Preparation of a Hybrid Zirconium Phytate and Its Application for the Removal of Fluorine in Metal-Precoating Effluent

Abrstract A hybrid zirconium phytate (ZrxIP6) with porous surface was synthesized via the direct precipitation method. The as-prepared ZrxIP6 was characterized by SEM, EDS, FT-IR. The sample obtained from P/Zr=3:1(mol/mol) indicated that a hybrid material might be formed through coordination of the zirconium ions with the –COPO3 groups in phytic acid molecules. An attempt to employ the hybrid ZrxIP6 as a sorbent in the removal of fluoride ions in metal-precoating wastewater was performed. The adsorption capacity qe and the residual F ions concentration Ce using the ZrxIP6 (P/Zr=3:1) as an adsorbent, were 1.21 mg.g and 1.47 mg.L, respectively.


Introduction
In the past decade, one acceptable and well-developed metal-precoating method has been the application of zirconium oxide on metal surfaces by immersion in or rinsing with hexafluorozirconic acid solution or by the sol-gel method [1]. The treatment is free of heavy metals, such as chromates, zinc and nickel, which are subject to restrictions due to worker's safety or waste disposal and is now widely adopted by metal coating factories in many countries. However, Fluoride is present in the wastewater after this industrial pretreatment activity, and the discharge of such industrial wastewater leads to fluoride contamination. Thus, fluoride elimination has to be addressed in the event that the fluorine concentration is above the established wastewater disposal standards in metal pre-coating process.
Several methods have been developed to remove fluoride from wastewater. These methods are based on the principles of ion exchange [2],chemical precipitation [3], adsorption [4], electrodialysis and reverse osmosis [5]. Generally, selective adsorption is a more attractive method. Recently, considerable works have been conducted in developing new adsorbents loaded for the purpose of the adsorptive removal of fluoride. Among such adsorbents, trivalent and tetravalent metal fixed onto porous adsorbents or carrier materials have shown promising results [6]. adsorptive resin based adsorbents that use rare earth elements or Zr(IV) ions are receiving increased attention because of their selective affinity to F -, high adsorption capacity, and ease of operation. However, there are some trivial defections, especially complicated preparation for these assembled resins. Therefore, developing new adsorbents for selective separation of fluoride from aqueous solutions is still of great significance.
Phytic acid (IP 6 H 12 or IP 6 ) is safely used in many industrial fields and regarded as a nontoxic and "green" reagent for the environment. Each IP 6 molecule contained 6 -O 3 PO 2groups with 12 acidic protons and this feature made IP 6 a strong chelating agent, as a result of intermolecular hydrogen bonding between phosphate groups. Complex compounds or hybrid materials prepared from IP 6 often show attractive properties. For example, conducting polymer hydrogels had been synthesized by polymerizing conductive polymer monomers with IP 6 . The excellent electrode performance of this hydrogel made it a possible candidate for bioelectronics and future-generation energy storage electrodes [7].The nanoporous film of zirconium phytate could be employed as a substrate for making biosensors and displayed good bioelectrocatalytic ability toward the reduction of H 2 O 2 [8].
Whereas stoichiometric zirconium phytate has been prepared and is recognized as a potential substrate for sensor assembly, there is still a lack in knowledge concerning the relationship between the morphology and the initial molar ratio of zirconium ions to IP 6 in solution. We proposed that the tendency of cross-linked Zr x IP 6 particles (Zr x IP 6 , the subscript x means that Zr coordinated by IP 6 is not the theoretical stoichiometric ratio), as reported, should lead to the possible preparation of promising coordinated hybrid materials for anion adsorption,, similar to the reported higher fluorine ions removal capacity of ZrP [9]. In this study, hybrid Zr x IP 6 particles with porous surfaces were prepared and characterized at low molar ratios of Zr 4+ and IP 6 12-(higher P/Zr ratio ) in the initial solution .The probability of Zr x IP 6 material as adsorbents for removing Ffrom metalprecoating effluent is also investigated.

