酸性矿山废水对钠基膨润土渗透性的影响

The effect of acid rock drainage on the permeability of sodium bentonite

钠基膨润土在环境岩土工程中被广泛应用,是垂直阻隔墙等防渗屏障的主要防渗材料。本文通过沉淀试验、自由膨胀试验和改进滤失试验对比了4种钠基膨润土在酸性矿山废水中的分散性、膨胀性和渗透性的差异。其中,改进滤失试验探究了滤失压力和酸性废水浓度对膨润土渗透系数的影响,钠基膨润土在酸性废水中的渗透系数相比去离子水中提高了2~3个数量级,而在50~150kPa范围内,渗透系数随滤失压力的提高逐渐降低,但提高滤失压力仍不足以抵消酸性废水的劣化作用。本文通过扫描电镜试验、X射线衍射试验和滤出液化学成分测试探究了酸性废水对钠基膨润土的劣化机理,得到结论:酸性废水对钠基膨润土的劣化作用包括离子交换和矿物溶解。其中,离子交换主要为酸性废水中三价阳离子(Fe3+为主)置换膨润土蒙脱石中一价阳离子(Na+为主)的过程;矿物溶解主要是酸性废水的强酸性导致蒙脱石中的Na被转化为离子的形式而使蒙脱石结构发生破坏的现象。通过对比4种钠基膨润土的微观试验结果得到了4种钠基膨润土的劣化机理差异,并归纳了影响劣化作用差异的关键因素。

Sodium bentonite holds significant utility in environmental geotechnical engineering as a primary constituent of anti-seepage structures,notably vertical cutoff walls.This study assesses the dispersion,expansibility,and permeability of four sodium bentonite varieties in acid rock drainage(ARD)bysedimentation,free expansion,and modified filtrate loss tests.Results from the modified filtrate loss tests indicate a permeability coefficient for sodium bentonite in acid rock drainage ranging 2-3 orders of magnitude higher than that observed in deionized water.Despite a decrease in the permeability coefficient of sodium bentonite in acid rock drainage with increasing filtration pressure within the 50-150 kPa range,such pressure increments fail to offset the deterioration of the acid rock drainage.Furthermore,this investigation delves into the mechanisms governing the deterioration of sodium bentonite induced by acid rock drainage,employing scanning electron microscopytest,X-ray diffractiontest,and chemical composition analyses of effluents of filtrateloss test.The study concludes that the deterioration of sodium bentonite due to acid rock drainage involves both ion exchange and mineral dissolution.The ion exchange primarily entails the replacement of monovalent cations(mainly Na+)in bentonite’s montmorillonite by trivalent cations(primarily Fe3+)in the acidrock drainage.Mineral dissolution occurs mainly due to the strong acidity of the acid rock drainage,which converts Na in the montmorillonite into ionic form,thereby disrupting the structure of montmorillonite.By comparing the microstructural test results of four types of sodium bentonite,the study elucidates the differences in their deterioration mechanisms and identifies the key factors influencing these differences.