Highly efficient metal-free electrochemical membrane enables zero-valent sulfur recovery from thiourea wastewater

Electrochemical recovery of zero-valent sulfur (S⁰) from thiourea (TU) wastewater offers a promising waste-to-value strategy that expects to promote the sulfur resource cycle in water treatment but still suffer from electrode poisoning and sulfur over-oxidation. Herein, we designed a metal-free CNT electrochemical membrane for selective oxidation of thiourea and recovery of S⁰. We found that defect sites on the carbon nanotube surface enable direct electron transfer for thiourea oxidation and may form carbon-sulfur bridge bonds, thereby facilitating the generation of S⁰ and urea. When treating real industrial wastewater with a concentration of 83.1 mg L^-1 thiourea, the CNT membrane system could achieve a thiourea removal efficiency of 95.4% and a S⁰ recovery rate of 83.8% at 150 L m^-2 h^-1. The corresponding treatment energy consumption and S⁰ recovery energy consumption are 3.3 ± 0.2 kWh kg_-TU^-1 and 9.9 ± 1.0 kWh kg_-S^-1, respectively, lower than that of existing commercial electrodes such as Ti/SnO₂-Sb, PbO₂, Ru/IrO₂, and BDD. More importantly, the membrane system exhibits a desirable durability and stability during a 48-h continuous flow operation. Our findings advance the electrochemical membrane design for efficient S⁰ recovery from thiourea wastewater, contributing to the realization of economical and sustainable waste management and reclaiming resource.