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

垃圾填埋场沥出物含有多种有毒有机污染物，包括必须防止进入地下水的若干所列物质。为此，《垃圾填埋场条例》规定，新的垃圾填埋场必须采用低渗透性衬垫系统，包括工程粘土层或合成土工膜和粘土的复合物，并位于低渗透性地质构造的下方。除了减少垃圾填埋场的渗滤液流量外，天然和工程粘土还必须通过吸附和生物降解等自然衰减过程降低到达地下水的污染物浓度。地质层（粘土衬垫屏障）保留或阻滞污染物的潜力可纳入填埋风险评估。填埋风险评估必须表明，在填埋场的运营期和关闭后的生命周期内，不会有不可接受的所列物质排放到地下水中，这段时间可能持续数百年，如果不是数千年的话。如果不能以有力的方式证明这一点，将成为拒绝颁发运营许可证的理由。在风险评估中使用预测模型来证明所列物质向地下水的排放（或其他情况），但本研究提案的前提是，目前使用的模型存在严重缺陷，没有考虑到科学的最新发展（下文概述了这方面的证据）。由于预测需要在未来进行，因此必须使用最准确的过程模型描述。不正确的预测的后果是双重的：（1）垃圾填埋场可能会被允许，实际上会在未来的某个时候造成污染或（2）垃圾填埋场可能会被拒绝许可，当使用的粘土类型会阻止所列物质的排放时，只有有限的信息关于英国粘土材料上不同化合物的衰减。已经开展了一些工作，但需要进一步研究，以提供更可靠的数据，供Landsim等模型使用。对于疏水性有机污染物（HOCs），如芳烃或脂肪烃，通常使用与土壤或沉积物有机碳（OC）含量相关的吸附模型，但这些模型没有考虑OC的不同化学性质。研究表明，现有的相关性预测的吸附能力和实验确定的一系列土壤和沉积物之间的重要偏差。最近的研究表明，吸附不仅受有机碳的数量控制，而且还受其组成（例如有机质（OM），来源，年龄和成岩蚀变历史的组合）的控制。目前使用的风险评估模型没有考虑到这一点，因此存在缺陷。此外，溶解有机碳（DOC）会对HOCs的吸附产生影响。DOC在合成渗滤液中的存在已被证明会减少HOCs吸附到矿物衬里材料上。考虑到《填埋指令》（1999/31/EC）的实施，这一现象尤为重要;《填埋指令》实施后，填埋场形成的新沥滤液中溶解的有机和无机化合物的浓度将不同于目前填埋场沥滤液中的溶解有机和无机化合物的浓度，并将含有不同浓度的优先污染物。因此，目前有关垃圾填埋场衬垫中HOC自然衰减的信息可能对新型沥滤物的应用有限。因此，本研究的目的是增加的溶解相和固相有机碳的作用和机制的理解在垃圾填埋场衬垫粘土中的吸附/解吸上市HOCs，并提供更可靠的数据和改进的模型，垃圾填埋场的监管风险评估。

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.