Modelling long-term release of chlorinated hydrocarbons from a contaminated brown coalfield, Bitterfeld, Germany

Abstract

Three kinetic and equilibrium partitioning column experiments were performed to determine the rates and extent of desorption of five chlorinated hydrocarbons (1,2 cis-dichloroethylene (DCE), 1,2,2-trichloroethane (TCA), trichloroethylene (TCE), 1,2,3,4-tetrachloroethylene (PCA) and chlorobenzene) found in the contaminated sediments in a former brown coal mine. A high fraction (more than 70%) of the contaminants DCE, TCA and TCE desorbed in a single-step column test performed for a period of 50–70 days, whereas the remaining 30–40% of the initial PCA and chlorobenzene load desorbed from the contaminated sediments. When the desorption test was conducted with contaminated sediments filled columns through continuous replenishment of the deionized water, the desorbed TCE fraction increased to as high as 95% after 70 successive days of leaching and as low as 30% for PCA. It turns out that the flux rate of the desorbed contaminant fractions is strongly correlated to the organic carbon-based partition coefficient (Koc) of contaminants. A coupled Ogata-Banks and the intraparticle diffusion equation analytical solution were used to predict the long-term release rates and desorption of these chlorinated hydrocarbon compounds from the contaminated sediment. The laboratory measured flux rate data were used to constrain model predictions of five effective diffusion coefficients ranging from 0.99 × 10−9 to 1.43 × 10−9 m2 s−1. The estimated effective diffusion coefficients correspond well with water partition coefficient (log Kow) of the five chlorinated compounds ranging from 1.86 to 2. The model, using two distinct flux rates, successfully described the long-term release and desorption of the five chlorinated hydrocarbons. The results underscore the fact that small-scale laboratory experiments and pilot-scale field trials are necessary before designing and implementing large-scale remediation of chlorinated hydrocarbon contaminated sites, which requires considerable financial commitments.