Sulfate removal from mine-impacted water by electrocoagulation: statistical study, factorial design, and kinetics

This work aimed to remove sulfate and acidity from mine-impacted water (MIW) via electrocoagulation (EC), a technique which stands as an advanced alternative to chemical coagulation in pollutant removal from wastewaters. The multiple electrochemical reactions occurring in the aluminum anode and the stainless steel cathode surfaces can form unstable flakes of metal hydroxysulfate complexes, causing coagulation, flocculation, and floatation; or, adsorption of sulfate on sorbents originated from the electrochemical process can occur, depending on pH value. Batch experiments in the continuous mode of exposition using different current densities (35, 50, and 65 A m^-2) were tested, and a statistical difference between their sulfate removals was detected. Furthermore, the intermittent mode of exposure was also tested by performing a 2²-factorial design to verify the combination with different current densities, concluding that better efficiencies of sulfate removal were obtained in the continuous mode of exposition, even with lower current densities. After 5 h of electrocoagulation, sulfate could be removed from MIW with a mean efficiency of 70.95% (in continuous mode of exposition and 65 A m^-2 current density), and this sulfate removal follows probable third-order decay kinetics in accordance with the quick drop in sulfate concentration until 3 h of exposure time, remaining virtually constant at longer times. Graphical abstract.