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In situ ATR-FTIR spectroscopic study of the co-adsorption of myo-inositol hexakisphosphate and Zn(II) on goethite.

Introduction

Numerous studies have shown that oxyanions can substantially affect adsorption of metals onto mineral surfaces, and vice versa (Elzinga et al. 2001; Zhang and Peak 2007; Elzinga and Kretzschmar 2013). Metal adsorption may be enhanced or reduced in the presence of anion ligands, depending on the physicochemical properties of the adsorbent, pH levels, surface coverage, as well as type and concentration of the metal ions and ligands involved (Elzinga et al. 2001; Elzinga and Kretzschmar 2013). Reduced metal adsorption is generally ascribed to the competition between metal-ligand complexation in solution and metal complexation at the surface, or to the direct competition between ligand anions and metal cations for coordination at mineral surface sites (Elzinga et al. 2001; Elzinga and Kretzschmar 2013). The favourable effects of co-adsorption on metal adsorption are generally ascribed to one or a combination of the following mechanisms: (1) surface electrostatic effects; (2) formation of ternary metal-ligand complexes at the mineral surface; and (3) formation of metal-ligand precipitates (Elzinga et al. 2001; Elzinga and Kretzschmar 2013).

The co-adsorption of metals (e.g. Zn(II), Pb(II), Cu(II), and Cd(II)) and ligands (e.g. sulfate, phosphate, oxalate, and glyphosate (GPS)) on minerals and colloid surfaces has been widely studied. For instance, formation of [gamma]-alumina-GPS-Zn ternary surface complexes was suggested on the basis of [sup.31]P solid-state nuclear magnetic resonance (NMR) data, which indicated that the GPS bound to [gamma]-alumina via a phosphonate group bridging the mineral surface and Zn ions; moreover, the sequence in which GPS and Zn are added can affect the adsorption mechanism (Li et al. 2013). Co-adsorbed sulfate promotes the adsorption of Pb(II) (Elzinga et al. 2001), Cu(II) (Beattie et al. 2008), and Cd(II) (Collins et al. 1999; Zhang and Peak 2007) on goethite (Gt; [alpha]-FeOOH). Phosphate, an important ligand widely found in the environment, also affects the mobility, transformation, and fate of metal ions on mineral surfaces. It has been reported that the addition of phosphate leads to a pH increase and thus enhances the formation of readily adsorbed Cu-P species in solution (Perez-Novo et al. 2009). The increased adsorption of Zn as a result of the presence of phosphate is primarily ascribed to the formation of P-Zn complexes on colloid surfaces (Perez-Novo et al. 2011).

Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) is an in situ effective technique for characterising oxyanion complexes (e.g. phosphate) on iron oxide surfaces in the presence and absence of transition metals (Elzinga and Kretzschmar 2013; Hinkle et al. 2015; Liu et al. 2017). For instance, the addition of aqueous Cd(Il) led to an increase in the amount of adsorbed phosphate on haematite (Hm) across the pH range 4.5-9.0, with the favourable effects of Cd increasing with increasing pH. Two structurally distinct ternary Cd-P complexes were simultaneously present at the Hm surface, in relative proportions varying with the pH, as indicated by ATR-FTIR data (Elzinga and Kretzschmar 2013). Furthermore, ATR-FTIR measurements showed that the cooperative adsorption behaviour of phosphate and aqueous Fe(II) likely results from a combination of ternary complexation and electrostatic interactions; surface complexation modelling studies required the inclusion of ternary complexes to simulate all conditions of the macroscopic data, further suggesting that phosphate and Fe(Il) formed ternary complexes on Gt and Hm surfaces (Hinkle et al. 2015). Recently, it was reported that Fe-P-Cd (i.e. phosphate-bridged) ternary complexes were found in the co-adsorption system of phosphate and Cd(II) on ferrihydrite, and electrostatic interaction also contributed to the co-adsorption process (Liu et al. 2017).

