The Formation Mechanism Of Biogenic Schwertmannite And Effects Of Seed Crystal On Its Growth Process And Properties

Acid mine drainage（AMD）refers to the drainage water with high acidity and high concentration of toxic heavy metals（iron,aluminium,manganese,zinc and other heavy metals）,metalloids（As）and sulfate produced by sulfur-containing minerals.It is a threat for surrounding soils,water resources and aquatic communities due to the arbitrary discharge of AMD.Under acidic conditions,Acidithiobacillus ferrooxidans（A.ferrooxidans）can oxidize Fe²⁺into Fe³⁺,and then combine with sulfate to form schwertmannite with poor crystallinity.Then the waste water is neutralized by the Cao to deal with AMD.The biomineralization-neutralization method can significantly reduce the lime demand and sediment amount,and the obtanied minerals can be used as excellent environmental functional materials,which has attracted much attention because of its many advantages.However,the efficiency of biomineralization in this method is usually low,and the primary mineralization rate（or conversion rate）of ferrous is only about 30%～40%.It is necessary to reduce and reoxidize the remaining Fe³⁺for multiple cycles to achieve a higher mineralization rate.Moreover,the pathway of crytal growth,the formation mechanism of needles,and how to effectively regulate the crystal growth（especially the mineralization rate）and morphology of schwertmannite are still rare.In this study,the effects of solution chemical properties such as OH^-and Fe²⁺on the primary mineralization rate and morphology of simulated AMD,especially the effects of A.ferrooxidans on needles formation by Fe²⁺catalyzed schwertmannite transformation,were studied by a series of characterization including atomic force microscopy,scanning electron microscopy,XRD,Mossbauer spectroscopy.The effects of exogenous solids as seed crystal on the growth process of schwertmannite and mineralization rate of schwertmannite synthesized by biological method was explored;the simultaneous adsorption of As（III）and Cd（II）on montmorillonite/schwertmannite was studied.In order to improve the efficiency of AMD biomineralization and promote the environmental application of mineral materials,its necessary to imporve the scientific basis.The main results are as follows:（1）It was found that the formation of schwertmannite could be divided into three stages.In the first nucleation stage,the crystallite presented as nonaggregation or aggregative forms via a successive polymerization process.The second stage was the formation of ellipsoidal aggregates,which were identified as ferrihydrite and/or schwertmannite.In the third stage,needles appeared on the surface of ellipsoidal aggregates,which was caused by the phase transformation of ferrihydrite or schwertmannite to lepidocrocite and goethite through an Fe²⁺（aq）catalysis-driven pathway.After the three stages,a typical characteristic“hedgehog”morphology was finally appeared.Meanwhile,A.ferrooxidans could significantly speed up the mineral transformation.Solution pH would affect the morphology of schwertmannite by acid leaching.（2）Adjusting the pH of reaction system（i.e.mainly contolling the concentration of OH^-）had a great influence on the precipitation rate or prodution of mineraliztion.The total iron precipitation rate of the treatment with constant pH=2.5 reached 96%after 72 h,which is much higher than that of the treatment with initial pH=2.5 without further adjustment（31.5%）.Further experimental results also revealed that the formation of schwertmannite depended on the content of hydroxyl complexes or the transformation of monomers to polymers,which were greatly affected by solution pH.In addition,the three-dimensinal structure of schwertmannite can ben enhanced by keeping the pH constant.The porosity of the obtained minerals was increased by 104.5%,and the pore size was 9.10 nm,which was higher 177.4%than that of control.（3）At low pH,the rate of schwertmannite transformed to goethite following addition of Fe²⁺（0～40 m M）was increased,behaving as more needles appeared on the surface of ellipsoidal aggregates.And the biomass of A.ferrooxidans was positively correlated with the transformation rate of schwertmannite catalyzed by ferrous ion.Further experiments demonstrated that the precipitation morphology of schwertmannite-aerobic treatments presented as more needles formed than schwertmannite-anaerobic by SEM,while ESR spectra proved that hydroxyl radical（^●OH）was produced only in the presence of oxygen.This indicated that there were two possible pathway for the formation of needles formation on the surface of ellipsoid aggregates.