| The insolubility of norganic arsenic compounds was the basis of removing arsenic from water. At the same time, the insoluble products were usually the final disposition materials, so their stability in the environment was particularly important. These insoluble arsenate compounds contain impurities, usually in the form of solid-solution. The nature of AsO 34? and PO 34? were similar, so there often formed various types of arsenate-phosphate solid solutions. In order to understand the stability, migrates and transformates of arsenic wastes in environment, we had to study arsenate solid-solution on the base of simple arsenate to understand its stability and solubility. This study was completed on the basis of the National Natural Science Foundation of China (NSFC40773059 and 40263001)"Dissolution of arsenate solid solution and its environmental effects".The study included two aspects, precipitation and dissolution. The dissolubility and stability of Ca5(AsxP1-xO4)3F and Ca5(AsxP1-xO4)3OH solid solution was test in different conditions, and the program PHREEQC was used for simulation calculation and thermodynamics analysis. The main results were as follows:①The powders of Ca5(AsxP1-xO4)3F and Ca5(AsxP1-xO4)3OH solid solution was generated by precipitation. The crystal shape, structure and chemical composition of Ca5(AsxP1-xO4)3F before and after of dissolution was analysised by SEM, X-ray diffraction and IR.②The dissolution of apatite and svabite were studied at different temperatures and pH. The concentration of phosphate, fluoride, calcium and arsenic in the solution were determined by ion chromatography and atomic absorption spectrometer. The two processes were compared. The results showed that: the process of svabite dissolution affected by temperature greatly, the solubility was 25℃>35℃>45℃at the same pH. When tempreture were same, the Ksp was the maximum at pH 2. The dissolution process of apatite was complex in the water, the interaction between solid-phase and liquid-phase should be studied further.③The influence of P:As mole ratio and differte conditions to stability and solubility was studied. The results showed that the dissolution of solid solution was impacted by pH, especially in acidic conditions, the concentration of arsenic was as ten times as it in neutral or alkaline solution. Therefore, pH control was very important to dispose this kind of risk solid waste like svabite- apatite solid solution.④Program PHREEQC was employed to calculate the solubility Ksp and free energy of apatite, svabite and solid solutions . The results showed that at standard conditions the Ksp of Ca5(PO4)3F was 10-54.92, Ca5(PO4)3OH was 10-53.21 ; the Ksp of Ca5(AsO4)3F was 10-39.39, Ca5(AsO4)3OH was 10-40.86. According to the results, the free energy was got,ΔGfo[Ca5(PO4)3F(c)]=-6434.03kJ/mol,ΔGfo[Ca5(AsO4)3F (c)]= -5302.65kJ/mol,ΔGfo [Ca5(PO4)3(OH)(c)]=-6327.87 kJ/mol, andΔGfo [Ca5(AsO4)3 (OH)(c)]=-5063.53 kJ/mol. The solubility and free energy of solid solution decreased with the P:As ratio rising.The results of this study provided important guidance to environmental protection and treatment decision-making in the process of arsenic waste disposal sites and methods selection.
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