-first and pseudo-second order kinetic models showed that the phosphate sorption fit the pseudosecond order model better. The small deviations observed in the estimated qe values by fitting the data by the pseudo-second order kinetic model may be attributed to uncertainties inherent in obtaining the experimental qe values. The application of this kinetic model is visualized in Fig. 5. Different steps are involved in the adsorption process, e. g. mass transfer from the solution to the sorbent surface, diffusion through the boundary layer surrounding the sorbent, surface adsorption and internal diffusion within the sorbent (Olu-Owolabi et al., 2014). The Elovich and intra-particle diffusion (IPD) kinetic models were

highlights their anionic exchange selectivity, is in the following order (Cavani et al., 1991; Scott et al., 2004): NO3− < Br− < Cl− < F− < OH– < SO42− < CrO42− < HAsO42− < HPO42− < CO32−, with preferential affinity for the anions with higher valence rather than monovalent state. Accordingly, the chlorine and especially the nitrate containing LDH will easily exchange the phosphate, while carbonate will limit the anion exchange property of the LDH materials. This is indeed observed for the CO3-containing sample for which the measured sorption capacity at equilibrium is only 14 mg-P/g and is probably due to the sorption on the external surface of the material. The data included in Table 1 indicate that the carbonate sample has a higher porosity among the non-calcined samples, and this parameter is probably playing the role in the phosphate sorption in this particular case. For the nitrate or chlorine containing correspondents, the anionic exchange mechanism is responsible for the phosphate uptake, where the measured sorption capacity is higher for the LDH-NO3 as compared with the LDH-Cl sample, which is in good agreement with the above order of anionic exchange preference as well as the higher interlayer spacing in the case of the LDH-NO3 material (d003 of 0.896 nm) as compared with the LDH-Cl correspondent (d003 of 0.776 nm). The anionic exchange mechanism is confirmed using analyses of X-ray diffractograms and in detail discussed in a later section. The adsorption profiles for the calcined samples (Fig. 4C) indicate that the phosphate adsorption via the reconstruction mechanism occurs slower (within 360 min) and is also dependent on the calcination temperature. The results reveal that the phosphate uptake increases when calcining at 325 °C and then lowers upon calcination up to 500 °C. The improvement of the adsorption capacity by Zn-Al LDH calcination up to 300 °C followed by a decrease in the adsorption capacity upon calcination at higher temperatures was also addressed previously (Mandal and Mayadevi, 2008; Cheng et al., 2009) as a consequence of spinel-type ZnAl2O4 formation upon increased calcination temperature. In the present experimental conditions by calcination at 500 °C the structure is completely transformed into a mixed metal oxide lattice with predominant ZnO phase. This is also confirmed by XRD patterns recorded on each calcination step (data shown in Fig. 9 which is discussed in a later section). Secondly, it should be noted that the phosphate adsorption process occurs at pH between 6 and 7.5 (data provided in SI) which is close to the isoelectric point (IEP) of the ZnO (reported IEP values between 6.4 and 7.13). Although the Zn1.25Alc500°C is endowed with a higher specific surface area as compared with the Zn1.25Alc325°C correspondent, the lowering of the adsorption capacity upon calcination up to 500 °C may be also assigned to some degree of agglomeration of the Zn1.25Alc500°C particles in solution. 3.3. Kinetic modeling of phosphate adsorption The kinetic study is correlated with the effect of time on the 239

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Fig. 5. Application of pseudo-second order kinetic model to the experimental data obtained on (A) non-calcined ZnAl-LDH and (B) calcined ZnAl-LDO.

