COMPETITIVE SORPTION BETWEEN 1,3-DICHLOROBENZENE OR 2,4-DICHLOROPHENOL AND NATURAL AROMATIC-ACIDS IN SOIL ORGANIC-MATTER

Previously, we showed that sorption of anthropogenic organic compounds in soils is competitive as a result of the heterogeneous sorption pot ential of soil organic matter (SOM). We hypothesize that naturally occ urring, low molecular weight organic molecules or ions could compete w ith anthropogenic molecules for sorption sites in SOM. We show here th at natural aromatic acids suppress sorption from water of 1,3-dichloro benzene (1,3-DCB) or 2,4-dichlorophenol (2,4-DCP) by up to similar to 40% to two soils, a Pahokee peat (93% SOM) and a Cheshire fine sandy l oam (3% SOM). The experiments were performed at the natural pH of the soil suspension (5.0 and 5.6, respectively), at which the aromatic aci ds were mostly ionized. In a screen test at 100 mu g/mL aromatic acid concentration, 10 out of 11 competed significantly with 2,4-DCP. Five were selected for further study: vanillic, p-hydroxybenzoic, trans-cin namic, p-coumaric, and ferulic acids. The selected five aromatic acids as well as the two anthropogenic compounds gave nonlinear isotherms ( Freundlich exponent, 0.507-0.852), indicating a distribution of site e nergies in SOM. The pattern in the competitive effect among the aromat ic acids was similar for the two soils; since the peat soil is almost mineral-free, this indicates that competition occurred in the SOM phas e. In nearly all cases, suppression of sorption decreased exponentiall y with increasing aromatic acid concentration, as would he expected fo r direct competition for sites. These results are in accord with the d ual-mode model of sorption to SOM we proposed earlier, in which both s olid-phase dissolution and hole (site)-filling mechanisms take place a nd in which competition takes place only in the hole-filling domain. T he results show that aromatic acids and possibly other small natural o rganic molecules can occupy and block adsorption sites in SOM, potenti ally affecting the fate, transport, and bioavailability of anthropogen ic organic compounds in the environment.