Efficient mercury removal from water by using modified natural zeolites and comparison to commercial adsorbents

The reduction of mercury use in industrial activities and its elimination from waste streams is of great importance to the circular economy approach. In this study a natural zeolite was impregnated with silver to derive Ag+, Ag2O and Ag0 forms. The natural and Ag-zeolites were effectively used for the removal of mercury from Hg(NO3)2 and HgCl2 aqueous solutions. Kinetic experiments, aqueous phase speciation modeling and post-sorption characterizations were used to interpret the sorption behavior of the zeolites. Natural zeolite is effective in removing mercury from Hg(NO3)2 solutions but sorption is negligible from HgCl2 solutions. Silver modification transformed the natural zeolite into an effective nanocomposite sorbent for mercury removal from HgCl2 solutions, the effectiveness being affected by the Ag form. However, Ag modification had no effect on the removal from Hg(NO3)2 solutions. These results demonstrate the strong interplay between ion exchange, surface Ag–Hg reactions and the aqueous phase Hg speciation which was further supported by complementary experiments with iodide under the same conditions. For comparison, Hg removal by commercial adsorbents, namely an activated carbon, a cation exchange resin and synthetic zeolite Y is presented. The results demonstrate that natural zeolite and Ag0 form are effective adsorbents performing better than commercial ion exchangers and are comparable to activated carbon. The use of abundant, cheap, environmental-friendly and easy to modify natural zeolite is an attractive alternative to commercial and other unsustainable and difficult to synthesize adsorbent materials.