Coagulation-flocculation of natural organic matter with Al salts: Speciation and structure of the aggregates

The flocs formed by coagulation of natural organic matter (NOM) of a lake w ater by Al sails were studied at a molecular level by Al-27 nuclear magneti c resonance (NMR) and small-angle X-ray scattering (SAXS). The speciation o f Al evolves with time: polynuclear Al species are depolymerized by the org anic ligands. In aged flocs, uncondensed monomers are the predominant Al sp ecies. Modeling of the SAXS curves indicates that the Al monomers in the fl ocs are evenly separated by 8-9 Angstrom, which suggests the existence of s pecific binding sites for Al on an "ordered" structure within the NOM. A ce llulose type structure is proposed as host for this locally ordered arrange ment of Al atoms. The fractal dimensions of the aggregates range from appro ximate to 2.3 at pH = 6.0 to 2.1 at pH = 8.0. This evolution of the structu re can be attributed to pH induced changes of the conformation of the organ ic macromolecules.