土壤-地下水中微塑料迁移的影响因素及机制研究进展

微塑料在环境中广泛分布,世界范围内的农业土壤及地下水中都已发现微塑料污染,生态环境和人体健康受到严重威胁。研究土壤-地下水中微塑料迁移的影响因素及机制,对于准确评价其分布归趋及环境风险具有重要意义。该研究通过文献调研,对土壤-地下水环境中微塑料的来源、团聚及迁移研究进行梳理、归纳和总结,系统阐明了土壤-地下水中微塑料迁移的影响因素,剖析了影响微塑料迁移的机制,并对未来研究进行展望。土壤中微塑料的来源可分为原位型微塑料和外源输入型微塑料2种,地下水中的微塑料一般源自于土壤中微塑料的垂直迁移及地表-地下水微塑料交换。水体中微塑料的团聚受多种水环境因素的影响,其团聚程度与迁移能力密切相关,是迁移行为的基础和前提。土壤-地下水中影响微塑料迁移的因素可分为化学、物理、生物3类。水化学条件、介质成分、水流条件、介质物理条件、植物生长发育、小型动物及微生物的生命活动均会影响土壤-地下水中微塑料的迁移行为,且影响机制各不相同。目前,土壤-地下水中微塑料的迁移研究处于起步阶段,在进一步的研究中,野外尺度微塑料迁移、多元化微塑料迁移、微塑料特性对其迁移行为影响、微塑料迁移过程中的转化等研究值得重点关注。

Review on impact factors and mechanisms of microplastic transport in soil and groundwater

Abstract: Microplastic (the small plastic with the diameter lower than 5 mm) has been widely detected in soil and groundwater worldwide, which draws high attention of researchers and publics as an emerging contaminant. For better predicting and evaluating the environmental risk of microplastic in soil and groundwater environment, mechanism studies focus on microplastic transport are necessary. Nevertheless, transport and retention processes of microplastic in soil and groundwater environment are far from comprehensively known, which should receive more attention. This review summarized the current studies of microplastic source, aggregation, and transport in soil and groundwater environment. Important chemical, physical and biotic factors affecting microplastic transport in soil and groundwater environment were detailly identified and analyzed. The main chemical factors were the hydrochemical condition (e.g. ionic strength, electrolyte type, pH and dissolved organic matter) and media chemical composition (e.g. Fe/Al oxide ratio and cation release). The main physical factors were the solution flow condition (e.g. flow rate) and media physical composition (e.g. grain size, surface roughness, saturation and heterogeneity). The main biotic factors were the plants (e.g. root development), small soil animals (e.g. movement and ingestion), and microorganisms (e.g. vital activity and basic property). The aggregation of microplastic in water environment was improved with increasing ionic strength, decreasing pH value and dissolved organic matter concentration, and the presence of high valent cations. The transport of microplastic in soil and groundwater environment was enhanced with increasing pH value, media grain size, flow rate and moisture content, however, it was inhibited with increasing ionic strength, surface roughness and electrolyte valence. With the presence of dissolved organic matter, the mobility of microplastic was significantly increased. Microplastic showed a low mobility in porous media containing the high amount of Fe/Al oxide. The transport of microplastic was also decreased if the porous media released cations into aqueous phase. In addition, preferential flow generated in structured heterogeneous media may dominant the transport of microplastic. Root growth and development produced crack and hole in soils, providing preferential channels for microplastic to vertically migrate. Small soil animals (e.g. earthworm and collembolans) may capture, ingest or carry microplastic and influence its transport behavior subsequently. Microorganisms (e.g. bacteria and algae) living on the surface of porous media may capture or adsorb moving microplastic, which then may increase the retention and decrease the transport of microplastic in soil and groundwater environment. The related impact mechanism of the factors mentioned above were systematically discussed and visually illustrated by schematic diagrams. At the end of this review, current knowledge gaps and prospective topics needed to be promoted were highlighted for further developing and investigating the fate and transport of microplastic in soil and groundwater environment. Given current researches were almost limited to laboratory scale and polystyrene microsphere, field researches (e.g. lysimeter experiment), diversified type, the basic property and transform researches should be emphasized in future microplastic transport studies. This review contributes to expanding our knowledge of fate and transport of microplastics in soil and groundwater environment.