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

垃圾填埋场渗滤液含有多种有毒有机污染物，包括一些必须防止进入地下水的清单物质。为此，《堆填区规例》规定，新的堆填区须设计有低渗透性衬垫系统，包括工程黏土层或合成土工膜和黏土的混合物，并位于低渗透性地质地层之下的地区。除了减少垃圾填埋场的渗滤液流量外，天然粘土和工程粘土还必须通过吸附和生物降解等自然衰减过程来降低到达地下水的污染物浓度。可将地质层(粘土衬垫屏障)保留或延缓污染物的潜力纳入垃圾填埋场风险评估。垃圾填埋场风险评估必须证明，在该填埋场的运营和关闭后的生命周期内，不会有不可接受的清单物质排放到地下水中，这段时间可能会持续数百年，如果不是数千年的话。如果不能以有力的方式证明这一点，将导致拒绝发放运营许可证。预测模型用于风险评估，以证明所列物质向地下水排放(或以其他方式排放)，但本研究提案的前提是，目前使用的模型存在严重缺陷，没有考虑到科学上的最新发展(证据概述如下)。由于预测需要向前推进到未来，因此必须使用对该过程的最准确的模型描述。错误预测的后果是双重的：(1)垃圾填埋场可能被允许在未来某个时候实际造成污染，或者(2)垃圾填埋场可能被拒绝获得许可，当所使用的粘土类型将阻止列出的物质排放时，关于英国粘土材料上不同化合物的衰减信息有限。已经开展了一些工作，但还需要进一步研究，以提供更可靠的数据，以便在Landsim等模型中使用。对于疏水性有机污染物(HOCs)，如芳香烃或脂肪族碳氢化合物，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，对于一系列土壤和沉积物，现有关联式预测的吸附容量与实验确定的吸附容量之间存在重大偏差。最近的研究表明，吸附不仅受有机碳的数量控制，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。合成渗滤液中DOC的存在已被证明降低了HOCs在矿物衬里材料上的吸附。鉴于《垃圾填埋指令》(1999/31/EC)的实施，这一现象尤其重要；在垃圾填埋指令后形成的新渗滤液将具有不同浓度的溶解有机和无机化合物，并将含有不同浓度的优先污染物。因此，目前可获得的关于垃圾填埋场衬里中HOCs自然衰减的信息对新型渗滤液的应用可能有限。因此，这项研究的目的是加深对溶解和固相有机碳在垃圾填埋场衬垫粘土中列出的HOCs吸附/解吸过程中的作用和机理的了解，并为垃圾填埋场的监管风险评估提供更可靠的数据和改进的模型。

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.