磷酸盐、腐殖酸与粉煤灰联合钝化处理模拟铅镉污染土壤

本研究利用过磷酸钙、腐殖酸、粉煤灰及其复配组合钝化处理人工模拟Pb、Cd污染土壤,并采用CaCl_2浸取法、三乙三胺五乙酸(DTPA)浸取法以及BCR形态分级实验评价钝化效果,利用X射线衍射仪(XRD)和扫描电子显微镜能谱(SEMEDS)分析土壤表面微观形态与结构,进一步探究其钝化机制.结果表明,除了腐殖酸单一处理,其他不同钝化处理均能降低土壤CaCl_2和DTPA提取态Pb、Cd含量,其中先添加过磷酸钙和腐殖酸,然后再添加粉煤灰的联合处理实验组效果最佳.土壤pH值与CaCl_2、DTPA提取态Pb含量存在微弱的正相关关系,与CaCl_2、DTPA提取态Cd含量存在负相关关系,速效磷含量与二者都存在显著的负相关关系,说明速效磷含量是控制土壤Pb、Cd活性的主要因素.施用磷酸盐、腐殖酸和粉煤灰可以促进Pb、Cd由活性较高的弱酸提取态向活性低的残渣态转化,从而有效降低Pb、Cd的迁移能力.XRD和SEM-EDS分析表明,过磷酸钙钝化重金属的机制主要是通过离子交换作用将重金属转化为难溶的Ca-重金属混合磷酸盐,3种钝化剂联合作用机制主要通过溶解/沉淀以及表面吸附作用将重金属转化为稳定的磷酸铅沉淀[Pb_3(PO_4)_2]或者混合重金属矿物[PbFe_3(SO_4)(PO_4)(OH)_6],从而有效钝化重金属.

Passivation of Simulated Pb-and Cd-Contaminated Soil by Applying Combined Treatment of Phosphate, Humic Acid, and Fly Ash

In this study, three kinds of amendments including superphosphate, humic acid, and fly ash and their complex combination were adopted to passivate the artificially simulated Pb-and Cd-containing soils. The passivation efficiency evaluation was performed via the CaCl₂ and triethylenetriaminepentaacetic acid (DTPA) extraction method as well as a BCR morphological classification experiment. The microstructures and structures of the soil were explored further via X-ray diffraction (XRD) and scanning electron microscopy with X-ray energy dispersive spectroscopy (SEM-EDS) to elaborate the passivation mechanism. The results demonstrated that all passivation processes, excluding single humic acid addition, could reduce the CaCl₂ and DTPA extraction contents of Pb and Cd in soils, where the optimal efficiency could be achieved by the sequential addition of superphosphate and humic acid, followed by fly ash. There was a weakly positive correlation between soil pH and CaCl₂/DTPA extraction content of Pb, a negative correlation between soil pH and CaCl₂/DTPA extraction content of Cd, and a significantly negative correlation between available phosphorous content and CaCl₂/DTPA extraction contents of Pb and Cd, suggesting the crucial role of available phosphorous contents to control the activities of Pb and Cd. In the presence of phosphate, humic acid, and fly ash, the Pb and Cd could convert from active weak acid extraction to low-activity residual speciation, resulting in effectively reducing Pb and Cd transferability. Throughout the XRD and SEM-EDS analyses, it was found that ion exchange was the predominant mechanism in heavy metal passivation by single superphosphate, wherein the heavy metals were transformed into an insoluble Ca-containing phosphate mixture. The dissolving/precipitation or surface adsorption could be concluded as the main mechanism in the combination of the three passivation agents that converted heavy metals to lead phosphate precipitate[(Pb₃(PO₄)₂] or mixed heavy metal mineral[PbFe₃(SO₄)(PO₄)(OH)₆], so as to obtain superior heavy metal passivation achievement.