Preparation and characterization of smectite-model humic acid complexes

Abstract

A humic acid analog was prepared by oxidative polymerisation of catechol in the presence of triglycine (PCT) and reacted in unbuffered medium with montmorillonite (M) homoionic to Na +, Ca 2+, Mn 2+, Cu 2+, Fe 3+ and Al 3+, and with two additional M samples treated with polynuclear hydroxyaluminium (Al-hydr) and manganese oxide (Mn-ox). PCT had an average molecular weight of 30 000 Da, a carboxylic acidity of 2.6 meq g −1 and a triglycine: catechol ratio of 0.65. Adsorption isotherms showed that the amounts of PCT adsorbed and bound to M increased according to the saturating cation, in the sequence: Ca 2+ < Na 2+ < Mn 2+ < Cu 2+ < Fe 3+ < La 3+ < Al-hydr. Electrophoretic mobility (EPM), bulk pH in water (pH b) and X-ray diffraction spectra were determined for the M-PCT complexes. The M-PCT bonds were not reversible at pH values lower than that of their formation. The surface acidity (as indicated by pH b) and the negative charge density (as indicated by EPM) of the complexes were higher than those of the respective homoionic M. This suggested that the binding of PCT involved complexation of surface clay cations by carboxyl groups and that the exchanged protons were held less strongly by the clay surface than they were by PCT. The expansion of M in M-PCT complexes was limited, and not related to the amounts of PCT bound, suggesting that interactions with external surfaces of M predominated in most cases.