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.