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The massive amount of sludge generated by the classic Fenton process,which has often been hypothesized to consist of ferric hydroxide,remains a major obstacle to its large-scale application.Therefore,reutilization of Fenton sludge has recently gained more attention.Understanding the formation,transformation,and properties of Fenton sludge combined with the stages of the Fenton reaction is pivotal,but not well illustrated yet.In this study,SEM-EDS,FT-IR,XRD,and XPS were applied to study the morphology,crystallinity,elemental composition,and valence state of Fenton sludge.The authors report that schwertmannite and 2-line ferrihydrite were generated and transformed in the oxidation phase and the neutralization phase of the Fenton process.SO24-in the solution decreased by 8.7％-26.0％ at different molar ratios of Fe(Ⅱ) to H2O2;meanwhile,iron ion precipitated completely at pH 3.70 with the formation of schwertmannite containing sulfate groups in the Fenton sludge.The structural sulfate (Fe-SO4) in schwertmannite was released from the precipitate with the addition of OH-,and the production of Fenton sludge decreased with increasing pH when pH ＞ 3.70.Goethite was found to form when the final pH was adjusted to 12 or at a reaction temperature of 80℃.Moreover,the possible thermal transformation to goethite and hematite indicated that Fenton sludge can be reused as a raw material for synthesizing more stable iron (hydro)oxides.The results provide useful insights into the formation and transformation of Fenton sludge,with implications for regulating the crystal type of Fenton sludge for further reuse.