Enhanced paper sludge dewatering and in-depth mechanism by oxalic acid/Fe 2+ /persulfate process.

As a typical advanced oxidation process, Fe ²⁺ -persulfate (PDS) oxidation technology has been widely and efficiently reported for enhancing sludge dewaterability. However, higher dosage of Fe ²⁺ must be added, which will restrain the oxidation efficiency of Fe ²⁺ -PDS process. In this work, the oxalic acid (OA)/Fe ²⁺ -PDS process was studied to improve paper sludge dewatering. With the OA dosage of 6 μmol (g total solid (TS)) ^-1 , Fe ²⁺ dosage of 0.3 mmol (g TS) ^-1 , and PDS dosage of 0.6 mmol (g TS) ^-1 , sludge dewaterability was improved more efficiently. The specific resistance to filtration and water content of sludge cake were decreased by 70.7% and 8.0%, respectively. In comparison with Fe ²⁺ -PDS process, the viscosities of sludge suspension and supernatant were further reduced by 3.73% and 51.77%, respectively, and the contents of extracellular polymeric substances fractions were increased. The improved sludge dewaterability in OA/Fe ²⁺ -PDS process was mainly contributed by the synergistic effect of oxidative disintegration by free radicals and flocs re-flocculation, the contributions of which were the orders: metal cations > sulfate radical > hydroxyl radical. OA enhanced the efficient disintegration of sludge flocs, released more bound water, generated more Fe ³⁺ -oxalate complexes, and finally increased the sludge particle size significantly, forming a larger aggregation and obvious cracks. Additionally, the stabilization of several heavy metals was improved due to the chelating capacity of OA. These works will provide a novel approach for sludge dewatering in the PDS oxidation process.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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