^●OH could oxidize the Fe²⁺to form Fe³⁺and further induced the formation of geothite with the presence of oxygen.Meanwhile,the three-dimensional excitation matrix fluorescence spectrum analysis of A.ferrooxidans proved that tryptophan was mainly composition,which could accelerate the dissolution of schwertmannite.This implied that A.ferrooxidans could acclerate the dissolution of schwertmannite and promote the transformation of schwertmannite to goethite catalyzed by Fe²⁺via dissolution/reprecipition mechanism.Besides,tryptophan could chelated with Fe³⁺to form Fe³⁺-A.ferrooxidans complex.However,the result of TEM-map showed that the absence of elements C in region of needles,implied that the A.ferrooxidans would release to solution during the formation goethite.And it was also proved that ferrous ion was the key factor to the needles formation on the surface of ellipsoidal aggregates and A.ferrooxidans played similar roles with H₂O₂ in the transformation of schwertmannite.Furthermore,electrochemical experiments demonstrated that the the interface potential between ferrous ions and schwertmannite was shirfed from 0.390 v(Fe²⁺+schwertamnnite)to 0.415 V(A.ferrooxidans+Fe²⁺+schwertmannite)following addition of A.ferrooxidans to electrolyte,which was higher than 0.387 V of ferrous ions.This was favored the electron sped up across the interface and facilitating schwertmannite transformation.These findings have important implications for understanding the role of microorganism on mineral transformation in AMD systems.（4）The experimental results showed that the oxidation rate of ferrous ions,total iron removal rate and mineral yield could be significantly imporved by using schwertmanite obtained from different stages as seed crystals.After 72 h,the total iron precipitation rate for the treatment of 6 h schwertmannite as seed crystal was 1.21～1.12 times of that control and24～72 h schwertmannite as seed crystal treatment.Compared with control treatment,the production of schwertmanite increased by 19.28%～14.20%after adding 6～72 h schwertmannite as seed crystal.And the iduction period of schwertmannite could be shortened.In addition,the experimental results showed that the nucleation rate of schwertmannite was negatively correlated with the initial concentration of ferrous sulfate and the addition of exogenous schwertmannite seed crystal could shorted the induction period of schwertmannite.However,the pH,oxidation rate of ferrous and total iron removal rate were not signifiantly changed after adding ferric oxide,magnetite and biochar.In contrast,the oxidation of ferrous ion was only 49.84%and 7.45%after adding iron phosphate and activated carbon,which was signicantly lower than 93.34%of the control treatment.This indicated that iron phosphate and activated carbon could significantly inhibite the oxidation of ferrous ions by A.ferrooxidans.And it was observed that schwertmannite grown on the surface of external solid behaving as needles,and its aggregation degree was obviously weakend by observing the morphology of Fe₃O₄,Fe₃PO₄ and the minerals after reaction.Due to the different hydrophobic force of the solid,different amounts of A.ferrooxidans was adsorbed to the surface.Generally,the solid with higher hydrophobic force had a greater adsorption capacity of A.ferrooxidans.Besides,the ability of A.ferrooxidans adsorbed on the surface of the minerals and the morphology of new formed schwertmannite did not change significantly.This suggusted that the formation process of schwertmannite was a simple crystal growth process,which had little relationship with the location of A.ferrooxidans.In view of the funtional of schwertmannite seeds crystal,the design of reflux could be used to improve the growth rate of schwertmannite in partical water treatment projects.（5）The SEM results showed that schwertmannite formed on the surface of montmorillonite.Compared with the pure schwertmannite,the particle size and the dispersion of new formed schwertmannite on the surface of montmorillonite were decreased obviously.Meanwhile,the presence of montmorillonite also led to a increase in the dispersion of schwertmannite.When the initial concentration of Fe²⁺was 20 m M,the specific surface area of schwertmannite-montmorillonite aggregates reached 79.37 m² g^-1,which was remarkably larger than that of schwertmannite(46.78 m² g^-1)and montmorillonite(9.24 m²g^-1).The schwertmannite-montmorillonite aggregates showed the maximum adsorption capacity for single As（III）(110.66 mg As（III）g^-1)and Cd（II）(78.81 mg Cd（II）g^-1)was much higher than single-phase schwertmannite(103.12 mg As（III）g^-1)and montmorillonite(5.19mg Cd（II）g^-1),at pH=6,respectively.And the adsorption capacities of schwertmannite-montmorillonite aggregates（Sch20Mt）was positively correlated with pH.The adsorption isotherms of As（III）and Cd（II）can be well described by the Langmuir model.In the As（III）-Cd（II）binary system,there is a competitive adsorption between As（III）and Cd（II）at pH=6.The adsorption capacity of Sch20Mt for As（III）(114.46 mg As（III）g^-1)in binary system was higher than that for single As（III）(110.66 mg As（III）g^-1),while that for Cd（II）(7.77 mg Cd（II）g^-1)in binary system was lower than for single Cd（II）(78.81 mg Cd（II）g^-1).The above results indicate that a potential application of the schwertmannite-montmorillonite aggregates as a promising remediation material for water and soil polluetd by multiple composite metals.In summary,it is contributed for imporving the total iron precipitation rate and the production of schwertmanite,as well as the regulating for mineral morphology,when the growth model of biogenic schwertmannite was established.The study of the interation between A.ferrooxidans and schwertmannite could help to better understand the behavior of schwertmannite in AMD environment.The exogenous solid added as seed crystal to the system of biogenic schwertmannite realized that deal with AMD and obtain environmental materials with excellent performance.