砷酸钙化合物的溶解度及其稳定性随pH值的变化

Dependence of solubility and stability of calcium arsenates on pH value

通过混合沉淀和溶解实验详细研究了pH值和Ca/As摩尔比对石灰沉淀法处理高浓度含砷废水的影响.沉淀后溶液中的As浓度随pH值的变化趋势与混合液的Ca/As摩尔比有关,在Ca/As=1·50和1·67时,沉淀后溶液中的As浓度在pH=7附近达到最低,然后随pH值的升高而呈现出增大的趋势;在Ca/As=2·00和4·00时,沉淀后溶液中的As浓度随pH值的升高而降低,且2种Ca/As摩尔比条件下的实验结果基本一致.混合沉淀过程中生成的砷酸钙化合物的组成与结构取决于溶液的Ca/As摩尔比和pH值.在低pH值和低Ca/As摩尔比条件下形成的沉淀物要比在高pH值和高Ca/As摩尔比条件下形成的沉淀物的颗粒要大,而且结晶程度要好.在Ca/As摩尔比为1·0~4·0和不同pH值条件下,主要生成Ca_3(AsO_4)_2·3H_2O、Ca_3(AsO_4)_2·2·25H_2O、Ca_5(AsO-4)_3OH和Ca_4(OH)-2(AsO_4)2·4H_2O,溶度积分别等于10^(-21·14)、10^(-21·40)、10^(-40·12)和10^(-27·49).

The effect of pH and 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 pH values. The residual arsenic concentration after precipitation under Ca/As = 2.0 and 4.0 decreased quickly with the pH increase. 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 pH and the Ca/As molar ratio. The particle size of the solids precipitated at high pH and high Ca/As ratio was much finer than for the low-pH and low-Ca/As precipitated solids. Ca3 （ AsO4 ）2 · 3H2 O, Ca3 （ AsO4 ）2 · 2.25H2 O, Ca5 （ AsO4 ）3 OH and Ca4 （OH）2 （AsO4）2 ·4H2O were identified in the experiment over a wide range of pH and 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.