Abstract: Arsenic is a toxic metalloid that poses significant health risks, including chronic arsenic poisoning, cardiovascular diseases, and cancer. Due to both human activities and naturally high arsenic levels, arsenic contamination has become a global environmental and health concern. The World Health Organization (WHO) lists arsenic among the ten chemicals of major public health concern. In the environment, arsenic primarily originates from arsenic-bearing minerals, and its mobility is governed by their dissolution and precipitation processes. Understanding the thermodynamic and kinetic properties governing the dissolution and precipitation of common arsenic minerals is essential for quantitatively assessing arsenic migration and transformation processes in natural environments. Calcium arsenate minerals, commonly found in calcium-rich and iron-poor environments such as karst regions, are secondary arsenic minerals that regulate arsenic mobility in these settings. Thermodynamic parameters, particularly solubility product constants (Ksp), are critical for assessing their stability and determining potential risks of secondary arsenic pollution. Despite their importance, existing studies report substantial inconsistencies in the solubility product (Ksp) values for the three most common calcium arsenate minerals: weilite (CaHAsO4), haidingerite (CaHAsO4·H2O), and pharmacolite (CaHAsO4·2H2O). These inconsistencies may arise from: (1) the similar formation conditions of these minerals, leading to their frequent co-occurrence in natural settings and the challenges in achieving pure synthesis in laboratory conditions; (2) interconversion among the three minerals during dissolution, along with phase transitions or the formation of other calcium arsenate phases, resulting in non-uniform dissolution behavior; (3) neglectance of calcium-arsenic complexation reactions in Ksp calculations. This study addresses these challenges by synthesizing pure weilite, haidingerite, and pharmacolite through controlled adjustments of pH, temperature, Ca/As ratios, and Mg/Ca ratios. Consistent dissolution experiments were then performed under ambient temperatures and pressures with real-time control of solution conditions. Using equilibrium solution parameters and accounting for calcium-arsenic complexation reactions, coupled with an updated Ca-As-H2O thermodynamic model, the standard solubility constants (Ksp) for weilite, haidingerite, and pharmacolite were determined as 10^-4.90, 10^-4.64, and 10^-4.66, respectively. 

Key words: calcium arsenate minerals, pure synthesis, dissolution, thermodynamics, solubility product (Ksp)