In addition to inorganic phosphate, organic phosphates (OPs) are important P pools widely involved in several biogeochemical processes (Turner et al. 2005), and account for 20-80% of the total P content in soils and sediments (Turner et al. 2002). In particular, little information is available on the cycling, mobility, and bioavailability of inositol phosphates, a prevalent group of OPs regarded as the 'link between agriculture and the environment' (Turner et al. 2002, 2007). Mvo-inositol hexakisphosphate (IHP), the most abundant OP in many soils (Turner et al. 2002; Doolette et al. 2009; Murphy et al. 2009; Jorgensen et al. 2011; Liu et al. 2014), strongly interacts with iron/aluminium oxides (Celi et al. 2001; Guan et al. 2006; Johnson et al. 2012; Yan et al. 2014a, 2014c) and metal ions (Bebot-Brigaud et al. 1999; Crea et al. 2008, 2009). However, to the best of our knowledge, very limited spectroscopic data on the co-adsorption of IHP and metal ions on iron oxides are available (Ruyter-Hooley et al. 2016, 2017; Wan et al. 2017).

In the present study, we investigated the co-adsorption of IHP and Zn(II) on Gt across a wide pH range (3.0-7.0), under aqueous chemical conditions, with undersaturated reactant concentrations with respect to zinc phytate (Zn-IHP) precipitate phases. Batch adsorption experiments, zeta potential measurements, and in situ ATR-FTIR spectroscopy were used to elucidate the mechanism of the co-adsorption behaviours of IHP and Cd(II) on Gt. This study could provide a comprehensive understanding of the fate of OPs and heavy metal cations in the soil and sediment environment.

Material and methods

Material

Analytical reagent-grade chemicals and ultra-pure water (p > 18 MQ cm) were used to prepare all solutions and suspensions used in this study. Dipotassium IHP ([greater than or equal to] 95%) was obtained from Sigma-Aldrich (St Louis, MO, USA). Stock solutions of IHP were prepared in ultra-pure water and refrigerated. The hydrolysis of IHP in stock solutions was monitored by measuring the orthophosphate level in solution before each use. The same Gt sample used in our previous work (Yan et al. 2015), with a specific surface area of 46.4 [m.sup.2] [g.sup.-1], was employed in this study.

Zeta potential measurements

The surface charge of Gt was measured in zeta potential mode using a Malvern Zetasizer ZEN 3600 analyser equipped with a 4 mW He-Ne laser ([lambda], = 633 nm). The Gt was prepared with a solid loading of 0.5 g [L.sup.-1] and a fixed ionic strength of 0.01 M KC1. The suspensions were allowed to equilibrate for 24 h and purged with [N.sub.2] overnight to remove C[O.sub.2]. Measurements were performed within pH 3.0-10.0. The zeta potential of Gt after adsorption of IHP, Zn(II), or both IHP and Zn(Il) was also determined as a function of pH (as described in the pH adsorption edge experiments below). All measurements were run in triplicate.

pH adsorption edge experiments

Before adsorption, a 1.0 g [L.sup.-1] Gt suspension was dispersed for 24 h in 0.01 M KC1 solution at various pHs (3.0-7.0) and purged with [N.sub.2] overnight to remove C C[O.sub.2]. Samples containing 50.0 mL of suspension (corresponding to 50 mg of Gt) were then added to 50.0 mL of 60 [micro]M IHP solution, 50.0 mL of 120 [micro]M Zn(II) solution, or 50.0 mL of a solution of 60 [micro]M IHP and 120 [micro]M Zn(II) at various pHs (3.0-7.0) in 0.01 M KCl. After equilibrating at 298 K by shaking at 200 rpm for 6, 12, and 24 h, the pH of each batch sample was measured and if necessary adjusted to the desired value with 0.1 M HCl (or KOH). The suspensions were shaken for 24 h at 298 K, centrifuged (at 9500 g for 15 min), and filtered through a 0.22-[micro]m membrane. The adsorption experiments were performed in triplicate. Before the colourimetric determination of phosphate by the phosphomolybdate blue method (Murphy and Riley 1962), IHP was hydrolysed to inorganic phosphate by digestion with concentrated sulfuric and perchloric acids (Martin et al. 1999). The loss of IHP by hydrolysis and adsorption on the container walls was negligible. The [Zn.sup.2+] concentration in the solutions was determined by atomic absorption spectrometry (AAS, Varian AAS 240FS).

ATR-FTIR spectroscopy

The ATR-FTIR spectra were recorded on a Bruker Vertex 70 spectrometer equipped with a deuterated triglycine sulfate detector. A single-reflection diamond ATR unit (Pike Technologies Inc., Madison, Wisconsin, USA) was used to record the spectra of 6 mM phytic acid solutions of different pHs (2.0, 3.0, 4.0, 5.0, and 7.0) and Zn-IHP (wet sample). The spectrum of the Zn-IHP precipitate synthesised at pH 6.0 was collected for reference. The picture of the single-reflection diamond ATR unit was provided in Fig. SI, available as Supplementary Material.

