Hg removal and the effects of coexisting metals in forward osmosis and membrane distillation

In this study, we investigate the rejection of Hg, Cd, and Pb and the effect of coexisting metals on Hg removal through forward osmosis (FO) and membrane distillation (MD) in order to establish a more effective water treatment process. The results of our laboratory experiment indicate that more than 97% of the rejection for each metal is achieved through the FO system, and this rejection is the highest among previous studies using membrane filtrations. Moreover, we examine the matrix effect of the coexisting Cd and Pb on the rejection of Hg in the FO system. Hg²⁺ rejection increases with increase in the concentration of the coexisting metals. Furthermore, we study the effect of the Hg concentration and the water temperature on rejection of Hg²⁺. Indeed, the rejection of Hg²⁺ is achieved above 95% under any condition. However, approximately 1–10 ppb Hg from the feed solution remains in the draw solution due to permeation. Therefore, we use a FO–MD hybrid system. Approximately 100% rejection of Hg²⁺ and a stable water flux are achieved. Thus, the FO–MD hybrid system is considered an important alternative to previous studies using membrane filtration for heavy metals removal.