Synthesis, Structure, and Stability of Gallium Arsenate Dihydrate, Indium Arsenate Dihydrate, and Lanthanum Arsenate

This paper reports on the hydrothermal synthesis, structural characterization, and chemical stability−leachability of three metal arsenates, namely gallium arsenate dihydrate (GaAsO 4 ·2H 2 O), indium arsenate dihydrate (InAsO 4 ·2H 2 O), and lanthanum arsenate (LaAsO 4 ). The new standard synthesis method involves hydrothermal precipitation at 433 K (160 °C) from equimolar (0.3 M) M(III)−As(V) nitrate solutions over a period of 24 h. The produced materials were found to be essentially stoichiometric and to exhibit very good crystallinity. The two dihydrates were found further to be made up of uniformly grown crystallites either aggregated (GaAsO 4 ·2H 2 O) or nonaggregated (InAsO 4 ·2H 2 O), reflecting their common orthorhombic crystal habit, while LaAsO 4 consisted of large aggregated particles with monoclinic habit features. In terms of stability, InAsO 4 ·2H 2 O and LaAsO 4 were found to release less than 1 mg/L arsenic when subjected to a TCLP-like leachability test (24 h contact at pH 5) while GaAsO 4 ·2H 2 O released 2.4 mg/L arsenic. An extended leachability study over a period of several weeks resulted in higher concentrations of arsenic released via an incongruent dissolution mechanism. Of the three compounds, LaAsO 4 was determined to be the most stable with arsenic equilibrium solubility equal to 4 and 13 mg/L, respectively, at pH 5 and 7 at 22 °C.