The adsorption of IHP as well as co-adsorption of IHP and [Zn.sup.2+] on the Gt surface was probed by a multiple-reflection horizontal ATR cell unit with a ZnSe crystal. The picture of this unit was provided in Fig. S2. For the surface adsorption studies, an evenly coated Gt thin film was deposited onto a ZnSe crystal element in a horizontal 45[degrees] ATR cell (Pike Technologies Inc.). The film was prepared by placing a finely dispersed suspension of Gt (2 mg of Gt in 1 mL of water) on the crystal surface and drying overnight. The coated crystal was sealed in a flow cell, placed on the ATR stage inside the IR spectrometer, and connected to a reaction vessel containing 500 mL of 0.01 M KC1 electrolyte adjusted to pH 5.0. A peristaltic pump was used to circulate the solute from the reaction vessel through the flow cell at a rate of 0.5 mL [min.sup.-1]. The flow cell effluent was collected as waste. The Gt deposit was equilibrated with the background solution for ~2h. A background spectrum consisting of the combined signals corresponding to the ZnSe crystal, the Gt deposit, and the reaction electrolyte was then collected as the average of 512 scans at a 4 [cm.sup.-1] resolution, and the adsorption experiment was started by pumping 30 [micro]M IHP in 0.01 M KCl electrolyte adjusted to pH 5 into the reaction vessel at a flow rate of 0.5 mL [min.sup.-1]. The sample spectra were also collected at different times as an average of 512 scans at 4 [cm.sup.-1] resolution.

After a 2-h injection of IHP, the final spectrum was collected and [Zn.sup.2+] was added to the reaction vessel in stepwise increments, with [Zn.sup.2+] concentrations increasing from 5 to 500 [micro]M at pH 5.0. For every concentration of [Zn.sup.2] 4 in solution, the IHP adsorption on the Gt deposit was monitored by inspecting the IR spectra of adsorbed IHP over 2 h. After collecting the final spectrum, the [Zn.sup.2+] concentration in the reaction vessel was increased to the following level, and the IHP was allowed to reach a new adsorption equilibrium with the Gt deposit in the flow cell. These experiments were also conducted at pH 4.0, 6.0, and 7.0

In addition, to investigate the effect of the addition sequence on the IR spectra of adsorbed IHP over time, the adsorption experiment was started by pumping 100 [micro]mol [L.sup.-1] Zn(II) in 0.01 M KCl electrolyte at pH 7.0 into the reaction vessel, at a flow rate of 0.5 mL [min.sup.-1], before the further injection of IHP and [Zn.sup.2+]. Then, 30 [micro]mol [L.sup.-1] IHP and the [Zn.sup.2+] solution were injected at a flow rate of 0.5 mL [min.sup.-1], with[Zn.sup.2+] concentrations increasing from 5 to 100 [micro]M at pH 7.0.

Results and discussion

Co-adsorption of Zn(II) and IHP

The pH adsorption edges for IHP in the absence and presence of 60 [micro]M Zn(II) are shown in Fig. la. Without Zn(II), the adsorption density of IHP decreased with increasing pH. At lower pH values, the Gt surface had a higher positive charge, resulting in a stronger electrostatic attraction for the IHP anions, which facilitated IHP adsorption. In addition, covalent interaction between the Gt surface and IHP anions could also exist. The adsorption density of IHP on Gt decreased from 0.88 [micro]mol [m.sup.-2] at pH 3.0 to 0.75 [micro]mol m 2 at pH 7.0, consistent with the literature (Johnson et al. 2012). The IHP adsorption was dramatically enhanced by addition of Zn(II). The IHP and Zn(II) adsorption densities in the IHP and Zn(II) coexistent system exhibited similar trends (Fig. 1). The adsorption density of Zn(II) on Gt increased at higher pH, resulting in a greater enhancement of IHP adsorption.

Fig. 1 b shows the pH adsorption edges for Zn(II) in the absence and presence of 30 [micro]M IHP. Without IHP, the adsorption density of Zn(II) increased with increasing pH. The introduction of IHP enhanced the Zn(II) adsorption. The extensive adsorption of Zn(II) and the limited IHP adsorption at high pH enhanced the favourable effects of Zn(II) on IHP adsorption under alkaline conditions, whereas these effects became less pronounced under acidic pH, where limited adsorption of Zn(II) was combined with enhanced IHP adsorption. This is similar to the phosphate and Cd co-adsorption system in Hm (Elzinga and Kretzschmar 2013). A similar result was reported by Ruyter-Hooley et al. (2016), who found that the presence of IHP significantly enhanced the adsorption of Cd(II) on gibbsite below pH 8.0, especially at higher concentrations of Cd(II) and IHP; the presence of IHP also led to a marked increase in the adsorption density of Cd(II) to kaolinite in the pH range of 4.0-8.5 (Ruyter-Hooley et al. 2017).

