The Study On Transport Of Heavy Metal In Red Soil And The Remediation Technology

Soil is an important part of ecological environment, and is one of the main resources which humanbeings must depend upon for survival. In the recent years, the heavy metal pollution of soil hasincreased with the development of mining industry, metallurgical industry, foundry industry andelectronic industry. According to statistics, in the past50years, the emission amount of cadmium was2.2×104tons, copper was9.39×105tons, lead was7.83×105tons, and zinc was1.35×106tons. Aconsiderable portion of those heavy metals had been absorbed into the system of soil and undergroundwater, which caused heavy metal pollutions in the world. That means the heavy metal pollution has ledto deterioration of soil quality, and caused frequent heavy metal contaminations in human society. Theheavy metal pollution of soil has been a threat to the circulation of soil ecosystem and the human beingsurvival. Therefore, the heavy metal remediation technology has become a research hotspot.The physical technology, the chemical technology, the phytoremediation technology, the microbialremediation technology and the electro-chemical technology are the traditional technologies to restorethe soil heavy metal contaminnation. Although the researchers are working for developing the heavymetal remediation technology, those remediation technologies still have some problems, such as, thehigh operating cost, the pollution of underground water, the secondary pollution to the environment,and the feasibility. So few technologies can be applied in engineerings.According to the drawbacks, the new soil remediation technology—CH-PRB (biodegradable chelatingagent+permeable reactive barrier) was raised in the project. In this project, the red soil was the object of thestudy, and the copper and the nickel were the specific pollutants. Leaching method is the traditional soilremediation technology and the researchers have enough engineering experiences on it. The technology ofPRB (permeable reactive barrier) is a new groundwater treatment technology, which has been widely used inforeign countries. The two techniques are combined in the CH-PRB technology, which is able to restore thesoil and the polluted groundwater which contaminated by heavy metal at the same time. And thegreen-chelating agents are used in the CH-PRB technology as the eluents to remove the heavy metal in thesoil, while avoiding secondary pollution in the environment.The study of the copper and nickel speciations in the artificial-contaminated soil were performed toevaluate the heavy metals distribution in the red soil. The analyzed results have indicated: the exchangeablefraction of copper was the high proportion in the copper-contaminated soil, which was31.98%, while inthe nickel-contaminated soil the most portion was observed in the residual form, up to57.49%, and theexchangeable fraction was29.53%.Conducting the soil column experiment(diameter was20cm, height was100cm), with the solution ofCuSO4slowly filting down into the soil column simulating the diffusion of copper in the red soil, themigration regularity of copper in the red soil was studied and the hydrodynamic diffusion coefficient in thedifferent layers were calculated using the instantaneous profile method. Basing the advection dispersion model and using the finite-difference time-domain method, the heavy metal migration model wasestablished, which coupled with the hydrodynamic diffusion coefficient.By the beaker test, with the copper and the nickel were the specific pollutants, the heavy metalsextraction efficiencies of chitosan, ethylene diamine citric acid, and sodium citrate from the red soilwere compared in the project. The studies have shown that the biological chelating agent-chitosancould be used as the eluent in the leaching process to remediate the heavy metal contaminated red soil,and the coexistence of a variety of heavy metals would make the heavy metals show good synergisticproperties in the process of extraction.With the model of the soil remediation process of CH-PRB, the contaminated soil columnexperiment, which diameter was20cm, height was100cm, and copper content was2.340mg/g, wasstudied. Using the0.2g/L chitosan solution as the eluent, the removal efficieny of the tested soil columnwas98.29%, and the remaining copper content in the soil was only0.04mg/g, which was lower thanthe upper limit of copper content in the second class soil in Environmental Quality Standards for Soils(GB15618-1995). By parallel experiments, the results have shown that the reaction filler which wasconsisted of the iron scrap, activated carbon powder, and cinder with the volume rate of1:1:1, was themost effective matter filled in the permeable reactive barrier. This kind of reaction filler could removethe heavy metal in the exudates of the contaminated soil column and in the underground water. Thefinal influent with [Cu]=0mg/L, COD=9.80mg/L, pH=5.11-5.12was obtained. The studies havedemonstrated that the soil remediation process of CH-PRB could restore the heavy metal contaminatedred soil efficiently and steadily, and this process was sustainable.Referring to the continuous stirred tank reactor (CSTR), the reaction kinetics model of thebiodegradable chelating agent leaching system in the CH-PRB was established. Furthermore, therelation between the extraction efficiency of the heavy metal, the volume of contaminated soil and theresidence time of eluent, the leaching time were discussed. At the same time, the initial construction ofthe CH-PRB soil remediation process design system was set up, which will offer references for theapplication of this new process.