Dissolution and Solubility of the Arsenate-Phosphate
Total Page：16
File Type：pdf, Size：1020Kb
CSIRO PUBLISHING Research Paper www.publish.csiro.au/journals/env X. Zhang et al., Environ. Chem. 2011, 8, 133–145. doi:10.1071/EN10102 Dissolution and solubility of the arsenate–phosphate hydroxylapatite solid solution [Ca5(PxAs1ÀxO4)3(OH)] at 258C Xuehong Zhang,A Yinian Zhu,A,B Honghu Zeng,A Dunqiu Wang,A Jie Liu,A Huili Liu,A Meifang QianA and Liwei XuA ACollege of Environmental Science and Engineering, Guilin University of Technology, Jian-Gan Road 12, Guilin 541004, P.R. China. BCorresponding author. Email: [email protected] Environmental context. Apatites form a large family of minerals and compounds that can incorporate a variety of ions, including arsenate that can substitute for phosphate. Apatites may therefore control the concentration of arsenic in some aqueous environments. This manuscript describes the synthesis and characterisation of the arsenate–phosphate hydroxylapatite solid solution and the solid solution–aqueous solution interaction. Abstract. Nine different members of the arsenate–phosphate hydroxylapatite solid solution [Ca5(PxAs1ÀxO4)3(OH)] were prepared and characterised by various techniques, and then dissolution of the synthetic solids was studied at 258C and pH 2 in a series of batch experiments. The concentrations of aqueous arsenate species increased rapidly at the beginning of the dissolution and reached a steady-state after 480 h. The concentrations of aqueous phosphate species increased very fast initially and reached a peak value within the first hour of dissolution and then declined slowly with time and remained constant after 240–360 h. The solubility of the Ca5(PxAs1ÀxO4)3(OH) solid solution increased and its stability decreased with an increase in the mole fraction of Ca5(AsO4)3(OH). The dissolution followed or slightly overshot the Lippmann solutus curve, then approached the solutus curve. The Ca5(AsO4)3(OH)-poor solid solution was in equilibrium with the arsenate-rich aqueous solution. Additional keywords: aqueous solution, evolution, Lippmann diagram, reaction path. Introduction Arsenate is isostructural and isoelectronic with phosphate, Calcium phosphate hydroxylapatite [Ca5(PO4)3(OH)] is a nat- which may facilitate arsenic(V) removal from water and waste- urally occurring material as a major component of animal bones water using hydroxylapatite.[13] The existing literature con- and teeth.[1] On the industrial front it is being used as lumino- firms the occurrence of this substitution reaction spread over phosphors and as raw materials for the manufacture of several the entire compositional range and can be utilised to throw light phosphatic fertilisers.[2,3] Hydroxylapatite shows a high capac- on its mechanism to arrive at the possibility of removal of 3À [2] ity for substitutions of Ca and PO4 by other cations and anions incorporated arsenic. Arsenic sequestration by compound due to its crystal structure and chemical composition,[4] signif- formation produces a low-solubility apatite-like structure of [14] icant among which from the point of view of arsenic poisoning the general form Ca10(AsxPyO4)6(OH)2. 3À 3À being the substitution of PO4 by AsO4 resulting in the for- Solid solutions are geochemically and environmentally mation of its isomorph, arsenate hydroxylapatite (AsHAP).[5–7] important because the interaction of dissolved toxic metals with Arsenic toxicity is a global health problem affecting many minerals frequently results in the precipitation of metal-bearing millions of people.[8] Occurrences of dissolved arsenic in surface solid solutions on the mineral surfaces or in rock or sediment and ground waters and observed adverse health effects have pores. As a result, metals can be removed from natural waters emphasised the need for better understanding of reactions that and the thermodynamic properties of these solid solutions have a govern arsenic mobility in the environment.[9] The presence of vital influence on the transport and fate of toxic metals in the dissolved phosphate in many natural settings raises the possibil- environment.[15] Some synthesis experiments at ambient tem- ity that intermediate compositions between Ca5(PO4)3(OH) and peratures were conducted to evaluate the conditions of forma- [11,16,17] Ca5(AsO4)3(OH) may form in preference to endmember com- tion and solubilities of different calcium arsenates. positions.[9] Arsenic concentrations in phosphate minerals are Hydroxylapatite (HAP) is one of the most stable forms of variable but can reach high concentrations, for example up to calcium phosphate and exhibits a low solubility (Ksp ¼ 1000 mg kgÀ1 in apatite.