Effect Of Cu(?) On The Dissolution And Transformation Of Oxyanion-substituted Schwertmannite Under Acid Mine Drainage Conditions

Schwertmannite is a secondary Fe???-oxyhydroxysulfate mineral formed in acidic?pH3⁴?,iron-and sulfate-rich acid mine drainage?AMD?environments.Schwertmannite poses an important potential to adsorb and scavenge toxic heavy metals in AMD environments,such as arsenate,arsenite,chromate,and copper.However,it tends to undergo phase transformations with changes in p H and redox conditions,and causes the release of toxtic heavy metals.In the present study,the effects of Cu???and pH on the stability of pure synthetic schwertmannite and oxyanion-subtituted schwertmannite prepared in the presence of chromate?Cr-Sch?,molybdate?Mo-Sch?,and arsenate?As-Sch?were investigated.The main results are as follows:?1?The pure synthetic schwertmannite and oxyanion-substituted schwertmannite have the characteristics of the typical schwertmannite,and the co-precipitation of oxyanions have no significant effect on the synthetic schwertmannite.The amount of oxyanion precipitated into the structure of schwertmannite followed the order of MoO₄^2->As O₄^3->CrO₄^2-,while the affinity of schwertmannite to the three oxyanion was AsO₄^3->MoO₄^2->CrO₄^2-.?2?The amount of Cu???adsorbed on pure synthetic schwertmannite and oxyanion-substituted schwertmannite was small,and only less than 10%Cu???was adsorbed on all schwertmannites under various Cu???concentration.The amount of Cu???adsorbed on various schwertmannites increased significantly as the pH increased to⁵.Cu???adsorption on various schwertmannites at both pH³ and pH⁵ was mainly through the complexation between Cu²⁺and the surface hydroxyl groups,or the exchange between Cu²⁺and H⁺.At pH³,the substitution of Fe???by Cu???was also possible,thus promoting the dissolution of Fe³⁺.Additionally,high concentrations of Cu???favored the precipitation of Cu?OH?₂ solid on the surface of schwertmannite at pH⁵,affecting the surface reactive sites and charges.?3?At pH³,the release of Fe³⁺and SO₄^2-was accelerated to some extent because of the substitution of Fe???by Cu???.However,the substitution of Fe???by Cu???also changed the structural properties,enhencing the stability of pure and oxyanion-substituted schwertmannites.At pH⁵,the large amount of adsorbed Cu???covered the surface active sites of various schwertmannites and hindered the dissolution of Fe³⁺,SO₄^2-and oxyanions.At both pH values,pure schwertmannite and chromate-substituted schwertmannite underwent transformation to goethite and ferrihydrite over a 2-month period.The presence of Cu???delayed the transformation rates and extent of both mierals,and the effect increased with the increasing of initial Cu???concentration.The structures of arsenate-and molybdate-substituted schwertmannites showed no significant changes in the presence or absence of Cu???at both pH values.?4?During the phase transformation process,the amount of released oxyanions followed the sequence of CrO₄^2->MoO₄^2->AsO₄^3-.Whlie the effect of oxyanions on stabilizing the structure of schwertmannite followed this order,AsO₄^3-?MoO₄^2->CrO₄^2-.The present study concluded that Cu???had an important influence on the dissolution and transformation of schwertmannite and revealed the fixation and release of heavy metals by schwertmannite in AMD conditions.These results have important environmental implications for the stability of schwertmannite and its potential to immobilize contaminant heavy metals under AMD environments.