Coagulation-Flocculation of Natural Organic Matter with Al Salts:  Speciation and Structure of the Aggregates

Abstract

The flocs formed by coagulation of natural organic matter (NOM) of a lake water by Al salts were studied at a molecular level by 27Al 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.