speciation of arsenic in sulfidic waters物种形成的砷sulfidic水域.pdf

Speciation of arsenic in sulfidic waters Richard T. Wilkin,a Dirk Wallschla¨gerb and Robert G. Forda a U.S. Environmental Protection Agency, Office of Research and Development, National Risk Management Research Laboratory, 919 Kerr Research Drive, Ada, OK 74820, USA bEnvironmental and Resource Studies Program, Trent University, 1600 West Bank Drive, Peterborough, ON K9J 7B8, Canada Received 13th November 2002, Accepted 6th March 2003 Article Paper First published as an Advance Article on the web 18th March 2003 Formation constants for thioarsenite species have been determined in dilute solutions at 25 uC, SH S from 2 1027.5 to 1023.0 M, SAs from 1025.6 to 1024.8 M, and pH 7 and 10. The principal inorganic arsenic species in anoxic aquatic systems are arsenite, As(OH)30, 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 w 1024.3 M at neutral pH. Conversion from neutral As(OH)30 to anionic thioarsenite species may regulate the transport and fate of arsenic in sulfate-reducing environments by governing sorption and mineral precipitation reactions. 1. Introduction in aqueous solution. The switch in arsenic speciation from oxyanionic to sulfoxyanionic forms was reversible and trig- The chemical speciation of arsen