As the point of zero charge ([pH.sub.PZC]) of Gt is 8.7, the surface is positively charged at pH <7.0, and it adsorbs neutral and negatively charged Zn-IHP aqueous complexes more readily than positively charged Zn(II) ions. According to stability constants of [Zn.sub.i][H.sub.j][L.sup.(12-2i-j)-] species (I = 1 and 2; j = 1, 2, 3, 4, 5, and 6), different Zn-IHP aqueous complexes are formed with varying ratios as a function of pH (Crea et al. 2009). Fig. 2 shows the zeta potentials of pristine Gt and of Gt with adsorbed Zn(II) and/or IHP. The zeta potential of pristine Gt decreased with increasing pH, and showed little variation in the presence of Zn(II), implying that Zn(II) had no remarkable effect on the surface charge. Due to the high negative charge of IHP, its adsorption caused a significant reduction in zeta potential, which became negative (below -40 mV) at all pH levels (3.0-10.0), consistent with the literature (Celi et al. 2001). The formation of inner-sphere surface complexes generally alters the zeta potential and the [pH.sub.pzc] values (Goldberg and Johnston 2001). The present zeta potential results indirectly support the formation of inner-sphere surface complexes. The zeta potential of Gt was slightly higher in the ternary system than in the binary Gt-IHP system, indicating that the presence of Zn(II) reduced the negative charge on the Gt surface.

ATR-FTIR spectra of IFIP solution

The ATR-FTIR spectra of the IHP solution as a function of pH are shown in Fig. 3. Different solution pHs induce protonation or deprotonation of the oxyacid ligand, thus modifying its symmetry (Guan et al. 2006; Yan et al. 20146). At pH 7.0, four bands at ~1177, 1130, 1067, and 990 [cm.sup.-1] dominated the IR spectrum of IHP. The bands at 1177 and 1067 [cm.sup.-1] were assigned to the asymmetric ([v.sub.a]s) and symmetric (vs) P-0 stretching vibration in the terminal HP[O.sub.3.sup.-1] groups respectively, whereas those at 1130 and 990 [cm.sup.-1] were attributed to asymmetric and symmetric P-O stretching in the terminal P[O.sub.3.sup.2-] groups respectively (Guan et al. 2006). A new band appeared at 935 [cm.sup.-1] at lower pH, corresponding to the stretching vibration of P-OH in [C.sub.6][H.sub.6][(HP[O.sub.4.sup.-]).sub.6] (Guan et al. 2006; Johnson et al. 2012; Yan et al. 20146).

IHP-Gt pH edge spectra

The time dependence of the in situ ATR-FTIR spectra of IHP adsorbed on the Gt surface at pH 4.0 is illustrated in Fig. 4a. The intensities of the IR absorption bands originating from the IHP surface complexes increased over time, indicating enhanced IHP adsorption on the Gt surface. The bands at 1070 and 1133-1136 [cm.sup.-1] were close to those at 1068 and 1130 [cm.sup.-1] in the spectra of the IHP solution respectively, but the bands at 1167-1169 [cm.sup.-1] were substantially shifted with respect to the band at 1185 [cm.sup.-1] in the IHP solution spectra (Fig. 4a), suggesting that IHP probably coordinated the surface through some of its six phosphate groups to form inner-sphere complexes, consistent with our previous study (Yan et al. 20146). With increasing reaction time, the bands at 1133 and 1169 [cm.sup.-1] shifted to 1136 and 1167 [cm.sup.-1] respectively--mainly because, due to the higher adsorbed amount of IHP, the extent of protonation of the adsorbed IHP increased to balance the excess negative charge, as indicated in Yan et al. (20146). It has been reported that the adsorption on aluminium hydroxide facilitated IHP dissociation, and that no more than four phosphate groups are involved in the complexation with the aluminium hydroxide surface (Guan et al. 2006).

