Removal characteristics of arsenic with various cationic surfactants using micellar-enhanced ultrafiltration(MEUF)

Abstract

The removal characteristics of arsenate using micellar-enhanced ultrafiltration (MEUF) were investigated with four different cationic surfactants - Hexadecylpyridinium chloride (CPC), Hexadecyltrimethyl-ammonium bromide (CTAB), Octadecylamine acetate (ODA) and Benzalkonium chloride (BC). CPC showed the highest removal efficiency of arsenic (96 %), and the removal efficiency with CTAB was 94 %. Over 80 % of arsenic was removed with ODA. Because the critical micelle concentration (CMC) of BC was higher than those of other surfactants, the removal efficiency of arsenic was lowest (57 %). The removal of arsenate was affected by the solution pH. The valance of arsenate decreases from trivalent to monovalent as pH decreases. As the removal mechanism of arsenate of MEUF is the electrostatic interaction between the micelle and the arsenate, so the valance of the arsenate is directly related to its removal efficiency. Due to the change of valence, the removal of arsenate reduced at lower pH. The presence of 0.45 mM of nitrate and 0.01 mM of phosphate reduced the removal efficiency by 5 ~ 8%. This decrease was because of the competition between the arsenate, nitrate and phosphate for the binding sites of the surfactant micelle. Similar decrease in the removal of arsenate was observed with CPC, CTAB and ODA in the presence of these anions. In cross-flow filtration, the removal efficiency of arsenic was similar to that in the dead-end system. However, the decline in flux was less than that in dead-end filtration. In order to lower the concentration of the surfactant in the effluent, the effluent was treated with powdered activated carbon (PAC) before discharging to the environment. Over 98% surfactant was removed with 1g-PAC/L. In conclusions, the MEUF is a feasible process to remove the arsenate from groundwater. CPC and CTAB showed higher removal efficiency. To overcome the hindrance of pH or other anions, the surfactant concentration should be higher than 10 mM.