老化过程对生物炭钝化复合污染土壤中镉铅稳定性的影响机制研究

Biochar is an important immobilization material for soil heavy metal contamination remediation. After added to the soil, biochar undergoes ageing process due to the long-term environmental changes. The investigation on the effect of ageing process on heavy metal re-mobilization in the biochar-amended soil is of great practical significance to the assessment of long-term effect of the remediation. An accelerated ageing experiment is conducted in the present project to investigate the effect of ageing process on the physicochemical properties and microstructure of different biochars. The relationship between the changes in biochar’s features and their raw materials and pyrolysis temperatures is explored. A batch sorption experiment is carried out to study the effect of ageing process on the single and competitive adsorption characteristics of Cd2+ and Pb2+ ions on biochars. The biochars which have adsorbed Cd2+ or Pb2+ ions are subjected to accelerated ageing, followed by the exploration of the effect of ageing process on the stability of Cd and Pb adsorbed onto the biochars. The difference between the results of short-term natural ageing and accelerated ageing of contrasting types of Cd- and Pb-contaminated soils amended by biochars is compared. The chemical fractions and migration and transformation rules of Cd and Pb in the soils under the condition of accelerated ageing are then studied. The effect of soil environmental factors on the aged biochar’s functionality is examined. The effect of ageing process on the stability of Cd and Pb immobilized by biochars in the soils is elucidated. The research outcome can provide a scientific basis for the prediction of long-term effect of in situ immobilization of heavy metals by biochar in contaminated soils.

生物炭是土壤重金属污染修复的一类重要钝化材料。生物炭施入土壤后受到外界长期环境变化的影响而发生老化现象。研究老化过程对生物炭修复后土壤重金属再迁移性的影响对评估修复的长期效果有着重要的实际意义。本项目通过人工加速模拟老化实验研究老化过程对不同生物炭理化性质和微观结构的影响机制，探寻老化生物炭表观特性变化与其母源物质及热解温度之间的相互关系。通过静态吸附实验研究老化过程对镉和铅在生物炭上单一及竞争吸附特性的影响机制。对已吸附镉或铅的生物炭进行人工加速老化，探究老化过程对生物炭已吸附镉和铅稳定性的影响机制。通过对比施加生物炭的不同类型镉铅复合污染土壤短期自然老化和人工加速老化结果的差异性，探讨加速老化条件下土壤中镉和铅的形态特征及迁移转化规律，考察土壤环境因子对老化生物炭功效的影响，阐释老化过程对生物炭钝化土壤中镉和铅稳定性的影响机制，为预测生物炭原位钝化污染土壤重金属的长期效果提供科学依据。

生物炭在土壤中的老化会导致其理化性质和结构特征改变，从而影响其对重金属的固持能力。本项目选用不同生物质原料制备的生物炭，通过干湿交替和冻融循环2种加速老化方法模拟生物炭的自然老化过程，研究老化过程对生物炭自身特性及其固定土壤重金属稳定性的影响。结果表明：. 干湿交替和冻融循环老化过程导致300～500℃下制备的小麦秸秆和玉米秸秆生物炭pH值下降，生物炭老化后pH值降幅随热解温度升高而降低（除了碱性最弱的300℃玉米秸秆生物炭以外）。老化生物炭中氧元素含量增加（除了氧元素含量最高的300℃小麦秸秆生物炭以外），芳香性减弱，极性和亲水性增强，并有新的碳酸盐矿物形成。2种老化过程导致500℃小麦秸秆和玉米秸秆生物炭比表面积显著增大，但对300℃和400℃生物炭比表面积的影响无明显规律。. 小麦秸秆生物炭较玉米秸秆生物炭对镉和铅显示出更强的吸附能力，这种吸附能力随热解温度升高而增强。干湿交替和冻融循环老化过程将300℃小麦秸秆和玉米秸秆生物炭上已吸附的镉和铅的有效态和潜在有效态转化为非有效态，而对500℃小麦秸秆和玉米秸秆生物炭上已吸附的镉和铅的形态分布影响不大。2种老化过程主要促进了上述生物炭上已吸附的镉与其表面含氧官能团的络合作用和已吸附的铅在其表面的沉淀作用，形成碳酸盐和磷酸盐等矿物沉淀。. 干湿交替和冻融循环老化条件下，添加5%（w/w）500℃小麦秸秆生物炭较添加等量500℃玉米秸秆生物炭更显著地降低了镉镍复合污染碱性土中镉的植物有效性，添加上述2种生物炭使不稳定态镉向残渣态转化；添加2.5%（w/w）500℃鸡粪生物炭通过沉淀作用和络合作用实现了对镉铅复合污染中性土中镉和铅的联合钝化，添加等量500℃小麦秸秆生物炭主要通过络合作用钝化该土壤中铅，但可能是由于铅能与镉竞争生物炭表面上有限的吸附点位，所以小麦秸秆生物炭未能降低该土壤中镉的植物有效性；2种老化过程中生物炭和土壤中溶解性有机碳的释放对生物炭固定土壤重金属具有抑制作用。. 本项目有助于评估生物炭在土壤修复中长期应用效果。