Electrocoagulation using aluminium anodes activated with Mg, In and Zn alloying elements

The simultaneous removal of phosphates, Zn²⁺ and Orange II, in two synthetic wastewaters was achieved using Al-Mg and Al-Zn-In alloys as anodes at 11.7 mA cm^-2 and a surface area to volume ratio of 19.0 m^-1. Higher removal efficiencies were obtained with Al-Zn-In, attaining values of 95-96% for phosphate, 99% for Zn²⁺ and 88-96% for Orange II, while somewhat lower values were seen with Al-Mg, with 89-93% for phosphate, 96% for Zn²⁺ and 50-60% for Orange II, depending on the solution. The higher efficiency with Al-Zn-In was attributed to its less passive behaviour, which was evident from polarisation plots. Numerous shallow pits, resembling general-like dissolution, were seen with Al-Zn-In, while fewer and larger pits were observed with Al-Mg. The energy demand for the removal of the pollutants was computed as 1.30 and 2.55 kWh m^-3 for the Al-Zn-In and Al-Mg anodes, respectively. The removal of phosphates and Orange II was explained in terms of the generation of cationic polynuclear aluminium species that provide electrostatic interactions with the anionic phosphates and Orange II. The removal of Zn²⁺ was attributed to the formation of insoluble Zn(OH)₂.