Binding of organic pollutants to humic and fulvic acids: Influence of pH and the structure of humic material

In order to compare their binding capacities, humic acids (HA) and fulvic acids (FA) were isolated from water and extracted from sediments, and used to measure partition coefficients (koc) for the model compounds benzo(a)pyrene (BaP) and pentachlorophenol (PCP). To study the effect of pH on the binding capacity of humic substances, koc values were measured at pH 5.0, 6.5 and 8.0. An equilibrium dialysis method was used to determine the koc and elemental analysis, 13C-NMR and FT-IR spectra were carried out on HA and FA to examine the relationship between the structure of humic material and the partition coefficients. The results show that HA have a greater affinity for binding hydrophobic compounds than FA. Change in pH only weakly affects koc values for the neutral chemical BaP while koc values for PCP (about 102 times lower than values of BaP) decrease radically with an increasing pH from 5.0 to 8.0. This suggest that only the unionized form of a weak organic acid like PCP can interact with the humic material. Moreover, elemental analysis and spectra indicate that high koc values can be related to a large aromatic content and to a rather low content of functional groups in the humic structures. This, at least partly, explains why the more hydropholic HA bind the model chemicals more than FA, and why the sedimentary humic substances show a higher binding capacity in comparison with the largely functionalised soluble humic compounds from the water phase.