Dependence of solubility and stability of calcium arsenates on pH value
砷酸钙化合物的溶解度及其稳定性随pH值的变化

The effect of pHand Ca/As ratio on the lime precipitation of arsenic-containing wastewater was studied in detail by both precipitation and dissolution experiments. For the precipitation under Ca/As=1.5 and 1.67, the residual arsenic concentration reached the lowest value at pH 7.0, and then increased slowly with the increasing pHvalues. The residual arsenic concentration after precipitation under Ca/As=2.0 and 4.0 decreased quickly with the pHincrease. There was no great difference between the experimental results for Ca/As=2.0 and 4.0. The composition and structure of the calcium arsenate compounds, which precipitated from the mixing of As solution with Ca(OH)2 slurry, depended on the pHand the Ca/As molar ratio. The particle size of the solids precipitated at high pHand high Ca/As ratio was much finer than for the low-pHand low-Ca/As precipitated solids. Ca3(AsO4)2·3H2O, Ca3(AsO4)2·2.25H2O, Ca5(AsO4)3OH and (Ca4(OH)2(AsO4)2·4H2O) were identified in the experiment over a wide range of pHand for Ca/As molar ratios between 1.0 and 4.0. Based on the analytical results,their solubility products were calculated as Ksp of 10-21.14, 10-21.40, 10-40.12 and 10-27.49, respectively.