Speciation of arsenic in sulfidic waters

Richard T Wilkin; Dirk Wallschläger; Robert G Ford

doi:10.1186/1467-4866-4-1

Speciation of arsenic in sulfidic waters

Geochem Trans. 2003 Mar 18:4:1. doi: 10.1186/1467-4866-4-1. eCollection 2003.

Authors

Richard T Wilkin¹, Dirk Wallschläger², Robert G Ford¹

Affiliations

¹ U.S. Environmental Protection Agency, Office of Research and Development, National Risk Management Research Laboratory, 919 Kerr Research Drive, Ada, OK 74820, USA.
² Environmental and Resource Studies Program, Trent University, 1600 West Bank Drive, Peterborough, ON K9J 7B8, Canada.

Abstract

Formation constants for thioarsenite species have been determined in dilute solutions at 25°C, ΣH₂S from 10^-7.5 to 10^-3.0 M, ΣAs from 10^-5.6 to 10^-4.8 M, and pH 7 and 10. The principal inorganic arsenic species in anoxic aquatic systems are arsenite, As(OH)₃ ⁰, and a mononuclear thioarsenite with an S/As ratio of 3:1. Thioarsenic species with S/As ratios of 1 : 1,2 : 1, and 4 : 1 are lesser components in sulfidic solutions that might be encountered in natural aquatic environments. Thioarsenites dominate arsenic speciation at sulfide concentrations > 10^-4.3 M at neutral pH. Conversion from neutral As(OH)₃ ⁰ to anionic thioarsenite species may regulate the transport and fate of arsenic in sulfate-reducing environments by governing sorption and mineral precipitation reactions.