Complexation of uranyl(VI) by aqueous orthosilicic acid

The complexation reaction of uranyl(VI) with orthosilicic acid, Si(OH)4, was studied by spectrophotometry in aqueous solutions of 0.1 M NaClO4 using competitive complexation with pyrocatechol violet to measure log K1 for the reaction UO22+ + Si(OH)4 <=> UO2(OSi(OH)3)+ + H+. The value of log K1 = -2.92 ± 0.06 obtained was used to estimate uranyl speciation in groundwater that contained only silicic and carbonic acids. The results of these speciation calculations indicate that the formation of UO2(OSi(OH)3)+ is greatest at pH 6 and that UO2(OSi(OH)3)+ comprises less than 50% of the uranyl species in groundwater that is undersaturated with respect to amorphous silica. (orig.)