Components, molecular conformation, and intermolecular forces of extracellular proteins with potential implications...

Chemical conditioning improves sludge dewaterability, but its molecular mechanism remains unclear. Herein, alterations in the components, conformation, and intermolecular forces of extracellular proteins under ferric chloride (FeCl₃) conditioning were investigated. After FeCl₃ conditioning at 100 mg/g dry solids, the content of hydrophilic amino acids in extracellular proteins decreased significantly, and the proportion of α-helices decreased from 19.9 ± 1.2 % to 13.2 ± 1.2 %, resulting in a 1.8-fold increase in the hydrophobicity of extracellular proteins. FeCl₃ decomposed extracellular proteins and disrupted their structure through oxidation. Notably, FeCl₃ destroyed the ionic, hydrogen, and disulfide bonds within proteins and formed a gel hollow structure within extracellular proteins, enhancing sludge dewaterability. Moreover, the flocculation-driven binding between Fe³⁺ and amino acid residues in extracellular proteins by electrostatic and hydrophobic interactions, changed the extracellular protein microenvironment. These findings fill a gap in our knowledge of the molecular mechanisms of chemically conditioned deep sludge dewatering.