Redistribution Mechanism Of Previously-bound Heavy Metals During The Fe(?)-induced Recrystallization Of Schwertmannite

The characteristics of extremely acid and rich in Fe³⁺,SO₄^2-,heavy metals of acid mine drainage?AMD?results in the formation of Fe???-oxyhydroxysulfate mineral-schwertmannite.Schwertmannite can mobilize heavy metals in AMD and sediments by adsorption or co-precipitation,thus reducing the mobility and bioavailability of heavy metals.However,Fe???/Fe???cycles in AMD environments could induce metastable schwertmannite transforming to more stable minerals,leading to the remigration and redistribution of previously-bound heavy metals.Previous studies have focused on the effect of heavy metal loading on the morphology and stability of schwertmannite,but the reductive dissolution of schwertmannite coupled to the behavior of redox-active metals are less known.Clarifying the redistribution of typical heavy metals in AMD during transformation of schwertmannite can provide theoretical support for avoiding secondary pollution of heavy metal in AMD polluted areas.Therefore,this study focus on the redistribution mechanism of heavy metals during the Fe???-induced recrystallization of schwertmannite.First,schwertmannite adsorbed with different heavy metals(including cadmium(Cd???,and redox-active chromium(Cr?VI?,arsenic?As?V??,manganese(Mn???,cobalt?Co????had been synthesized.The analysis methods of ion chromatography?IC?,X-ray powder diffraction?XRD?,Fourier infrared spectrum?FTIR?,scanning electron microscope?SEM?,X-ray photoelectron spectroscopy?XPS?,Mossbauer spectroscopy,and X-ray fine structure spectroscopy?XAFS?were used.Changes in ion aqueous concentrations,composition of transformation products,valence state and local coordination environment of heavy metals on the secondary minerals were analyzed.Results show that"schwertmannite?lepidocrocite?goethite"is the dominant transformation pathway during the Fe???-induced recrystallization of schwertmannite under the nearly neutral slightly acidic conditions?pH 6.5?.During the transformation,the cationic heavy metals?Cd,Mn,Co?were first desorbed and then mobilized on the transformation products,while the oxyanions?Cr,As?were hardly migrated.The main conclusions are listed as follows:?1?The transformation of cadmium-associated schwertmannite and subsequent element repartitioning behaviors.The way of schwertmannite associated with Cd was predominant by absorption and formation of inner-sphere complexes.The diffuse layer model?DLM?showed that Cd existed as monodentate complexes?Fe_?1?OCd⁺and?Fe_?2?OCd⁺on schwertmannite surfaces.The shrinking core model revealed that Fe???-induced process would affect mineral dissolution by changing surface reaction-controlled step to internal diffusion-controlled step,whilst significantly promoting the dissolution rate of Cd-adsorbed schwertmannite.Transformation products of Cd-schwertmannite were goethite and lepidocrocite.The transformation rates decreased and more lepidocrocite aggregated with increasing adsorbed Cd levels.Adsorbed Cd blocked the surface sites for later Fe???adsorption and the Fe???-Fe???electron transfer,then resulted in the decelerated transformation and the accumulation of intermediate phase lepidocrocite."Schwertmannite?lepidocrocite?goethite"was the dominant transformation pathway.Redistribution of Cd was controlled by transformation degree.During the rapid dissolution?Fe???1mM?,adsorbed-Cd was first released significantly and then trapped again by the bulk minerals.Recrystallization of schwertmannite favored the re-mobilization of Cd,and finally Cd was mainly associated with the secondary minerals by complexation with surficial OH groups.?2?The behavior of chromium and arsenic during the Fe???-induced recrystallization of schwertmannite.The pre-equilibrated period removed all the added arsenate and>81%of the added chromate via sorptive interactions with schwertmannite.When chromate was present,adsorption mainly occurred through anion exchange with SO₄^2-groups,while for arsenate,adsorption involved both complexation with mineral surface hydroxyl groups and SO₄^2--exchange.Adsorbed chromate and arsenate both inhibited the Fe???-induced recrystallization of schwertmannite,cause schwertmannite structure was reinforced after the substitution of SO₄^2-by chromate and arsenate.CrO₄ ^2--schwertmannite persisted as the dominant mineral phase for up to 30 d as a consequence of Fe???consumption and coverage of Cr???-Fe???hydroxide.In contrast,AsO₄^3--schwertmannite transformed to poorly crystalline goethite via an intermediate lepidocrocite in 4 mM Fe???treatment.The adsorbed Cr and As were rarely released during the transformation.In addition,transformation of AsO₄^3--schwertmannite caused no re-mobilization of arsenic into solution,indicating the repartition to secondary minerals.?3?The behavior of manganese and cobalt during the Fe???-induced recrystallization of schwertmannite.During the pre-equilibrated period,Mn???and Co???adsorbed to schwertmannite surfaces by complexing with-OH groups,and Mn???,Co???had no valence state change.As log K_Co>logK_Mn,the adsorption amount of Co???on the surface of schwertmannite was greater than that of Mn???,and the inhibition effect of Co on the transformation was also stronger;Correspondingly,Co was less mobile during the transformation.Competitive adsorption of Mn???,Co???and Fe???on schwertmannite surface reduced the adsorption capacity of Fe???.Meanwhile,the coverage of Mn???/Co???hydroxyl complexes weaken the electron transfer between Fe???-Fe???,which led to the retardation of schwertmannite transformation.Adsorbed Mn???and Co???led to different transformation pathways.Mn???-schwertmannite transformed into goethite via an intermediate lepidocrocite,while Co???-schwertmannite were directly transformed into goethite.The migration of Mn and Co was related to the transformation degree.The rapid dissolution of schwertmannite induced by Fe???promoted the re-mobilization of Mn and Co,and the formation of secondary minerals helped to re-immobilize Mn and Co.Results of XANES showed that Mn???and Co???were oxidized to Mn???and Co???,respectively,during the Fe???-induced recrystallization of schwertmannite.The fitting results of EXAFS of Mn,Co K-edge showed that the surrounding environment of Mn and Co atoms has changed during 30 d.Finally,Mn and Co were mainly combined with new minerals in the form of monodentate mononuclear and edge-sharing bidentate binuclear.