Phase Transformation Of Schwertmannite Mediated By Dissolved Organic Matter In Acid Mine Drainage And Its Effects On Environmental Behaviour Of Heavy Metals

Schwertmannite is a secondary iron hydroxy sulfate mineral commonly found in acid mine drainage?AMD?environments.Owing to its characteristic properties including hydroxyl and high surface area,schwertmannite is considered an important sink for trace metals,and it plays a vital role in the fate and transport of heavy metals.Dissolved organic matter?DOM?can alter the characteristic properties of the mineral such as surface charge,and stability.Therefore,phase transformation of schwertmannite will occur when mediated by DOM,causing redistribution or even releasing of bound heavy metals in the mining area.It is an important process of heavy metal environmental behavior in mining area.Nonetheless,it has been explored in only few studies.The present study investigated the mineralogical characteristics of sediments and their roles in heavy metals retention associated with AMD systems in Shaoguan Dabaoshan Mine.And experiments were devised to investigate the interaction between DOM?L-tryptophan,fulvic acid,desferrioxamine B?DFOB??and schwertmannite.Therefore,the goals of this study were to elucidate the role of DOM plays in the release of Cr from schwertmannite;to assess the processes of schwertmannite dissolution and transformation;as well as to better understand the fate and transport of heavy metals.The main results were listed as follows:?1?Results showed that schwertmannite,jarosite,goethite and ferrihydrite were the dominant Fe-oxyhydroxide minerals which were detected alternately in the surface sediment with the increasing pH from 2.50 to 6.93 along the Hengshi River.The concentrations of heavy metals decreased along the river.And decreasing contents of schwertmannite ranging from 35 wt%to 6.5 wt%were detected along the Hengshi River,which was corresponding to the decreasing metal contents.The main DOM in the river is established to be tryptophan-like substances.?2?Results of X-ray diffraction?XRD?showed that the minerals synthesized by the rapid chemical methods were schwertmannite.No new mineral was found after the addition of Cr.However,compared with pure schwertmannite,CrO₄^2--schwertmannite had broader and somewhat shorter peaks suggesting that the incorporation of CrO₄^2-caused a slight degeneration of the schwertmannite structure.Scanning electron microscopy?SEM?images show that synthetic schwertmannite consists of nearly spherical particle aggregates with a diameter of about 300-400 nm,but granulum was found on the surface of CrO₄^2--schwertmannite particles.Results of Fourier transform infrared?FTIR?spectra showed that CrO₄^2-can be found in synthetic CrO₄^2--schwertmannite.?3?The concentrations of total Fe,Cr and SO₄^2-in solution reached a maximum when the pH was 6.5 and the concentration of L-tryptophan was 5 mM,but the concentrations of total Fe and Cr decreased after 40 d.The newly formed minerals were confirmed to be mixtures involving residual schwertmannite,goethite,ferrihydrite and jarosite by XRD,SEM and Transmission Electron Microscopy?TEM?.Based on the shrinking core model,chemical reaction is a rate-controlling step in the first period and leaching of Cr from schwertmannite in the presence of L-trp is controlled by diffusion through the?product?layer during the second kinetic phase.A possible mechanism is proposed to be a ligand-controlled binary system,accompanied by possible reduction at acidic pH conditions?3.2?,including mass transfer and charge transfer processes.?4?Fulvic acid could affect the dissolution rate of schwertmannntie.It will promote the dissolution at the FA concentrations of 1 or 10 mg/L,while inhibited at the FA concentrations of 50 mg/L.The characterization on the solid phase by XRD,SEM and X-ray photo electron spectroscopy?XPS?technologies revealed that goethite was the dominant newly secondary mineral phases which played a vital role in controlling the fate and transport of Fe and Cr.The addition of Cr inhibited the phase transformation of schwertmannite under the same conditions.Most of the Cr was associated with fractions of iron oxide surface-bound phase?F2?and amorphous iron oxide-bound phase?F3?,with the percentage ranging from 87.5%to54.5%for pH 3.2 and 86.4%to 69.7%for pH 6.5.It indicated the resorption of Cr on to the secondary Fe minerals.The possible mechanism was proposed to be synergistic effect including ligand-promoted and reduction.?5?The liberation of ions in the solution increased with the increasing concentration of desferrioxamine B.The concentrations of Fe and sulfate were determined to be maximum for pure-schwertmannite and CrO₄^2--schwertmannite at pH of 6.5 in the presence of 100 mg/L desferrioxamine B.Based on the X-ray absorption near edge spectroscopy and FTIR analysis,results indicated that schwertmannite transformation proceeded through a ligand-induced and reductive pathway that generated goethite.The experimental data was analyzed using two kinetic models Kabai and shrinking core models,Cr appeared to suppress the schwertmannite to goethite phase transformation and the liberation of Cr were kinetically controlled by diffusion through the secondary minerals layer of dissolution.In short,the dominated product was verified to be goethite and the pathways of schwertmannite transformation were ligand-induced and reductive.The higher reduction of ligand organic compounds,higher stability constant of complexes formed with heavy metals and the simpler of the molecular structure will be beneficial to the transformation of schwertmannite under the same pH condition.In addition,it suggested that in AMD environment,there exists dynamic interaction between schwertmannite and heavy metals mediated by DOM.The speculation on the reason of field investigation about heavy metal concentrations sharply decreased and then increasing in Hengshi River was proved to be true by experimentations.Thus,the formation and phase transformation of Fe-minerals are the critical factor in controlling the mobility of heavy metals in mining area.The results of this study present new perspectives for understanding the mobility and fate of heavy metal in AMD system,and are of use to the design of remediation strategies for AMD contaminated.