Studies On The Adsorption And Redox Behaviors Between Arsenic-containing Pyrites And As(?,?)

Adsorption and redox reactions occur between arsenic-containing pyrite and arsenic,which affect the migration and conversion of arsenic in supergene environments.However,the influence of arsenic in pyrite on the interaction processes is still enigmatic.The present study investigates the interaction mechanisms between arsenic-containing pyrite and arsenic and the effect of different conditions.In this work,arsenic-containing pyrites were hydrothermally synthesized with similar composition as the naturally surface-oxidized pyrites in supergene environments.The effect of arsenic incorporation on the chemical composition and physicochemical properties was analyzed,and the interaction mechanism between arsenic-containing pyrite and aqueous arsenic was also studied.The results are as follows.1.The pristine pyrite was successfully synthesized by hydrothermal methord with FeSO₄,CS?NH₂?2and S0.Arsenic-containing pyrites with the arsenic contents of 0?Apy0?,4.4?Apy5?and 9.9 wt.%?Apy10?were produced when the intitial molar ratio of As:Fe was controlled by 0.10 and 0.22.As the major arsenic species,As???and As?-I?respectively substituted Fe?II?and S2?-II?in the pyrite.The relative contents of As???and As?-I?respectively reached 76.6% and 17.2% in Apy5,and91.0% and 8.0% in Apy10.Arsenic substitution resulted in a high content of Fe???and a decrease in pyrite crystallinity.2.During the redox processes of arsenic-containing pyrites and As???,S0 and SO₄2-were formed as the main products accompanied by the adsorption of As???and As???at all pH.As???was released into solution system due to the collapse of the crystal structure of pyrite and the oxidation of As?-I?.Different redox mechanisms were observed with pH increasing from 3.0 to 11.0 in the reaction system.At pH 3.0-6.0,Fe???in crystal structures contributed much to the oxidation of arsenic-containing pyrite,and arsenate and released As???were adsorbed on the surface of solid products.At pH 7.0-11.0,aqueous arsenate worked as the major oxidant,and its oxidation capacity increased with increasing pH.When the pH was increased from 3.0 to 7.0 and 11.0,the release ratio of arsenic from the Apy10 structure increased from 34.1% to 41.5% and 56.8%,respectively.The oxidation of arsenic-containing pyrites was enhanced by O₂,the steady concentration of SO₄2-,S0 andreleased As???increased,the goethite on the mineral surface facilitated to the adsorption of arsenic.3.Under anoxic conditions,the aqueous As???was oxidized to As???which was released to solution during the redox processes of arsenic-containing pyrites and As???.·OH was produced from the oxidation of H₂ O on the sulfur-deficient sites on arsenic-containing pyrites surface,showing an increased production with the increase content of structural As,which indicated that the oxidation of aqueous As???was enhanced by the sulfur-deficient sites.Under oxic conditions,the equilibrium concentration of aqueous As???,SO₄2-and S0 increased significantly,the formation of ·OH proceeds predominantly via reduction of O₂ on pyrite surface along with a minor contribution from the oxidation of H₂ O on surface sulfur-defects.Under acidic conditions,for the generation of ·OH,H₂O₂ was the predominant intermediate,which subsequently transformed to ·OH through Fenton mechanism,under alkaline conditions,H₂O₂ was main oxidant for the oxidation of arsenic-containing pyrites.