Experimental Study On Properties Of Phosphate Rock-Soil-Bentonite Cutoff Wall Material

Vertical anti-fouling barrier isolation technology is currently the most widely used and effective way to block the transport of pollutants in groundwater soil pollution control projects at home and abroad,with the advantages of wide range of pollutant treatment objects,strong impermeability and low engineering costs.In this paper,phosphate powder is used to modify soil-bentonite,and its basic soil properties parameters are tested by basic geotechnical tests,and the problems arising from the tests are studied and verified;the heavy metal Pb²⁺is used as the representative pollutant ion,the adsorption characteristics and mechanism of Pb²⁺by Xiangtan clayey sand（S）,clayey sand-bentonite（SB）and phosphorite powder-soil-bentonite（PSB）under different influencing factors were investigated by Batch test and combined with different adsorption models and microscopic testing means in comparison.The adsorption characteristics and mechanism of Pb²⁺by PSB under different influencing factors;then considering the engineering practice,the PSB material is prepared into soil column specimens for model tests to study their compression and permeability characteristics,and the prediction and evaluation of the breakdown time of the barrier under different working conditions and the soil column tests carried out in this paper are combined with the existing one-dimensional analytical model.The main results are as follows:（1）The expression of K is derived from the perspective of the law of energy conservation,which is not only well verified theoretically,but more importantly,can get a good agreement for the test results of different scholars and the data of different geotechnical test standards.This formula is then used to discuss the liquid limit index of different specifications and to suggest how different specifications and standards can be relatively unified.In addition,the influence of moisture absorption behavior of drying soil samples on the results of specific gravity determination of soil particles is discussed.And on this basis,a modified specific gravity bottle method which can directly adopt natural air-drying soil samples for specific gravity testing is proposed.The method can avoid the influence of moisture absorption behavior of drying soil samples on specific gravity testing,and the error of the modified method mainly comes from the determination of water content.（2）The adsorption process of Pb²⁺by PSB is consistent with the proposed first-order kinetic model,the proposed second-order kinetic model,the intergranular diffusion model and the liquid film diffusion model.Among them,the proposed second-order kinetic model fitted the experimental data at the three test concentrations best.The intergranular diffusion model and the liquid film diffusion model showed that both intergranular diffusion and liquid film diffusion were involved in controlling the adsorption process.（3）Thermodynamic analysis showed that the adsorption of PSB on Pb²⁺was exothermic,and the Langmuir model could better describe the adsorption process of PSB on Pb²⁺than the Freundlich model.The analysis of thermodynamic parameters showed that the degrees of freedom and entropy values of the whole solid-liquid system decreased and decreased,respectively,as the adsorption of Pb²⁺by PSB proceeded.In addition,the incorporated phosphorite powder gave the soil sample a certain buffering capacity in acidic environment,which enhanced the adsorption performance of the soil sample on Pb²⁺under acidic conditions,and the solution p H had a great influence on the adsorption of Pb²⁺by PSB,and the adsorption amount of Pb²⁺by PSB increased with the increase of the initial p H.（4）Microscopic analysis（XRD,XRF,SEM and FT-IR）showed that minerals such as quartz,calcite and hydroxyapatite in PSB were involved in the adsorption process of Pb²⁺,and the adsorption of Pb²⁺by PSB was significantly higher than that of S and SB,which was mainly due to the reaction of PO₄^3-released from the dissolved phosphate powder with Pb²⁺in solution to form phosphate precipitates on the adsorbent surface or between layers of-OH and PO₄^3-are involved in the adsorption process of Pb²⁺.（5）Considering the engineering practice,the PSB mixture was prepared into soil columns and compression and permeation tests and contaminant migration tests were carried out.It was found that the consistency of soil sample preparation was good and the permeability coefficient of soil column was<1×10^-7cm/s,which satisfied the specification requirements.Based on some of the basic parameters measured from the tests conducted and the comparison and generalization of the research results of different scholars to determine the basic parameters of the model,the existing one-dimensional analytical model was combined with the existing one-dimensional analytical model to predict and evaluate the breakdown time of the wall under different working conditions and the soil column tests conducted in this paper.