The Influence of Natural Dissolved Organic Matter Found in Groundwater on the Adsorption and Mobility of Pentachlorophenol

Abstract

The effect of natural dissolved organic matter (NDOM) found in groundwater on the adsorption and mobility of the pesticide pentachlorophenol (PCP) was investigated in a laboratory. The low NDOM concentrations found in groundwater as well as the technical difficulties associated to its extraction generally limit environmental studies conducted with natural materials. In this study, a ultrafiltration procedure using a reverse osmosis apparatus was used to concentrate NDOM and to obtain two water fractions respectively with a low (< 0.1 mg L^-1) and a high (15 mg L^-1) content in NDOM. Kinetics studies, equilibrium sorption isotherms and transport experiments in water-saturated soil columns have been performed to quantify the soil adsorption and mobility of PCP within these two water fractions. A one-dimensional advection-dispersion transport model that assumes linear equilibrium adsorption isotherm was used to describe the experimental breakthrough curves (BTC’s). Adsorption rate experiments (pH 5.4 and 6.3) showed that the soil partition equilibrium for PCP was achieved in one hour and was not affected by the presence of NDOM. Sorption isotherms showed an enhancement of the soil distribution coefficient (K_d) for PCP in the presence of NDOM at pH 5.4 and 6.1, while at pH 5.2 no significant effect was observed. Also, pH was found to greatly affect the soil sorption of this weak acid (pK_a = 4.75). The BTC’s were fitted to the analytical solution of the transport model to determine the dispersivity and the retardation factor. The BTC’s (pH 6.2–6.5) showed that NDOM do not significantly affect the mobility of PCP (expressed in terms of K_d and retardation factor) for an influent PCP concentration of 50 μg L^-1. After the soil column elution, the recovery efficiency of 96% and 103% for the PCP injected indicates a complete reversible sorption process. In natural conditions where NDOM is found in low concentration (< 15 mg L^-1), the effect of NDOM on the adsorption and mobility of PCP should be negligible.