New insight into modification of extracellular polymeric substances extracted from waste activated sludge by homogeneous Fe(II)/persulfate process

Abstract

Fe(II)/persulfate process has been proven to be a promising technique for disintegrating sludge, while during the Fe(II)/persulfate treatment the direct information about the variation of extracellular polymeric substances (EPS) properties, which is a key factor affecting sludge dewatering, is still lacked. In this work, different dosages of Fe(II)/persulfate were employed to treat EPS fractions extracted from waste activated sludge. The experiment results showed that EPS fractions were modified by Fe(II)/persulfate process. An oxidation/flocculation process was raised to unveil how Fe(II)/persulfate modified EPS fractions: Firstly, SO4– and OH were formed by chemical reactions of Fe(II) activating persulfate and radical interconversion, respectively. Then these species nonselectively fragmented EPS fractions through decomposing their components, which facilitated decrement of the hydrophilic components and high/mid molecular weight of organics in EPS fractions. Furthermore, these radicals transformed the secondary structure of EPS proteins by affecting the hydrogen bonds at specific positions and inducing the cleavage of the S–S bonds in cysteine residues of proteins, which led to loose layout of protein molecules and thus increased exposure of the hydrophobic groups hidden in EPS protein molecules. Secondly, Fe(III), i.e., the oxidation product of Fe(II), assembled the ruptured colloids particles through lessening electronegativity. Consequently, Fe(II)/persulfate process elevated the flocculability and hydrophobicity of EPS fractions, which would improve physicochemical and rheological properties of sludge to facilitate its dewaterability.