Fate of coagulant species and conformational effects during the aggregation of a model of a humic substance with Al13 polycations

A model of a humic substance (MHS) obtained from auto-oxidation of catechol and glycine, was aggregated at pH 6 and 8 with Al13 polycations. The fate of Al13 coagulant species upon association with MHS functional groups was studied using solid state 27Al Magic-angle spinning (MAS) NMR and CP-MAS 13C NMR. Electrophoretic measurements and steady-state fluorescence spectroscopy with pyrene as a fluoroprobe, were combined to investigate structural re-organization of humic material with aluminum concentration. MAS 27Al NMR revealed that the coagulant species are Al13 polycations or oligomers of Al13 units at both pHs. CP MAS 13C spectra indicated that, at low Al concentration, hydrolyzed aluminum species bind selectively to carboxylic groups at pH 6 and to phenolic moieties at pH 8. At higher coagulant concentrations, the remaining functional groups also interact with hydrolyzed Al to yield similar CP MAS 13C spectra in the optimum concentration range. Negative values of electrophoretic mobility were obtained at optimum coagulant concentrations even though an overall charge balance was achieved between MHS anionic charge and Al13 cationic charge at pH 6. The polarity-sensitive fluorescence of pyrene revealed that the interaction of Al13 coagulant species with MHS functional groups induces the formation of intramolecular hydrophobic microenvironments. Such structural changes were reversed upon further addition of Al13 polycations.