The Reaction Mechanism And Kinetics Of Manganite And Dissolvable Sulfide (S^2-)/Fe²⁺

Manganese（hydr）oxides are widely distributed in soils and sediments.Due to the properties of large specific surface area and high adsorption and oxidation activity,they can markedly affect the migration,transformation and fate of pollutants in the environment.The oxidation of slag and tailings left by sulfide mineral mining causes acid rock drainage,thereby leading to soil acidification.Thus,the oxidation process has gained more and more attention.It is known that oxidation processes of sulfide and aicd producing are kinetically controlled by the reactivity of manganese oxides,and it also plays an important role in the anaerobic oxidation process of solid sulfide minaral with Fe²⁺.Therefore,knowledge of the reaction process and kinetics of Fe²⁺and S^2–with manganese oxides is essential to an understanding of the redox behavior of them in the environment.It’s useful to provide theoretical basis for interaction mechanism of pyrite and manganese oxides,and guidance for sulfide minerals pollution control.Current researches are focused on the redox mechanism and dynamics of high valance manganese oxides with sulfide minerals,and less concerned about the low valance ones,its influence factors is still an open question.In this work,manganite was prepared by hydrothermal method in lab,and the redox mechanism and dynamics of manganite with S^2-and Fe²⁺were studied,using X-ray diffraction（XRD）,scanning electron microscope（SEM）,ion chromatography（IC）and spectrophotography.The influence of the amount of manganite,temperature,pH and gas atomosphere on the oxidation process and kinetics was considered.The main topics and results of the research are summarized as follows:1.γ-Mn OOH was the oxidant for test,and the mechanism and kinetics of reaction of Mn OOH/Na₂S interface was determined as a function of minaral concentration of0.625,1.250,1.875 and 2.500 g/L,temperatures in 20,30 and 40 ^oC,pH at 4,6,8,10and 12,and gas atomosphere under anaerobic and aerobic conditons.Under anaerobic condition,when oxidant manganite was excessively used in experiments,the oxidation products of S^2-were identified as elemental sulfur and small amounts of sulfate,sulphite,thiosulfate,additionally,the S₈ was found to be accounted for 80%of the removed sulfide.At high pH,manganite was reduced to Mn（OH）₂,which was easily oxided to Mn₃O₄ andβ-Mn OOH in the air,it can be concluded that S₈ can also react with Mn OOH,leading to S₂O₃^2-.At low pH,Mn²⁺was formed and released.Polysulfides formation via S₈ with S^2-could not be discarded when the minaral was insufficient,and the polysulfide formation was favored at high pH values,being quite unstable at the low pH range.Under anaerobic atmosphere,the oxidation rate of S^2-in the initial period followed a pseudo first-order kinetic law,The rate increased with an increase in mineral concentration,which was indicative of a surface controlled process since the specific area was proportional to minaral quantity;The rate increased with temperature,and the energy of system activation Ea was calculated to be 46.71 k J/mol.When the pH was adjusted from 4.0 to 12.0,the rate increased from pH 4.0 to a maximum at a pH of about 7.0 and then decreased at higher pH.In contrast to anaerobic condition,the initial oxidation rate of S^2-was relatively slower under aerobic atmosphere,O₂ participated in the oxidation reaction of S^2-,together with Mn OOH,and the air was conductive to the formation of S₂O₃^2-by the further oxidation of elemental S.The initial oxidation rate of S^2-was in accordance with first order kinetics law,at the initial pH of 6-12,the initial oxidation rate of S^2-decreased with an increase in pH,and elemental S was prior to be generated at lower pH,otherwise,S₂O₃^2-was favored instead.2.Measurement of the oxidation mechanism and kinetics of Mn OOH with Fe²⁺in different conditions was conducted,considering Fe²⁺concentration of 5,10 and 15mmol/L,pH at 3,5 and 7,and environment atomosphere under anaerobic and aerobic conditons,respectivley.Under anaerobic atmosphere,the result indicated the feasibility of the reduction of Mn OOH to Mn²⁺,resulting in the oxidation of Fe（II）to amorphous Fe（OH）₃ catalyzed by Mn²⁺and Fe²⁺to formγ-Fe OOH andα-Fe OOH.The oxidation rate of Fe²⁺increased with an increase of Fe²⁺concentration.It also suggested that the reaction rate increased with pH resulted from the reactivity of Fe²⁺increasing with pH.Under aerobic atmosphere,the products were similar to that of anaerobic atmosphere,what’s more,the oxidation rate of Fe²⁺increased with an increase of Fe²⁺concentration and pH.Compared with the oxidation rate of anaerobic atmosphere,the rate was remarkably accelerated in the presence of O₂.