Arsenate removal from water by Al electrocoagulation with graphite as cathodes: Efficiency, cost, and potential reasons superior to aluminum cathodes

The effects of typical cathodes including Al, Ti, Cu, stainless steel, and graphite on Al electrocoagulation (EC) are investigated for arsenate removal. The results indicate that while arsenic removal efficiencies were similar across different cathodes (>97 %, pH_i=7), the Al-Graphite (Al-G) EC system demonstrated the lowest residual Al^3 +, electrolyte resistance, charge transfer resistance, and operating costs, including energy consumption, electrode loss, and sludge production. Compared to Al-Al EC, Al-G EC reduced electrode loss by 50 %, energy consumption by 27 %, wet sludge production by 40 %, and residual Al³⁺ by 52 % at 2.5 A/m². Al-G EC with NaCl displayed the lowest charge transfer resistance and energy consumption. Coexisting Ca²⁺ and Mg²⁺ distinctly decreased the energy consumption and residual Al³⁺. Fe²⁺, Mn²⁺, and humic acid showed insignificant effects on the operating costs and residual Al³⁺ (p value > 0.05). Al_b and Al_c represented the predominant aluminum forms in Al-G EC and Al-Al EC, respectively. The flocs from Al-G EC exhibited higher zeta potentials (5.40 −19.24 mV) and smaller pore diameter (8.98 nm), enhancing As(V) adsorption. The lower HER activity of graphite cathode prevented a rapid increase in pH, thereby regulating the predominant aluminum forms. The flocs from Al-G EC had a better dehydration property, and lower weight (wet: 1.6 g/L, dry: 0.03 g/L) and water percentage (98.3 %), which was favorable for the subsequent sludge treatment. The continuous-flow experiment (30 h) further demonstrated the advantage of Al-G EC over Al-Al EC.