experiments. This model gives indication a sorption process is controlled by diffusion phenomena when the qt vs t1/2 results in a linear plot. Secondly, if this plot passes through the origin, it might be concluded that only the diffusion phenomenon plays a role in the adsorption process. The present sorption data fitted by this model suggest that the phosphate sorption process is controlled by more than one mechanism because the plotted qt vs t1/2 is multi-linear. Two stage adsorption process is observed for the LDH-type materials while a three step adsorption process occurs in the case of the calcined materials (LDOs) (Fig. 7). The model allows the calculation of the constant related with the thickness of the boundary level θ. In general, a larger value of the θ indicates a greater boundary layer effect, and this was calculated for each step in the adsorption process (see Table 3). The first step in the adsorption profile observed in case of the noncalcined ZnAl-LDH samples (Fig. 7A) may be related with the diffusion of the phosphate anions from the solution to the surface and anionic exchange at the edges of the LDH particles, a so-called external surface adsorption or film diffusion. The second step may be due to the intraparticle diffusion, e. g. interlayer diffusion, which can be associated with the anionic exchange between the nitrate with phosphate anions in the interlayer region. The first step is characterized by smaller k3 and θ values than observed for the second step indicating that the surface

used in order to investigate if the phosphate adsorption onto the LDH or LDO occurs via chemical interaction or that diffusion may be the rate limiting step. The correlation coefficients obtained by fitting the data with the Elovich model (Table 2, plotted curves are given in Fig. 6) are low except for the calcined materials, which may suggest that there occurs a chemical interaction between the phosphate molecules and the LDOs surface. This leads us to conclude that, apart from the re-hydroxylation of the layers and re-population of the interlayer gallery, a chemical bonding of phosphate on the surface sites during the sorption process may occur. This conclusion is also confirmed by X-ray diffraction measurements recorded after the sorption tests which indicated the formation of hopeite (Zn3(PO4)2·4H2O) phase during the reconstruction mechanism as it will be in detail discussed in a later section. The internal diffusion in case of the LDH materials is controlled by the anion affinity for the brucite-layers while the reconstruction mechanism of the mixed oxides is dependent on the applied heat treatment. Surface or film diffusion (at the surface and edges of the LDH particles) or intra-particle diffusion (interlayer diffusion towards the anionic exchange mechanism on LDH or towards repopulation of the interlayer gallery on LDOs) may occur for this type of materials. Therefore, the intra-particle diffusion model (Weber and Morris, 1963) was used to fit the data obtained from the phosphate sorption

Fig. 6. Application of Elovich kinetic model to the experimental data obtained on (A) non-calcined ZnAl-LDH and (B) calcined Zn1.25Al-LDOs. 240

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Fig. 7. Application of intraparticle diffusion kinetic model to the experimental data obtained on (A) non-calcined ZnAl-LDH and (B) calcined Zn1.25Al-LDO.

indicated by the Elovich model (1st step) (as also demonstrated by XRD analysis in a later section), re-hydroxylation of the brucite-like sheets (2nd step) and re-population of the interlayer gallery with the phosphate anions (3rd step). The calculated rate constants are the highest for the 2nd step indicating that the re-hydroxylation of the brucite-like sheets occurs very fast, followed by the slower re-population of the interlayer gallery with phosphate anions. The high values calculated for the thickness of the boundary layer θ indicate that the intra-particle diffusion associated with the phosphate intercalation plays the main role in the overall sorption process.

diffusion plays less role as rate-limiting step in the overall sorption process. The phosphate anions are adsorbed onto the surface and edges of the LDH particles being afterwards transported via intra-particle (interlayer) diffusion and retained at the exchange sites. Given the higher values of the thickness of the boundary layer θ, the second step may be considered as rate controlling step in the adsorption process. Interlayer diffusion in LDH is also dependent on the interlayer distances and anion size and is further promoted depending on the anions affinity. This effect can be observed by comparing the data obtained for the NO3-LDH material with the Cl-LDH and CO3-LDH correspondents having the same cationic ratio of 1.25. The higher unit cell parameter c characteristic for the LDH-NO3 (c of ~2.698 nm) favours the diffusion of the phosphate anions from the edges in the interlayer gallery as compared with its LDH-Cl correspondent (c of ~2.335 nm). The thickness of the boundary layer creates a higher gradient (θ2-θ1) between interlayer space and the particles edges, therefore increasing the rate of interlayer diffusion from the first to the second stage of adsorption as observed from the data included in Table 3. A more complex adsorption process is observed when fitting the adsorption data obtained on the calcined Zn1.25Al-LDOs (Fig. 7B). The three steps may be correlated with the hopeite precipitation as also

