Research On Impermeability Mechanism Of Bentonite-Polymer Composite Geosynthetic Clay Liners In Red Mud Disposal Yard

The red mud disposal yard is specially used to treat and dispose of the red mud solid waste generated during the alumina production process.The high ionic strength of the bauxite liquor can suppress swelling of montmorillonite,so sodium bentonite geosynthetic clay liners（Na B GCL）is too permeable to be effective as liners in red mud disposal facilities.In view of the poor impermeability of Na B GCL,bentonite-polymer composite geosynthetic clay liners（BPC GCL）have been developed.The hydraulic conductivity experiment and swelling index experiment were conducted to explore the permeation behavior and the impermeability mechanism of bauxite liquor and BPC GCL was revealed.The application model of BPC GCL based on numerical simulation was established,which laid the foundation for the theoretical feasibility and practical application of BPC GCL as an impermeability material for red mud disposal yard.The main research contents and conclusions are as follows:（1）Through a nationwide survey of domestic red mud disposal yard,it was concluded that the bauxite liquor was highly alkaline（p H>12）,the ionic strength ranged from 77.3mmol/L to 620.3 mmol/L,and the main elements and anions were Al and Na,Cl^-and SO₄^2-,the content of the main heavy metal elements were Cr and Pb,and their highest concentrations are 5.9 mg/L and 1.1 mg/L.All of them exceed the 0.05 mg/L and 0.01 mg/L specified in Class III in the domestic groundwater quality standards（GB/T 14848-2017）.Through hydraulic conductivity experiment of bauxite liquor and BPC GCL found that the hydraulic conductivity of BPC GCL to the bauxite liquor is 10^-8 m/s-10^-12 m/s,which is significantly higher than the hydraulic conductivity when it interacts with deionized water(10^-12 m/s-10^-13 m/s),but lower than the hydraulic conductivity(10^-7 m/s-10^-8 m/s)of the traditional Na B GCL under the action of the same bauxite liquor.The results showed that the BPC GCL had good impermeability properties.（2）By analyzing two kinds of bauxite liquor as the permeating solution,the influencing factors of the impermeability characteristics of the BPC GCL were investigated.The hydraulic conductivity of BPC GCL was not only dependent on the chemical properties of the permeable liquid,but also related to the characteristics of bentonite and the content of polymer.The hydraulic conductivity of the sample BPC GCL CP6.5 permeated in deionized water is1.7×10^-13 m/s,and when it permeates in the Guangxi sampled bauxite liquor with an ionic strength of 224.5 mmol/L,the hydraulic conductivity is 3.1×10^-11 m/s,the hydraulic conductivity is 5.0×10^-9 m/s when it penetrates into the bauxite liquor sampled from Shandong with an ionic strength of 620.3 mmol/L.It can be seen that the hydraulic conductivity increases as the ionic strength of the permeating solution increases.At the same time,the swelling index of bentonite also affects the hydraulic conductivity.The same sample BPC GCL CP6.5 permeates in deionized water with a swelling index of 29.0 m L/2g.When permeated in Guangxi sampled bauxite liquor with an ionic strength of 224.5 mmol/L The medium swelling index is 18.0 m L/2g.When the swelling index is 8.2 m L/2g when it penetrates into the bauxite liquor sampled from Shandong with an ionic strength of 620.3mmol/L,it can be seen that as the bentonite swelling index increases,the hydraulic conductivity decreases.trend.In addition,with the increase in the amount of polymer added,the hydraulic conductivity also showed a downward trend.When the amount of polymer added was 6.5,7.5 and 10.8,the hydraulic conductivity of the sample permeated into the red mud leachate sampled in Shandong were 5.0×10^-9 m/s,6.7×10^-11 m/s and 6.6×10^-12 m/s.The hydraulic conductivity decreased as the amount of polymer increased,indicating that sufficient amount of polymer was needed to ensure a sufficiently low hydraulic conductivity of the BPC GCL.Moreover,the amount of polymer necessary depended on the chemical properties of the liquid and the type of polymer.（3）After discussing the impermeability characteristics of the BPC GCL,the impermeability mechanism was further explored.The impermeability mechanism of BPC GCL on bauxite liquor was not only related to the ion exchange property and osmotic expansion property of montmorillonite in bentonite,but also related to the amount of polymer,the structure of polymer and the elution.When permeated with deionized water or dilute solution,the particles would expand and closed the pores between the particles due to the permeable swelling in the montmorillonite,resulting in low hydraulic conductivity.The permeation of high concentration solution could prevent the permeable swelling of the montmorillonite interlayer,which leaded to the swelling of bentonite particles and the closure of intergranular pores,thus leading to the higher hydraulic conductivity and the lower impermeability effect.The addition of polymer and the gradual elution also had a great influence on the hydraulic conductivity of the BPC GCL.The addition of polymer mainly fills the intergranular pores through the polymer hydrogel,which further played a positive role in the impermeability effect.When hydrated with deionized water,the linear structure and the cross-linked network structure were composed of hydrogels,which filled the gaps between bentonite particles and cover the surface of bentonite and played a good impermeability effect.However,as the permeability continued,the structure of the hydrogel was destroyed to promote the elution of the polymer.The elution of the polymer hydrogel opened the pores and provided more flow paths,leading to higher hydraulic conductivity and lower impermeability effect.（4）On the basis of discussing the permeability behavior and impermeability mechanism of the BPC GCL,a model for the migration of pollutants in the liner of three composite red mud disposal yard based on numerical simulation was constructed.The main pollutant Al³⁺and the migration model of heavy metal pollutants Cr⁶⁺and Pb²⁺was established.According to the migration diagram of Al³⁺in each interface in CP6.5,the color of the migration diagram of Al³⁺in each interface of BPC GCL within 10 years changes from orange-red to green to blue.It can be seen from the concentration range indicated by the legend.As the color changes,the concentration of Al³⁺gradually decreases,and Cr⁶⁺and Pb²⁺also show the same law.The attenuation behavior of the pollutants in the BPC GCL proved that the BPC GCL had a blocking effect on pollutants,thus showing a good impermeability effect.Furthermore,comparing the concentration changes of Al³⁺,Cr⁶⁺and Pb²⁺in the same interface of BPC GCLCP6.5,CP7.5 and CP10.8,we can see that,if we observe that CP6.5,CP7.5 and CP10.8are all in the L1 layer BPC GCL three points The concentration of one interface shows that the concentration of the three pollutants gradually decreases,showing a declining trend.With the increase of polymer addition,the BPC GCL had a strong effect on Al³⁺,Cr⁶⁺and Pb²⁺.The blocking effect was gradually strengthened,and its impermeability effect was gradually strengthened accordingly.