Effects of anions on calcium arsenate crystalline structure and arsenic stability

Based on the crystallization reaction between calcium and arsenate, three anions, namely, chloride (Cl−), sulfate (SO42−), and carbonate (CO32−), were introduced during the formation of calcium arsenate in this study. The effects of these anions on arsenic stability and calcium arsenate crystalline properties were characterized using TCLP, XRF, XRD, SEM-EDS, TEM and particle size distribution (PSD). The results show that these anions significantly affect the calcium arsenate crystalline structure and arsenic-leachate concentration. The effect of chloride on arsenic stabilization was mainly a result of the formation and growth of calcium arsenate apatite (Ca5(AsO4)3OH and Ca5(AsO4)3Cl) crystal. Sulfate is propitious to improve the formation of Ca4(OH)2(AsO4)2·4H2O crystals and enhance their stability effectively under corresponding sulfate concentration treatments. A small amount of carbonate is beneficial to the formation and growth of Ca4(OH)2(AsO4)2·4H2O crystals. However, when the CO32- concentration was higher than 0.5 mol/L, Ca-As compounds were not precipitated. Instead, CaCO3 and Na3Ca(CO3)2·5H2O were generated successively, inhibiting calcium-arsenate crystallization and thus releasing more arsenic into aqueous solution. The trends of grain size are opposite to that of arsenic-leachate concentration, indicating that larger calcium arsenate crystals are more stable in the TCLP test. The internal structure of different Ca-As compounds can be changed and transformed by adding the three anions within a certain range. In summary, the results of this paper provide some meaningful information for the treatment of arsenic-containing wastewater or arsenic-bearing sludges using lime precipitation.