3.4. Adsorption isotherms (effect of initial P concentration) Adsorption isotherms (Fig. 8) were obtained on the Zn1.25Al-NO3 and its calcined products by measuring the equilibrium adsorption capacity at increased initial phosphate concentrations ranging from 5 mgP/L to 200 mg-P/L, respectively. The apparent distribution coefficient (Kd), which is related with the distribution of the ions between the solid sorbent and the aqueous phase, was calculated as: Kd = qe/ Ci (L/g) (Torres-Dorante et al., 2009).

Table 3 Intra-particle diffusion parameters. qt = k3t0.5 + θ 1st step

Zn1.25Al-NO3 Zn1.5Al-NO3 Zn2Al-NO3 Zn1.25Al-Cl Zn1.25Al-CO3 Zn1.25Alc325 Zn1.25Alc500

2nd step

3rd step

k3.1

θ1

R2

k3.2

θ2

R2

k3.3

θ3

R2

0.055(5) 0.0572(2) 0.077(7) 0.134(5) 0.309(4) 0.194(5) 0.849(1)

46.211(2) 37.143(6) 15.963(5) 28.991(7) 1.861(1) 5.881(2) 5.260(5)

0.925(2) 0.974(3) 0.999(6) 0.976(0) 0.993(9) 0.956(4) 0.914(1)

0.704(8) 1.233(5) 0.260(2) 1.012(1) 0.548(6) 8.223(3) 7.939(3)

79.052(8) 78.676(1) 54.242(1) 42.195(1) 9.966(4) 6.174(1) −10.191(5)

0.720(6) 0.970(7) 0.875(1) 0.991(5) 0.999(7) 0.990(3) 0.985(8)

– – – – – 2.048(6) 2.042(6)

– – – – – 65.527(6) 54.928(7)

– – – – – 0.982(3) 0.999(9)

k3.x - the adsorption rate constant (mg/g.min-0.5) determined for each step observed in the adsorption profile, θ - the thickness of the boundary layer calculated for the first, second and third stage of adsorption, respectively.

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Fig. 8. Phosphate adsorption isotherms (A) and variation of RL values (B) of the Zn1.25Al-NO3 and calcined products.

orientation at low phosphate concentrations towards more perpendicular orientation at higher phosphate concentrations. For the calcined products, the Kd values start to decrease at lower initial phosphate concentrations suggesting that the reconstruction of the layered network is affected by the initial concentration of anions in solution. This is also confirmed by X-ray diffraction measurements on the recovered solids after the sorption tests (Fig. 9 B and C). It shows that the reconstruction degree increases with the increase of the initial phosphate concentration as the intensity of the (003) basal reflection in the XRD pattern increases in the same trend. It is important to note that the unit cell parameter increases only up to ~2.765 nm and remains constant for the entire range of concentrations, suggesting that only a small part of phosphate and possible carbonate anions present as impurity in the aqueous solutions are intercalated in the interlayer galleries. Further, the phosphate anions are precipitating on the surface of the brucite-like sheets in the form of hopeite (Zn3(PO4)2·4H2O) due to the high availability of zinc (Bhatnagar, 2013). Both LDH and hopeite phases are observed to form gradually on the account of the ZnO phase. The ZnO phase is observed to reduce in the same trend. It is reported that the hopeite formation during phosphate sorption on calcined Zn-containing LDH begins at a pH below 7.5, while a pH above 8 to 9 would inhibit the hopeite formation (Koilraj and Kannan, 2010). In the present experimental conditions, the pH was also monitored (data provided in SI) but not adjusted to high values (above 8 or 9) because this would represent a disadvantage in further technology transfer to real environmental conditions. The pH of the phosphate solution is ~5.5 and remains at values below 6 in case of using the noncalcined structures, but the formation of hopeite is never detected in this case for any phosphate concentration. This observation, together with the measurements on the metal traces in the effluent solutions of the sorption tests (Fig. 9D), highlight the great stability of the cations within the LDH structure, representing an additional advantage for their use in real environmental depollution applications. For the calcined products, the pH increases up to values of 7–7.5 but never above 8, therefore facilitating the hopeite formation together with the reconstructed LDH. Some traces of Zn2+ are detected after the sorption tests (Fig. 9D), but these amounts are always below the maximum limits accepted in the surface waters (Agency, 1991; Organization, 2006). Further to investigate the sorption mechanism on the different sites, e.g. anionic exchange or electrostatic interaction and on the Al-enriched domains, micro-Raman spectra were recorded (Fig. 10). At low concentration, phosphates are distributing between the surface hydroxyl groups of both LDH and Al-enrich phases as the intensity of the M-OH

