高稳定性多孔秸秆炭表面氮基团活性点的定向调控及其对土壤铅镉汞的选择固定机制

The low selectivity and uncertain stability nature of biochar remediation is a key risk to widen its application in practice, even though it can remove heavy metal pollution from contaminated soil effectively. In order to modify the shortcomings of current techniques, this project aims to investigate the immobilization of lead, cadmium and mercury ions by using nitrogen-modified hierarchical porous straw biochars. The key issues in this study are: (1) the preparation of novel nitrogen modified biochars based on the design of constructive mode of nitrogen functional groups; (2) the application of such nitrogen-modified biochars in the immobilization of heavy metal ions; (3) improvement of biochar stability by controlling mode of nitrogen-carbon binding; (4) enhancement of synergy between pore and nitrogen groups in remediation of heavy metal-contaminated soil by controlling the pore structure of biochars. The interactions between the nitrogen functional groups and the soil organic matter and minerals will be investigated to analyze the difference of the stability of various nitrogen functional groups. The mathematical model will be formulated based on the relationship of physicochemical properties (nitrogen functional groups and pore characteristics) and immobilization efficiency (the selectivity, immobilization capacity and biochemical stability). In the meantime, the desired nitrogen modified biochar will be obtained through the optimization of preparation process. The accomplishment of this task will provide a theoretical reference for the synthesis of novel biochar with high selectivity and stability and mechanism for the targeting remediation of heavy metals contaminated farmland soil.

生物质炭修复是治理土壤重金属污染的有效方法，但存在选择性不强与稳定性不确定等缺陷，成为该技术推广的主要风险。增强生物炭基团活性位，提高其对目标重金属的选择固定与长效性是克服该风险的关键。本项目深入研究秸秆炭氮官能化活性点定向调控及其对铅镉汞的选择作用机制及稳定性问题，主要包括：从秸秆炭氮基团构建模式设计入手，增加氮官能化活性点位，提升其对铅镉汞的固定效率；通过氮基团结构调控，赋予其对铅镉汞的选择性；通过控制氮碳键合方式，增强秸秆炭氮基团活性点稳定性；控制孔结构，发挥其与氮活性点对目标重金属固定的协同作用；研究秸秆炭氮基团同土壤有机质、矿质元素的作用，确定影响氮基团固定重金属的稳定性因素与机制；建立模型，分析氮活性基团与炭孔结构协同固定重金属的内在联系，建立构效关系，进而构建出对目标重金属强选择、高固定、稳定强的氮改性多孔秸秆炭，为新型秸秆炭的设计与定向高效修复土壤重金属污染提供依据。

实现生物炭对重金属的高选择稳定钝化修复，使生物炭农用潜在风险降到最低，是关系生物炭土壤修复安全利用的核心问题。以三聚氰胺等常见含氮物为改性剂，通过低温液化、原位聚合、控温热解等技术，制备了富含吡咯、吡啶、氨基和石墨氮秸秆炭，通过调控预聚合条件、碳化温度、氮改性剂种类与剂量等关键参数，对秸秆炭氮形态、结构与含量实现了定量控制，合成了高固定铅镉汞的氮改性秸秆炭。在此基础上，探晰了钾、钠、钙、镁等矿质元素和有机物TC秸秆炭活性基团的相互作用，揭示了不同形态氮结构掺杂秸秆炭对铅汞镉吸附-固定机制，明晰了秸秆炭氮官能团构建对土壤铅汞镉的固定性能和修复效果的影响。主要结果如下:1）通过秸秆液化、三聚氰胺聚合和控温热解合成了氮含量与结构可调的高比表面氮掺杂生物炭，并通过调节三嵌段共聚物模板种类和三聚氰胺用量，实现了氮掺杂生物炭的孔结构调控；2）在秸秆生物质与磷酸低温聚合过程中加入不同氮前驱体组装可控合成了不同结构氮活性位，明确了磷酸及三聚氰胺控制在原位聚合热解掺氮的作用机制，获得了富含吡咯或氨基氮活性位的氮掺杂多级孔秸秆炭一步合成方法，为高固定重金属的氮掺杂生物炭制备提供一条新策略；3）掺杂氮的形态结构与比例含量直接影响生物炭对重金属的选择钝化容量及效率，石墨氮、吡啶氮和氨基氮易于结合铅汞，吡咯氮更易于亲和镉，而修饰在球形短距离微介孔上的氮结构更有利于秸秆炭吸附性能提升。4）制备炭亲和重金属的氮形态以络合配位为主导，被固定的重金属以残渣态为主，受环境因素影响较小，可稳定存在环境中；5）施用研究制备的氮掺杂秸秆炭明显降低土壤中重金属可交换态量和小白菜地上部铅汞镉富集量，提升了小白菜地上部的产量和品质。研究成果可为高稳定性多孔秸秆炭表面氮基团活性点的定向调控及铅汞镉污染农田钝化修复提供理论和实践依据。