Experimental Study On The Removal Of Cr (?) From Groundwater By Modified Fly Ash And Microorganism

Groundwater has long been the source of production and living water for human beings With the rapid development of social economy,groundwater environmental security has increasingly become the focus of human attention,and groundwater pollution remediation has become an urgent problem in today's society.Among them,the most representative is the heavy metal pollution of groundwater.For example,chromium pollution of groundwater is a kind of waste water that is difficult to be directly treated and has the disadvantages of slow degradation rate,serious harm and easy to cause secondary pollution.Current groundwater pollution repair technology is widely used in the in-situ remediation technology,also called lm-prb osmotic reaction wall(permeable reactive barrier)is one of them,the removal efficiency of pollutants in groundwater depends largely on the medium inside the wall,the choice of medium and recycling in the wall is a hot issue studied by current academic circleIn this paper,fly ash,chitosan and acetic acid were used as raw materials to prepare modified materials.The ratio of raw materials was optimized by orthogonal experiment.By means of characterization analysis(sem,laser particle size analysis)and composition analysis(electron spectroscopy,X-ray diffraction),the characteristics and pollutant removal rate before and after modification of fly ash were explored.The adsorption mechanism of modified materials was revealed by the adsorption model and diffusion model.The repair experiment was carried out in two parts:PRB was used in the first-grade repair experiment,and the modified materials prepared by the experiment acted as the medium in the wall,considering the influence of different factors on the removal rate of hexavalent chromium;The secondary remediation experiment aimed at the remediation of hexavalent chromium in the medium,isolated the bacterial strain from the soil sample containing chromium in daqing oilfield,screened the high-hexavalent chromium reducing strain,and completed the recovery treatment of hexavalent chromium in the medium to avoid secondary pollution.Specific research contents and results are as follows:(1)Modified materials were prepared by crosslinking chitosan,fly ash and acetic acid.The optimum raw material ratio was obtained by three factors and three levels of L9(33)orthogonal experiment.The experimental results show that under the condition of 298k and pH=5,when the concentration of H2SO4 is 2mol/L,chitosan:acetic acid:acid fly ash=2g:3%:15g,the adsorption capacity of the modified material reaches the maximum value of 0.48mg/kg.The optimal ratio was named cwf-2c15(2)Scanning electron microscope and laser particle size analysis test results show that the modified fly ash is evenly distributed with chitosan particles on the surface,forming chitosan coating,the specific surface area of fly ash increases,the number of adsorption holes increases,and the adsorption effect is good.Electron energy spectrum and X-ray diffraction analysis show that the basic material composition before and after the modification of fly ash does not change,and the crosslinking between fly ash and chitosan is essentially a chain formed by the physical and chemical action between the silicon matrix and the polysaccharide structure,and the chitosan is bound to the fly ash surface by bond cooperation(3)Experimental data fitting results show that the adsorption conforms to the Freundlich isothermal model,that is,the adsorption process of cwf-2c15 on hexavalent chromium belongs to physical adsorption,accompanied by a trace chemical reaction.The diffusion of hexavalent chromium is mainly membrane diffusion.Of the absolute value of the free energy of adsorption reaction A G has nothing to do with temperature,A H<0,A G<0,since the heat in the process of adsorption,A S>0,the adsorption process is not reversible entropy increase reaction(4)The leaching toxicity of fillers is less than the safe value.PRB model was established in the first-stage restoration experiment to design the hexavalent chromium removal rate experiment.The five factors of sewage concentration,temperature,pH value,solution velocity and medium dosage are considered.The experimental results showed that the concentration of hexavalent chromium was 20 mg/L,the temperature was 20-25?,the pH value was 5-6,the flow rate of the solution was about 1.5?2 m/h,and the adsorption effect was the best.The removal rate of hexavalent chromium was higher than 80%.Meanwhile,with the increase of the amount of cwf-2c15,the removal rate of hexavalent chromium increased significantly(5)Microbial remediation technology was used in the secondary remediation experiment,mainly for the recovery treatment of the reactive medium.In this study,a high hexavalent chromium reducing strain was isolated from the chromium bearing soil of daqing oilfield and named Pseudomonas balearica DQ-2.Using this strain to carry out intermittent experiments on hexavalent chromium,the true reduction rate of hexavalent chromium can reach 93.7%,and the removal rate of hexavalent chromium in the reaction medium is 89.71%(close to 90%)This study has important reference significance for the remediation of hexavalent chromium in groundwater.At the end of the reaction system,a secondary microbial repair stage is added,which can reduce hexavalent chromium to achieve the true hexavalent chromium removal.The recovery of the medium is completed while the hexavalent chromium in PRB medium is removed.