Effects of Humic Acid Purification on Interactions with Hydrophobic Organic Matter: Evidence from Fluorescence Behavior

Conventional isolation of humic materials from natural matrixes includes demineralization by treatment with HF/HCl. The possible effect of this on the structural integrity of humic acid (HA) was investigated by comparing the interactions of two aqueous HAs, one produced by the conventional procedure (CHA) and the other by a gentler resin method (RHA), with a hydrophobic probe (pyrene). Although RHA had a substantially higher ash content, it was found that this had relatively little effect on its interaction with the probe molecule. In view of the micellar model of dissolved HAs, however, it was noted that the HF-treated HA had a greater apparent ability to sequester pyrene and isolate it from the aqueous bulk. This was revealed by the kinetics of probe fluorescence enhancement when Mg 2+ was added to a solution also containing HA and by the anisotropy excitation spectra of the native humic fluorophores. The effect was ascribed to a lesser degree of conjugation in the HA demineralized with HF, producing polymers of greater flexibility that formed micelle-like structures more quickly and effectively.