Reagents and apparatus
All chemicals were of analytical grade and were used without further puri¿cation. Industrial grade phytic acid (60% wt.) was obtained from Ruifeng biochemicals reagent Co. Buffer solutions with different pH values were prepared using HCl or CH 3 COONa purchased from Shenyang 2nd chemical reagent Co.
Fourier transform infrared (FT-IR) spectrometer analysis was recorded in the region of 4000-500 cm í1 (PerkinElmer®;USA) using a KBr wafer. The microstructure,morphological and elemental composition of the Zr x IP 6 sample was characterized with a JSM-6460 LV scanning electron microscope (SEM) along with Energy dispersive X-ray spectroscopy (EDS) . N 2 adsorption-desorption measurement (NOVA-2200E Quantachrome®;USA) of the dried powers was performed at the temperature of liquid nitrogen (77 K).

Synthesis of hybrid ZrxIP6
The hybrid Zr x IP 6 particles were prepared by the reaction of ZrOCl 2 ·8H 2 O with phytic acid. The concentration of the ZrOCl 2 ·8H 2 O solution was kept constant at 0.1 mol.L -1 . The introduced phytic acid varied from 0.05 mol.L -1 to 0.15 mol.L -1 to prepare hybrid Zr x IP 6 material. After stirring for 30 min, the white precipitated products were collected from the solution by centrifuging at 7000 rpm for 15 min, rinsing with deionized water several times and then drying at 70 º C for 24 h .

Fluoride adsorption experiments
The adsorption abilities of fluoride onto sorbents were performed by shaking 1 g Zr x IP 6 with 100 mL collected wastewater sample in a polyethylene beaker with at 200 rpm at 25 . The specific amount of fluoride adsorbed was calculated from the usual mass balance [10] .Equation (1) With q t being the sorbate loading (mg.g -1 ) at time t, C 0 and C t (mg.L -1 ) are the liguid phase concentrations of fluoride at initial and time t, V the volume of liquid in the experiment (L) and m b the mass of the sorbent (g). Moreover, the subscript of e preceded by q and C used in this article, q e and C e , indexes the corresponding quantities at adsorption equilibrium.

SEM images morphology of ZrXIP6 compounds
Zr x IP 6 (x 3) porous particles were obtained in a wide concentration range of phytic acid to Zr(OCl) 4 ·8H 2 O. An increasing molar ratio of P/Zr from 2:1 to 3:1 gave similarly porous Zr x IP 6 . However, the higher P/Zr (>3:1) leaded to the agglomerated porous particles were shown in Figure 1(a) and Figure 1(b) due to insufficient electrostatic stabilization and oligo-clusters assembly by adjacent IP 6 or Zr x IP 6 molecules. The BET surface area of the hybrid Zr x IP 6 was 112.3m 2 .g -1 , similar to reported cited reference. EDS spectra recorded shown that the P/Zr molar ratio of sample was 2.25:1(figure.S1), a little higher than the theoretical stoichiometry of Zr 3 IP 6 (P/Zr=2:1), but lower than that of initial stoichiometry of the solution(P/Zr=3:1). Thus, we proposed that a Zr 4+ ion coordinates with four -OH in two or more separate -COPO 3 groups in one or more IP 6 molecules, rather than with four -OH in one single IP 6 molecule. This ratio of P in Zr x IP 6 could be partly explained by the integer of IP 6 molecules in this oligo-cluster structure. It can be supposed that when the amount of IP 6 introduced is sufficiently high, the growth of Zr x IP 6 should result in a deeply interlinked network. As shown in Figure 1

