A systematic study of arsenic removal from drinking water using coagulation-filtration and electrocoagulation-filtration

This research studied chemical coagulation (CC) with titanium, zirconium, iron, and aluminum salts and iron electrocoagulation (EC) in search for innovative and effective processes for arsenic removal.;In the first phase of the research, ferric-chloride and alum were compared with titanium and zirconium coagulants in representative challenge water. As(V) adsorption was significantly greater than As(III) and the highest As(V) loadings both on mass and molar bases were observed with ferric-chloride. As(V) removal increased with decreasing pH with all coagulants, while As(III) removal was generally independent of pH. Alum did not remove As(III). The highest As(III) loadings were observed with Ti(III), which indirectly oxidized As(III). All the coagulant sludges passed TCLP and WET tests. Based on technical and economic comparisons, ferric-chloride proved to be the best coagulant.;The effects of silica, phosphate, and vanadate on As(III)/(V) adsorption in the challenge water tested with FeCl3, Alum, TiCl4, TiCl3, and ZrCl4 coagulants produced similar interferences with few exceptions. Silica significantly reduced As(V)/As(III) adsorption at all pHs. Phosphate reduced As(V) adsorption at low pHs, however not affecting As(III) adsorption. Vanadate did not affect As(V) or As(III) adsorption.;In the second phase, As(III) and As(V) removals were studied during EC compared with FeCl3 CC. Fe2+, not Fe3+, was produced at the iron anode regardless of current, dissolved-oxygen, pH, or rod purity. Fe2+ generated decreased with increasing pH due to Fe2+ oxidation by dissolved-oxygen.;As(V) removal increased with decreasing pH during both CC and EC, however EC at pH 6.5 was anomalous because of poor Fe2+ oxidation. The best As(V) adsorption capacity was observed with CC at pH 6.5, while capacities remained similar at pH 7.5 and 8.5. In contrast to literature, As(III) was not oxidized nor was Cl2 generated during EC. The competing ions, silica and phosphate interfered with As(V) adsorption during both CC and EC depending on the concentration of competing ions. FeCl3 CC was found to be the most effective process for arsenic removal.;In the final phase, the effectiveness of FeCl3 CC was studied in depth. As(V) adsorption capacity was 5-15 times higher than As(III). In-situ formed hydroxides had 25 times higher adsorption capacities than granular-media. Mixing time and competing ions--silicate, phosphate, and vanadate--affected arsenic adsorption, the level of interference depended on the presence/absence of competing ions. Silica hindered the phosphate and vanadate effects. Mineql+ software could not simulate As(III) or As(V) adsorption.