Solubility-product constant of an amorphous aluminum-arsenate phase (AlAsO4·3.5H2O) AT 25 °C

• Thermodynamic data and chemical formula for amorphous Al-arsenate are provided.
• The pH ranges of both congruent and incongruent dissolutions were identified.
• The Al–As association may occur in mining sites, contaminated soils and sediments.

The solubility-product constant of an Al(III)–As(V) amorphous phase synthesized at 25 °C was determined. The phase was found to dissolve congruently at pH 2.7 and incongruently above this pH value. From the congruent dissolution data corrected by activity coefficients, a solubility-product constant (Ksp) of 10− 18.06 ± 0.05 (25 °C, 1 atm) was determined. The standard Gibbs free energy of formation (ΔG°f) calculated for this phase was − 2068.37 kJ·mol− 1. The solid was identified by the chemical formula AlAsO4·3.5H2O, according to a combination of chemical and thermogravimetric analyses. The thermodynamic data on amorphous aluminum arsenate is relevant to the investigations of arsenic immobilization in environments where As–Al association occurs, such as those occurring in mining sites, contaminated soils and sediments.