The intensities of the IR absorption bands of the IHP surface complexes increased with decreasing pH, indicating increased accumulation of IHP at the Gt surface as pH was lowered (Fig. S3). This is consistent with the results of macroscopic experiments, which showed that IHP adsorption on Gt increased with decreasing pH (Fig. 1a). A second aspect emerging from the spectra in Fig. S3 was a gradual change in the appearance of the spectra as the pH was lowered from 7.0 to 4.0. This indicates that not only the extent of adsorption, but also the speciation of adsorbed IHP varied with pH, leading to the possible presence of multiple IHP complexes on the Gt surface over most of the pH range considered here, consistent with the results of previous IR studies (Yan et al. 20146).

Ternary Zn(II)-IHP-Ct systems

The effect of Zn(II) on the adsorption of IHP on the Gt surface is illustrated in Figs 4b and 5, which show the in situ ATR-FTIR results of the experiments on the ternary systems, conducted at pH 4.0-7.0. The data show that, at all considered pH values, addition of Zn(II) promoted IHP adsorption, as demonstrated by the increasing intensities of the IR vibrational bands of the IHP surface complexes with increasing concentrations of aqueous Zn(II). These results are consistent with those of quantitative adsorption (Fig. 1). A similar phenomenon was observed in Hm-P-Cd (Elzinga and Kretzschmar 2013) and Hm-IHP-Cd ternary systems (Wan et al. 2017). In particular, at pH 4.0, upon addition of aqueous Zn(II), the intensity of the peak at 1011 [cm.sup.-1] increased and the band at 1070 [cm.sup.-1] gradually shifted to 1085 [cm.sup.-1], similar to the bands of Zn-IHP precipitates at 1003 and 1080 [cm.sup.-1] respectively (Fig. 6). However, in the Gt-IHP-Zn(II) ternary system, the band at 1133 [cm.sup.-1], which exhibited the highest intensity, differed from the shoulder-like band of the synthesised Zn-IHP precipitates at 1124 [cm.sup.-1] (Fig. 46). It is likely that the favourable electrostatic and/or chemical interactions between Zn(II) and IHP on the surface of Gt enhanced IHP adsorption.

Concerning the mechanisms involved, three distinct processes may produce the favourable effect of Zn(II) co-adsorption on IHP adsorption on the Gt surface, observed here. (1) Surface electrostatic effects, where inner-sphere adsorption of Zn(II) increases the positive charge at the Gt surface (at pH < [pH.sub.PZC]), making the interaction between IHP anions and the surface more favourable. In the pH range of 3-7, the major species of Zn in the presence of IHP ([H.sub.12L]) are Zn[H.sub.2L.sup.8-], Zn[H.sub.3L.sup.7-], Zn[H.sub.4L.sup.6-], Zn[H.sub.5L.sup.5-], Zn[H.sub.6L.sup.4-], [Zn.sub.2]H[L.sup.7-], [Zn.sub.2][H.sub.2L.sup.6-], [Zn.sub.2][H.sub.3L.sup.5-] and [Zn.sub.2][H.sub.4L.sup.4-] (Crea et al. 2009). (2) Formation of ternary Zn(II)-IHP complexes, where adsorption is promoted by direct physical or chemical bonds between adsorbed Zn(II) and IHP. (3) Formation of Zn-IHP precipitate phases (Elzinga and Kretzschmar 2013). Surface precipitation often involves the formation of a three-dimensional surface phase containing both the metal ions and the ligand (Sheals et al. 2003; Elzinga and Kretzschmar 2013). To determine which of these processes are effective in the systems studied here, we characterised the IHP complexes formed as a result of Zn(Il) co-adsorption in the Gt-IHP-Zn(II) ternary systems. To elucidate the effect of Zn(II) addition on the IR spectra of the IHP complexes, we calculated the difference between the spectra collected with and without Zn(II) addition, at each pH.

If surface electrostatic interaction is the dominant mechanism for IHP adsorption in the presence of aqueous Zn(II), it is reasonable to assume that the IHP surface complexes formed upon Zn(II) addition would resemble those formed in the absence of Zn(II). However, comparison of the spectra of the ternary systems to those of IHP surface complexes formed in the absence of Zn(II) at different pHs highlights significant differences in both the degree of splitting and position of the 1R bands (Figs 4b and 5). Based on this observation, the possibility that strictly electrostatic effects determined the increase in IHP adsorption upon addition of aqueous Zn(II) could be ruled out. It was also proposed that surface electrostatic interaction was not the primary factor controlling the increase in P or IHP sorption in ternary systems (Elzinga and Kretzschmar 2013; Wan et al. 2017).

