生物炭孔径与负载纳米零价铁粒径间的耦合影响其对土壤砷/铬修复效率的机理

Although simultaneous adsorptive and reductive removal of arsenic (As) and chromium (Cr) from soil has been achieved by using nanosized zero valent iron (nZVI), the heavy metal removal capacity has often been compromised due to high tendency of aggregation of nZVI. Immobilization of nZVI on support materials can stabilize nZVI and supported nanoparticles exhibit enhanced removal capacity for As and Cr. However, coupling effects between pore size of support materials and particle size of nZVI on heavy metal removal by nZVI are still not well understood. To achieve this, first, the particle size threshold of nZVI (Do) at which nanoscale effects occur will be determined by using pentavalaent As(AsV) and hexavalent Cr(CrⅥ) as probes. The AsV and CrⅥ are used to indicate the sorptive and reductive capacity of nZVI, respectively. Next, the correlation will be established between particle size of nZVI and pore size of ordered porous carbonaceous materials. Then, nZVI of different particle size will be impregnated in different sized pores of supported carbonaceous materials. The effects of this coupling on oxidation of nZVI as well as AsV and CrⅥ removal mechanisms by nZVI will be evaluated with quartz crystal microbalance and batch sorption experiments, respectively. Last, Do-sized nZVI will be synthesized on disordered porous biochars, and tested for removal of AsV and CrⅥ from both aqueous solution and a sandy soil. To understand heavy metal removal mechanisms by nZVI, both supported and unsupported nZVI will be characterized mainly with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDS), transmission electron microscope (TEM). The heavy metal speciation can be analyzed with inductively coupled plasma-atomic emission spectrometry-mass spectrometry (ICP-MS) and high-pressure liquid chromatography (HPLC). This work can provide theoretical basis for preparation of supported nZVI for remediation of AsV and CrⅥ contaminated soils.

利用纳米零价铁（nZVI）吸附还原重金属是当前污染土壤修复的热点，但nZVI易凝聚成大颗粒使效率降低。负载到多孔材料后可有效降低其粒径和明显提高对重金属的吸附和还原能力，但目前缺少对材料孔性与nZVI粒径耦合会影响nZVI效率的研究，本项目将主要围绕该问题进行。首先确定nZVI吸附与还原砷/铬的临界粒径，并采用具有规则孔隙的碳材料获得材料孔径与nZVI粒径的对应关系；然后研究铁离子投入量与不同孔隙中临界粒径的nZVI形成间的联系，并对nZVI被负载到不同孔隙碳材料后的氧化动力学和nZVI吸附与还原重金属的机制和能力进行探讨；最后借助于以上nZVI临界颗粒形成的规律与调控方法，采用不同孔隙分布的生物炭材料负载nZVI，对理论指导下所制备材料中的nZVI临界颗粒形成进行验证和参数调整，并检验该材料在土壤中实际吸附和还原砷/铬的能力，为基于土壤重金属污染治理的nZVI材料制备提供理论依据。

生物炭（BC）可有效降低nZVI团聚并提高其对重金属的去除，但目前缺少对材料孔性与nZVI粒径耦合影响nZVI去除土壤和水体重金属机理的研究。本项目利用湿化学法研究了nZVI颗粒在碳材料孔隙中的形成机制和所表现出的化学特性，通过土壤培养实验研究了BC/nZVI修复土壤中砷铬的效果和分子机制。主要结果如下：（1）掌握了制备粒径8-50nm的nZVI的技术参数，发现降低还原速率和Fe2+浓度以及提高表面活性剂浓度可降低nZVI粒径：当羧甲基纤维素浓度为4g/L，Fe2+浓度为0.05g/L时，nZVI粒径为约8.6nm；（2）nZVI颗粒优先分配到碳材料较大孔径孔隙中，且孔隙内nZVI粒径受Fe2+浓度影响：在主要孔径为2.5和17.5nm的介孔碳上负载nZVI后，17.5nm孔道所占比例大幅降低；Fe2+浓度增加10倍可使nZVI平均粒径从13.3nm增至75.4nm；（3）掌握了生物炭孔径的化学调控技术：发现双氧水和硝酸可分别增加松木BC的微孔和介孔比例，KOH、KOH耦合氮掺杂可使BC的比表面积增大约2.6和3.6倍；（4）碳负载nZVI粒径与砷铬去除效率间呈负相关关系：介孔碳负载nZVI粒径为75.42nm—11.42nm时所对应的单位质量铁去除Cr(VI)量为90.4—1946mg/g；BC负载粒径为41.5—6.1nm nZVI对As(V)的去除量为58.86—109.11g/kg；（5）生物炭性质和合成条件的优化可提高nZVI对砷铬的去除：较其他生物炭，松木BC/nZVI具有更佳的失电子能力和Cr(VI)还原能力；热解温度升高（300-700oC）提高了BC的电子传递速率和复合材料去除Cr(VI)能力（2.5倍）；硫掺杂可通过改善nZVI电子利用效率提高Cr(VI)去除量3.5倍；（6）BC/nZVI促进土壤中As(V)和Cr(VI)向稳定态转化，提高了土壤游离铁和可溶性有机质含量；nZVI降低了土壤细菌的多样性，而BC/nZVI刺激了土壤铁还原菌所属、使得厚壁菌门和变形菌门成为优势群落。研究结果阐明了nZVI在多孔碳材料中的分配规律和化学特性，明确了复合材料修复土壤中砷铬的化学和微生物学机制，为基于土壤重金属污染治理的nZVI材料制备提供参考。已发表基金标注论文19篇。