Formation-Transformation Of Schwertmannite And Its Interfacial Reaction Behavior And Mechanism With Heavy Metals

Schwertmannite（Sch）is the ubiquitous Fe（III）-oxyhydroxysulfate mineral in acid mine drainage（AMD）,with weak crystallinity,large specific surface area,high hydroxyl site density and high sulfate(SO₄^2-)content.During the schwertmannite formation,lots of heavy metals（HMs）would be enriched on/in its surface/structure through adsorption,co-precipitation or isomorphic substitution.However,metastable schwertmannite transforms into crystalline iron oxides easily,resulting in the release and redistribution of HMs.Therefore,the formation and transformation of schwertmannite both control and influence the speciation,mobility,and environmental fate of associated HMs.However,the schwertmannite formation conditions and transformation laws,as well as the reaction behavior and mechanism of heavy metals on schwertmannite surface are still unclear.Thus,after clarifying the Fe mineralogy and chemical fractions of HMs in the typical AMD sediments,schwertmannite was chosen as the major research subject in our works.The formation-transformation process and properties of schwertmannite via direct Fe³⁺hydrolysis,the transformation characteristics and Cu speciation of Cu-bearing schwertmannite,and the adsorption-oxidation behavior and mechanisms of As（III）on Cr（VI）-incorporated schwertmannite were systematically explored by combining solution chemical experiments,（in-situ）quick scanning X-ray absorption spectroscopy（Q-XAS）,powder X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FTIR）,X-ray photoelectron spectroscopy（XPS）,scanning/transmission electron microscopy（SEM/TEM）,and so on.These studies benefit a better understanding of mineralogical properties of schwertmannite and provide important evidences for predicting and evaluating the behaviors and risk of HMs in related environments.The main results are:1.The impacts of aging p H on Fe mineralogy and chemical fractions of HMs in AMD sediments were explored,combined with natural sediment sample analysis and AMD solution aging experiments.Jarosite and goethite are the major Fe minerals in the sediment sample S1with solution p H 2.68,while schwertmannite and minor ferrihydrite are dominant in sample S2with solution p H 6.78.With increasing the AMD solution p H,the total contents of HMs（expect for As）and the reducible fraction of HMs（expect for Pb）in the sediments both increase obviously.In the sediments,Mn,Zn,Ni and Cd are mainly associated with Fe minerals,while Pb possibly exists as Pb-bearing minerals（e.g.,Pb SO₄）.The aging experiments of AMD solution of sample S1 at different p Hs showed that schwertmannite is dominant initially,with a higher crystallinity being at a lower p H.With increasing aging time,the pre-formed schwertmannite transforms to goethite and jarosite at p H≤3,while it keeps stable at p H 5 and7 due to the accumulation of more HMs.2.The formation-transformation process and properties of schwertmannite under various geochemical conditions were systematically revealed via direct Fe³⁺hydrolysis pathway,including Fe³⁺hydrolysis-dialysis and Fe³⁺hydrolysis by adding OH^-.Pure schwertmannite is obtained through Fe³⁺hydrolysis at 25-60°C and dialysis for 1-15 days,while minor goethite appears at higher hydrolysis temperatures.A shorter dialysis time and the presence of K⁺or NH₄⁺during Fe³⁺hydrolysis both slightly increase schwertmannite crystallinity.During Fe³⁺hydrolysis by adding OH^-,sulfate-bearing ferrihydrite initially forms and then transforms to schwertmannite quickly.In contrast,the pre-formed ferrihydrite does not transform into schwertmannite under the same solution conditions,despite SO₄^2-adsorption.With decreasing Fe³⁺hydrolysis rate,schwertmannite crystallinity increases clearly and its morphology of“network”structure becomes larger.As to schwertmannite transformation,high temperature,high p H,and the presence of Fe²⁺favor its transformation to goethite,while a low Fe³⁺hydrolysis rate and a high Cl^-concentration hinder the transformation.In contrast,the presence of K⁺or high NH₄⁺concentration induces schwertmannite transformation into jarosite.3.The transformation regularity of Cu-bearing schwertmannite and the corresponding chemical fate of Cu under acidic and neutral conditions was in-depth discussed.The p H value affects the transformation rate and products of Cu-bearing schwertmannite.At p H 3,schwertmannite transforms into goethite quickly at 60 ^oC,and the presence of Cu increases goethite crystallinity,while its transformation rate was slow at room temperature.At p H 7,60^oC,the transformation rate of Cu-bearing schwertmannite declines obviously,with a slower rate at higher Cu content.A mixture of goethite and hematite is formed finally in the absence of Cu,while the presence of Cu induces the transformation into only hematite.In addition,the chemical speciation of Cu is closely related to p H and mineral types.Cu is almost in solution at p H 3,while the adsorbed fraction on schwertmannite is dominant at p H 7 initially.During the transformation,the dissolved Cu enters to the grain defects and pores of formed goethite at p H 3,thus leading to a significant increasing precipitated fraction of Cu.At p H 7,the adsorbed fraction of Cu on schwertmannite decreases with aging time,while the adsorbed fraction on hematite and precipitated fraction both increase accordingly.During which,part of Cu can enter the hematite lattice by isomorphous substitution.4.The behavior and mechanisms of As（III）adsorption-oxidation on Cr（VI）-incorporated schwertmannite were elucidated,combined with macroscopic solution chemistry experiments and in-situ Q-XAS.Compared with the Langmuir equation,As（III）adsorption isotherms can be better described by Freundlich equation,ascribed to the presence of multiple heterogeneity adsorption sites.As（III）adsorption and oxidation both increase remarkably with increasing Cr（VI）incorporation.When p H increases from 3 to 7,As（III）adsorption increases,whereas its oxidation increases up to p H 5 and then decreases at higher p H due to the surface passivation by the more generated Cr（III）precipitates.The Cr（VI）speciation（aqueous vs.structural）seems no significant effects on As（III）adsorption oxidation if structural Cr（VI）is available.The As（III）adsorption-oxidation pathways and mechanisms on Cr（VI）-incorporated schwertmannite include that As（III）firstly adsorbs through surface complexation and anionic exchange,followed by a direct redox between As（III）and Cr（VI）on the surface;the newly generated As（V）and Cr（III）then co-adsorb or precipitate on the schwertmannite surface,simultaneously decreasing their toxicity and mobility.