| The widespread use of organic pollutants has led to considerable environmental contamination and cause severe health problems for humans and other wildlife. The environmental pollution has been one of the most serious social problems. Prevention of the pollution, and remediation of groundwater, soils contaminated with organic pollutants have been challenging. Zero-valent metal, especially zero-valent iron, which is very promising for removal of contaminants and can be used to nearly all organic compounds, has been identified as a leading potential technology, and received increasing attentions in recent years.So, two organic pollutants,γ-HCH and 1,2,3,4-TCDD chosen as the main model compounds, a series of batch experiments with 7ml amber glass vials capped with PTFE lined septa employed as the reactors were designed in the lab to study the degradation of organic pollutants by iron, zinc, and bimetal Fe/Pd, Fe/Ag under anaerobic conditions. The main conclusions are as follows: (1) The dechlorination ofγ-HCH by iron proceeded successfully and large benzene with a little chlorobenzene were identified as the final products. A possible main degradation pathway which involves two intermediate products of TeCCH and DCCD and final product of benzene formed via dichloroelimination is proposed based on the identified final products, though no intermediates identified. The reaction is preudo-first-order in respect toγ-HCH concentration, and the initial solution pH, reaction temperature and iron content were the important factors for the transformation rate ofγ-HCH by iron in the reaction. In general, the extent ofγ-HCH degradation increases with pH growth, temperature increment, and increasing the dosage of iron.(2) Zinc was shown to be thermodynamically more favorable and kinetically more reactive than iron which is the preferred metallic species utilized in most zero-valent metal applications. The successful treatment of PCDDs by zinc powder was achieved in 0.1M phosphate buffer solution at pH 6.85 and at room temperature and under anaerobic condition, and reductive dechlorination was observed that 1,2,3,4-TCDD can be stepwise and completely dechlorinated to DD mainly both via 1,2,4-TrCDD, 1,3-DCDD, 1-MCDD to DD and via 1,2,4-TrCDD, 2,3-DCDD, 2-MCDD to DD based on the species and distribution of products in the 7 separate systems. The rate phenomena of PCDD dechlorination kinetics for these give reaction systems can be described using preudo-first-order reaction model. The more chlorinated dioxin congeners are degraded at faster rate, that is, the tendency of rate constant follows TCDD>TrCDD>DCDD. In each separate systems, the observed half-lives of 1,2,3,4-TCDD, 1,2,3-TrCDD, 1,2,4-TrCDD, 1,2-DCDD, 1,3-DCDD, 1,4-DCDD and 2,3-DCDD are 0.56, 2.62, 5.71, 24.93,41.53,93.67 and 169.06 hours respectively.(3) Bimetallic Fe/Pd and Fe/Ag are very effective material at remediation of PCDD and the 1,2,3,4-TCDD can be stepwise and completely dechlorinated to DD in phosphate buffer solution at pH 6.85 by them. It is proposed that the pathway of 1,2,3,4-TCDD dechlorination can be mainly via 1,2,3-TrCDD, 1,2-DCDD, 1-MCDD to DD in Fe/Pd bimetallic system while via 1,2,4-TrCDD, 1,2-DCDD, 1-MCDD to DD in Fe/Ag according to the species and distribution of products in both the systems. In the process of 1,2,3,4-TCDD dechlorination, the final product, DD, was formed larger and more rapidly in Fe/Pd system than in Fe/Ag system in reaction time. The rate phenomena of PCDD dechlorination kinetics for both given reaction systems can be also described using preudo-first-order reaction model and the k(?),obs is 0.14hr-1 in Fe/Pd bimetallic system and 0.62-1 in Fe/Ag bimetallic system respectively. Compared the results of 1,2,3,4-TCDD dechlorination in bimetallic Fe/Pd and Fe/Ag systems with that in zero-valent zinc system, three differences were obvious: (i) the dechlorination pathways of 1,2,3,4-TCDD were different; (ii) 1,2-DCDD, the intermediate of 1,2,3,4-TCDD dechlorination, is the major DCDD in bimetallic Fe/Pd and Fe/Ag systems, while 2,3-DCDD and 1,3-DCDD are the major in the zinc system; and (iii) 1,2,3.4-TCDD can be completely dechlorinated to form DD more rapidly in the Fe/Pd and Fe/Ag systems than in the zinc system in shorter time.The results enrich the theory of environmental application of zero-valent iron, zinc, and bimetallic Fe/Pd, Fe/Ag for treatment of HCH and dioxins, and also lay a foundation for the application of the technology to remediation of polluted areas. |
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