填埋场覆盖土-植被-大气间汞的交换机制对汞的形态分布及释放通量的影响研究

A large amount of Hg-bearing solid waste is collected and disposed of in landfills due to the unseparated solid waste collection system widely adopted in China. With the degraded of organic substances, the mercury accumulated in landfill will be converted as elemental mercury or methyl mercury and emit into the atmosphere accompanied with landfill gas, which become an important source of mercury and methyl mercury and bring about a heavy pollution to the soil, aquatic and atmospheric environment..The phycial, chemical and biological factors play an very important role on governing the transformation and emission mechanism of mercury. The objective of this study is to investigate the physical, chemical and biological characteristics of cover soil and the vegetation species distribution, morphological character and transformation regularity, to evaluate the role of the interaction between the cover soil and vegetation on the morphological distribution of mercury, to disclose the mercury transfer and emission mechanism in the landfill gas, to clarify the influence of complex effect of vegetation physiological characteristic and irradiance on governing the flux of mercury emission from landfill to the atmosphere， to discuss the interchange mechanism of mercury between soil, vegetation and air. Directive adjust and inhibit the mercury transfer and emission path through optimum designing the cover soil and vegetation technology to provide the theory evidence and technical support of mercury pollution control in landfill.

我国传统的垃圾混合收集和填埋处理方式，导致大量的有害重金属汞进入填埋场。在填埋场这一特殊的环境条件下，伴随着有机垃圾的降解，长期积累在填埋场的汞会转化为挥发性的单质汞和毒性更强的甲基汞，并伴随着填埋气体扩散至大气，对周围的大气、地下水以及土壤造成严重污染，填埋气成为大气中汞的一个重要污染释放源。填埋场汞的形态分布、迁移转化与释放规律受到物理、化学、生物等因素的共同影响，本课题通过对填埋场覆盖层理化、生物特性以及植被类型、覆盖度和迁移规律系统调查分析，深入解析填埋场覆盖土与植被之间的相互作用对汞的形态分布的影响；揭示填埋气中汞的传输及释放途径；阐明植被生理特性与光照条件的复合效应对汞的释放通量的影响规律；深入探讨填埋场覆盖土-植被-大气之间汞的交换机制；通过优化设计填埋场覆土植被工艺，定向调节和抑制填埋场中汞的传输和释放途径，为有效控制填埋场带来的汞污染提供理论依据和技术支持。

填埋场作为大气汞的重要人为释放源之一，已严重影响当地或区域大气的汞负荷。本课题研究了填埋场覆盖层植被特征、不同基质（覆盖土、植被、填埋气）中汞的分布特征及其影响因素及不同植物覆盖条件下植物-气象参数对汞释放的影响。研究发现，不同填埋场的植物种类、优势种各异，但群落分布、优势种随时间变化规律一致，即随着填埋时间的延长，植被的覆盖度增加，优势种均从一年生植被向多年生植被转变，这主要垃圾组成、填埋工艺和气候条件的影响。填埋气中的汞在填埋初期含量较高，但随着填埋时间的延长，填埋气中汞含量呈下降趋势；植被覆盖区填埋气中汞浓度低于裸露区域；填埋气中甲基汞含量高，表面填埋场存在甲基汞污染。在不同覆盖条件下，汞的通量呈现出明显的昼夜变化，植被覆盖条件下汞的通量远小于植被割除条件下汞的通量，表明植被可有效抑制汞的释放；在碱地肤覆盖条件下，发生汞沉降现象，这主要是由于碱地肤的遮荫效应和对大气汞的吸收所造成；在所有覆盖条件下（除了碱地肤覆盖条件）汞的通量与太阳辐射、气温、土壤温度呈正相关，而与相对湿度呈负相关，表明在不同条件下气象参数对汞的通量的影响具有一定差异。因此，碱地肤可作为被植物生态修复的最适物种选择，以控制填埋场汞的释放。