赤泥渗滤液对GCL多尺度孔隙结构及防渗性能影响

Influence of bauxite liquors on multi⁃scale pore structure and impermeability of GCLs

赤泥渗滤液会对赤泥堆场使用的土工合成黏土衬垫(Geosynthetic Clay Liner, GCL)防渗性能造成负面影响,易产生渗滤液渗漏风险,严重威胁地下水和土壤环境安全。为了从多尺度孔隙结构角度揭示GCL对赤泥渗滤液的防渗机理,通过柔性壁渗透试验获取GCL对赤泥渗滤液的渗透系数,利用XRD试验分析蒙脱石晶体结构和质量分数的变化,描述固液相互作用,并开展压汞试验和氮气吸附-脱附试验,定量探究渗透液体对GCL中膨润土多尺度孔隙结构的影响。结果显示,具有高离子强度的赤泥渗滤液可以压缩GCL中膨润土的扩散双电层厚度,抑制渗透膨胀过程,进而导致作为渗流通道的集合体内和集合体间孔隙(渗流孔隙)体积增大,GCL渗透系数大幅增加;强碱性的赤泥渗滤液能溶解具有吸水膨胀性的蒙脱石,生成非膨胀性的次生矿物。通过降低膨润土膨胀性能和堵塞孔隙作用可改变膨润土多尺度孔隙结构,进而影响GCL渗透系数。天然钠基GCL的防渗性能好于人工钠化GCL,这是因为前者蒙脱石质量分数较高,具有更好的膨胀性能,相同赤泥渗滤液渗透后,天然钠基GCL依旧可以保持较小的渗流孔隙体积。具有更高离子强度和更强碱性的赤泥渗滤液会因剧烈的固液相互作用,导致渗透后的GCL具有较大的渗流孔隙体积和较高的渗透系数。

The object of this study is to reveal the mechanism controlling the hydraulic conductivity of geosynthetic clay liners to bauxite liquors based on multi⁃scale pore structures.Flexible wall permeation tests were conducted on two GCLs(GCL1 with artificial sodium bentonite and GCL2 with natural sodium bentonite)with DI water and bauxite liquors as permeant liquids.The mineralogy changes were determined by X ray diffraction tests to analyze the solid⁃liquid interactions between GCLs and bauxite liquors.Noteworthy,this study combined the mercury intrusion porosimetry and nitrogen gas adsorption tests to characterize the multi⁃scale pore structure of bentonite in GCLs after permeation.The results indicate that the intensity of the characteristic reflection and interlayer distance of montmorillonite decreases after permeation because of solid⁃liquid interactions.Moreover,the hyper⁃alkaline bauxite liquors reduce the montmorillonite content in GCLs owing to the dissolution.These solid⁃liquid interactions control the hydraulic conductivity of GCLs to bauxite liquors by changing the multi⁃scale pore structures of bentonite.Bauxite liquor with higher ionic strength can suppress the thickness of the double diffusing layer in bentonite and inhibit the osmotic swelling process of bentonite,leading to the increase of flow path volume(inter⁃aggregate and intra⁃aggregate pores)and hydraulic conductivity.The hyper⁃alkalinity of bauxite liquors also causes the montmorillonite dissolution and the generation of non⁃expansive secondary minerals.The dissolution weakens the swelling performance of bentonite,resulting in more flow path volume and higher hydraulic conductivity.However,the secondary minerals might clog the flow paths,leading to lower hydraulic conductivity.GCL2 has better chemical compatibility with bauxite liquors than GCL1 due to more montmorillonite content,considering that the better swelling performance made GCL2 less flow path volume.When GCL is permeated with bauxite liquor with higher ionic strength and alkalinity,GCL has less interlayer pore volume and more flow path volume,resulting in higher hydraulic conductivity.