Phase Transformation Of Jarosite And Redistribution Mechanism Of Coprecipitated Arsenate Mediated By Dissolved Organic Matter

Acid mine drainage?AMD?is characterized with extremely low p H and rich in SO₄^2-,Fe³⁺,as well as metal?loid?s,resulting in the formation of Fe???-oxyhydroxysulfate mineral–jarosite.Due to its properties of remarkable isomorphism and hydroxyl-rich,jarosite is considered as an effective scavenger for metal?loid?s?such as As?by co-precipitation and adsorption,reducing the mobility and bioavailability of metal?loid?s.Dissolved organic matter?DOM?is commonly found in natural environment and exhibits a prominent contribution to the dissolution of iron minerals,thereby affecting the distribution of loaded metal?loid?s.Therefore,the present study investigated the interaction between DOM?oxalate and gallic acid?and jarosite.The main conclusions were summarized as follows:?1?The synthetic minerals were verified as typical jarosite by X-ray diffraction?XRD?,and no discernible changes in XRD peak positions were observed.However,compared with pure jarosite,the characteristic peak intensity of As?V?-bearing jarosite was enhanced,indicating that the incorporation of As?V?stabilized the structure of jarosite.Fourier transform infrared?FTIR?and Scanning electron microscopy?SEM?showed that As?V?substituting in jarosite had no significant effect on the crystal structure and surface feature.?2?The formation of Fe???-oxalate complex was the first and key step to promote the dissolution of As?V?-bearing jarosite.Under p H 2.5 condition,the dissolution rate of As?V?-bearing jarosite increased with the increase concentration of oxalate,a large amount of As?V?releasing into solution subsequently.The insignificant re-adsorption of arsenate could be explained by the competition for Fe???active sites on the jarosite surface with oxalate.In addition,XRD indicated that As?V?-bearing jarosite transformed to goethite and lepidocrocite at p H 6.5 and oxalate enhanced the crystallization of secondary minerals.After 60 d reaction,almost all of the liberated As?V?re-adsorbed to the solid phase.It indicated that newly formed goethite and lepidocrocite played a vital role in controlling the fate of As?V?.The proposed mechanism was that oxalate promoted dissolution?p H 2.5?and recrystallization?p H 6.5?of jarosite.?3?The liberation of Fe???,K⁺and SO₄^2-ions in the solution increased gradually during gallic acid-induced dissolution of jarosite,as solid-phase As transformed to aqueous and surface-complexed As.As?V?bearing into jarosite stabilized the structure of mineral and inhibited the transformation rate.Jarosite persisted as the dominant mineral phase at p H 2.5 up to 60 d,though a large amount of structural Fe???was reduced by gallic acid.However,at p H5.5,jarosite mainly transformed to ferrohexahydrite?Fe SO₄?6H₂O?with gallic acid addition.The mechanism was proposed to be the synergistic effect of ligand and reduction.The present study concluded that DOM had an important influence on the dissolution and transformation of jarosite and revealed the repartitioning behavior of arsenic in AMD environment.The result is of great significance for arsenic pollution control.