Chemistry and structure of aggregates formed with Fe-salts and natural organic matter

The flocs formed by coagulation of the NOM contained in two natural waters by iron chloride were studied by X-ray absorption spectroscopy (XANES and EXAFS) and small-angle X-ray scattering (SAXS). The Fe K-edge XANES data clearly showed that the Fe within the Aocs is octahedralIy coordinated and that its oxidation state is III. The EXAFS results revealed that the NOM is strongly complexed with Fe(III), thus limiting the Fe hydrolysis to the oligomeric stage (typically trimers). The Fe speciation determined from SAXS data confirmed these findings: single-corner-sharing trimers are the predominant Fe species within the flocs. The pH and the nature of the NOM did not influence the Fe speciation or the level of complexation, but played a major role in the structuring of the flocs. (C) 1999 Elsevier Science B.V. All rights reserved.