Study On Quantitative Relationship Between Manganese Average Oxidation State And Redox Activity Of Birnessite

The application of manganese oxide materials in environmental restoration has received extensive attention in recent years.The redox potential?Eh?of manganese oxide is different under different environmental conditions,which has a great relationship with the average oxidation degree of manganese?Mn AOS?.By studying the quantitative relationship between Mn AOS/Eh and the reactivity,the physicochemical properties and environmental behavior of manganese oxide can be better understood.Therefore,the birnessite material was synthesized by chemical reduction method and characterized.The birnessite with different Mn AOS was obtained by adjusting the redox properties of the solution with Mn²⁺.Hydroquinone was used as a chemical probe to characterize the Eh of birnessite,and the quantitative relationship between Mn AOS and Eh was established.The quantitative relationship between Mn AOS and the reactivity of birnessite was established by thermodynamic and kinetic analysis of bisphenol A?BPA?.The effects of Eh and Mn AOS on the reaction between birnessite and BPA were also investigated.The main conclusions are as follows:?1?The birnessite is a nanosphere structure composed of nanosheets,each of which has a size of about 200-300 nm.Its specific surface area is 45.5 m²/g.The birnessite having different Mn AOS is obtained by adjusting the redox properties of the solution with Mn²⁺.When the pH was 5.5 in the reaction,Mn AOS became 3.98,3.87,3.75,3.64 and 3.53,respectively,as the initial Mn²⁺concentration increased.?2?The Mn AOS of the birnessite is directly proportional to its Eh.Using hydroquinone as a chemical probe,according to the Nernst equation,the higher the Mn AOS of the birnessite,the higher the redox potential,and the same trend as the electrochemical method.The oxidation of hydroquinone by birnessite conforms to the pseudo second-order kinetics,and the oxidation reaction rate decreases with the decrease of Mn AOS.?3?The Mn AOS of birnessite is directly proportional to its redox activity.The higher initial birnessite concentration is favorable for the reaction.The removal rate of BPA decreases with the increase of pH.The coexisting metal ions inhibit the oxidative degradation of BPA by birnessite,and the inhibition effect of Mn²⁺is most obvious.Because the concentration of birnessite is positively correlated with Eh.The lower the pH,the higher the Eh of the birnessite.The addition of Mn²⁺reduces the Eh of the birnessite.In addition,the apparent reaction rate constant?k?of the initial reaction stages increases with the increase of Mn AOS,because the redox potential of birnessite increases with the increase of Mn AOS.These findings revealed the structural change of birnessite has a significant effect on its reactivity,which is important for the development of high-efficiency oxidant materials in wastewater treatment,and has broad application prospect in actual water treatment and soil remediation.