Arsenic speciation in picromerite K2Mg(SO4)2•6H2O: Electron paramagnetic resonance and synchrotron X-ray absorption spectroscopic characterizations and implications for organic fertilizers

Picromerite K 2 Mg(SO 4 ) 2 •6H 2 O (also known as schoenite, schönite, or sulfate of potash) formed from alkaline lakes by evaporation is an increasingly important chlorine-free fertilizer and has been used to produce other organic fertilizers such as arcanite K 2 SO 4 and boussingaultite (NH 4 ) 2 Mg(SO 4 ) 2 •6H 2 O. Picromerite and boussingaultite, two Tutton’s salts, are also common secondary solid phases in diverse types of mine tailings and play important roles in controlling the mobility and bioavailability of various heavy metal(loid)s, including arsenic. In this study, picromerite crystals containing 64 ppm As have been synthesized at ambient conditions from aqueous solutions by slow evaporation. Arsenic K-edge X-ray absorption near-edge-structure and extended X-ray absorption fine structure data show that the dominant oxidation state is +5 and that As 5+ occupies the S site in picromerite. Single-crystal and powder electron paramagnetic resonance spectra of gamma-ray-irradiated picromerite reveal three arsenic-associated oxyradicals: AsO 4 2− , AsO 3 2− , and AsO 2 2− . The orientations of the principal 75 As hyperfine directions of the AsO 3 2− and AsO 2 2− radicals match the bonding directions of the SO 4 2− groups in the picromerite structure, further supporting substitutions of their respective diamagnetic precursors AsO 4 3− and AsO 3 3− for the sulfate group. These AsO 4 2− , AsO 3 2− , and AsO 2 2− radicals in picromerite are similar but not identical to their counterparts in boussingaultite and gypsum, suggesting that sulfate minerals are capable of sequestrating both arsenate and arsenite, with important implications for understanding the fate and bioavailability of arsenic associated with agricultural applications of organic fertilizers.