电化学调控生物炭反应墙修复土壤重金属的机制研究

Soil heavy metal pollution is one of the major environmental concern in China. Our previous results revealed that the oxygen contained functional groups (OCFGs) on biochar can be mediated by external electric field, and the types and quantities of the obtained OCFGs depended highly on the conditions of electric field, such as voltage, duration time, which provides new insight for the controllable tuning of these OCFGs. In this proposal, we want to establish an effective reactor to immobilize heavy metal ions in soil by using biochar as the materials of permeable reactive barrier. In this reactor, the electric field is supposed to fulfill multiple functions: mediating the OCFGs in biochar, promoting the release of dissolved organic matters, and inducing the directional migration of heavy metal ions. Biochar samples prepared from woody and herbaceous plants will be subjected to electro-chemical treatment at first, and the structure, surface composition, the types and quantities of OCFGs, electron accepting/donating capacities and the release of DOM will be characterized to identify the laws of electric conditions (such as voltage, time and the type of anion in electrolyte) to exert above effects. By employed Pb(II) and Cr(VI) as representatives, the removal behavior of these heavy metals by biochar at simulated (kaolin earth) and real soil under external electric field will be explored, with particular emphasis on the identification of the mechanism underlying heavy metals removal by biochar promoted by the mediation of OCFGs as well as the discharge of DOM under external electric field. The results of this study will provide fundamental knowledge in the high-effective and in-situ remediation of soil heavy metals by biochar mediated by external electric field.

土壤重金属污染是我国面临的主要环境问题之一。课题组前期探索发现电场对生物炭表面有改性作用，改性所得表面官能团的种类/数量与电场条件密切相关。本申请提出将生物炭作为可渗透反应墙的材料，利用外电场调控改性生物炭表面官能团，并促进生物炭可溶性有机物(DOM)的释放，从而强化重金属离子在电场下的定向迁移作用及在生物炭表面的高效固定，最终实现土壤重金属的绿色修复；本申请拟以不同类型生物质制备生物炭，研究电场下生物炭表面结构、元素组成、表面官能团种类/数量、氧化还原性及DOM释放的规律，揭示电场条件(电压、时间，极性交换、土壤阴离子等)对生物炭表面性质的影响和控制机制；以Pb(II)和Cr(VI)为代表，研究电场作用下生物炭对高岭土和实际土壤中重金属离子的固定行为，解析表面官能团改性及可溶性有机物释放对生物炭去除土壤中重金属的促进效应及作用机制，为电场调控生物炭高效固定土壤重金属提供基础。

针对电场改性生物炭可渗透反应墙强化重金属离子的定向迁移问题，本项目重点研究了生物炭在电场下结构、性质的变化对重金属铅（Pb）和铬（Cr）在定向迁移能力的影响，探索了原位调控下生物炭释放DOM对重金属离子的固定能力，解析了DOM在EK-PRB修复过程中对变价重金属迁移转化的作用机制，为电场调控生物炭高效固定土壤重金属提供新思路。.研究结果表明DOM与土壤重金属具有较好的络合能力，可以分别将实际土壤中93%的铅、93.56%的铬溶解至液相，使其在电场中迁移去除。热解温度和电场电压对生物炭在电场作用下释放DOM影响较大。不同制备温度的生物炭所需最优电压条件不同，而热解温度越高生物炭释放DOM的能力越弱，这与生物炭本身制备温度存在较大关系，随着制备温度的上升，生物质中的有机物质发生脱水、断裂，生物炭炭化程度增加，DOM释放能力会随之降低。与此同时，DOM随着生物炭热解温度的提高，TOC值快速下降，表面结构复杂多样且变得光滑，主要矿物组成以钾盐为主，官能团种类相似，荧光强度逐渐降低。项目通过三维荧光光谱分析、平行因子分析和2D-COS结合同步荧光光谱分析，探究了生物炭释放DOM的组成成分，各成分与重金属离子的结合能力以及结合顺序。生物炭DOM主要包含3种类腐殖质荧光组分，分别为C1（Ex: 320-420 nm，Em: 380-525 nm）、C2（Ex: 280-370 nm，Em: 340-530 nm）、C3（Ex: 310-440 nm，Em: 425-550 nm）和1种类富里酸荧光组分C4（Ex: 250-330 nm，Em: 280-550 nm）。随着热解温度上升，组分C1和C4占比逐渐减少，组分C2和组分C3占比逐渐增加。项目研究发现生物炭DOM对于变价金属铬的结合顺序不同，与Cr(VI)的结合顺序为C4→C3→C2→C1；与Cr(III)的结合顺序为C3→C1→C4→C2。此外，项目研究发现相较于原位释放DOM而言，将生物炭PRB衍生的DOM提取后作为土壤添加剂异位添加可以对电动修复引起的土壤酸碱化起到缓冲效果，进一步提高PRB中生物炭对土壤重金属的吸附量。这些研究为电场调控下生物炭释放DOM修复固定土壤重金属的调控和环境应用提供了理论基础。