Dissolved and solid phase organic carbon influences on the sorption/desorption of hydrophobic organic contaminants in clay barriers

Lead Research Organisation: University of Southampton
Department Name: Faculty of Engineering & the Environment

Abstract

Landfill leachates contain a wide range of toxic organic contaminants, including a number of listed substances which must be prevented from reaching groundwater. To this end, the Landfill Regulations require that new landfills are designed with low permeability liner systems, comprising engineered clay layers or a composite of synthetic geomembranes and clay soils, and are located in areas underlain by low permeability geological formations. In addition to reducing the flow of leachate from the landfill, natural and engineered clays must reduce the concentration of contaminants reaching groundwater by natural attenuation processes such as sorption and biodegradation. The potential for geological layers (clay liner barriers) to retain or retard contaminants may be incorporated into landfill risk assessments. A landfill risk assessment must demonstrate that there will be no unacceptable discharge of listed substances to groundwater over the operational and post closure life of the site, a period of time that could last for hundreds if not thousands of year. Failure to demonstrate this in a robust way will provide cause for the refusal of an operational permit. Predictive models are used in risk assessments to demonstrate the discharge (or otherwise) of listed substances to groundwater, but it is the premise of this research proposal that the models currently in use are seriously flawed and do not take into account recent developments in the science (the evidence for this is outlined below). As predictions need to be carried so far ahead into the future, it is imperative that the most accurate model description of the process is used. The consequences of incorrect predictions are twofold: (1) landfill sites may be permitted that will actually cause pollution at some time in the future or (2) landfill sites may be refused a permit, when the type of clay being used would have prevented discharge of listed substances, There is only limited information about attenuation of different compounds on UK clay materials. Some work has been carried out but further research is required to provide more reliable data for use in models such as Landsim. For hydrophobic organic contaminants (HOCs) such as aromatic or aliphatic hydrocarbons, models which relate sorption to soil or sediment organic carbon (OC) content are often used, but these models do not take account of the diverse chemical nature of OC. Studies have shown important deviations between the sorption capacity predicted by existing correlations and that determined experimentally for a range of soils and sediments. Recent research has demonstrated that sorption is controlled not just by the quantity of organic carbon but also by its composition (e.g. a combination of organic matter (OM), source, age, and diagenetic alteration history). The risk assessment models currently in use do not take this into account and are therefore flawed. In addition, dissolved organic carbon (DOC) can have an impact on the sorption of HOCs. The presence of DOC in synthetic leachate has been shown to decrease the sorption of HOCs onto mineral liner materials. This phenomenon is particularly important in view of the implementation of the Landfill Directive (1999/31/EC); the new leachates formed in post-Landfill Directive landfills will have different concentrations of dissolved organic and inorganic compounds than leachates from current landfills, and will contain different concentrations of priority pollutants. Information currently available on natural attenuation of HOCs in landfill liners may therefore have limited application to the new types of leachates. The aims of this research are therefore to increase understanding of the role and mechanisms of both dissolved phase and solid phase organic carbon in sorption/desorption of listed HOCs in landfill liner clays, and, to provide more reliable data and improved models for regulatory risk assessment of landfills.

Publications

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Anika Yunus (Author) (2011) Leachate dissolved organic matter characterization using spectroscopic methods. in Journal of Water Reuse and Desalination

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Simoes A (2011) Sorption of organic contaminants by Oxford Clay and Mercia Mudstone landfill liners in Quarterly Journal of Engineering Geology and Hydrogeology

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Stringfellow A (2011) Sorption of Mecoprop by two clay landfill liner materials: Oxford Clay and Mercia Mudstone in Quarterly Journal of Engineering Geology and Hydrogeology

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Woodman N (2011) Transport of Mecoprop through Mercia Mudstone and Oxford Clay at the laboratory scale in Quarterly Journal of Engineering Geology and Hydrogeology

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Yunus A (2011) Characterisation of the recalcitrant organic compounds in leachates formed during the anaerobic biodegradation of waste. in Water science and technology : a journal of the International Association on Water Pollution Research

 
Description Different types of organic matter present in landfill liner clays show significantly different impacts on the sorption and desorption of hydrophobic organic contaminants (HOCs). In this study, toluene and naphthalene sorption capacities of the clay barrier materials (the Oxford Clay, Kimmeridge Clay and Wittering Formation) were high in comparison to literature values due to the characteristics of the organic carbon present in the clays.

Amorphous organic matter ((AOM) degraded phytoplankton or higher plant material) present in these samples showed the highest sorption capacity for HOCs, whereas phytoclasts (fossil plant fragments) exhibited less sorption capacity, although higher than observed by other investigators. %AOM had the greatest influence on sorption isotherm non-linearity although O/C atomic ratio and %phytoclasts also played a part. Although sorption of HOCs has previously been found to be a function of H/C and H/O, this study demonstrated that %AOM was the major influence on sorption. The study also demonstrated the association of heavy metals with different types of organic carbon in the Oxford Clay.

Dissolved organic carbon (DOC) in landfill leachates is a complex mixture of compounds which ranges from small aliphatic hydrocarbons and proteins to large macromolecules such as humic and fulvic acids. Leachates with five differing DOC characters and origins were used at various dilutions to establish the effect of DOC on HOC sorption behaviour. The results showed that leachate DOC interacts with both the clay and HOCs in solution leading to increases or decreases in sorption relative to literature sorption values.
Exploitation Route The study demonstrated that both leachate DOC and the organic carbon characteristics of barrier clays affect sorption of HOCs and further work is needed to fully understand the mechanisms of sorption in a complex system.

The research demonstrated that commonly used literature sorption coefficients are inadequate in landfill risk assessments. Site specific values are more reliable and the use of specific organic carbon parameters such as %AOM could improve risk assessment models.
Sectors Environment