Unravelling sequence-dependent molecular transformation pathways of dissolved organic matter in chemical cleaning wastewater during combined Fenton oxidation and activated carbon adsorption processes

This study investigates the molecular transformations of dissolved organic matter (DOM) in industrial wastewater by combining Fenton oxidation and powdered activated carbon (PAC) adsorption processes. Industrial effluents, particularly those rich in refractory DOM, pose significant challenges for treatment and reuse. Using chemical cleaning wastewater (CCW) as a model sample, the study evaluates the impact of two coupling sequences—Fenton-PAC and PAC-Fenton—on DOM removal and transformation. The molecular-level behavior of DOM is explored using high-resolution mass spectrometry to analyze chemical changes, which reveals that each process sequence leads to distinct molecular pathways, significantly influencing the oxidation state, size, and heteroatom content of the remaining DOM. In the PAC-Fenton process, less-adsorbed substances, primarily lipids and proteins/peptides, undergo preferential mineralization or oxidation by Fenton treatment. This results in their transformation into CRAM/lignin-like compounds. In contrast, during the Fenton-PAC process, oxygen addition, dealkylation, and decarboxylation drive the depolymerization of DOM in CCW, producing smaller molecules such as amino sugars and carbohydrates. These smaller molecules exhibit weaker subsequent adsorption affinity. The findings demonstrate that the PAC-Fenton process achieves superior removal of complex DOM and incurs over 50% lower chemical costs compared to the Fenton-PAC approach, enhancing cost-efficiency. Moreover, these results provide valuable insights into optimizing industrial wastewater treatment strategies, contributing to enhanced environmental sustainability.