The obtained values are listed in Table 4. The calculated Kd values are constant at low initial phosphate concentrations and decrease at higher initial phosphate concentrations indicating that the saturation of the adsorption sites occurs only at higher concentrations. Good agreement with the high adsorption capacities measured for the studied samples. For the non-calcined Zn1.25Al-NO3 sample, the Kd value of 1 remains constant up to initial concentrations of ~75 mg-P/g indicating that the distribution coefficient is independent of surface adsorption due to the phosphate adsorption into the interlayer gallery through the anionic exchange mechanism. The greater Kd at low concentrations (e. g. 5 up to 100 mg-P/ L) indicates the remarkable affinity of phosphate for the synthesized LDH materials in this range of concentrations as compared with their corresponding calcined products (LDOs). The different mechanisms, e.g. electrostatic interaction at surface or interlayer by anionic exchange and surface sorption at the Al-enriched domains were investigated by post-sorption characterization on the recovered solids. First, the anionic exchange mechanism was confirmed by analyses of X-ray diffractograms (Fig. 9A). It is observed that the unit cell parameter c, which is related to the interlayer gallery thickness and stacking of the brucite-like sheets, increases from ~2.697 nm (NO3LDH) to values between ~3.375 nm and ~3.494 nm corresponding to phosphate exchanged LDH (Badreddine et al., 1999). It is interesting to note that the interlayer basal spacing increases with increasing phosphate concentrations. This may be related with a different orientation of the phosphate anions in the interlayer gallery, e. g. from more tilted

Table 4 Apparent distribution coefficient for phosphate sorption on the Zn1.25Al-NO3 and calcined products. Ci (mg-P/L)

5 15 24.6 50.7 75.9 101 152 196

Zn1.25Al-NO3

Zn1.25Alc325

Zn1.25Alc500

Kd (L/g)

Kd (L/g)

Kd (L/g)

1 1 1 1 0.988(9) 0.831(6) 0.582(8) 0.433(6)

1 1 1 0.928(0) 0.866(9) 0.746(5) 0.633(5) 0.624(6)

1 0.934(6) 0.839(8) 0.794(8) 0.681(2) 0.612(8) 0.544(7) 0.522(1)

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Fig. 9. (A), (B) and (C) XRD patterns after phosphate uptake at different initial phosphate concentrations on the Zn1.25Al-NO3 and calcined products with main reflections of (°) LDH phase, (*) ZnO phase and (Δ) Zn3(PO4)2·4H2O hopeite phase; (D) metals evolution during sorption tests as a function of the calcination temperature.

