Investigation of the combined use of capping and oxidizing agents in the immobilization of arsenic in sediments.

The combined use of capping (lanthanum modified bentonite; LMB) and oxidizing (calcium nitrate; CN) agents was investigated to immobilize arsenic (As) in sediments. The vertical changes in labile As and dissolved As were measured using diffusive gradients in thin films (DGT) and Rhizon devices. The results showed that the combined application of LMB and CN had the optimal effect on the immobilization of both DGT-labile As and dissolved As, compared to single treatments using LMB or CN. After 60 days of incubation, the maximum reduction efficiencies of DGT-labile As at sediment depths were 76.4%, 70.8%, and 44.9% of those treated with LMB + CN, CN, and LMB, respectively. After 32 days of incubation, the average concentrations of dissolved total As throughout the depths decreased from 7.71 μg/L after the control treatment without any amendments to 5.25, 4.03, and 3.15 μg/L after the addition of LMB, CN, and LMB + CN, respectively. The larger part of exchangeable As at sediment depths was converted into the reducible As mainly bound Fe/Mn oxide-hydroxides after combining LMB and CN. Due to the As(III) existing mainly in the form of electrically neutral H3AsO3 in sediments, it is hard to adsorb As(III) for the LMB and iron/manganese oxide-hydroxides formed by the oxidation effect of calcium nitrate. Thus, the single or combined LMB and CN use had much weaker effect on the immobilization of As(III) compared with As(V). The results of current study indicated that the combined use of LMB and CN could be a promising method to control the potential release of As from the sediment to the overlying water. However, this method needs further improvement to achieve a better immobilization effect on As(III) in sediments.