Biosynthesis Of Schwertmannite And Its Efficiency And Mechanism Of Arsenite Removal From Aqueous Solutions

During the last decade,the bioleaching technique,originated from biohydrometallurgy industry for extracting rare metals from sulfide minerals,has been successfully applied in municipal sludge treatment for removal of toxic heavy metals and proved to be an efficient and cost-effective alternative to physical or chemical technologies.Acidithiobacillus ferrooxidans(A.ferrooxidans) and Acidithiobacillus thiooxidans(A.thiooxidans) have been employed as the most significant microorganisms involved in bioleaching processes. However,it was observed that Cr could not be effectively solubilized and Cu solubilization was weakly correlated to mean hydraulic residence time in sewage sludge bioleaching experiments with inoculation of Acidithiobacillus ferrooxidans.Some researchers also reported that the bioleaching efficiencies of Cr and/or Fe were much lower than corresponding values in the chemical leaching process,and even that the solubilized Cu from sludge decreased again if the bioleaching time prolonged in the medium with pH ranged from 2 to 3.Some researchers hypothesized that secondary iron minerals including iron hydroxide and jarosite likely formed in sludge during bioleaching process,and that the adsorption of heavy metals on these secondary iron minerals resulted in the above-mentioned lower solubilization efficiencies.In recent years,the application of bioleaching approach in tannery sludge treatment in terms of the removal or recovery of Cr, a dominant toxic metal in tannery sludge,has been studied extensively by our research group.In the present work,the secondary iron mineral was separated for the first time from bioleached tannery sludge when sludge bioleaching experiments were performed in an air-lift bioreactor mainly by Acidithiobacillus ferrooxidans LX5 and Acidithiobacillus thiooxidans TS6 at pilot scale.The secondary iron mineral was identified to be a sole secondary hydroxyl iron sulfate mineral—schwertmannite,but not a mixture of ferric hydroxide and jarosite as hypothesized in earlier studies.Enlightened by the formation of schwertmannite in sludge during bioleaching in the bioreactor,synthesis of schwertmannite through oxidation of FeSO₄ by Acidithiobacillus ferrooxidans strains(i.e.A.ferrooxidans LX5) was studied systematically and the synthesis conditions were optimized.Furthermore, the stability or phase transformation of the biogenic schwertmannite was investigated. Moreover,the removal of As(â…¢) in aqueous medium by schwertmannite was studied extensively.The main results were presented as follows:The bioleached tannery sludge filtrate with pH～3 was rich in dissolved organic matter (DOM),Fe²⁺,Cr³⁺,SO₄^2- and A.ferrooxidans LX5.A large amount of ocherous precipitate was formed in the filtrate within 40 h during the sludge filtrate was incubated in a gyratory shaker at 28℃and 180rpm.The ocherous precipitate was identified as pure schwertmannite by X-ray diffraction(XRD) and Fourier transform infrared(FT-IR) spectroscopy.Results of scanning electron microscopy(SEM) and energy-dispersive X-ray spectroscopy(EDS) analysis showed that the schwertmannite particles were spheroids of uniform size with a diameter of approximately 1μm and their chemical composition could be expressed as Fe₈O₈(OH)_4.60(SO₄)_1.70,but Cr(â…¢) that had already solubilized from sludge by bioleaching incorporated into schwertmannite was about 2.43% by weight.It has well been accepted that the generation of acid mine drainage(AMD) is closely linked to the effects of Acidithiobacillus ferrooxidans and a schwertmannite-jarosite-goethite mineral paragenesis always occurs in AMD and waters or sediments impacted by AMD.However,there is only a sole secondary iron mineral(i.e. schwertmannite) formed in sludge bioleaching system.The presence of schwertmannite as monominerallic phase in sludge bioleaching environment should be predominantly attributed to extremely high content of DOM(＞300 mg C/L),short reaction time(＜40 h), high sulfate concentration(～9000 mg/L) and low pH value in this system.Biosynthesis of pure schwertmannite with a large quantity could be easily achieved through oxidation of FeSO₄ solution mediated by A.ferrooxidans LX5 resting cells.The yield of schwertmannite was affected mainly by bacterial density,the starting pH of reaction solution and reaction time,etc.The optimized reaction conditions were:1×10⁷cells/mL of bacterial density,8-12 g/L of the initial concentration of Fe²⁺(added as FeSO₄·7H₂O),about 3.2 of the starting pH(without addition of any acid or base solution) was incubated in gyratory shaker at 28℃and 180rpm for 48-60 hours.The biosynthesized product was spherical schwertmannite of uniform size with a diameter of about 2μm, having no obvious characteristic pin-cushion morphology,and its chemical composition could be expressed as Fe₈O₈(OH)_4.42(SO₄)_1.79.The precipitation rate of iron in the course of synthesis reaction was 37.43%. The stability and phase transformation of the biogenic schwertmannite and As(â…¢)-containing schwertmannite were investigated in the present work.The synthetic schwertmannite suspension(1 g/L of mass concentration) was allowed to stand at 25℃in a incubator with occasional shaking.The solids sampled from the schwertmannite suspension were analyzed for mineralogical composition.Experimental results showed that the biogenic schwertmannite and As(â…¢)-containing schwertmannite were more stable than those synthesized by chemical method or naturally formed in acid mine drainage,as indicating that no any phase transformation occurred at pH 6 and pH 8.5 environments over a period of 90 days.Batch adsorption experiments were conducted to explore the adsorption process and related mechanism of As(â…¢) from aqueous solution on synthetic biogenic schwertmannite. Results indicated that biogenic schwertmannite has a strong adsorption for As(â…¢) and the adsorption capacity attained to almost 95%of equilibrium capacity within the first 60 minutes.The adsorption kinetic data could be described by the Lagergren pseudo-second order rate equation.Arsenite elimination was favored at pH=7-10.The changes of solution ionic strength(0.0001-0.1M) and the normally presented anions in natural environment had no obvious side effect on arsenite removal efficiency by schwertmannite except for phosphate(＞0.001M) and sulfate with a concentration of above 0.01M.Adsorption isotherm data for As(â…¢) was found to fit well to Langmuir isotherm equation(R²ï¼ž0.99) with a maximum adsorption capacities of 114 mg/g at room temperature.The adsorption of arsenite on schwertmannite could be achieved spontaneously and it was a endothermic process,but temperature changes within experiment range had no significant impact on the adsorption capacity.The pH_pzc(pH of the point of zero charge) for schwertmannite was found to be initially 5.4,but when As(â…¢) was adsorbed,it decreased from this value to about 4.3.Moreover,FT-IR analysis results suggested that direct surface complexation between surface OH groups and As(â…¢) occurred.It was concluded that As(â…¢) was specially adsorbed onto schwertmannite by an inner-sphere mechanism through surface complexing action between surface hydroxyl and As(â…¢) and by ligand exchange between SO₄^2- and As(â…¢).It was concluded that synthesis biogenic schwertmannite with large quantity could be easily achieved through oxidation of FeSO₄ by A.ferrooxidans LX5 simulating the sludge bioleaching reaction conditions.Arsenite in aqueous solution can be specifically adsorbed onto the biogenic schwertmannite.It was indicated that biogenic schwertmannite,as a potentially excellent adsorbent,would play an important role in treatment of arsenic-contaminated groundwater in terms of arsenite removal.