The Intereaction Mechanism And Kinetics Of Todorokite And Sulfide Minerals

Sulfide minerals,including dissolvable sulfide and solid sulfide minerals,are abundant in content and widely distributed in the supergene environments.The oxidation of sulfide minerals often leads to the soil acidification and the release of heavy metal ions in the open environment,threatening the environment and human health.As one of the important component of soils and sediments,manganese oxides have some excellent physicochemical properties,such as adsorption,oxidation and catalysis,and affect the migration,transformation and fate of sulfide minerals in the environment.The intereaction meachanism and kinetics of todorokite and sulfide have been studied systematacially,which help to clarify the oxidation process of sulfide minerals in the environment and could offer the basic date for treatment of sulfide minerals pollution.In this work,the electrochemical behaviors of S^2-,S2O3^2-and SO₃^2-on platinum electrode have been studied by Electrochemical analysis methods,such as cyclic voltammetry?CV?and constant potential electrolysis?chronoamperometry?,and cyclic voltammetry was used for the in situ measurements of intermediates in aqueous systems of dissolvable sulfide and manganese oxides.In addition,XRD,FTIR,AAS,SEM and TEM were used to characterize the reaction products of todorokite and FeS,and the influence of the amount of todorokite,pH and Fe²⁺ on the redox process was considered.The electrochemical system of FeS and manganese oxides was established,and the output power,current,potential and the influencing factors were analyzed.It was preliminary discussed that the effects of air,oxygen,pH and reactant concentration on the collaborative symbiosis of iron and manganese?hyro?oxide have been studied,with the coexitence of Fe²⁺ and Mn²⁺ in the reaction system.The main results are as follows:1 A new electrochemical method was built for the in situ analysis of the concentration of S2",S2O3^2-and SO₃^2-.The different kinds dissolvable sulfide exhibits different electrochemical oxidation behavior on platinum electrode at pH 12.S^2-was first oxidized to elemental S at 0 V?vs SCE?,and then was oxidized to SO4^2-at 0.8 V,respectively.The peak current for the oxidation of S^2-to elemental S has the positive linear correlation with the concentration of S^2-and the square root of the scan rate.Na₂S2O3 and Na₂SO₃ are directly electrochemically oxidized to SO4^2-at 0.9 V and 0.8 V,and oxidation peak current has a positive linear correlation with the square root of the scan rate.Cyclic voltammetry was conducted in the redox processes of dissolvable sulfide and manganese oxides including birnessite,todorokite and manganite as well as the intermediates,and the results show that S^2-was oxidized to SO4^2-and S2O3^2-.The decrease rates current of S^2-to elemental S decreased,and the oxidation current peaks of S2O3^2-to SO4^2-were observed and increased as reaction proceeded,suggesting the formation of S2O3^2-intermediate in the redox processes.The difference in the decrease rates for the oxidation peak current of S^2-indicated that the oxidation activity followed this trend:birnessite>todorokite>manganite.2 The oxidation behaviors and kinetics of FeS solution with different manganese oxides were investigated using electrochemical method at pH ^2-6.5 in aqueous system.Elemental S and a small amount of SO4^2-were the main oxidation products of FeS,und released Fe.Wwas released in the presence of air.When pH were adjusted to 2.0,solid iron hydroxide was not formed;lepidocrocite was formed at pH 3.5 and 5.0.When pH was 6.5,FeS was exposed to air and kept stable after 30 d.When pHs were adjusted as 2,3.5,5 and 6.5,the release ratio of Fe²⁺ were 53.5%,21.8%,1.3%and 0.17%,respectively.The results showed that the dissolution of FeS increased with a decreased in pH,which accelerated the oxidation of FeS.In the reaction system of FeS and todorokite,the main oxidation products were elemental S and the SO4^2-,and the content elemental S is less than 10%.With the increase of the amount of todorokite from 0.1 to 0.2 and 0.4 g in aqueous system,the oxidation rate increased first and then decreased.When pH decreased,the oxidation rate of FeS was accelerated.The effect of initial pH on the reaction rate was further investigated.When the initial pH was 2.0,pH gradually increased to 5.5 after reaction for 10 d,the reaction rate was significantly decreased,and a small amount of goethite was formed.When the initia pH was adjusted to 3.5,5 and 6.5,pH was changed to about 7.5 in the initial stage and kept stable in the later one.After reaction for 30 d,FeS and todorokite were not completely consumed.In the reaction system of FeS and todorokite,the addition of Fe²⁺ contributed to the oxidation of FeS and the reaction rate increased with the increase of the concentration of Fe²⁺.The discharge process and reaction products were investigated using the electrochemical method in FeS and manganese oxide aqueous system.When FeS was oxidized in electrochemical system,a small amount of sulfur and goethite was formed on the surface.In addition,a small amount of Fe²⁺ and Mn²⁺ was dissolved to reaction system.The oxidation activity follows the order of birnessite>todorokite>manganite.As the reaction proceeds,the output power was increased first and then decreased.In the range of pH from 2 to 6.5,low pH facilitated the rapid reaction,and promoted the dissolutioI of Fe²⁺ and Mn²⁺,which further improved the reactivity of manganese oxide.3 Mixing Fe²⁺ and Mn²⁺ in aqueous system promoted the formation of iron/manganese hydroxide in the presence of air or oxygen.In single Mn²⁺ solution,when pHs were adjusted to 5-9,haussmanite and manganite were only formed in air and the mixture of buserite were generated in oxygen atmosphere at pH 9.In only Fe²⁺ aqueous solution,goethite was formed at pH 5 and a mixture of lepidocrocite and goethite was formed at pH 7 and 9 in the presence of air or oxygen.In the mixed system of Fe²⁺and Mn²⁺,the oxidation product was goethite at pH 5.When pH was controlled at 7 and 9,lepidocrocite was formed as the main product with a small amount of goethite.The results showed that Fe²⁺ was more likely to be oxidized to produce iron hydroxide,and the adsorption of Mn²⁺ on the surface can promote the formation of iron oxides in higher aqueous system.It should be pointed out that the adsorption of Mn²⁺reduced the crystallinity of the newly formed iron hydroxide and inhibited the transformation to high crystallinity lepidocrocite or goethite.