as irreversible, for 0 < RL < 1 it is favourable reversible adsorption, linear in case of RL = 1 and unfavourable for RL > 1 (see Fig. 8B). The Freundlich adsorption model (Foo and Hameed, 2010) assumes that the adsorption occurs on a heterogeneous surface involving a multilayer adsorption mechanism, and that the adsorbed amount increases with increasing initial adsorbate concentration. The empirical dimensionless parameter n represents the energetic heterogeneity of the adsorption sites, and the adsorption is considered as satisfactory when 1 < n < 10. The isotherms fit better with the Langmuir model for the non-calcined Zn1.25Al-NO3 material, as highlighted by the high R2 value and calculated qm close to the value determined by the experiment. This is in accordance with the anionic exchange mechanism involved in the phosphate sorption onto LDH, the adsorption occurs on a finite number of sites corresponding with the maximum adsorption capacity of the LDH material. The adsorption is reversible as highlighted by the RL values ranging between 0 and 1 (see Fig. 8B). In the case of the calcined products, the isotherms fit better by applying the Freundlich model with the multilayer adsorption mechanism, with the adsorbed amount increasing as the initial

vibrations are decreasing in the same order. The exchange with the nitrates in the interlayer gallery is accomplished as the concentration increases. Thus, full anionic exchange occurs when phosphates reach concentration of 75 mg-P/L which is in well agreement with the variation of the Kd value as well as the increase in the interlayer distance observed by XRD as described above. The different mechanisms are schematically illustrated in Fig. 10. The equilibrium adsorption data were fitted by two different isotherm models, e. g. Langmuir and Freundlich adsorption models (Foo and Hameed, 2010), and the values of the adsorption parameters obtained in both models are listed in Table 5. The Langmuir adsorption model (Langmuir, 1918; Foo and Hameed, 2010) assumes the existence of a maximum adsorption limit corresponding to a complete monolayer formation of the adsorbate molecules on the adsorbent surface. This model considers the following assumptions: the adsorption is localized, all the adsorption sites on the sorbent have similar energies, there is no interaction between the adsorbed molecules and the limiting reaction step is the surface reaction. The dimensionless equilibrium parameter RL predicts the efficiency of the adsorption process, when for an RL = 0 the adsorption is considered 243

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Fig. 10. micro-Raman spectra recorded after phosphate uptake at different initial phosphate concentrations on the Zn1.25Al-NO3 and schematic illustration of the adsorption sites.

The presence of bicarbonate anions is not affecting the sorption behavior of the Zn1.25Al-NO3 sorbent, while its calcined product shows a reduction in efficiency towards phosphate adsorption with 25% or 28% when present as individual component or in combination with chlorine, nitrate and sulphate, respectively. Further, carbonate anions show the most interfering effect due to the high affinity for the LDHtype structures. The presence of bicarbonate anions is interfering more during the P sorption process when using the Zn1.25Al-Cl material as compared with the Zn1.25Al-NO3 correspondent. This might be related with the d003 interlayer spacings (see Table 1), e. g. the interlayer region where the anionic exchange occurs. The d003 value of 0.778(1) nm calculated for the Zn1.25Al-Cl is closer to the anion size reported for the carbonate or bicarbonate anions and therefore is favourable for exchanging these anions rather than phosphate. The higher interlayer spacing of the nitrate containing LDH (d003 of ~0.899 nm) facilitates the phosphate exchange rather than the carbonate or bicarbonate anions. Furthermore, in the case of calcined materials the selectivity towards phosphate may arise from the multiple mechanisms involved in anion uptake. For the Zn-containing LDOs, the formation of hopeite by precipitation also favours phosphate uptake rather than the other competitors. On the other hand, even if the calcined products show higher sorption capacities as compared with the non-calcined LDH, the slower uptake of the phosphate anions together with the formation of hopeite by phosphate precipitation may represent a disadvantage with regard to the possibility to regenerate the material for a subsequent sorption cycle. In any case, at this concentration, the phosphate adsorption efficiency is still high enough for practical applications due to the high anion exchange capacity of Zn-containing LDH material. Note that the aquatic systems which may be threatened by eutrophication may contain C in the HCO3– anionic form in much lower concentrations. Higher levels of bicarbonate up to 120 mg/L may be observed in wastewaters or secondary wastewaters (Park et al., 2010).

