Macroscopic studies of the effects of selenate and selenite on cobalt sorption to gamma-Al2O3.

Metal ion sorption can be significantly impacted by the presence of other solutes or complexing species. In this research, macroscopic sorption studies were conducted to evaluate the effect of strongly sorbing Se(IV) and weakly sorbing Se(VI) oxyanions on cobalt(II) sorption to gamma-Al2O3. Se(IV) was found to significantly alter Co(ll) sorption as a function of Co(II) surface coverage, while Se(VI) was found to have no effect on Co(II) sorption. Under low Co(II) surface loadings (<0.1 micromol/m2), Se(IV) increased Co(II) sorption as a function of the Se(IV) coverage. At low Se(IV) surface coverages, no change in Co(II) sorption was detectable, while at high Se(IV) loadings Co(II) sorption was significantly increased. The increase in Co(ll) sorption in the bisorbate systems can be explained by either an electrostatic enhancement mechanism or byternary complex formation. Se(IV) decreased Co(II) sorption at high Co(ll) surface loadings (>0.5 micromol/m2) where coprecipitation of Co(II) and A(III) in the form of layered double hydroxides (LDH) is expected to be the dominant sorption mechanism for the single-sorbate case. The extent of the Co(ll) sorption reduction in Co(III)/Se(IV) bisorbate systems compared to the corresponding single-sorbate systems increased with increasing Co(II) surface coverage. The rate of Co(II) desorption was reduced in the presence of Se(IV) compared to the single-sorbate case, indicating a direct interaction between Co(II) and Se(IV). A reaction between Co(II) and Se(IV) is further supported by an increase in Se(IV) sorption in the same bisorbate samples where Co(II) sorption is decreased. Thus, the macroscopic data indicates Se(IV) may be altering the mechanism of Co(II) sorption, potentially forming a ternary surface complex or different surface precipitate.