Coupled Sorption And Oxidation Of Dissolved Organic Matter On Manganese Oxides: Reaction Mechanisms

Dissolved organic matter（DOM）,an active substance widely distributed in the environment,involves numerous biogeochemical processes such as carbon cycling,and transport and transformation of heavy metals.Manganese（Mn）oxides are also widely distributed in the environment,and the coupled reactions of sorption and oxidation may occur when DOM sorbs onto Mn oxides.It is expected that the interactions between Mn oxides and DOM will change the molecular composition of DOM,which will further affect the reactions between DOM and heavy metals.Therefore,it is of great environmental significance to elucidate the mechanisms and impact of the coupled sorption and oxidation of DOM on Mn oxides,which will help to accurately predict the carbon cycling and the environmental behaviors of heavy metals.In this study,birnessite,a typical Mn oxide,was selected to study DOM-Mn oxide interactions.By employing the spherical aberration corrected scanning transmission electron microscopy（Cs-STEM）,Fourier transform ion cyclotron resonance mass spectrometry（FT-ICR-MS）,and other relevant analytical techniques,the interactions between Mn oxides and DOM and the impact of iron（Fe）oxides on the interactions were investigated at the sub-nano scale and molecular level.Moreover,the binding characteristics of Cu²⁺to DOM before and after reactions with Mn oxides and Fe oxides were further investigated.The major findings of this study are as follows,（1）Through Cs-STEM and FT-ICR-MS analysis,the microscopic distribution and molecular transformation of DOM on birnessite were investigated at sub-nano scale and molecular level,respectively.Results of Cs-STEM experiments showed that birnessite exhibited a"nanoflower"morphology with a size of tens to more than one hundred nanometers（diameter direction）,in which its surfaces,inter-flakes,and pores between aggregates can act as the places for DOM sorption and oxidation.The oxidation state of organic carbon located in the inter-flakes was higher than that on the surface due to the abundant reactive sites within birnessite minerals,which highlighted the importance of mineral microstructure in controlling the reactivity of organic matter.According to FT-ICR-MS analysis,the oxygen content and molecular weight of DOM increased after reactions with Mn oxides,which may be due to the aromatic ring-opening and polymerization of phenolic compounds,respectively.By comparing the molecular weights of oxidized and newly formed molecules,potential products corresponding to the aromatic ring-opening and polymerization reactions can be identified.The results highlighted the importance of phenolic compounds in the molecular transformation of DOM on Mn oxides.Above findings offered direct visual evidences for the microscopic distribution of DOM on birnessite and provided molecular evidences for the potential molecular transformation pathways of DOM.（2）Based on above results on DOM-birnessite interactions,we further investigated the influence of ferrihydrite on the coupled reactions of sorption and oxidation of DOM on birnessite.Sorption experiments showed that ferrihydrite was acted as a competitor of birnessite to sorb DOM,and its DOM sorption capacity was higher than that of birnessite.Thus,the existence of ferrihydrite is beneficial to the sequestration of DOM.However,the sorption of DOM by ferrihydrite inhibited the redox reaction between birnessite and DOM,which resulted in the weakened reductive dissolution of birnessite and the inhibition of oxygen addition reactions of DOM（including aromatic ring-opening and carboxylation of substituted groups on aromatic rings）.Interestingly,the presence of ferrihydrite promoted the polymerization of phenolic compounds,especially in the mixed systems of Fe and Mn oxides with high content of ferrihydrite.For the microscopic distribution of organic carbon,Cs-STEM analysis demonstrated that the region with high birnessite content had a higher oxidation state of organic carbon.The relative abundance of carboxylic and phenolic carbon had a positive correlation with Mn content,while the relative abundance of aromatic carbon showed a negative correlation with the Mn content.However,the ferrihydrite content had no significant correlation with the oxidation state of organic carbon,and the relative abundance of different carbon species had a poor correlation with the ferrihydrite content.Above results can be explained by the different reactivities of Fe and Mn oxides since birnessite can both sorb and oxidize DOM while ferrihydrite mainly sorbed DOM.Our findings elucidated the influence of ferrihydrite on the coupled sorption-oxidation reactions of DOM on birnessite,and emphasized the difference in the microscopic distribution of organic carbon on birnessite and ferrihydrite.（3）The impact of the sorption and oxidation of DOM on the binding characteristic of DOM with Cu²⁺was investigated using fluorescence spectroscopy and infrared spectroscopy combined with two-dimensional correlation spectroscopy.The results showed that birnessite preferentially sorbed and oxidized compounds with high Cu²⁺binding ability,including compounds rich in carboxylic and phenolic groups in humic-like fractions,which resulted in a decrease in Cu²⁺binding ability of DOM in the supernatant samples and made the sequence of carboxylic group binding with Cu²⁺become later.Owing to the higher sorption capacity than that of birnessite under our experimental conditions,ferrihydrite sorbed not only reactive compounds in humic-like fractions,but also compounds with high Cu²⁺binding ability in fulic-like fractions,which further decreased the Cu²⁺binding ability of DOM in the supernatant samples.As a result,the sequence of phenolic groups,aryl groups,C-O of polysaccharides and aliphatic C-H binding with Cu²⁺became much less distinguishable.Birnessite and ferrihydrite reacted with DOM together can further decrease the Cu²⁺binding ability of DOM in the supernatant samples,and the degree of decrease depended on the concentration of both birnessite and ferrihydrite.In this study,sorption and oxidation of DOM did not alter the order of the sensitivity of Cu²⁺binding to different fluorescent components or functional groups,but,the sequence of Cu²⁺binding to different fluorescent components or functional groups was affected by their concentrations.Therefore,sorption and oxidation of DOM changed the concentrations of fluorescent components and functional groups in DOM,which further altered the sequence of Cu²⁺binding to different fluorescent components or different functional groups.Above findings improved our understanding of the reactivity of natural organic matter with heavy metals under the impacts of minerals,which would be helpful for developing accurate reaction models of natural organic matter with heavy metals.