A capacitive deionization and electro-oxidation hybrid system for simultaneous removal of heavy metals and organics from wastewater

Abstract

Capacitive deionization is an appealing technology to remove and recover heavy metal ions from wastewater. However, the co-existing organic pollutants in actual wastewater greatly inhibit its practical application, by causing electrodes fouling and deterioration issues. Herein, a novel system combing the capacitive deionization and electro-oxidation (CDI-EO) processes in a single apparatus was developed for simultaneous removal of heavy metals and organics. This hybrid system consists of a carbon-coated graphite paper as the cathode for heavy metals removal, and a RuO₂-IrO₂ protected titanium plate as the anode for organics degradation. The synthetic and actual textile wastewater were both treated to explore the purification mechanisms and evaluate the system stability. When copper (Cu²⁺) or Acid Orange 7 (AO7) was present individually in the synthetic wastewater, the CDI-EO cell achieved 83% and 91% of removal efficiencies, respectively, at an applied current density of 5 mA/cm². When Cu²⁺ and AO7 were mixed, their removal efficiencies still maintained at 80% and 89%, respectively, with little mutual interference. Cu²⁺ ions were removed through cathodic electrosorption and electrodeposition. By reversing the electrode polarities at 3 V for 30 min, over 71% of the deposited Cu was re-dissolved for potential recovery, with the cathode re-generation. Active chlorine species were identified as the predominant oxidants generated by the anode for AO7 degradation, and a possible degradation pathway was determined. Cycling tests indicated that the CDI-EO system exhibited good stability and reliable pollutants removal. It performed equally well when treating the actual textile wastewater collected from an industrial park. The energy consumption of the CDI-EO system for textile wastewater treatment was 4.66 kWh/m³-wastewater, which is much lower than that of other advanced oxidation processes.