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 ( 1 O 2 ), known for its selective oxidation and environmental stability, can break down protein structures even at low levels. 1 O 2 -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 1 O 2 -based electrochemical process with a catalytic cathode of pyrite-modified graphite (electro-FeS 2 @graphite) for sludge dewatering under neutral conditions. Results showed that under optimized conditions, the electro-FeS 2 @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 FeS 2 @graphite catalytic cathode mainly generated 1 O 2 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-FeS 2 @graphite system: (i) in situ high H 2 O 2 yield at FeS 2 @graphite cathode, (ii) considerable 1 O 2 generation within the system, and (iii) selective attack on hydrophilic aromatic structures by 1 O 2 . 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 1 O 2 -based electrochemical process in achieving stable and cost-effective sludge dewatering without acidification.