Enhanced removal of arsenic and fouling mitigation of nanofiltration process via coagulation pretreatment

Arsenic contamination has been considered as one of the serious environmental issues around the world. In this study, the conventional chemical coagulation was applied as pretreatment of the nanofiltration process to enhance the removal efficiency of arsenite (As(III)) and arsenate (As(V)) and mitigate the membrane fouling. Three types of coagulants including aluminum chloride (AlCl3), polyaluminium chloride and ferric chloride (FeCl3) were compared. More than 80% of As(V) could be removed by coagulation, nanofiltration, and their combined process, whereas nanofiltration or coagulation using aluminium-based coagulants only achieved approximately 10% of removal efficiency of As(III). FeCl3 was proven to be the most effective coagulant to remove both As(V) and As(III) with the removal efficiency of 99% and 95%, respectively. The coagulation pretreatment was able to significantly reduce the declining trend of water flux. The formed fouling layer on the membrane surface after the treatment was investigated by a field-emission scanning electron microscopy. Compared to the case of direct filtration of humic acid solution, a more loose and permeable fouling layer could be formed when coagulation was used as a pretreatment process.