Dissolved arsenic and antimony in the Black Sea

Abstract

Water samples from the 1988 Black Sea Expedition were analysed for dissolved, inorganic arsenic and antimony speciation, and methylarsonic, methylstibonic, dimethylarsinic and dimethylstibinic acids. Inorganic arsenic was depleted in surface waters, increased rapidly with depth in the oxic water column, and then decreased in the anoxic waters. In oxygenated waters arsenate [As(V)] was the predominant species, while arsenite [As(III)] was the major form near the sulfide interface. Both arsenate and arsenite were found in the deep anoxic water. In contrast to arsenic, inorganic antimony displayed highest concentrations at the surface, decreased to the sulfide interface, and increased in the deeper anoxic water column. The speciation of antimony was dominated by Sb(V) in oxygenated waters, Sb(III) near the sulfide interface, and Sb(III) and (V) in the deep anoxic zone. Inorganic arsenic and antimony displayed apparent removal just above the sediment-water interface. The methylated arsenic and antimony acids were only detected in surface waters. These data demonstrate that arsenic and antimony are controlled by both biological and redox reactions. Like phosphate, arsenic appears to be linked with the cycling of organic matter in surface waters. However, at and below the sulfide interface their behaviors diverge. Antimony and phosphate are linked only by their apparent participation in the cycling of metal oxides near the sulfide interface. The abundance of thermodynamically unstable As(V) and Sb(V) in the deep anoxic waters of the Black Sea can be attributed primarily to the delivery of the pentavalent ions via detrital fluxes and the formation of deep waters, with subsequent slow rates of reduction.