Effects of Four Kinds of Oxide Nanoparticles on Proteins in Extracellular Polymeric Substances of Sludge

Proteins are the most important component in sludge extracellular polymeric substances (EPS) and play a crucial role in the formation of sludge flocs, adsorption performance of sludge, and flocculation ability of sludge. This research is aimed at exploring the changes in proteins in EPS extracted from concentrated sludge after various nanoparticle (NP) treatments. The results showed that the protein content in EPS decreased by 40% after nanoalumina (Al(2)O(3) NPs) treatment but increased at varying degrees after nanoferric oxide (Fe(3)O(4) NPs), nanozinc oxide (ZnO NPs), and nanotitanium dioxide (TiO(2) NPs) treatments. The four kinds of nanoparticles not only affected the protein content in EPS but also influenced the types and structures of proteins. The results of three-dimensional fluorescence spectroscopy showed that the tyrosine-like protein content in soluble EPS (SEPS) decreased after treatments with four kinds of NPs. Infrared spectroscopy analysis revealed that the absorption intensity of amide I and amide II weakened after Al(2)O(3) NP treatment, whereas that of amide I enhanced after Fe(3)O(4) NP, ZnO NP, and TiO(2) NP treatments. Further analysis of the secondary structure of proteins in the infrared range of 1700–1600 cm(−1) demonstrated that the value of α-helix/(β-sheet+random coil) decreased from 0.513 to 0.383 in SEPS after TiO(2) NP treatment. For the samples treated by Fe(3)O(4) NPs, the percentage of α-helix significantly increased and that of β-sheet slightly decreased in proteins from SEPS and loosely bound EPS.