Infrared characteristic of hybrid ZrxIP6
The FT-IR spectra of the Zr x IP 6 compounds with different initial P/Zr ratios were presented in figure 2. Whereas all of the prepared Zr x IP 6 compounds shown a weak absorption band approximately 1630 cm -1 and a broad absorption band at 3400 cm -1 , these bands were not shown. Both of the adsorption bands could be due to the unbounded P-O-H bonds, and crystalline H 2 O. FT-IR spectra of the as-prepared Zr x IP 6 compounds. The bands of the phosphate group presenting at 962 cm -1 and 1060 cm -1 in the FT-IR spectrum of IP 6 disappeared and was replaced with a broad one at 1048 cm -1 with less trailing character (P/Zr=3:1). The 510 cm -1 and 1260 cm -1 adsorption bands for PO 4 3and the P=O double bond stretching are still present in all samples [11]. In the spectra of the higher molar ratio of P/Zr (6:1 and 9:1), the broad band gradually split into two overlapped but distinguishable bands at 1032 cm -1 and 1096 cm -1 . Similar to the spectral characteristics of trivalent metal compounds (Fe or Al phytate), there was a strong band at 1092 cm -1 , and the adsorption bands at 1096 cm -1 of hybrid. The minor absorption bands in the 880-700 cm -1 region were more similar to doublet bands and were also shifted upward 10-30 cm -1 compared to the reported FT-IR spectra of trivalent, divalent or monovalent metal phytate compounds [12].
Scheme.1 Schematic view of the structure of the hybrid Zr x IP 6 coordinated on the surface and internally. (P) represented the -COPO 3 Hgroup The tentative mechanism was depicted in Scheme.1. Stable hybrid material with a nanoporous surface were obtained in a wide range of phytic acid to Zr(OCl) 4 . A molar ratio of P/Zr between 3:1 and 6:1 gave the best results. If the ratio of Zr 4+ ions to IP 6 was lower, only an interlinked oligomer-like Zr x IP 6 with a regular surface was found, most likely due to coating of the polymerized IP 6 molecules on the hybrid Zr x IP 6 particles. In the optimal range, the excess IP 6 molecules could be interlinked with the same Zr 4+ ions by the di-, tri-, and tetra-coordination method. We proposed that tetra-coordination occurred at the internal of the hybrid Zr x IP 6 , whereas unsaturated coordination occurred at the surface of hybrid material.

Fluoride adsorption experiment results
Zr x IP 6 as Fions adsorbents in metal-precoating wastewater is attractive because of two advantages. If dissociation of hybrid Zr x IP 6 solids happened: i) a small amount of introduced IP 6 anions in recycled wastewater could act as a corrosion inhibitor for Zr-based nanofilms coated on a metal surface [13] ii) free Zr(IV) cations could be used as one supplementary component of diluted hexafluorozirconic acid metal-precoating agents.    (Figure 3). The q e and C e by taking Zr x IP 6 (P/Zr=3:1) as adsorbent were 1.21 mg.g -1 and 1.47 mg.L -1 . For oligomer-like Zr x IP 6 (P/Zr=6:1) hybrid materials, q e was decreased to 1.754 mg.g -1 and C e was increased to 7.06mg.L -1 . The variation could be ascribed to the decreased chelating Zr(IV) ions exposed to surrounding environmental due to small specific surface area of Zr x IP 6 (P/Zr=6:1).The influence of other anions on the fluoride ion removal efficiency has been researched previously for complicated practical water recycling systems. Thus, the presence of other anions in pre-treatment wastewater might have a negative interference to fluoride adsorption process. experiments: a) P/Zr=3:1; b) P/Zr=6:1. Zr x IP 6 powder with adsorbed Fions was separated, filter-collected and dried at 70ºC for 24h. SEM images shown that there was no morphology change of the hybrid Zr x IP 6 ( Figure  4). It was supported by the results of the EDS analysis of the recollected Zr x IP 6 (P/Zr=3:1). The quantitative ratio of P/Zr was 2.30:1(figure.S2), similar to that of fresh Zr x IP 6 (2.25:1). Thus, the high adsorption capacity of the Zr x IP 6 clusters for fluoride ions cannot be interpreted simply by an ion exchange mechanism, which was based on the electrostatic attraction force.

Conclusions
A novel, adsorbent hybrid Zr x IP 6 material with porous surface was synthesized. The DOI: 10.1051/ 06039 (2016) , matecconf/2016 MATEC Web of Conferences 6 results revealed that Zr 4+ ions might interact with the -OH of the same or another phosphate groups in various IP 6 molecules. The external-linked interaction meant that many phytic acid molecules were either covalently bonded to the phytate complexes or were polymerized, forming oligophytate clusters through P-O-P bands in higher P/Zr ratio Zr x IP 6 . A schematic view of the mechanism of coordination was suggested according to the FT-IR, SEM and EDS data. Fluoride removal experiments revealed that the adsorption capacity q e and residual Fions concentration C e using hybrid Zr x IP 6 (P/Zr=3:1) as adsorbent were 1.21 mg.g -1 and 1.47 mg.L -1 , respectively. The experimental results confirmed that the asprepared Zr x IP 6 was an attractive adsorbent for hexafluorozirconic acid metal-precoating wastewater treatment.