Unexpected effect of buffer solution on removal of selenite and selenate by zerovalent iron

Many studies used buffers to control pH during the reaction of ZVI, but the effect of buffer solutions received little attention. In this study, the effect of buffers on selenite (Se(IV)) and selenate (Se(VI)) removal by ZVI was systematically investigated. It was found that the addition of buffers can hinder the corrosion of ZVI, and then affect Se removal. Although the removal of Se(IV) by ZVI at initial pH (pH(0)) 4.0 was accelerated due to the presence of buffer, less Se(IV) was removed in buffered systems than that in unbuffered systems at pH(0)= 6.0. Buffers also dramatically inhibited Se(VI) sequestration at pH(0) 4.0-10.0. These effects were caused by the buffers instead of their influence on pH variation during the reaction, evidenced by the experiments that decoupled pH effect from overall buffer influences. The zeta potentials of the suspension in buffered systems were more negative than those in the unbuffered systems, implying the adsorption of buffer onto the ZVI surface. Accordingly, the effect of buffers observed in this study was mainly caused by its adsorption on ZVI surface, which may interfere the corrosion of ZVI as well as the adsorption of Se(IV)/Se(VI) on the surface of corroded ZVI, the first step of Se(IV)/Se(VI) sequestration by ZVI. These results suggest that it requires careful evaluation for the suitability of buffers in some reactions to study the kinetics and mechanisms.