Abstract: The mineral phase transformation of amorphous magnesium-calcium carbonate affects the environmental behavior of the coexisting arsenic (As) in mining environments. However, the mineral phase transformation process of amorphous magnesiumcalcium carbonate and the partitioning behavior of the coexisting arsenic remain unclear. In this study, we examined the effects of pH condition (7.5 and 9.5), As valence state (As(III) or As(V)), and concentration (1 and 10 mg·L^-1) on the phase changes and arsenic partitioning during arsenic-participated amorphous magnesium-calcium carbonate transformation. X-ray diffraction, Raman spectroscopy, Fourier transforms infrared spectroscopy, and scanning electron microscope techniques were used to characterize the solid samples. The results showed that amorphous magnesium-calcium carbonate firstly was transformed into Mg-bearing vaterite (CaCO₃, hexagonal system), then to Mg-bearing CaCO₃·H₂O and calcite (CaCO₃, trigonal system), and finally to aragonite (CaCO₃, orthorhombic system) at different pH conditions and As(III) or As(V) concentrations. The conversion rate of amorphous magnesium-calcium carbonate highly depended on pH conditions, As valence states, and its concentrations. The transformation rate of amorphous magnesium-calcium carbonate at pH 7.5 was higher than that at pH 9.5. The transformation rate decreased with increasing As concentration. The Mg-bearing aragonite and calcite formed at pH 9.5 can immobilize more As(As(III) or As(V)) than those formed at pH 7.5 and showed a better fixation ability for As(V). Our results have implications for understanding the mobility and transformation behavior of arsenic in mining-impacted environments.

Key words: amorphous magnesium calcium carbonate, crystallization transformation, mobility, arsenic fixation