Influences of landfill leachate dissolved organic carbon on the attenuation of toluene and naphthalene by Oxford clay

Abstract

The uncontrolled release of landfill leachate poses a significant threat to the environment and is prevented in part by the landfill liner. The attenuation of contaminants by the liner material may be crucial in limiting environmental damage should the containment of the landfill fail. While the sorption of hydrophobic organic contaminants (HOCs) to clay materials is well documented in the literature in terms of the organic carbon content of the sorbent in a variety of solution phases, a knowledge gap exists in respect to the influence of landfill leachate dissolved organic carbon (DOC) on the attenuation of HOCs. A laboratory based batch sorption study was undertaken, using leachates with five differing DOC characters and origins, at various dilutions, to establish the influence that DOC present in landfill leachates had on the attenuation of the HOCs toluene and naphthalene by Oxford Clay. The aim of this work was to increase the understanding of attenuation of toluene and naphthalene to Oxford Clay in the presence of leachate DOC and to elucidate the role of DOC chemical composition and concentration in landfill leachates on the sorption of HOCs to the Oxford Clay. The objective of this work was to provide more reliable data for predicting the contribution of attenuation of HOCs by sorption in the risk assessment of landfills and recommending updates to the selection process of Kd values for landfill risk assessment models such as LandSim.

The chemical character of the DOC was found to affect how the DOC interacted with the solid phase and humic substance-like (HSL) DOC was observed to preferentially sorb to the Oxford Clay. The sorption of DOC was reversible, indicating that DOC, and so DOC-HOC complexes, had the potential to desorb from the clay. The addition of DOC to the aqueous phase resulted in deviation from the sorption and desorption isotherms in a DOC free leachate in the case of both toluene and naphthalene. While DOC effects on toluene sorption and desorption behaviour were not clear cut, with both increases and decreases in sorption and desorption observed with the addition of DOC; a uniform decrease in sorption and an increase in desorption relative to the DOC free leachate was observed for naphthalene sorption and desorption behaviour. No desorption was observed for toluene in three of the leachates. In all other DOC containing leachates sorption was reversible and exhibited hysteresis. The alteration in sorption and desorption behaviour of HOCs due to DOC could not be explained solely by the maturity or the concentration of DOC in solution, and was attributed to a complex interaction of both factors. Increasing the hydrophobicity of the HOC resulted in increased sorption (naphthalene > toluene). Recommendations were made for the selection of a Kd value to model HOC attenuation on the basis of a ‘worst case’ scenario. Implications for the post closure monitoring and containment of the landfill and the relevance of commercially available humic acid as a leachate DOC surrogate are also presented.

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ORCID for Anne Stringfellow: orcid.org/0000-0002-8873-0010

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