Whereas secondary Zn(II)-IHP precipitates could reasonably be expected to resemble bulk phases precipitated from oversaturated solutions, ternary complexes would have different ATR-FTIR spectra compared with those of both typical IHP surface complexes in the binary Gt IHP system and Zn-IHP precipitates. Fig. 4c shows the difference spectra at pH 4.0 calculated by subtracting the spectrum of adsorbed IHP before Zn(II) addition from the spectra of IHP adsorbed in the presence of increasing levels of aqueous Zn(II). Because Zn(II) promotes IHP adsorption, the intensity of the difference spectra increased with the Zn(II) concentration (Fig. 4c). Fig. 7 displays the difference spectra calculated in the same way at pH 5.0, 6.0, and 7.0. At all three pH values, the intensities of the bands in the difference spectra became more pronounced with increasing Zn (II) concentrations, reflecting the increase in amounts of IHP adsorbed at higher Zn(II) concentrations. The difference spectra obtained for different Zn(II) concentrations at each pH were rather similar to each other (Figs 4c and 7), indicating that similar IHP surface species were formed at low and high Zn(II) concentrations. Minor differences emerged between the difference spectra obtained at pH 4.0, 5.0, 6.0, and 7.0, with peaks at ~1005, 1089, and 1133 [cm.sup.-1] respectively. These observations suggest that the pH did not markedly influence the types of complexes formed in the ternary systems involving co-adsorption of IHP with Zn(II) on Gt. This differs from the Hm-P-Cd ternary systems. Systematic and gradual change were observed in the ternary ATR-FTIR difference spectra as pH decreased from 9.0 to 4.5, suggesting the presence of at least two different Cd-P ternary complexes changing in proportion with pH (Elzinga and Kretzschmar 2013). For the Hm-IHP-Cd ternary systems, the ATR-FTIR difference spectra for increasing concentrations of Cd(II) from pH 5.0 to 9.0 were quite similar, but differed from those at pH 4.0 (Wan et al. 2017). These results suggested that the co-adsorption mechanisms involved in ternary systems are dependent on the nature of inorganic phosphate or OP, types of metals, pH, and minerals.

The clear differences between the IR spectra of the Zn-IHP precipitates (Fig. 6) and the difference spectra of the ternary systems (Figs 4c and 7) indicate that bulk precipitation did not occur in these systems. Our previous study showed that two Zn-IHP complexes, containing 1.67 and 0.83 mM IHP and with IHP/[Zn.sup.2+] ratios of 1 :1 and 1:2 respectively, were sufficiently stable at pH 7.0 (Feng et al. 2016). It can thus be concluded that the solutions considered in this study were undersaturated with respect to the Zn(II)-IHP solid phase. The difference spectra of the ternary systems did not change with Zn(Il) concentration in a given experiment (Figs 4c and 7) indicating that surface precipitation did not occur. Surface precipitation of Zn (II)-IHP phases would likely be accompanied by formation of adsorption complexes and become more prominent at higher levels of co-adsorbed Zn(II). The evolution from adsorption to precipitation with increasing Zn(II) concentrations would result in a change in the IR difference spectra of the ternary systems, which was, however, not observed in this study. Similarly, it was also reported that bulk precipitation was not the main mechanism for the co-adsorption of P or IHP and Cd(II) on Hm (Elzinga and Kretzschmar 2013; Wan et al. 2017). The formation of Cd(II)-IHP precipitates has been reported to occur at higher concentration of Cd(II) in the ternary IHP-Cd (II)-kaolinite system (Ruyter-Hooley et al. 2017).

In addition, to investigate the effect of the addition sequence on the IR spectra of adsorbed IHP, 100 [micro]mol [L.sup.-1] Zn(II) in 0.01 M KCl electrolyte at pH 7.0 was injected alone, to react with the Gt before the addition of IHP and aqueous Zn(II). The spectra shown in Fig. 8 display bands at ~1003, 1083, and 1133 [cm.sup.-1], similar to those of the spectra obtained with the injection of IHP at the beginning of the process (Figs 4 b and 5). These results reveal that the sequence in which IHP or Zn(II) were added did not affect the speciation of surface complexes.

