| Heavy metal contamination in groundwater is a worldwide environmental problem.This problem is becoming more and more serious,resulting in water quality damage in many areas,greatly reducing the availability of water resources.In order to meet the increasing demand for clean water resource,it is very important to develop effective method to treat with heavy metal contaminated groundwater.Lead,cadmium and copper are typical heavy metals and it's very difficult for them to degrade in groundwater.When organisms and humans get in touch with these heavy metals,there will be significant impact on their health.Permeable reactive barrier(PRB)is a widely used in-situ treatment technology for remediation of groundwater pollution.In this research,PRB was used as an economical and effective remediation method for heavy metal pollution treatment.The aim of this research is to develop a new type of PRB reaction medium for treating the heavy metal contaminated groundwater.Peanutshell biochar was obtained by pyrolysis method.The adsorption characteristics of solid wastes(sawdust?peanut shell?oyster shell powder)and biochar were investigated,a novel adsorption medium for permeable reaction barrier was developed.Using solid waste as PRB adsorbent can increase the utilization rate of waste and reduce the cost to a great extent.Firstly,solid wastes such as oyster shell powder,sawdust and peanut shell were selected as adsorption alternative materials,their adsorption characteristics were investigated.Secondly,peanut shell biochar was obtained by pyrolysis method and its adsorption characteristics were studied.Then,the adsorption mechanism of oyster shell powder and peanut shell biochar were revealed by means of electron microscopy(SEM),X-ray diffraction(XRD),Fourier transform infrared spectroscopy(FTIR).Finally,the mixing ratio of PRB reaction medium was determined,and the feasibility of the reaction medium was studied through column and tank tests.The main results are as follow:(1)Sawdust,peanut shell and oyster shell powder were selected as alternative materials for heavy metal adsorption.Lead,cadmium and copper were selected as typical heavy metal ions.The adsorption characteristics of these alternative materials for heavy metals were investigated by batch adsorption experiments.For lead and cadmium,oyster shell powder showed better adsorption capacity than peanut shell and sawdust;however,for copper,peanut shell showed better adsorption capacity than oyster shell powder and sawdust.Oyster shell powder and peanut shell were proved to be economical andeffective adsorbent for heavy metal ions.(2)Peanut shell biochar was obtained by pyrolysis method.The carbonization mechanism of peanut shell biochar was revealed by analysis of the characteristics of peanut shell biochar,the carbonization process can be roughly divided into several processes: proto-biomass,excessive carbon,amorphous carbon,composite carbon and disordered carbon.The durability and adsorption characteristics of peanut shell biochar were improved by pyrolysis method.Peanut shell biochar can be used as PRB medium material for treatment of heavy metal contaminated groundwater.(3)The mixing ratio of oyster shell powder and peanut shell biochar was determined to be 5:1 as PRB material.The mixture showed good adsorption capacity and durability.Considering the effect of particle size on the permeability and adsorption efficiency of the adsorbent,oyster shell powder in the range of 0.85~1.18 mm was selected as the adsorption material for heavy metal treatment.(4)The adsorption mechanism of heavy metal adsorption by oyster shell powder and peanut shell biochar was revealed.The adsorption process of heavy metals by oyster shell powder included pore filling,electrostatic attraction,non-electrostatic attraction,surface precipitation and ion exchange;the adsorption of heavy metals by peanut shell biochar mainly included precipitation,complexation,ion exchange and physical adsorption.(5)Through the dynamic column simulation test of oyster shellpowder-peanut shell biochar mixture,the breakthrough curves of three heavy metal solutions under different conditions were obtained.Thomas model,Adams-Bohart model and Yoon-Nelson model were used to fit the column experimental results,and some dynamic parameters of heavy metals adsorption by mixture were obtained.Based on the limitation of Thomas model,the modified model was developed to well predict the breakthrough curves of column results.The prediction formula of column operation time is obtained taking the factors such as solution flow rate,initial solution concentration and the height of adsorbent in column into consideration,and its validity is proved.(6)Soil in Guanyue Mountain was selected for soil tank experiment to simulate the practical application of permeable reaction barrier.The basic physical,mechanical properties and adsorption characteristics of soil were investigated.The relationship between thickness and life of PRB was deduced to predict the feasibility of oyster shell powder-peanut shell biochar mixture.Soil tank test showed that the operation result of PRB materials in 6 months was in accordance with the expectation.Oyster shell powder and peanut shell biochar can be used as adsorbent for heavy metal ions with low cost and good adsorption capacity.It has broad application prospects to use oyster shell powder-peanut shell biochar mixture as PRB reaction medium for remediation of heavy metal contaminated groundwater in PRB. |
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