No abstract

Activated sludge extracellular polymers (ECP) were extracted either by sonication or a combination of sonication and cation exchange resin treatment (CER). The chemical composition of the aqueous extract was investigated by chromatographic analysis of amino acids and sugars after hydrolysis. Up to 70 to 80% of the total organic carbon (TOC) of ECP was characterized. Proteins were found to be the major constituent of ECP, which was confirmed by pyrolysis/GC/MS analysis. Sugar and protein analysis led to complementary information both on the origin of extracellular material and on sludge floc structure. The monosaccharide composition in ECP and sludge allowed the proposal of different origins for extracellular polysaccharides. The predominance of proteins in ECP underlined their key role in the floc structure, and led to hypotheses on their origin. Proteins were better extracted than sugars when the CER extraction was combined with sonication. This supposed that proteins are more involved than sugars in electrostatic bonds with multivalent cations. Electrostatic bonds were found to be uniformly distributed in the floc and closely combined with hydrophobic bonds. About 25% of ECP amino acids were negatively charged and 24% exhibited hydrophobic properties, highlighting the specific role of proteins in the floc structure.

The flocs formed by coagulation of natural organic matter (NOM) of a lake water by Al salts were studied at a molecular level by 27 Al nuclear magnetic resonance (NMR) and small-angle X-ray scattering (SAXS). The speciation of Al evolves with time: polynuclear Al species are depolymerized by the organic ligands. In aged flocs, uncondensed monomers are the predominant Al species. Modeling of the SAXS curves indicates that the Al monomers in the flocs are evenly separated by 8-9 Å, which suggests the existence of specific binding sites for Al on an "ordered" structure within the NOM. A cellulose type structure is proposed as host for this locally ordered arrangement of Al atoms. The fractal dimensions of the aggregates range from ≈2.3 at pH ) 6.0 to 2.1 at pH ) 8.0. This evolution of the structure can be attributed to pH induced changes of the conformation of the organic macromolecules.