Effect Of Impurity In Pyrite On The Adsorption And Reduction Of Selenium (?) At Pyrite-water Interface

Se is a micronutrient element that is essential for the human body.79 Se is one of the isotopes of Se which is radioactive with a half-life of up to 70,000 years.As one of the fission products,79 Se will enter the geological repository of high level radioactive waste.Pyrite can adsorb and reduce Se(Se(?))to the low-valent Se(0,-?,-?)precipitate,which is important to retard the migration of Se in environment.The theoretical formula of pyrite is FeS2,while it usually contain impurities such as Co,Ni,Cu and Zn.These impurities normally substitute Fe in pyrite structure,resulting the change of physical and chemical properties.Because the complexity of impurities in natural pyrite,it is difficult to study the effect of a special element on its properties quantitatively.On the contrary,synthetic pyrite can provide pyrite doped with special impurity.In addition,due to the complex environmental conditions of groundwater,many factors affect the chemical form of Se and the adsorption / reduction on the pyrite-water interface.Therefore,the effects of initial pH,buffer,pre-equilibration time and ionic strength on the adsorption and reduction of Se(?)at the interface of pyrite-water were investigated herein.After that,the effect of impurities on the adsorption and reduction of Se(?)at the interface of pyrite-water was studied using synthesized impurity-containing pyrite.Meanwhile,the synthesis products and reaction products were characterized using XRD,SEM,BET,XPS and XANES.Accordingly,the followed understanding can be concluded.XRD results demonstrated that the synthesized product were pure pyrite,indicating the successful synthesis of pyrite.ICP-AES results showed that the impurities contained in pyrite lattice were less than the proposed values.SEM observation showed that the synthesized pyrite was dominated in nanoparticle aggregates.The pure pyrite and Co-doped pyrite were similar with each other in morphology,which were tetrahedron bipyramids with a diameter up to 800 nm.The Ni-doped pyrite were around 50 nm,and they were easy to agglomerate into blocks.The Cu-doped pyrite were about 1-4 mm,with rough and irregular surface.Cu adoption didn't changed the morphology and particle size of pyrite.The Zn-doped pyrite were up to 50 nm,and agglomerated into a more regular large ball particles.The N2-BET confirmed that with the increase of impurities content,the specific surface area increased accordingly.For example,the specific surface areas of Ni-doped pyrite were up to 24.67-29.53 m2/g.The effects of initial pH,buffer,pre-equilibration time and ionic strength on the adsorption and reduction of Se(?)at the interface of pyrite-water were investigated.The results showed that with the increase of pH and pre-equilibration time,the adsorption of Se(?)by pyrite was promoted due to the Fe(II)/Fe(III)-hydroxide precipitation.CH3COOHCH3 COONa inhibited the adsorption,while NaH2PO4-Na2HPO4 enhanced the adsorption at pH=4.5.After 24 hours,the adsorbed Se(?)were 95%,25.6% and 17.2% in NaH2PO4-Na2HPO4,without buffer and CH3COOH-CH3 COONa buffer system respectively.However,the buffers affected the adsorption slightly at pH=6.0.The higher the ionic strength,the slower the adsorption of Se(?)by pyrite.The XPS and XANES results showed that Se(?)was adsorbed on the surface firstly and then reduced to Se(0),rather than thermodynamically stabled FeSe and FeSe2.The effect of impurities on the adsorption / reduction of Se(?)at the interface of pyritewater was studied using synthetic impurities-containing pyrite.The results showed that the adsorption of Se(?)was inhibited by 2.4% and 4.7% Co-doped pyrite,while it was enhanced by 13.4% Co-doped pyrite.At the initial pH = 6.0,the removal of Se(?)increased with the increase of Co content.Ni adoption can promote the adsorption of Se(?)by pyrite.At the initial pH = 4.5,nearly 100% of Se(?)was adsorbed by pyrite after 24 hours,while pure pyrite adsorbed only 43.3% of Se(?).The adsorption of Se(?)was inhibited by Cu adoption in the range from 3.6% to 6.9%,however,it was enhanced with less Cu adoption(2.3%).When the Zn content were 1.0% and 10.4%,the adsorption of Se(?)by pyrite were promoted,and the adsorption of Se(?)by pyrite was inhibited when the Zn content is 2.6%.The lower the initial pH,the faster the removal of Se(?)by impurities-containing pyrite.After 14 days of reaction,99.6% and 48.5% Se(?)were adsorbed by 2.4% Co-doped pyrite when initial pH were 4.5 and 6.0,respectively.XANES analysis showed that the reduction ability of Co and Ni-doped pyrite was excellent.The adsorption capacity of pyrite doped with 3.6% Cu was not so good as pyrite containing 2.3% Cu,however,its reduction ability was better.Therefore,synthetic pyrite has better adsorption capacity and reduction ability than natural pyrite.