土壤孔隙结构中铅被磷酸盐稳定化的过程及再释放特征

Phosphates were the most common targeted materials for the stabilization of lead contaminated soil with high concentrations. The remediation efficiency was often influenced by the mass transfer of the reaction reagents in the soil pore structure and could result in general questions of cost, dilatation and long term stability. Few studies were conducted in understanding factors of solute migration and mixing limitation influencing the remediation efficiency related with in soil multi-pore structure. This research tries to construct the emulated soil multi-pore structure fabricated based on standard photolithograph techniques, and simulate the inside precipitation reaction processes of phosphate and lead. Mass transformation and porosity occlusion with different impacted factors were verified by using microscopy imaging for real-time visualization, SEM to characterize the crystal polymorphs. Sediment stability and potential release risk were also assessed based on the strong acid-etch test, correlated the dissolution with the pore damage, and predicted by pore-scale numerical modelling. This research was proposed to reveal the interaction between phosphate and lead in soil-liquid interface, and hopefully provided the evidence for improving stabilization remediation efficiency.

磷酸盐作为专一性强的稳定化材料常用于处理铅污染土壤，但其稳定化修复效率受到反应物质在土壤孔隙中传质效果的影响，容易导致处理成本增高、增容及长期释放风险等常见问题。目前以土壤孔隙结构为基础，在土壤空间异质性导致的溶质迁移和混合受限影响修复效率方面鲜有研究。本研究试图通过构建土壤孔隙结构的模拟装置，探索存在可控孔隙水、污染物、修复药剂和矿质条件下复杂土壤孔隙环境中磷酸盐稳定化修复铅的过程；辅助光学成像和微观表征手段（明视场、荧光显微镜成像、扫描电镜等）观测，探明土壤孔隙结构中磷酸盐与铅的传质过程及对反应效率的影响；结合极端溶出条件设计，通过数值模拟手段预测稳定化产物的长期释放风险。最终为提高铅污染土壤稳定化修复效率提供理论依据。

固化稳定化是一种广泛使用的重金属污染土壤修复技术，但其微观作用机制一直是研究的难点。本课题以专一性强的磷酸盐为稳定材料，研究在土壤孔隙结构中的铅被磷酸盐稳定化过程的影响因素。研究表明，Re=0.1时，模拟结果与试验结果的平均均方根误差不超过0.0103m/s，模拟获得的流体速度分布结果与试验所获得的结果吻合度高，表明土壤孔隙网络模型能够很好地模拟土壤孔隙，可以描述其微观尺度上的流体运动特征；在一维凝胶逆扩散体系中，铅和磷酸盐两种反应物在更靠近阴离子一侧产生反应，稳定化反应速率受溶质扩散系数影响，随着稳定化产物的增多，扩散受限，反应速率逐渐降低。在二维多孔微流控体系中，随着稳定化反应的启动，呈现出显著的空间异质性格局。影响因子分析表明，多孔体系中铅稳定化反应受环境条件影响明显。pH值越高，产物结晶度越差、结晶颗粒越小，沉淀迁移性和分布范围也越大；偏酸性条件下产物结晶度高，堵塞程度也高，边界区闭合面积大。氯离子的添加在较宽pH值范围内形成磷氯铅矿，随着氯离子浓度升高产物稳定性也有较好的提升。腐殖酸的添加，造成沉淀产物破碎化，孔隙堵塞的情况变得不明显，整体结晶生长过程不显著。各因子对多孔介质中铅稳定化反应的影响机制有助于理解土壤等实际多孔介质中固化稳定化过程的发生、发展和修复效率。与K2HPO4和K3PO4相比，KH2PO4形成的具有较好的结晶特性和最大尺寸的Pb -磷酸盐矿物，且析出物比较稳定在pH 2.88的溶液中溶出率很低(<0.02%)。