表面修饰生物炭对土壤中Cr(VI)的还原-固定作用与机理

The contamination of heavy metals in soil is now getting an increasing attention. Biochar remediation has the advantages of in situ operation, easy handling and low cost, which has a broad prospect for practical application. The surface functional groups play an important role in the remediation of chromate contamination in soil. Hence, the modification of biochar surface, the solution of scientific questions related to chromate remediation, will support the optimization of process conditions and the improvement of remediation effects. In this subject, a phosphoric method is employed for the preparation and modification of biochar. The influences of various factors, including the biochar surface characteristics, the soil physicochemical properties and the operating conditions, on the reduction and stabilization of Cr(Ⅵ) will be investigated. The thermodynamics and kinetics of this process, and the migration and transformation of Cr(Ⅵ) will also be studied. On the basis of conditional tests, methods including XPS, ATR-FTIR and EXAFS will be applied for the illustration of complexation mechanism between surface functional groups and chromium. For this target, the microscopic structure parameters and the macroscopic test performance will be considered in combination. The implementation of this subject will solve an important theoretical problem in the field, and direct the preparation and application of high-efficient biochar. Consequently, this subject has notable theoretical values and practical meaning.

土壤重金属污染问题已经引起人们的日益重视。生物炭修复具备可实现原位修复、易于操作、成本低廉等优势，具有广阔的应用前景。表面官能团对土壤铬污染修复具有重要作用。对生物炭表面进行修饰，解决铬污染修复的科学问题，将为优化工艺条件、提高修复效果提供支持。本研究采用磷酸法制备与修饰生物炭，考察生物炭表面特性、土壤理化性质、操作条件等对Cr(Ⅵ)还原与固定的影响，研究过程的热力学、动力学及铬的迁移转化。在条件试验基础上，应用XPS、ATR-FTIR、EXAFS等分析手段，综合微观结构参数与宏观试验现象，揭示表面官能团与铬的络合机理。本课题的实施将解决该领域的关键科学问题，指导高效生物炭的制备与应用，具有显著的理论价值与实用意义。

铬是突出的土壤重金属污染物之一，通过食物链的传递与富集，对人类造成致癌、致畸形、致基因突变等危害，对生态系统也存在极大风险，其修复已成为一项紧迫任务。生物炭修复具备可实现原位修复、易于操作、成本低廉等优势。通过表面修饰手段，提高生物炭固定土壤中铬的效能，有助于推进该技术的实际应用。本研究首先进行生物炭制备原料的筛选，确定小麦秸是制备生物炭的合适原料。接下来通过无机酸预处理对生物炭进行表面修饰。所制备的表面修饰生物炭对Cr(VI)的还原-吸附容量提高2倍以上。生物炭对Cr(VI)的还原与吸附能力密切相关。Cr(VI)在生物炭表面的还原有利于转化后Cr(III)的吸附。序批实验表明，随着生物炭表面官能团的增加，土壤溶液中残留的总Cr量明显降低（<2 mg/L），且Cr基本以Cr(VI)的形态存在。优化了表面修饰生物炭的投加比例（4 wt%），确定了该投加量下适应的污染强度范围。生物炭对Cr(VI)的固定效果受土壤pH的影响不大。动力学的研究表明生物炭的投加降低了土壤Cr的初始释放量，且加速了土壤溶液中Cr(VI)的还原与固定速度。土柱淋洗实验表明投加生物炭提高了土壤对Cr(VI)的固定作用，减弱了其随溶液向下运移进而污染地下水的能力。已固定Cr难以重新释放，降低了其迁移转化风险及生物有效性。土壤中Cr形态分析表明，模拟污染土壤中的Cr(VI)被还原并原位吸附于生物炭表面。机理分析表明，表面修饰生物炭吸附Cr(VI)后O含量上升，含氧官能团参与了反应，在O含量与吸附与还原能力之间存在密切关联。对Cr的吸附存在表面络合作用，很可能在Cr与含C=O基团之间发生了络合反应。羧基参与了Cr(III)的络合反应，而并未扮演Cr(VI)还原的电子供体角色。此外，酚基的-OH也可能参与了Cr(III)的络合反应。本课题的实施建立了提高生物炭固定铬性能的技术途径，明确了工艺的主要影响因素，阐明了表面官能团与Cr相互作用的机理，可指导高效生物炭的制备与应用。