Effect of dissolved humic material (DHM) on bioavailability of some organic xenobiotics to Daphnia magna

The effects of dissolved humic material (DHM) on the bioconcentration of six organic xenobiotics—chlorobenzuron (CCU), diflubenzuron (DFB), terbutylazine (TBA), diazinon, 4-chloroaniline (4-CA) and pentachlorophenol (PCP)—to Daphnia magna (Straus) in laboratory experiments were investigated. Commercial Aldrich humic acids (A-DHM) were used as the sources of DHM. 24 h bioconcentrations (BC24) for these xenobiotics were determinated in the presence of 0, 0.5, 5.0, 10, 20 and 50 mg L−1TOC. With diazinon, 4-CA and PCP 48 h bioconcentrations (BC48) were determined as well.The bioconcentrations (BC) of CCU and diazinon, were significantly reduced, when DHM concentrations increased. The effect on the other chemicals was insignificant. With diazinon DHM exhibited a distinct time-dependency: BC48 lay significantly below BC24-values.The observed reduction of bioavailability to Daphnia magna was attributed to the binding of the organic chemicals to DHM, which was quantitatively measured for CCU by an equilibrium dialysis technique. The association curves obtained for CCU and DHM at concentrations of 10, 20 and 50 mg L−1 TOC were linear with decreasing slopes, when DHM increased. This indicates that the affinity of CCU for DHM increased with decreasing DHM concentration, most probably due to micelle formation of DHM at higher TOC concentrations, which reduces the available surface area.