生物炭修复重金属污染农田土壤的机制及应用研究进展

将生物质转化为生物炭并用于重金属污染农田土壤修复中,是有效利用生物质资源、保障粮食安全的有效途径之一。然而,生物炭的应用效率受其特性和土壤环境影响极大。该研究综述了生物炭特性,并探讨了生物质和热解温度对其影响规律,阐明了生物炭对重金属的直接固定作用,以及通过影响土壤p H值、阳离子交换量(Cation Exchange Capacity,CEC)、矿物组分和有机质等,进而间接固定重金属的作用机制。同时,该文系统总结了国内外生物炭在田间试验中的应用,从土壤重金属迁移性和生物有效性、作物累积重金属和作物产量等3个方面阐明了生物炭的应用效果和作用规律。针对田间试验条件区别于室内试验的特殊性,探讨了生物炭施撒方式及用量、施肥等田间管理和气候环境等现场条件对生物炭应用的影响,并对今后完善生物炭在土壤修复中作用机制、扩大研究尺度和长期土壤监测等方面研究进行了展望。

Mechanism for the application of biochar in remediation of heavy metal contaminated farmland and its research advances

Heavy metals contaminated soils pose a serious risk to human beings and animals via direct exposure and food chain. Biochar, a carbon-rich material, is used to remediate heavy metals contaminated farmland. This strategy provides an effective method for utilizing biomass resources and ensuring food safety. With increasing attention, the number of published articles concerning biochar has been increasing in the recent ten years, therefore providing researchers with a large amount of evidence and insights. In this study, the latest studies of biochar in the remediation of heavy metals contaminated farmland were reviewed, with the focus on possible mechanisms of biochar-heavy-metal interactions, related impact factors, and in-situ application of biochar at the field scale. Biochar showed a strong sorption ability, attributed to its physiochemical properties such as large specific surface area, abundant functional groups and high cation exchange capacity. The application effect of biochar was greatly influenced by its characteristics. After summarizing biochar''s physiochemical property data in recent years, the study discussed the changing law of biochar''s properties with the alteration of feedstocks and pyrolysis temperature, respectively. To modulate the properties of biochar for soil remediation, various modifiers with different concentrations were adopted, including acids, bases, oxidizing agents, organic solvents and metal salts or oxidizing agents. In general, the purposes of modification were to enlarge the surface area, to change the functional groups, and to increase the adsorption performance and catalytic capacity. Furthermore, the immobilization mechanisms of heavy metals by biochar were illustrated. The direct immobilization could be achieved through physical absorption, electrostatic attraction, ion exchange, complexation, precipitation, and redox reaction. Besides, the indirect effects of biochar on heavy-metal mobility and bioavailability, which could be achieved via impacting soil characteristics and thus heavy-metal-soil complexation, were less understood and could be largely underestimated. Biochar addition could alter many soil properties including pH value, dissolved organic carbon, mineral composition, and cation exchange capacity. These changes would affect heavy-metal-soil interactions and thus heavy-metal mobility and bioavailability. Many laboratory studies had demonstrated biochar''s effectiveness in decreasing the bioavailability of heavy metals as well as improving soil quality. However, the value of biochar in the remediation of contaminated land had not been well tested in the field. In different field trials, distinct results (beneficial, neutral or adverse effects) had been reported due to wide variations in field conditions and biochar characteristics. To better understand whether biochar application could provide a promising direction for soil remediation, this review was undertaken to assess the published field trial. The results of most previous field trials indicated that biochar could potentially reduce heavy-metal bioavailability in the field. Meanwhile, a significant decrease in the heavy-metal enrichment of the crops was observed. It was found that the use of biochar may help increase crop yields on polluted farmland and reduce the amount of mineral fertilizer used in the field. The application of biochar could inactivate heavy metals through improving soil physicochemical properties (pH, cation exchange capacity, water retention capacity etc.). In addition, it also could be used to enhance the uptake of soil nutrients for plant growth. However, according to a majority of studies, biochar''s effectiveness in reducing the impacts of heavy metals depended on a myriad of factors in the field, including biochar applying process (variety and dosage rate of the biochar, mixing depth), agronomic measure (nitrogen-phosphorus-potassium fertilizer application) and climatic conditions (air temperature and precipitation). In the last part, future research on the perfection of the mechanisms of soil remediation using biochar, the expansion of the scale, and the long-term monitoring on soil was prospected.