Use of a column leaching test to study the mobility of chlorinated HOCs from a contaminated soil and the distribution of compounds between soluble and colloid phases
Introduction
Contaminated soils may be secondary sources of hazardous compounds in the environment. Therefore, in order to perform thorough risk assessments of contaminated sites the mobility of hazardous compounds from the contaminated soils must be measured or estimated. For this purpose several leaching tests have been developed for analyzing mobile and available fractions of organic compounds in soils and other solid materials (Reemtsma and Mehrtens, 1997, Comans et al., 2001, Enell et al., 2004). However, international standard leaching procedures for organic compounds have not yet been established.
Hydrophobic organic compounds (HOCs) comprise a wide array of compounds, including (inter alia) polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polychlorinated dibenzofurans (PCDFs) and polychlorinated dibenzo-p-dioxins (PCDDs). These compounds are often toxic, persistent in the environment and can bioaccumulate (Safe, 1986, ATSDR, 1995). Reducing dispersal of these compounds is highly important to minimise human exposure to them. For example, PCDD/Fs are unintentionally formed by a number of production processes used in the chemical industry, including the production of chlorophenols, and the combustion of diverse materials. Persson et al. (2007) investigated five sawmill sites in Sweden with a historic use of chlorophenols and found levels of contaminants in the soil ranging from 0.1 to 4800, 0.2 to 940, 1.3 to 6.8 and 0.03 to 51 mg kg−1 dry weight (d.w.) of chlorophenols (CPs), polychlorinated phenoxyphenols (PCPPs), polychlorinated diphenyl ethers (PCDEs) and polychlorinated dibenzo-p-dioxins and furans (PCDD/F), respectively.
HOCs are largely associated with organic matter (both dissolved and particulate) in soil (Karickhoff et al., 1979, Schwarzenbach and Westall, 1981, Frankki et al., 2006). Dissolved organic carbon or fine particles may be transported through the soil profile as colloids (Laegdsmand et al., 2005) and may increase release rate of HOCs from soil and waste material (Liu and Amy, 1993, Mackay and Gschwend, 2001, Kim et al., 2002). Thus, if colloid-facilitated transport from contaminated soils is not considered there is a risk that the dispersal of HOCs will be underestimated.
The study presented here had three main aims. The first was to assess the utility of an equilibrium and recirculation column test for hydrophobic organic chemicals (ER-H test) developed by Gamst et al. (2007) for determining the partitioning and mobility of HOCs in chlorophenol-contaminated soil. This column test is designed to determine equilibrium solute concentrations of non-volatile organic compounds in contaminated soils, but its suitability for analyses of chlorinated compounds or highly hydrophobic compounds, e.g. PCDD/Fs, had not been previously tested. The second objective was to investigate the distributions of CPs, PCDEs and PCDD/Fs in colloids retained in the leachate and the solutes from the ER-H test. It has been suggested that the colloids present in the leachate from the ER-H test can be expected to be mobile under natural conditions (Hansen et al., 2004). The final objective was to study the method’s sensitivity to changes in pore-water velocity and whether such changes affect the colloid fraction in the leachate.
Section snippets
Characteristics of the contaminated soil
Three samples of soil, each weighing approximately 0.5–2 kg, from the surface down to 1 m depth, were collected at a contaminated sawmill site in Luleå, Northern Sweden (65° 35′ N, 22° 03′ E), where tetrachlorophenol preservatives had been used as fungicides between approximately 1960 and 1975. The three samples were pooled and mixed manually in a bucket using a spade in the laboratory, adding a quarter of the samples at a time, until the composite sample was believed to be homogenous (after
Composition of CPs, PCDEs and PCDD/Fs in the particulate fraction and water phase
The amount and composition of CPs, PCDEs and PCDD/Fs in the material retained by each filter (>2.7 μm, 2.7–0.7 μm and 0.7–0.2 μm) were evaluated. No significant differences were found amongst the three particulate fractions, thus the cumulative amounts in these fractions (>0.2 μm) are presented in Fig. 1. The leached CPs were primarily found in the water phase (44–81 mg l−1) and only minor proportions were found in the particle fractions (0.8–1.8 mg l−1). The reverse was found for the other compounds
Conclusions
The assessed ER-H method proved to be suitable for determining the leaching of CPs with acceptable repeatability. In addition, changing the pore water velocity from 20 ml h−1 (the published default rate) to 30 ml h−1 was shown to have little or no effect on the turbidity, pH, dissolved organic carbon content or concentrations of CPs, PCDEs, PCDFs and PCDDs in the leachate, and did not increase the mobilisation of colloids. However, the ER-H method seems to be less suitable for highly hydrophobic
Acknowledgements
Eva Narbrink, of the Swedish Geotechnical Institute, is gratefully acknowledged for help and support during the leaching tests. The project was supported by the Northern Sweden Soil Remediation Centre, which is funded by the EU Structural Funds and New Objective 1.
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