Boron removal from high sulfate-containing coal-fired power plant wastewater by an ultrasound/bipolar electrocoagulation process with aluminum electrodes

Boron in coal-fired power plant (CFPP) wastewater is difficult to remove by the electrocoagulation (EC) process due to its high sulfate concentration. Sulfate significantly decreases the aluminum ions released from aluminum anodes by forming low-solubility aluminum-sulfate-hydroxides on the surface of electrodes. Moreover, boron can cause repassivation after pitting corrosion initiated by chloride due to borate's alkalinity. Previously, adding chloride was perceived as a solution to induce pitting, thereby enhancing the boron removal. However, adding chloride, was ineffective in causing pitting to improve boron removal by EC in the presence of high sulfate concentrations. The aim of this study is to investigate whether the application of ultrasound and chloride could improve the efficiency of boron removal via electrocoagulation when present of high sulfate concentration. We combine ultrasound (US) and chloride addition to overcome the repassivation and evaluate the effect of restraining repassivation by current efficiency. The results demonstrated that the US application effectively eliminates the passive film, significantly improving the current efficiency from 52.4 % to 102 %. Integration of US and chloride addition could dramatically enhance boron removal by BEC, elevating the removal capacity from 11 mg/L to 29 mg/L. Through the synergistic effect of ultrasound and chloride ions, we can prevent repassivation and the associated decline in current efficiency, significantly extending the practical application range of electrocoagulation.