Transformation Of Phenolic Compounds By Manganese Oxide-coated Sands And Its Mechanism

Manganese oxides are ubiquitous in soils and sediments and exhibit versatility in regulating mobility, bioavailability, and fate of environmental contaminants via the processes of adsorption, catalytic transformation, abiotic oxidation, and phototransformation. Oxidative transformation of organic contaminants by manganese oxides was commonly investigated with pure MnO2suspension, which deviates from the fact that natural manganese oxides are seldom present as a pure form in the natural environment. In this study, we prepared manganese oxide-coated sands (MOCS) and chose using bisphenol A (BPA) and triclosan as the model compound. And we respectively studied the ability and the mechanism of oxidizing bisphenol A in water phase and oxidizing triclosan in drying condition, the main research results of this study included the following three parts:1) Surface characteristics showed that sample detection by X-ray powder diffraction (XRD) exhibit no No manganese oxide peaks were observed in the XRD pattern, confirming that the formed manganese oxide was amorphous. The SEM image showed an apparent difference in the surface morphology between MOCS and the original sand. A rougher surface for MOCS suggests the formation of manganese oxide coating. The amount of manganese oxide coating on the MOCS was estimated to be0.21mg Mn/g of sand through nitric acid dissolution and atomic absorption spectroscopy analysis. BPA was transformed by MOCS and the reaction followed an exponential decay model. in the process of transformation of bisphenol A by MOCS in oxidation, generating Mn2+, showed that the process was redox reaction, bisphenol A which adsorbed on the surface of MOCS accounted for only A fraction of the total, showed that the adsorption rate was equal to oxidation rate of bisphenol A basic equal The reaction was pH dependent and followed the order of pH4.5> pH5.5> pH6.5> pH7.5> pH8.6> pH9.6, indicated that acidic conditions facilitated BPA transformation while basic conditions disfavored the reaction. Coexisting metal ions exhibited inhibitory effects and followed the order of Fe3+> Zn2+> Cu2+> Ca2+> Mg2+> Na+. Transformation of BPA by MOCS was much slower than that by pure MnO2suspension. However, similar transformation products were identified in both studies, suggesting the same reaction pathways. Besides, the process generated dienone, monocyclic aromatic compounds and polycyclic aromatic compounds, etc.2) In drying condition, MOCS showed effective capacity of oxidating toward triclosan. After oxidized and decomposed, triclosan generated products of2,4-dichlorophenol,3-chlorophenol. After reaction between them for4days, the triclosan concentration was reduced to75.6%of the original in the experimental condition, and after32days, the persent of concentration reduced became32.5%. To sum up, the triclosan concentration gradually decreased with the increasing reaction time, and the rate of reducing decreased; the concentration of2,4-dichlorophenol increased with reaction time in the early stage and then decreased, however, the concentration of3-chlorophenol contined to increase with reaction time.This work suggests that the reactivity of MnO2in the environment might be overestimated if extrapolating the result from the pure MnO2suspension because natural MnO2is mainly present as coating on the surface of soils and sediments. In our research on the migration and transformation of envirmental organic pollutants, only consider the process of occur in the aquatic would be inadequate, we should consider the other interface reaction, such as interface of solid, because the oxidant remain shows effective transformation capacity toward envirmental organic compounds in drying condition.