Bioavailability of 2,4-dichlorophenol associated with soil water-soluble humic material

Interaction of organic xenobiotics with soil water-soluble humic material ( WSHM) may influence their environmental fate and bioavailability. We utiliz ed bacterial assays (lux-based toxicity and mineralization by Burkholderia sp. RASC) to assess temporal changes in the bioavailability of [C-14]-2,4-d ichlorophenol (2,4-DCP) in soil water extracts (29.5 pg mL(-1) 2,4-DCP; 840 .2 mug mL(-1) organic carbon). HPLC determined and bioavailable concentrati ons were compared. Gel permeation chromatography (GPC) was used to confirm the association of a fraction (>50%) of [C-14]-2,4-DCP with WSHM. Subtle di fferences in pa ra meters describing 2,4-DCP mineralization curves were rec orded for different soil-2,4-DCP contact times. Problems regarding the inte rpretation of mineralization data when assessing the bioavailability of tox ic compounds are discussed. The lux-bioassay revealed a time-dependent redu ction in 2,4-DCP bioavailability: after 7 d, less than 20% was bioavailable , However, GPC showed no quantitative difference in the amount of WSHM-asso ciated 2,4-DCP over this time. These data suggest qualitative changes in th e nature of the 2,4-DCP-WSHM association and that associated 2,4-DCP may ex ert a toxic effect. Although GPC distinguished between free- and WSHM-assoc iated 2,4-DCP, it did not resolve the temporal shift in bioavailability rev ealed by the lux biosensor. These results stress that assessment of risk po sed by chemicals must be considered using appropriate biological assays.