Influence Of Dissolved O₂ On Phenol Oxidation By δ-MnO₂

Manganese oxides are among the most active minerals in natural environment and play crucial roles in abiotic transformation of organic compounds.Although redox reactions of organics on manganese oxides have been extensively studied,the role of dissolved O₂ in these processes has been largely overlooked.Effects of dissolved O₂ on structural transformation and redox activity of manganese oxides during organics oxidation are still unclear.This study chose phenol as model organic compound to investigate the influence of molecular oxygen on phenol oxidation byδ-MnO₂ which is a typical manganese oxide.First,kinetics of phenol oxidation byδ-MnO₂ was investigated.The results suggested phenol degradation byδ-MnO₂ was rapid and followed pseudo-first-order kinetics at the early stage（<1 h）,but it became much slower and deviated from pseudo-first-order kinetics at later stage.The reaction rate decreased as pH increased.Besides,dissolved O₂ inhibited phenol oxidation at pH range 5.5⁸.5.By fitting the data of the reaction at the early stage,the reaction orders were determined to be 1.47（oxic）and1.33（anoxic）with respect to initial concentration of phenol（26.6⁷9.8μM）,1.37（oxic）and 1.32（anoxic）with respect toδ-MnO₂ dosage（472.7¹890.9μM）,and 0.18（oxic）and 0.27（anoxic）with respect to pH（5.5⁸.5）,respectively.The initial rate constants of phenol degradation under oxic and anoxic conditions were determined to be（4.64±0.23）×10^-6μM^-2.022.02 h^-1 and（1.53±0.18）×10^-5μM^-1.921.92 h^-1,respectively.In addition,a retarded rate model was used to describe the total kinetics of phenol degradation.The results showed more serious retarding effect on phenol degradation at pH 7.0 and 8.5.Conversely,the reaction rate deviated from pseudo-first-order kinetics more significantly under anoxic condition at pH 5.5,which is likely because the phenol oxidation at pH 5.5 was too fast that the retardation effect may be mainly determined by the consumption of reaction sites by phenol rather than by oxygen.To investigate the mechanism of the inhibitory effect of dissolved O₂ onδ-MnO₂redox activity,XPS,XRD,SEM,FTIR were used to determine the changes of valence,crystal structure,surface micro-morphology and surface functional group ofδ-MnO₂.The results showed more obvious Mn（III）accumulation on surface and more pronounced structure changes with the tendency to transform to feitknechtite,manganite and hausmannite,as well as higher degree of agglomeration and smaller particle surface area under oxic and neutral or alkaline conditions.Furthermore,according to analyses of phenol degradation products,the main products under the experimental conditions were p-benzoquinone,o-benzoquinone,hydroquinone and4,4’-biphenol,and there was no significant difference in the distribution of products under oxic and anoxic conditions.Based on the valence changes of manganese oxides and the degradation products of phenol,the net electron transfer from phenol toδ-MnO₂after reaction was estimated.It was concluded that dissolved O₂ promoted the oxidation of Mn（II）to Mn（III）on manganese oxides surface,resulting in higher degree of structure alteration,particle aggregation and surface passivation,hence slower phenol degradation rate.This study revealed the effect of dissolved O₂ on the reactivity ofδ-MnO₂ during oxidation of organic compounds,which is of great significance for understanding organic contaminants transformation in Mn-rich soil and water environment,and provided theoretical guidance for application of manganese oxides in organic pollution control as well.