Comparative study of arsenic removal by iron using electrocoagulation and chemical coagulation

摘要

This research studied As(III) and As(V) removal during electrocoagulation (EC) in comparison with FeCl_3 chemical coagulation (CC). The study also attempted to verify chlorine production and the reported oxidation of As(III) during EC. Results showed that As(V) removal during batch EC was erratic at pH 6.5 and the removal was higher-than-expected based on the generation of ferrous iron (Fe~(2+)) during EC. As(V) removal by batch EC was equal to or better than CC atpH 7.5 and 8.5, however soluble Fe~(2+) was observed in the 0.2-nm membrane nitrate at pH 7.5 (10-45%), and is a cause for concern. Continuous steady-state operation of the EC unit confirmed the deleterious presence of soluble Fe~(2+) in the treated water. The higher-than-expected As(V) removals during batch mode were presumed due to As(V) adsorption onto the iron rod oxyhydroxides surfaces prior to the attainment of steady-state operation. As(V) removal increased with decreasingpH duringboth CC and EC, however EC at pH 6.5 was anomalous because of erratic Fe~(2+) oxidation. The best adsorption capacity was observed with CC at pH 6.5, while lower but similar adsorption capacities were observed at pH 7.5 and 8.5 with CC and EC. A comparison of As(III) adsorption showed better removals during EC compared with CC possibly due to a temporary pH increase during EC. In contrast to literature reports, As(III) oxidation was not observed during EC, and As(III) adsorption onto iron hydroxides during EC was only 5-30% that of As(V) adsorption. Also in contrast to literature, significant Cl2 was not generated during EC, in fact, the rods actually produced a significant chlorine demand due to reduced iron oxides on the rod. Although Cl_2 generation and As(III) oxidation are possible using a graphite anode, a combination of graphite and iron rods in the same EC unit did not produce As (III) oxidation. However, a two-stage process (graphite anode followed by iron anode in separate chambers) was effective in As(III) oxidation and removal. The competing ions, silica and phosphate interfered with As(V) adsorption during both CC and EC. However, the degree of interference depends on the concentration and presence of other competing ions. In particular, the presence of silica lowered the effect of phosphate with increasing pH due to silica's own significant effect at high pHs.