Study On The Mechanism Of Photochemical Formation And Transformation Of Birnessite And As(?) Removal Under Solar Irradiation

Manganese oxide minerals are widely distributed in soils,sediments,rock coatings and ocean nodules.Natural manganese oxide minerals affect the transportation and fate of pollutants in the environments,due to their small particle size,low point of zero charge values and high redox activity.Natural manganese oxide minerals also participate in the redox reactions of organic matters and inorganic minerals,and play an important role in the geochemical cycle of C,N,S and other elements.Hence,its formation and transformation in the supergene environment and application in the removal of heavy metals in polluted water have attracted much attention.Many researches are focused on the abiotic and microbially mediated Mn²⁺oxidation to manganese oxide minerals,and little attention has been paid to the effect of photochemical reactions induced by solar irradiation on the formation of manganese oxide minerals.In this work,the catalytic oxidation mechanisms of Mn²⁺to manganese oxide minerals by reactive oxygen species which generated from the reaction induced by solar irradiation,and the influences of coexisting cations and pH were reaearched in the simulating natural aqueous environments.The transformation mechanisms and effects of Mn(?)intermediates during the formation process of manganese oxide minerals were also clarified.The photochemical oxidation of Mn²⁺and the formation process of manganese oxide minerals on the surface of ferrihydrite were further studied.The research about the application of manganese oxides in the removal of As(?)in polluted water was carried out based on the understanding of formation and transformation process of manganese oxide minerals.The main experiments and results are listed as follows.1.Mn²⁺was oxidized to hexagonal birnessite by NO₃^-with solar irradiation under anoxic and oxic conditions.The effects of concentrations and species of coexisting cations(Na⁺,Mg²⁺and K⁺)on birnessite micromorphology and crystal structure were investigated.The roles of adsorbed Mn²⁺and pH(4.0-8.0)in the transformation of the photosynthetic birnessite were further studied.The results indicated that Mn²⁺was oxidized to birnessite by superoxide radicals(O₂^·-)generated from the photolysis of NO₃^-under UV irradiation.The particle size and thickness of birnessite decreased with increasing cation concentration.The different micromorphologies of birnessites could be ascribed to the position of cations in the interlayer.The birnessite showed a plate-like morphology in the presence of K⁺,while exhibited a rumpled sheet-like morphology when Na⁺or Mg²⁺was used.The adsorbed Mn²⁺and high pH(?6.0)facilitated the reduction of birnessite to low-valence manganese oxides including hausmannite,feitknechtite,and manganite.This study suggests that interlayer cations and Mn²⁺play essential roles in the photochemical formation and further transformation of birnessite in aqueous environments.2.Mn(?)is an important intermediate during the photochemical formation of birnessite.Mn²⁺was photochemically oxidized to manganese(?)oxide minerals in aqueous system containing NO₃^-(1.0 mmol L^-1)and Mn²⁺(20?mol L^-1)with solar irradiation.Mn(?)intermediates,fromed via the oxidation of Mn²⁺by reactive oxygen species generated from the photolysis of NO₃^-,were then further oxidation and disproportionation to manganese(?)oxide minerals,with the latter being the main pathway.The photochemical oxidation rate of Mn²⁺decreased with increasing initial Mn²⁺concentration due to the lower disproportionation rate of Mn(?)intermediates.The increase in NO₃^-concentration,pH and temperature promoted Mn²⁺photochemical oxidation.Pyrophosphate,a kind of Mn(II,?)complexing ligand,affected the formation of manganese(?)oxide minerals by promoting the formation and reducing the reactivity of Mn(?)intermediates.The photochemical formation of manganese(?)oxide minerals was promted in the presence of low concentration pyrophosphate(5 and 10?mol L^-1),but inhibited by the addition of high concentration pyrophosphate(>20?mol L^-1).This study reveals the important role of Mn(?)intermediates in the formation of natural Mn oxide minerals.3.The oxidation of dissolved Mn²⁺to manganese oxide minerals was promoted by ferrihydrite with irradiation.The photochemical oxidation mechanisms of Mn²⁺were studied in aqueous system of Mn²⁺and ferrihydrite with irradiation,and the effects of initial Mn²⁺concentration and pH(6.0-8.0)on the species of manganese oxide minerals were investigated.The results indicated that Mn²⁺was oxidized to feitknechtite and manganite by O₂ under dark conditions.Solar and UV irradiation not only accelerated the oxidation of Mn²⁺,but also promoted the formation of birnessite.O₂^·-generated from the photochemical reaction of ferrihydrite was mainly responsible for the oxidation of Mn²⁺.The initial concentration of Mn²⁺had no significant effects on the proportion of feitknechtite,manganite and birnessite.The formation of birnessite was inhibited by increasing pH.The proportion of birnessite decreased from 78.9%to 18.2%with the increase of pH from 6.0 to 8.0.Mn²⁺,formed stable complexes with surface groups of ferrihydrite,was oxidized to Mn(?)intermediates which was stabilized and transformed to feitknechtite and manganite.Mn²⁺in solution was oxidized to Mn(?)intermediates which were further oxidized and disproportionated to birnessite.The changes of initial Mn²⁺concentration and pH had no obvious influence on the transformation of ferrihydrite.This study clarifies the formation mechanism of manganese oxide minerals on the iron oxides surface,which is expected to provide theoretical evidences for the research on the formation of natural complex iron/manganese oxide minerals.4.The removal of As(?)was accelerated when the nonferrous metals smelting plant wastewater containing Mn²⁺(14.23?mol L^-1),NO₃^-(209.84?mol L^-1)and As(?)(16.38?mol L^-1)was exposed to solar irradiation,and the mechanisms were further clarified.The photochemical oxidation of Mn²⁺to birnessite efficiently promoted the removal of As(?)in wastewater samples,and the ratio of removed As(?)and Mn²⁺was up to 1:3.51.The removal mechanisms of As(?)were researched by using simulated wastewater,and the effects of Mn²⁺,Ca²⁺and As(?)concentrations were studied.The results suggested that the adsorption of As(?)on the surface of birnessite and formation of Mn As O₄precipitates were responsible for As(?)removal.When Mn²⁺concentration was low(100?mol L^-1),the adsorption of As(?)on the surface of birnessite was the main pathyway for As(?)removal,while As(?)was removed mainly through the formation of Mn As O₄precipitates when Mn²⁺concentration was high(200?mol L^-1).The addition of Ca²⁺promoted the removal of As(?)in low Mn²⁺concentration solution.This study develops a method for the removal of As(?)in wastwater based on the photochemical oxidation of Mn²⁺,and provides basic data for understanding the chemical behavior of Mn²⁺,NO₃^-and As(?)with irradiation in environments.