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ligands have been reported to increase the aqueous solubility of Cr(III) over that of the hydroxide complex(Palmer 1991) ' Controls on Chromium Solubili Cr(VI) as the chromate anion is highly soluble, and, under certain conditions, quite mobile in the aquatic environment Formation of lead chromate (PbCr04), barium chromate (BaCr04), or a barium chromate-sulfate solid solution (Ba(SO4, Cr04)) may control aqueous chromate concentrations in ' environments enriched in lead or barium (Rai 1989, Richard 1991) In most aquatic environments, however, chromate is removed from solution by formation of surface complexes on mineral oxides (manganese, aluminum, iron) and clay minerals The adsorption reaction is pH dependent with ' adsorption decreasing as pH increases Adsorption is also decreased by the presence of other anions, such as phosphate, silicate, and sulfate, which compete with chromate for surface complexation sites (Rai 1989) ' Cr(III) is generally II much less mobile in aquatic environments than Cr(VI) Cr(III) can form complexes with organic matter bound to aquifer solids and thus be removed from solution (Richard 1991) The Cr(ill) hydroxide complex is relatively insoluble and will precipitate to form Cr(OH)3 solid (Palmer 1991) However, if Fe(III) is present, dissolved Cr(OH)3 does not form solid Cr(OH)3, but rather forms an amorphous (Cr,Fe)(OH)3 solid solution (Palmer 1991) Amorphous (Cr,Fe)(OH)3 is believed to control Cr(III) solubility in natural systems (Rai 1989, Richard 1991) This exact solubility of chromium in amorphous (Cr,Fe)(OH)3 is related to its concentration in the solid and is less than that of Cr(OH)3 solid (Palmer 1991) At the relatively low chromium mole fractions (< 01) expected in natural environments, precipitation of (Cr, Fe)(OH)3 would maintain dissolved chromium levels below the EPA drinking water MCL over a wide range of pH values (Rai 1991) References Alloway, B J (Editor) 1990 Heavy Metals in Soils John Wiley& Sons, New York Bartlett, R and B James 1979 Behavior of chromium in soils III Oxidation Journal of Environmental Quality 8 31-35 Eary, L E and D Rai 1991 Chromate reduction by subsurface soils under acidic conditions Soil Science Society of America Joumal 55 676-683 Kroschwitz, J I (executive editor) 1992 Encyclopedia of Chemical Technology Fourth Edition Volume 6 John Wiley& Sons, New York ' C\Documents and 155 November 2004 Satti ngs�cgohnsen\Desktop\Unocal\GeechemEval_D oc doc