In summary, the results of the adsorption experiments showed that the presence of IHP promoted Zn(II) adsorption, and vice versa, on the surface of Gt. The ATR-FTIR data suggested the possible formation of inner-sphere Gt-IHP-Zn ternary surface complexes, in which [Zn.sup.2+] was bound to free phosphate groups of the chemisorbed IHP species. At variance with these results, a previous [sup.31]P magic angle spinning NMR and surface complexation modelling study suggested that two outer-sphere ternary complexes can be formed on gibbsite in the presence of both IHP and Cd(II), and the predominant one is dependent on the adsorbate concentration (Ruyter-Hooley et al. 2016). Recently, Ruyter-Hooley et al. (2017) studied the cosorption of IHP and Cd(II) on kaolinite in the pH range of 3.5-12.0, and reported that IHP is bound to the surface through both inner- and outer-sphere complexes, but two extra ternary complexes are required to fit the IHP-Cd(II)-kaolinite data, as indicated by an extended constant capacitance model. In this study, we suggest that formation of outer-sphere complexes was not the dominant mechanism in the pH range of 4.0-7.0. Wan et al. (2017) studied the co-adsorption of IHP and Cd(II) on Hm by ATR-FTIR spectroscopy, and concluded that two structurally distinct ternary surface complexes were formed in the pH range of 4.0-9.0. The Hm-IHP-Cd ternary surface complex was formed with the sorbed IHP as the bridging molecule at low pH; and the Hm-Cd-IHP-Cd surface complex was formed with the sorbed Cd(II) as the bridging species at high pH (Wan et al. 2017). The results of our study show some differences from the Hm-IHP-Cd(II) system. The formation of Gt-IHP-Zn inner-sphere ternary complexes was proposed as the dominant mechanism for enhancing the co-adsorption of IHP and Zn(II) on Gt, as indicated by the ATR-FTIR difference spectra in the pH range of 4.0-7.0 (Figs 4c and 7). A similar sorption mechanism was suggested in the [gamma]-alumina-GPS-Zn ternary systems, in which GPS bound to [gamma]-alumina via a phosphonate group bridging the mineral surface and Zn(II) (Li et al. 2013). In addition, it was shown that the sequence of additional GPS and Zn(II) affected the sorption mechanism. At pH 8, Zn-Al layered double hydroxide precipitates formed if Zn(II) was added first, but no precipitates formed if GPS was added first or simultaneously with Zn(II). In contrast, at pH 5.5, only [gamma]-alumina GPS-Zn ternary surface complexes formed regardless of whether GPS or Zn(II) was added first or both were added simultaneously (Li et al. 2013). However, in our study, the sequence of addition of IHP or Zn(II) did not affect the speciation of surface complexes (Fig. 8). These results indicate that the presence of metal ions altered the adsorption mechanisms of IHP or other ligands on iron oxides and other minerals.

Conclusions

We studied the influence of co-adsorption of Zn(II) and IHP on the adsorption density and mechanism of these species on the surface of Gt. Addition of aqueous Zn(II) led to enhanced IHP adsorption across the pH range tested, with the favourable effects of co-adsorption increasingly pronounced with rising pH. The ATR-FTIR and zeta potential measurements on the ternary adsorption systems suggest the formation of Gt-IHP-Zn ternary surface complexes at the Gt-water interface. The results of this study are relevant for multicomponent mineral-water interfaces, where dissolved IHP and trace metals are simultaneously present. Future studies should focus on determining the detailed configuration and structure of Gt-IHP-Zn ternary complexes on iron oxides using other techniques, such as Zn K-edge extended X-ray absorption fine structure spectroscopy, surface complexation models, and density functional theory calculations.

Abbreviations: ATR-FTIR, attenuated total reflectance Fourier transform infrared spectroscopy; Gt, goethite; Hm, haematite; IHP, myo-inositol hexakisphosphate; Zn-IHP, zinc phytate.

https://doi.org/10.1071/SR17333

Conflicts of interest

The authors declare no conflicts of interest.

Supplementary material

More detailed information about (1) The single-reflection diamond ATR unit and the multiple-reflection horizontal ATR cell unit with a ZnSe crystal; and (2) ATR-FTIR spectra of IHP adsorbed on Gt at pH 5 and 6 as a function of time.

Acknowledgements

This research is supported by the National Natural Science Foundation of China (41603100), the National Key Research and Development Program of China (2017YFD0200201), and the Fundamental Research Funds for the Central Universities (2662017PY070). The authors report no conflict of interest.

