Containment Performances Of Sand-Polymer Amended Bentonite Vertical Barriers Exposed To Heavy Metal Contaminants

The main purpose of this study is to investigate the engeering properties and amended mechanism of sand-polymer amended bentonite backfills,the polymers include Polyanionic Cellulose and Xanthan Gum.A seires of laboratory experiments and contaminants transport model calculation are conducted to explore the basic characteristics,engeering properties,chemical compatibility and membrane behavior of sand-polymer amended bentonite backfills.The main works and conclusions are incorporated as follows:（1）The variations in engeering properties of bentonite amended by five different polymers are investigated.Polyanionic Cellulose and Xanthan Gum are selected as the amending agents based on the results of engeering peoperties.The basic characteristics and hydraulic performance of Polyanionic Cellulose amended bentonite and Xanthan Gum amended bentonite are evaluated.The results indicate that swell index,liquid limit and hydraulic performance of the amended betonites are siginificantly improved by the polymers,and the hydraulic conductivity of the amended betonites decrease with increasing dosage of polymer in bentonite.Under the same test condition,the sand-Polyanionic Cellulose amended bentonite backfills（PSB）and sand-Xanthan Gum amended bentonits backfills exhibit larger compression index and lower hydraulic conductivity compared to conventional sand-bentonite backfills.（2）The chemical compatibility of PSB and XSB are investigated,a series experiments are conducted to evaluate effects of calcium chloride,CaCl₂ solution nd Lead nitrate,Pb（NO₃）₂ solution on the basic characteristic,hydraulic performance and consolidation behavior of backfills.The results show that the Pb（NO₃）₂ solution has more significant impact on hydraulic performance of bentonite filter cake compared to CaCl₂ solution.PSB and XSB have greater liquid limit（w_L）and lower specific gravity（G_s）compared to convetntional backfills（SB）,and the three backfills exhibit lower w_L and higher G_s after exposed to Pb（NO₃）₂ solution.Both the hydraulic performance of PSB and XSB with polymer dosage 0.6%（dry weight to dry backfill mixture）can satisfy the requirement(≤10^-9 m/s)when eposed to 0-500 mmol/L Pb（NO₃）₂ solution.The hydraulic conductivity to Pb（NO₃）₂ solution relative hydraulic conductivity to Tap water（k_w）of PSB and XSB is lower than 10,indictating that the hydraulic performance polymer amended backfills are obviously improved compared to SB.The polymer elution and of polymer amended backfills prepared through liquid and oven dring,resulting in higher hydraulic conductivity after polymer eluted.The initial void ratio and compression index of PSB and XSB decrease with increasing Pb（NO₃）₂ concentration.（3）The X-ray diffractometer analysis（XRD）,scanning electron microscope（SEM）,Fourier Transform Infrared Spectroscopy（FTIR）,Zeta potential and particle size analysis of polymer amended bentonite to evaluate the amending mechanism of polymer.The results indicate that the amending mechanism including:1)Clogging the void pores between bentonites intergranules through polymer hydrogels and stitching bentonites particles through polymer chains.2)Chelationg adsorption,the carboxyl group and hydroxyl group adsorb heavy metal ions（i.e.,Lead ions）,which diminish the impacts of Lead ions on compression of bentonites diffusion double layer.3)the increased Zeta potential and decreased paticle size improve the adsorption capacity of polymer amended backfills.4)The amending mechanism of polymer amended backfills including the package of sand by polymer amended bentonite,the clogging void pores and the stitching intergranules by polymer chains.（4）The changes in adsorption capacity of PSB and XSB to Pb（II）and Zn（II）are investigated by Batch adsorption tests,pH and electric conductivity.The results indicate that the removal percent（RP）of backfills decerases with the initial concentration of solution,under the same test condition.the RP value of PSB and XSB are higher than that of conventional backfills SB.the isotherm adsorption results indicate that PSB and XSB exhibt higher the maxmum Pb（II）adsorption capacity compared to SB,The Freundlich exponent（N）of three backfills are lower than 1 impling stronger interaction between three backfills and Pb（II）.The adsorption free energy（E）of three backfills are in the range of 8.0 kJ/mol<|E|<16.0 kJ/mol,indicating that the adsorption is governed by chemical ion-exchange.The similar results of adsorption of backfills to Zn（II）are obtained.（5）The membrane and diffusion behavior of backfills to Pb（II）and Zn（II）are conducted.The chemical membrane coefficient（ω）and retardation factor（R_d）of three backfills decrease with the increasing Pb（NO₃）₂ concentration.Under the same test condition,PSB and XSB exhibits higherω,lower R_d,effective diffusion coefficient（D^*）compared to conventional backfills（SB）.The fitting result of analytical evaluation model using CMR method is better than that of using relative concentration method.The retardation factor obtatined from column chemical osmosis test is higher than that obtained from CMR method.