The sorption of hydrophobic organic compounds in the presence of environmental concentrations of dissolved humic and fulvic acids at variablepH values

Scope and method of study. Laboratory experiments were performed in order to evaluate the potential of dissolved organic material to mediate the transport of hydrophobic organic compounds in systems containing soil or sediment, and water, hydrophobic organic compounds, and dissolved organic matter under differing pH conditions. Batch reactors were used to equilibrate individual solutions of DDT, DDD and Dieldrin with set amounts of three different soils in the presence of dissolved organic matter. The organic matter was used at concentrations ranging from 0 mg/l to 12.5 mg/l as dissolved organic carbon. The tests used humic acid and fulvic acid as the dissolved organic matter in separate experiments. The experiments were conducted at pH values of 4, 7, and 10. Analysis of the target hydrophobic organic compounds in solution at equilibrium was performed using gas chromotography following microextraction with hexane.; Findings and conclusions. The predominant factor controlling the sorption of hydrophobic organic compounds is the organic carbon content of the soil, and the solubility and octanol-water partition coefficient of the compound. Sorption increases with an increase in soil organic carbon content, and with a decrease in compound solubility and an increase in octanol-water partition coefficient. On an equal organic carbon content basis, the A-horizon soil used in this study had a greater sorption capacity than the B-horizon soils. This result may be due to compositional changes of soil organic matter with depth which typically includes an increase in the fulvic-humic acid ratio. Fulvic acids have been noted to sorb hydrophobic organic compounds less than humic acids. This effect may be responsible for the lower sorption capacity found for the B-horizon soils. Next, the dissolved humic and fulvic acids used in this study were found to decrease sorption of the hydrophobic organic compounds by binding with the compounds in solution. Inhibition of compound sorption was found to increase as the dissolved organic matter concentration of the solution was increased. The inhibition of sorption was greater using the humic acid than for the fulvic acid. In addition, inhibition was more pronounced for the hydrophobic organic compounds with lower solubilities and greater octanol-water partition coefficients such as DDT and DDD. Sorption of Dieldrin in the presence of dissolved organic matter appeared to be unaffected due to its greater solubility. Also, sorption inhibition was slightly enhanced at lower pH values. Finally, the presence of dissolved organic matter in soils, surface waters, and ground waters could potentially facilitate the transport of hydrophobic organic compounds through these systems resulting in greater transport distances and faster travel times than would be expected without taking the dissolved organic matter into consideration.