Adsorption And Transformation Of Chlorophenols By Goethite And Its Mechanism

Iron oxides including goethite are common components in soil and sediments which have high specific surface area and redox potential and exceeding adsorptive and oxidative power for a range of organic compounds. The processes of adsorption and oxidative transformation on the surface of iron oxides play an important role in conversion and transformation of organic contaminants for changing their exhibiting forms and speciations even their mobility, toxicity and ecological benefits which significantly influence the whole environmental geochemistry. Chlorophenols are widely used in agriculture and industry as well as common byproducts yield in some production processes. Because of their high toxicity, bioaccumulativity and relatively recalcitrant, chlorophenols has become a serious environmental contaminants which have been designated as major priority pollutants. Thus, this article outlined surface reactivity of iron oxides and their adsorption and oxidation of organic pollutions, introduced the reantion mechanism of organic compounds by iron oxides and analyzed their influences on conversion and transformation of organic contaminants. On this basis, we mainly studied the adsorption and oxidative transformation of the three chlorophenols and its reaction mechanism of oxidative conversion. The main research results of this study included the following parts:(1) Adsorption of2,4,6-TCP,2,4-DCP and4-CP by goethite. The result showed a relative amount of adsorption of the three CPs to goethite and the adsorption increased rapidly within the initial4h and then leveled off. The adsorption of the three CPs was in the order4-CP>2,4-DCP>2,4,6-TCP. It was also observed that the adsorption capacity of a-FeOOH was lower than syn-FeOOH. Moreover, solution pH significantly influenced the adsorption of CPs to goethite. The adsorption of2,4,6-TCP achieved its maximum at pH4.5and decreased when pH was higher or lower than this range and the adsorption data more gradual decrease at the side of the pH above4.5while more slightly decrease at the other side. While the greatest adsorption amount of4-CP and2,4-DCP achieved at pH6.0. The percentage of adsorption increased rapidly with solution pH up to pH6.0and then decreased significantly on further increasing pH suggesting that neither too acidic nor alkaline conditions.In concluding, the optimum pH for the adsorption of2,4,6-TCP,2,4-DCP and4-CP are found to be4.0,6.0, and6.0, respectively.(2) Oxidative transformation of2,4,6-TCP,2,4-DCP and4-CP by goethite. We investigated both goethite samples can effectively induce the transformation of the three CPs. The oxidized amount of each chlorophenol was according to the order2,4,6-TCP>2,4-DCP>4-CP and the results of the oxidation studies also showed that the oxidation capacity was in the order a-FeOOH> syn-FeOOH for all of the chlorophenols investigated which were opposite to adsorption reaction. The solution pH also significantly affected the oxidative transformation rate of the three CPs by goethite. The maximum oxidative activity of goethite were observed at pH4.5,6.0, and4.5for2,4,6-TCP,2,4-DCP and4-CP respectively.(3) Transformation products and pathways of the three CPs. Details of the product characterization by chromatography-mass spectrometry were analysized in present study. In summary, products and intermediates from CPs oxidation including2,4,6-TCP,2,4-DCP and4-CP were detected. The result showed that chlorinated dimers were the major products for each chlorophenol among the identified oxidation products which can be matched to a series of OH-PCDEs. On the basis of identified products, a reaction scheme was proposed for the goethite-mediated transformation of CPs.The results of the present study suggested that Fe oxides especilly goethite in aquatic soil and sediments may well play an important role in the natural attenuation of CPs.