Overlooked role of singlet oxygen in electrochemical processes for efficient sludge dewatering at neutral pH

Conventional hydroxyl radical (•OH)-based treatments (i.e., iron-based advanced oxidation technologies (Fe-AOTs) and electrochemical processes) are generally considered efficient approaches for sludge dewatering via destroying the stable extracellular polymeric substances (EPS) structure in sludge. However, extremely acidic conditions (i.e., pH 2.8-3.2) in such processes hinder their commercial viability, primarily due to corrosive and environmental issues. Singlet oxygen (¹O₂), known for its selective oxidation and environmental stability, can break down protein structures even at low levels. ¹O₂-based processes may enable efficient sludge dewatering under neutral conditions by disrupting complex EPS structures. Therefore, in this study, we for the first time proposed a ¹O₂-based electrochemical process with a catalytic cathode of pyrite-modified graphite (electro-FeS₂@graphite) for sludge dewatering under neutral conditions. Results showed that under optimized conditions, the electro-FeS₂@graphite system achieved superior sludge dewatering (47.9 % water content over ten cycles) compared to existing Fe-AOTs and electrochemical methods. Electron paramagnetic resonance, quenching tests, and reactive oxygen species probes confirmed that the FeS₂@graphite catalytic cathode mainly generated ¹O₂ species for efficiently eliminating hydrophilic aromatic structures and enhancing sludge dewatering without pH regulation. A three-step mechanism was proposed for the enhanced sludge dewaterability in the electro-FeS₂@graphite system: (i) in situ high H₂O₂ yield at FeS₂@graphite cathode, (ii) considerable ¹O₂ generation within the system, and (iii) selective attack on hydrophilic aromatic structures by ¹O₂. The operational cost of the proposed system was also substantially lower than that of the above-mentioned treatments. Overall, these findings highlight the effectiveness of ¹O₂-based electrochemical process in achieving stable and cost-effective sludge dewatering without acidification.