Catalytic Oxidation Characteristics Of Birnessite With Various Structures For The Humification Of Hydroquinone

It was firstly studied by Huang et al. that polyphenols in soil can be catalyzed and oxidated by Mn oxides to form the humic substances. In general, Mn oxides used to study the formation of humic substances are limited to birnessite, the catalytic oxidation ability of which was mainly investigated through analyzing the effect of environmental factors, such as light, temperature, pH and O2 on the speed and degree of the humification. But the influence of the types of Mn oxides, the structure of crystal, the surface chemistry characteristics and the different substructures of the same mineral (average manganese oxidation state, vacant site, content of H+, Mn4+ and Mn3+) on the humifiaction have never been lucubrated. Therefore, the impact of Mn oxides on the approach of soil humification, dynmic process and the degree of humification can be systemically studied from mineral structures and substructures, which has significant meaning on clarifying the catalysis and mechanism of Mn oxides in the formation of humus, and also play a guidance role in revealing the environmental property and function of Mn oxides in global carbon circulation and environment protection.In this paper, acidic and alkaline birnessites with different AOS were synthesized. Both the acid and alkal birnessite with the lowest AOS were treated with Na4P2O7 of different concentration, which can produce birnessites that contain much Mn4+ and less Mn3+, and thereby birnessites with higher AOS. The influence of mineral structures and substructures of these synthetical and treated birnessites on the oxidation polymerization of hydroquinone, which were used as precusor of soil humification, were studied in detail. The results in this study were listed as follows:1. The type and substructures(average Mn oxidation state, vacant site, content of H+, Mn4+ and Mn3+) of the birnessites have important influence on the humification speed and degree of hydroquinone. The type of birnessites have much more influence than the substructures of birnessites on the humification speed and degree of hydroquinone. The acidic birnessite contains more H+ than alkaline birnessite, leading to a higher oxidation capacity of acid birnessite than alkaline birnessite. When the reaction reached equilibrium, the products of acid birnessite reacted with hydroquinone have a higher degree of humification. Therefore, the content of H+ have much more impact on the humification degree of hydroquinone than the content of Mn4+ and Mn3+. When birnessite with the same mineral type have higher AOS, they will abtain more electron, and thus possess stronger oxidation ability. The change trend of oxidation capacity of birnessites treated with Na4P2O7 of different concentration is consistent with the same type of birnessites with different AOS but not treated with Na4P2O7.2. During the process of humification of hydroquinone, alkaline birnessites can release much more Mn2+ from the mineral structure than acidic birnessites since alkaline birnessites have lower ratio of Mn4+/Mn3+ than acid birnessites. Because Mn4+ and Mn3+ have different capacity of receiveing electron, for birnessites with the same mineral type, the higher the AOS is, the less Mn2+ will be released. When a part of Mn3+ extracted by Na4P2O7, the content of vacant site and the ratio of Mn4+/Mn3+ will be enhanced. This will lead to much more electron obtained by birnessites, and higher oxidative ability provided by birnessites. Compared with the same type of birnessite not treated with Na4P2O7,,the content of Mn2+ released from the treated mineral is higher in the initial stage of reaction, and is lower when the reaction is completed.3. The FTIR spectra analysis of the separated solid substance and the liquid phase extractant during the reaction process of hydroquinone illuminate that humic-like substance were produced in the reaction system by oxidation polymerization of hydroquinone. The degree of polymerization enhanced with the increase of the reaction time. Acidic and alkaline birnessites do not clearly affect the functional group of the humification products of hydroquinone.4. The results of ultrafiltration analysis indicate that molecular weight of 75.7% of substance extracted from the liquid phase exceed 30 thousands after 3 months of reaction between hydroquinone and acidic birnessite with AOS of 4.0. The FTIR spectra of the liquid phase extractant is similar with the standard humic acid, which makes sure that the oxidation polymerization product of hydroquinone is humic acid-like substance.5. Intermediate products benzoquinone and formate were detected by SPME-GC and ion chromatography during the humification of hydroquinone. 6. The XRD analysis of the solid substance separated directly from the solution phase indicate that acid birnessites with AOS of 4.0 can be translated into rhodochrosite during the humidication process of hydroquinone, which is produced by the reaction of Mn2+ redoxed by Mn4+ and CO2 decomposed by hydroquinone during the humification process of hydroquinone.