concentration increases, suggesting the heterogeneity of the adsorption sites, e. g. interlayer re-population and surface precipitation, as it was also indicated by the Elovich kinetic model observations. 3.5. Competitive sorption The phosphate adsorption potential of non-calcined nitrate and chlorine LDH, e. g. Zn1.25Al-NO3 and Zn1.25Al-Cl respectively, was evaluated in the presence of individual or combined competitive ions. Cl−, NO3−, SO4−, HCO32– and CO32– were selected as representative anions present in phosphate containing wastewaters or at different ecosystem levels. The effect of competitive ions was evaluated for an initial concentration of 25 mg-P/L together with equal concentrations of 25mgCl/L, 25mgN/L, 25 mg-S/L and/or 25mgC/L (as described in the Section 2.3). As demonstrated by the data presented in Fig. 11, the presence of individual or combined chlorine, nitrate or sulphate anions shows minimal impact on phosphate adsorption which is due to the lower affinity of these anions for LDH materials. Table 5 Determined parameters of Langmuir and Freundlich isotherms for adsorption of phosphate by Zn1.25Al-NO3 and its calcined products. Adsorption isotherm model Langmuir qe = KLqmCe / (1 + KLCe) Freundlich qe = KFCen

qm (mg/g) KL(L/mg) R2 KF n R2

Zn1.25Al-NO3

Zn1.25Alc325

Zn1.25Alc500

90.912(1) 0.003(4) 0.999(3) 1.097(6) 1.038(4) 0.998(3)

172.154(2) 0.002(5) 0.949(2) 1.218(9) 1.090(8) 0.997(7)

124.882(6) 0.006(5) 0.961(8) 1.361(0) 1.185(7) 0.998(1)

KL and KF represent the Langmuir bonding term related to interaction energies (L/mg) and the Freundlich affinity coefficient (mg(1-n)Ln/g), respectively, qm denotes the Langmuir maximum capacity (mg/g), Ce is the equilibrium solution concentration (mg/L) of the sorbate and n is the Freundlich linearity constant.

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Fig. 11. Sorption efficiency of P in the presence of competitive anions (equilibration time 60 min).

4. Conclusions ZnAl-LDH with different cationic ratio and interlayer anions were synthesized by the co-precipitation method at constant pH. Detailed spectroscopic investigation revealed the presence of Al-enriched domains when the materials are prepared with high Al content. The feasibility of using both the as-synthesized as well as their calcined LDO products as adsorbents for phosphate sorption from aqueous solutions was evaluated. The sorption results in combination with structural investigation before and after sorption tests provided information on the relationship between the phosphate uptake mechanism and the physical-chemical properties of these materials. The results showed that the phosphate uptake occurs fast and that the adsorption kinetics depends on the structural properties and sorption mechanism, which is different for a non-calcined and a calcined material. The adsorption of phosphate onto the non-calcined LDH occurs via the anionic exchange mechanism, while both structural reconstruction and precipitation mechanisms were observed for the calcined LDO materials. This was also confirmed by fitting the adsorption data by Langmuir or Freundlich isotherm models. The Langmuir model fitted better for the non-calcined samples indicating that the adsorption occurs on a finite number of sites corresponding with the maximum adsorption capacity of the LDH material. The heterogeneity of the adsorption sites observed when using the calcined products was demonstrated by the applicability of the Freundlich isotherm model. The detailed study on competitive sorption suggests that the Zn containing LDH materials are suitable for phosphate recovery or removal from aqueous media in wastewater treatment processes.

Acknowledgements E. M. Seftel greatly acknowledges the Fund for Scientific Research – Flanders (FWO – Vlaanderen) for financial support (project no. 12D3815N). The authors acknowledge the project NUREDRAIN which is financed by the Interreg North Sea Region Programme with financial support of the European Regional Development Fund. Pegie Cool and Radu-George Ciocarlan acknowledge financial support by FWO Vlaanderen project no. G038215N.

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2018.08.018. 245

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