[10] Mining, milling, transporting of 10À53.02–10À53.51); however, the solubility of AsHAP phosphate ores, manufacturing of phosphate fertilisers and using [Ca5(AsO4)3(OH)] is significantly greater than that of phosphate fertilisers containing arsenic are ways in which the HAP,[11,18–22] which suggested that phosphate substitution in workers, public and the environment are exposed to the enhanced the arsenate compound could significantly reduce their natural toxicity hazard of arsenic and other trace metals.[11,12] solubilities.[21–24] Ó CSIRO 2011 Open Access 133 1448-2517/11/020133 X. Zhang et al. Table 1. Summary of synthesis and composition of the Ca5(PxAs12xO4)3(OH) solid solution Sample Volumes of the precursors (mL) Solid composition Residual solid composition after dissolution for 2880 h 0.5-M Ca(OH)2 0.5-M H3PO4 0.5-M H3AsO4 AsPHA-1 100 6.0 54.0 Ca5(P0.12As0.88O4)3(OH) Ca5.67(P0.27As0.73O4)3(OH)2.34 AsPHA-2 100 12.0 48.0 Ca5(P0.22As0.78O4)3(OH) Ca4.64(P0.35As0.65O4)3(OH)0.28 AsPHA-3 100 18.0 42.0 Ca5(P0.33As0.67O4)3(OH) Ca4.38H0.24(P0.47As0.53O4)3 AsPHA-4 100 24.0 36.0 Ca5(P0.43As0.57O4)3(OH) Ca4.40H0.20(P0.57As0.43O4)3 AsPHA-5 100 30.0 30.0 Ca5(P0.54As0.46O4)3(OH) Ca5.17(P0.78As0.22O4)3(OH)1.34 AsPHA-6 100 36.0 24.0 Ca5(P0.63As0.37O4)3(OH) Ca5.04(P0.82As0.18O4)3(OH)1.08 AsPHA-7 100 42.0 18.0 Ca5(P0.71As0.29O4)3(OH) Ca4.42H0.16(P0.87As0.13O4)3 AsPHA-8 100 48.0 12.0 Ca5(P0.82As0.18O4)3(OH) Ca4.70(P0.90As0.10O4)3(OH)0.40 AsPHA-9 100 54.0 6.0 Ca5(P0.92As0.08O4)3(OH) Ca4.65(P0.95As0.05O4)3(OH)0.30 The thermodynamic properties of solid solutions have a vital decanted solutions were analysed to calculate the compositions influence on the transport and fate of toxic metals in the of the precipitates. In addition, ,10 mg of sample was digested environment. Therefore, a thorough knowledge of the thermo- in 20 mL of 1-M HNO3 solution, and then diluted to 100 mL with dynamics of materials containing mixed crystals allows us ultrapure water. It was analysed for Ca and As using a PE Model to optimise passive and permeable remediation barriers AAnalyst 700 atomic absorption spectrometer (PerkinElmer, and other environmental technologies.[15] The study of solid Waltham, MA, USA), phosphate using an ion chromatography solution–aqueous solution (SSAS) systems has garnered much (Metrohm 790, Metrohm AG, Herisau, Switzerland) or a PE attention from environmental geochemists because they have Lambda 20 UV-visible spectrometer (PerkinElmer). All syn- been demonstrated to play major roles in the mobility of thetic solids were characterised by powder X-ray diffraction dissolved metals in contaminated environments.[25] Only a (XRD) with an X’Pert PRO diffractometer (PANalytical B.V., few studies have been carried out on the dissolution and stability Almelo, the Netherlands) using Cu Ka radiation (40 kV and of the Ca5(PxAs1ÀxO4)3(OH) solid solutions, for which little 40 mA). The XRD measurements for unit cell parameter calcu- information exists in the literature. Consequently, there are lation were performed within a 2y range 10–808 with a scanning insufficient data upon which to assess the true environmental rate of 0.108 minÀ1. Crystallographic identification of the risk of arsenic posed by these minerals.[5] synthesised apatites was accomplished by comparing the exper- In the present study, a series of the Ca5(PxAs1ÀxO4)3(OH) solid imental XRD patterns to standards compiled by the International solutions (AsPHA) with different P/(PþAs) atomic ratios were Center for Diffraction Data (ICDD), which were card prepared by a precipitation method. The resulting solid solution #00-009-0432 for hydroxylapatite and #00-033-0265 calcium particles were characterised by various techniques. This paper arsenate hydroxide. The morphology was analysed by scanning reports the results of a study that monitors the dissolution and electron microscopy (Jeol JSM-6380LV, Japan Electron Optics release of constituent elements from synthetic arsenate-phosphate Ltd., Tokyo). The samples were measured in a form of KBr hydroxylapatite solid solutions using batch dissolution experi- pellets over the range of 4000–400 cmÀ1 using a FT-IR spec- ments. The SSAS reaction paths are also discussed using the trophotometer (Nicolet Nexus 470, Thermo Fisher Scientific Lippmann diagram to evaluate the potential effect of such solid- Inc., Waltham, MA, USA). solutions on the mobility of arsenic in the environment. Dissolution experiments Experimental methods Half a gram of the synthetic solid was placed in a 250-mL Solid preparation and characterisation polypropylene bottle. HNO3 solution (150 mL, 0.01 M) was The experimental details for the preparation of the samples added into each bottle. The bottles were capped and placed 8 by precipitation were based on the following equation: 5Ca2þþ in a temperature-controlled water bath (25 C). Water samples 3À À (3 mL) were taken from each bottle on 16 occasions (1 h, 3 h, 6 h, 3XO4 þ OH ¼ Ca5(XO4)3(OH). Where X ¼ phosphorus or arsenic for the endmembers and (PþAs) for the solid solutions.