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Yupeng Yan (A), Biao Wan (A), Yanyi Zhang (B), Limei Zhang (A,C), Fan Liu (A), and Xionghan Feng (A)

(A) Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtse River), Ministry of Agriculture, College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, PR China.

(B) Environmental Monitoring Center of National High-tech Industrial Development Zone Branch, Qingdao Municipal Environmental Protection Bureau, Qingdao 266000, PR China.

(C) Corresponding author. Email: lmzhang@mail.hzau.edu.cn

Received 13 December 2017, accepted 28 April 2018, published online 6 July 2018

Caption: Fig. 1. The pH adsorption edges on goethite (0.5 g [L.sup.-1]) for myo-inositol hexakisphosphate (IHP) with and without Zn(II) (a) and for Zn(II) with and without IHP in 0.01 M KCl (b). The initial IHP and Zn(II) concentrations were 30 and 60 [micro]mol [L.sup.-1] respectively. When not shown, error bars ([+ or -] standard deviation) are smaller than the corresponding symbols.

Caption: Fig. 2. Zeta potential of goethite (Gt, 0.5 g [L.sup.-1]) with and without adsorbed myo-inositol hexakisphosphate (IHP) and/or Zn(II), plotted as a function of pH in 0.01 M KCl. The initial IHP and Zn(II) concentrations were 30 and 60 [micro]mol [L.sup.-1] respectively.

Caption: Fig. 3. The ATR-FTIR spectra of 6 mmol [L.sup.-1] IHP solutions in 0.01 mol [L.sup.-1] KCl.

Caption: Fig. 4. The ATR-FTIR spectra of 30 [micro]mol [L.sup.-1] myo-inositol hexakisphosphate (IHP) adsorbed on goethite at pH 4.0, at different times (min) (a) and at increasing levels of solution Zn(II) concentrations (0-500 [micro]mol [L.sup.-1]; IHP first, then IHP and Zn(II) together) (b). Difference spectra between the spectra collected for IHP surface complexes formed in the presence of increasing levels of solution Zn(II) and the spectrum of adsorbed IHP collected before Zn(II) addition for the experiment conducted at pH 4.0 (c). The numbers in the legend of the panel in Fig. 4a denote the adsorption time: 10-120 min. The numbers in the legend of each panel in Figs 4b and c represent the Zn(II) concentrations: 0, 5, 25, 50, 100, and 500 [micro]mol [L.sup.-1]. The numbers along each spectrum denote the corresponding wavenumber.

Caption: Fig. 5. The ATR-FTIR spectra of myo-inositol hexakisphosphate (IHP) surface species obtained with increasing Zn(II) concentrations at pH 5 (a), 6.0 (b), and 7.0 (e). The experiments were conducted in a 0.01 mol [L.sup.-1] KCl background electrolyte, at a total IHP concentration of 30 [micro]mol [L.sup.-1]. The numbers in the legend of each panel represent the Zn(II) concentration in [micro]mol [L.sup.-1]. The numbers along each spectrum denote the corresponding wavenumber.

Caption: Fig. 6. The ATR-FTIR spectrum of zinc phytate precipitate.

Caption: Fig. 7. Difference spectra between the spectra collected for myo-inositol hexakisphosphate (IHP) surface complexes formed in the presence of increasing levels of solution Zn(II) and the spectrum of adsorbed IHP collected before Zn(II) addition for the experiments conducted at pH 5.0 (a), 6.0 (b), and 7.0 (c). The numbers in the legend of each panel denote the corresponding Zn(II) concentrations in [micro]mol [L.sup.-1]. The numbers along each spectrum denote the corresponding wavenumber.

Caption: Fig. 8. The ATR-FTIR spectra of 30 [micro]mol [L.sup.-1] myo-inositol hexakisphosphate (IHP) adsorbed on goethite at pH 7 in the presence of Zn(II) (5-100 [micro]mol [L.sup.-1]). The Zn(II) solution (100 [micro]mol]L.sup.-1]) was injected first, before introduction of IHP and Zn(II). The numbers in the legend of the panel denote the corresponding Zn(II) concentrations in [micro]mol [L.sup.-1]. The numbers along each spectrum denote the corresponding wavenumber.
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Title Annotation:attenuated total reflectance Fourier transform infrared spectroscopy, Zinc
Author:Yan, Yupeng; Wan, Biao; Zhang, Yanyi; Zhang, Limei; Liu, Fan; Feng, Xionghan
Publication:Soil Research
Article Type:Report
Date:Aug 1, 2018
Words:7068
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