Ultraeffective ZnS nanocrystals sorbent for mercury(II) removal based on size-dependent cation exchange

We report a novel nanocrystals (NCs) sorbent, which shows an extraordinary adsorption capacity to aqueous Hg(2+) based on cation exchange and allows for the utmost removal of mercury from water. The NCs sorbent was synthesized by direct coating ZnS NCs on the surface of the α-Al2O3 nanoparticles. The as-prepared ZnS NCs sorbent can efficiently remove over 99.9% Hg(2+) in 1 min, and lower the Hg(2+) concentration from 297.5 mg/L (ppm) to below 1.0 μg/L (ppb) within 5 min. The saturated adsorption capacity of ZnS NCs for Hg(2+) is about 2000 mg/g, which is close to the theoretic saturated adsorption capacity. The mechanism of Hg(2+) removal by ZnS NCs sorbent, the influences of pH value and other cations on Hg(2+) removal were investigated, respectively. Meanwhile, it is found the size-dependent cation exchange plays a critical role in the removal of Hg(2+) by ZnS NCs. Small size ZnS NCs shows better performance than the big size ZnS NCs in the adsorption capacity and adsorption rate for Hg(2+). Furthermore, the mercury adsorbed by the ZnS NCs sorbent is readily recycled by extraction with aqueous sodium sulfide.