| Chlorinated solvents are widely used in industries such as dry cleaners, electronic equipment manufacturers, metal parts fabricators, insecticide and herbicide producers, military equipment manufacturers, etc. These solvents replaced petroleum derived mineral spirits and have distinct advantages because of their nonflammability. And volatile chlorinated hydrocarbons are the byproducts of sterilization using chlorine for drinking water. The persistence and mobility of these hydrocarbons in the subsurface was largely unanticipated, therefore historical disposal practices has lead to widespread groundwater contamination. Treatment of volatile chlorinated hydrocarbons contaminated groundwater is usually accomplished by pumping the groundwater to the surface and removing the contaminant through oxidation or air stripping. But the system is limited for volatile chlorinated hydrocarbons are immiscible with water and easy to be adsorbed on the soil. However, complete removal of the contaminants from the subsurface is rarely achieved, and the treatment usually only transfers the contamination to another medium. And the pumping and treat takes long time and is not very economical. A possible alternative to pump and treat is the containment of the contamination with in situ barriers using reactive media to degrade the contaminants present in the groundwater. The system is called permeable reactive barriers (PRBs). The concept of PRBs is relatively simple. Reactive material is placed in the subsurface to intercept a plume of contaminated groundwater that must move through it as it flows, typically under its natural gradient, thereby creating a passive treatment system. As the contaminant moves through the material, reactions occur that transform it to less harmful (nontoxic) or immobile species. The common materials for volatile chlorinated hydrocarbons are Fe? related Fe(II) mineral system and bimetallic system.The Fe?(iron) technology currently shows considerable promise as a cost-effective method of groundwater remediation. The iron is collected as a waste product from machining and foundry operations. The most promising iron is granular from auto-manufacture and the specific surface area should be over 1 m /g in order to make the reactive system holds higher ratio of surface area to solution volume.Considerable research indicates the degradation process is widely accepted as abiotic reductive dechlorination, involving corrosion of the Fe by the chlorinated hydrocarbons. Under aerobic conditions, the reaction of Fe0 and water is expressed as:2Fe0+2H2O+O2→2Fe2++4OH-Under anaerobic conditions, the reaction of Fe and water is expressed as:Fe0+2H2O→Fe3++H2+2OH-The reaction of chlorinated hydrocarbon and Fe can be expressed as:Fe0+ RCl +H+→Fe2++RH+Cl-DO, H+ results from the dissociation f water and RC1 are all oxidants in the reduction process. We know, the amount of DO is very limited in groundwater and the reaction that H+ changes to H2proceeds slowly under the anaerobic condition. Therefore, Fe is the source of electron and RC1 is electron acceptor.This study is a part of the project supported by NSFC (national science foundation of China). We conducted a series of batch experiments and column experiments using materials collected in China in laboratory. PCE and CT are the target contaminants in the study. The objectives of the paper are to study the mechanism of dechlorination, efficiency of degradation, the reliable analytical method of gas chromatography to detect volatile chlorinated hydrocarbons and half -lives of PCE and TCE. The half-life is the key factor to design PRBs because the scale of PRBs depends on it.During the study, the conclusions that are drawn are followed.1. The results of batch experiments prove that iron is effective to degrade volatile chlorinated aliphatic hydrocarbons. The rate of degradation appears to be directly proportional to the surface area to volume ratio. And perchlorinated hydrocarbons tend to be reduced more rapidly than their less chlorinated con...
|
PDF Full Text Request