Study On The Remediation Efficiency Of Permeable Reactive Barrier To Simulate Groundwater Contaminated By Lead And Cadmium

Groundwater,as an important natural resource on the earth,has a close influence on the production and life of human beings.Due to the unreasonable discharge of industrial wastewater,waste residue and the large amount of pesticide and chemical fertilizer,the groundwater pollution is becoming more and more serious.At present,heavy metal pollution of groundwater on the safety of the environment has become the focus of attention.Therefore,how to repair heavy metal contaminated groundwater has become a very important issue..Permeable reactive barrier(PRB)is an effective technique for in-situ repair of groundwater pollution,and its efficiency depends on the nature of the reaction filler to a large extent.In this study,the simulated heavy metal Pb/Cd contaminated groundwater was treated with corn straw,synthesized zeolite with fly ash,ferromanganese nodules derived from loess in northwestern China and fly ash as the reactive mediums.The response surface method was used to analyze the influencing factors.At the same time,the two factors of influent concentration and flow rate of Pb/Cd were selected for single factor and mixed orthogonal test,and the influence of the two factors on Pb/Cd repair effect was studied by SPSS regression model.Finally,the feasibility and persistence of PRB column were investigated by PRB column,and XRD,FTIR and SEM were used to study the effect of four kinds of reactive fillers on Pb/Cd repair.The main conclusions of this study are as follows:(1)The experiments were carried out with straw,zeolite,ferromanganese nodules and fly ash as fillers,and the effects of concentration,flow rate,temperature,p H and filler properties on the removal rates of Pb/Cd were studied by PRB.The experiment involves: The choice of the reaction filler,the test andanalysis of the safety of the reaction filler,the design of the PRB experimental device,the experiment of the influencing factors and so on.The results showed that the specific surface area of ferromanganese nodules were 121.07m2/g,which had a large specific surface area;and the zeolite CEC was 117mmol/100 g,indicating that the four kinds of fillers can be used as PRB filler for the experiment.The packing safety test showed that the leaching toxicity test values of the four fillers were lower than the standard values,indicating that the packing was safe and available.The results showed that the concentration,flow rate,pH and temperature had a great effect on Pb/Cd removal rates,while the removal rates of Pb/Cd were significantly increased after adding humification straw.(2)The response surface method was used to optimize the experiment of PRB column.According to the Box-Behnken design in Design-Expert software,the response surface analysis was carried out to complete the condition optimization.The results are as follows: the model of analysis of variance Pb/Cd regression model were significant(P<0.0001);the determination coefficient of the model were RPb2=0.9957 and RCd2=0.9839,which showed that it can explain the response of 99.57% and 98.39% of the variation.To sum up,we can conclude that the removal rates of Pb/Cd were the highest when the concentration of Pb and Cd were 7.11 and 0.699mg/L with the temperature of24~25? in weakly acidic solution.(3)Single factor and mixed orthogonal experiments were carried out.The results showed that the PRB system can withstand the maximum lead concentration was 20mg/L(removal rate reached more than 80%),and the maximum acceptable concentration of Cd was 2mg/L(removal rate reached more than 75%).PRB system was used to repair the simulated Pb/Cd contaminated groundwater,and the simulation results were analyzed,and the results showed that: there was a certain correlation between Pb/Cd removal rates,influent concentration and flow rate,and the correlation between Pb/Cd removal rates and concentration was higher than that of Pb/Cd removal rates and flow rate;binary linear regression model can better show the change trend of Pb/Cd removal rates with concentration and flow rate.(4)The durability and experiment of Pb/Cd were studied by using PRB column.The results showed that the four kinds of fillers played different roles inthe reaction process.According to the spectral analysis,there were a large number of functional groups on the surface of the straw,speculating that the coordination reaction between the straw and Pb/Cd were observed;the surface of ferromanganese nodules become rough,which might be caused by reactive precipitation;the adsorption effect of fly ash and fly ash zeolite were obvious because of its loose surface and large specific surface area.To sum up,the physical and chemical reactions were used by four kinds of filler to achieve the depth repair of groundwater of Pb/Cd compound pollution.