重金属阴离子在水-秸秆吸附剂微界面的形态转化机制研究

Crop straw based materials is becoming attractive for the adsorption of heavy metal pollutants from water and wastewater. During the removal process of Cr(VI), a typical heavy metal anion, not only adsorption but also speciation transformation occurs in the water-straw adsorbent micro-interface. However, in contrast to the adsorption process, it is not clear enough about the transformation process of Cr(VI) anions by now. Therefore, it deserves more research to understand the mechanism of Cr(VI) transformation in the water-straw adsorbent micro-interface. In this project, rice straw will be modified with quaternary ammonium to prepare straw adsorbent for Cr(VI) removal. The change of the surface micro-environment of straw adsorbent before and after adsorption of Cr(VI) will be characterized by FTIR, SEM-EDS, and element analysis. The Solide Surface ZETA Potential Analyser and ATR-FTIR will be employed to probe the water-straw adsorbent micro-interface under aqueous solution system, and the in situ characterization of the speciation transformation process of Cr(VI) will be carried out. The kinetics curves of reduction reaction of Cr(VI) to Cr(III) in the micro-interface will be measured. The impact of temperature, pH, functional groups of adsorbent, and co-existing matters such as copper ions , sulphate ions, and organic pollutants on the transformation of Cr(VI) will be discussed. The change of the composition of adsorption solution will be determined by means of IC, LC-MS, and UV-Vis spectrophotometer. The XPS, ICP will be used to detect the existence of Cr with different valence and their distribution between straw adsorbent and solution. According to the chemical analysis results, the main reaction products and electron donors need to be confirmed. The final aim of this research work is to deeply elucidate the speciation transformation mechanism of Cr(VI) in the water-straw adsorbent micro-interface, and to enhance the comprehension of removal principles of heavy metals by straw based water purification materials. The implementation of this project will theoretically support the development of new micro-interface process that is more effective for transformation and removal of Cr(VI).

农业秸秆去除废水中Cr(VI)重金属阴离子过程中不仅有吸附作用，还存在形态转化问题。本项目以季铵化改性稻草为对象，采用FTIR、SEM-EDS等考察秸秆吸附-还原Cr(VI)前后表面微环境的变化。利用固体表面ZETA电位仪和ATR-FTIR原位定性表征Cr(VI)转化过程中水-秸秆吸附剂微界面电荷性质与基团组成的变化。考察温度、pH、共存离子等因素对Cr(VI)形态转化的影响，探讨转化过程的关键驱动因素和动力学特性。借助IC、LC-MS等监测Cr(VI)转化过程中溶液有机和无机组成的变化；利用ICP、XPS等探明不同价态铬在液/固相的分配和赋存状态；明确 Cr(VI)转化过程主要反应产物和电子供体，揭示相关反应机理。从而较深入地阐明Cr(VI)在水-秸秆吸附剂微界面的形态转化机制，深化人们对秸秆材料去除水中重金属污染物原理的认识，并为构建更高效的Cr(VI)转化去除微界面过程提供理论支撑。

重金属阴离子六价铬（Cr(VI)）的生物吸附过程中不仅有吸附作用，还有形态转化问题。为了准确理解Cr(VI)去除机理，项目研究以季铵化改性秸秆吸附剂为对象，系统探讨了Cr(VI)吸附与还原两种机理，特别是还原转化的驱动因素和反应机制。采用固态核磁共振碳谱（13C-NMR）、X-射线光电子能谱（XPS）等表征发现改性后秸秆材料表面形貌粗糙，有断裂纤维束，N和Cl含量显著增加，有较多季铵基和羟基。吸附等温线测试表明改性秸秆对Cr(VI)吸附容量约37mg/g，去除性能良好。研究建立了一种Cr(VI)和三价铬（Cr(III)）同步快速解吸方法，即20mL 2 mol/L H3PO4 解吸剂，常温振荡约10min，总铬回收率大于95%，实现了对液、固相还原产物Cr(III)的定量分析。在此基础上，定量研究了Cr(VI)离子交换和还原转化两种去除机理，结果表明Cr(VI)去除方式主要是与材料表面Cl-进行离子交换，实验交换系数接近1，且参与交换的Cr(VI)主要是HCrO4-及Cr2O72-；在Cr(VI)吸附去除过程中明确证实了还原现象的存在，且发现只有少部分（<30%）被吸附Cr(VI)与材料表面电子供体基团反应转化为Cr(III)；而大部分（>65%）产物Cr(III)被固定在吸附材料表面，少量重新释放到吸附溶液中。研究考察了pH、初始浓度、吸附剂量和温度对Cr(VI)还原反应的影响，结果表明pH和初始浓度对Cr(VI)影响显著，低pH和高浓度可大幅提升Cr(VI)还原程度，而吸附剂量和温度对Cr(VI)还原也有一定促进作用。Cr(VI)还原动力学研究发现该反应初始反应速率较快，约30min可达到近平衡态；提出了一个新的速率方程来描述Cr(VI)还原动力学过程，拟合结果表明反应初始阶段符合零级反应，并估算了Cr(VI)还原反应的活化能为36.8kJ/mol。结合固相表征分析和液相还原产物定性，研究剖析了Cr(VI)还原转化的电子转移反应路径，认为Cr(VI)还原转化的电子供体主要来自N和C的氧化过程。本研究还利用改性秸秆填充柱和实际电镀废水，测试了动态吸附条件下Cr(VI)的去除效果与还原转化情况，并与树脂、活性炭等商业吸附剂进行了对比。通过项目研究，揭示了Cr(VI)在生物吸附过程中的还原转化机制，加深了对Cr在固-液界面行为的理解，为治理Cr重金属污染提供理论支持。