磷酸盐对铅污染土–膨润土竖向隔离墙材料沉降特性影响的试验研究

Experimental study on effect of phosphates on sedimentation behavior of lead-contaminated soil-bentonite slurry wall backfills

土–膨润土竖向隔离墙广泛应用于控制地下水中污染物的运移。将两种磷酸盐用于铅污染环境下传统隔离墙材料改良,以增强墙体材料分散性,提高其对污染物的阻滞效果。通过一系列沉降试验和微观扫描电镜试验,研究了不同铅浓度、磷酸盐掺量和种类下土–膨润土隔离墙材料的沉降特性及其变化规律,并给出相应的机理解释。试验结果表明,隔离墙材料在铅污染作用下发生絮凝,沉积物体积增加36%;磷酸盐作用下,铅污染试样沉降模式由絮凝沉降型转变为累积沉降型,同时沉积物体积明显减小,沉积物结构更致密;磷酸盐的空间位阻效应和具有增加土粒表面负电势的性能使得隔离墙材料的分散性得到显著改善,土颗粒保持为平行有序的排列结构;相同磷酸盐掺量下,掺加六偏磷酸钠试样的沉降体积较小,其对铅污染试样的分散效果优于焦磷酸钠;铅浓度为0～0.1,1～2和6 mmol时,六偏磷酸钠最优掺量分别为0.1%,0.5%和2%。研究成果可对改善受污染隔离墙材料的分散性提供一定参考。

Soil-bentonite（SB） slurry walls are widely used in controlling migration of the contaminants in groundwater. Adding two types of phosphate to lead contaminated SB backfills may be advantageous to maintain the deflocculated structure of the bentonite in backfill, which is beneficial to enhance contaminant retarding ability of the backfill. A series of sedimentation and scanning electron microscope（SEM） tests are conducted to investigate changes in sedimentation behavior of SB backfills with various lead concentrations and phosphate contents/types, and the relevant mechanism is given. The results reveal that lead results in flocculation of the SB backfill and a 36% increase in the sediment volume. A certain amount of phosphate makes sedimentation curve of the contaminated backfill change from flocculation sedimentation type to accumulation sedimentation type, and the sediment volume of the backfill decreases significantly. Addition of phosphate enhances dispersity of the SB backfill due to steric stabilization of the phosphate and increased negative charge of the clay particle surfaces. The soil particles present a paralleled arrangement. Sodium hexametaphosphate has higher dispersibility compared with sodium pyrophosphate, because the backfills amended with sodium hexametaphosphate have smaller sediment volume. Optimum content of the phosphate is 0.1%, 0.5% and 2% while lead concentration in the backfill ranges in 0~0.1, 1~2, 6 mmol, respectively. The results obtained in this study may provide a meaningful guidance for improving dispersity of contaminated SB backfills.