Electrochemical oxidation pathways and sludge valorization for detoxification of real chemical industry wastewater: Performance, energy consumption and circular economy perspective.

Industrial wastewater containing refractory organic pollutants represent a major environmental challenge due to their high toxicity, complex composition, and resistance to conventional treatment processes. This study systematically investigates electrochemical oxidation pathways and sludge valorization for the treatment of real chemical industry wastewater characterized by high organic load (COD 7556 mg L-1) and elevated salinity. Four electrochemical configurations were comparatively evaluated in a 250 mL batch reactor under continuous UV irradiation (100 W): electro-oxidation (EO, graphite-SS), electro-oxidation coupled with electrocoagulation (EO + EC, Al-SS), EO-assisted Fenton-like oxidation (Fe-SS + H2O2), and photo electrocatalytic oxidation (PECO, TiO2-SS). The effects of pH (3-7), temperature (20-40 °C), current density (10-50 mA cm-2), and electrolysis time (15-75 min) were optimized using response surface methodology based on a central composite design. Treatment performance was assessed through COD removal, UV254 reduction, and acute toxicity using the Daphnia magna immobilization assay. The TiO2-SS PECO system exhibited the highest oxidation performance, achieving 87% COD removal and 72% UV254 reduction with negligible residual toxicity (1% immobilization). The Fe-SS Fenton-like configuration achieved comparable COD removal (86%) but with slightly higher toxicity (6%). In contrast, GRP-SS EO and Al-SS EO + EC systems showed moderate yet stable efficiencies with COD removals of 82% and 72%, respectively. Energy analysis revealed specific energy consumption values ranging from 56.9 to 68.5 kWh kg-1 COD removed depending on the electrochemical configuration. In addition, electrochemically generated Fe- and Al-based sludges were recovered and reused as secondary coagulants, achieving up to 78% COD removal during subsequent treatment. These findings demonstrate that integrating electrochemical oxidation with sludge valorization enables simultaneous pollutant degradation, ecotoxicity reduction, and resource recovery, highlighting a promising circular treatment strategy for high-strength industrial wastewater.