A Study Of The Mechanism Of Zn Sorption And Zn Isotope Fractionation During Sorption At Al Oxide/Water Interface Using EXAFS Technology

Zinc(Zn)is a necessary nutrient for living organisms and is widely used in industrial production.Zinc cycling in surpergene environment is mostly cotrolled by mineral/solution interfacial sorption.Interfacial sorption not only controlles zinc transportation and transformation but also fractionate zinc isotopes.However,the relationship between zinc sorption mechanism and isotopic fractionation remains vague.Thus,this work selected ?-Al2O3 was as sorbent and use batch experiment to study the sorption behaviors of Zn and its isotopes with different reaction time(048h),initial Zn concentration(C0:0.1-0.8mM),co-existing anion and cation species(Ni2+,Co2+and SiO44-),background electrolytes(0.1 and 0.01M NaNO3).Extended X-ray fine structure(EXAFS)technology was applied to determine molecular-lever structure of Zn.The main findings were shows as follows:1).Zn sorption at ?-Al2O3/solution interface is independent of the background electrolyte concentration and the co-existing Ni2+ and Co2+,but is affected by the reaction time,pH,initial Zn concentration and coexisting silicic acid.Zn sorption has a positive correlation with reaction time,pH and initial Zn concentration.Zn sorption increases with reaction time and reach sorption equilibrium in the first 12 h.The equilibrium sorption increases from near zero(pH<6.0,C0=0.01 mM)to 3.3 ?mol m-2(about 100%)(pH?7.5,CO=0.8 mM)with pH and initial zinc concentration increasing.Co-existing SiO44-always inhibites Zn sorption that goes back with Si concentration(C0Si)increasing from 0.64mM to 0.8mM.2).Zn sorption at ?-Al2O3/solution interface fractionates Zn isotopes.The magnitude of Zn isotope fractionation between sorbed phase and residual aqueous(?66Znsorbed-aqueous)is affected by pH,initial Zn concentration and silicic acid.At pH?6.5 or C0?0.2mM,the sorbed phase prefers heavy Zn isotopes yielding ?66Znsorbed-aqueous of 0.47±0.03‰;at pH?7.0 and C0?0.4mM,no isotopic effect occurs(?66Znsorbed-aqueous=0.02 ±0.07 ‰).Co-existing SiO44-fractionate Zn isotopes more heavily.The magnitude of Zn isotope fractionation increases from-0.04±0.06‰ to 0.62±0.06‰ with Si concentration increasing from 0.08 mM to 0.8 mM.3).Zn sorption mechanisms also depend on pH,initial Zn concentration and silicic acid.At pH 6.0,Zn is sorbed as inner-sphere complexes regardless of co-existing Co2+and Ni2+.At pH of 6.5-8.0,Zn sorption mechanism is surface precipitation in which Zn is sorbed as ZnAl LDH and CoNiZn-Al LDH in Co and Ni absence and presence systems,respectively.Silicic acid addition results in more complexed sorption mechanism.In Si presence systems,inner-sphere complexes and Zn-Al LDHs can be exist simultaneously and proportion of this two sorbed phases changes with Si concentration.At low Si concentration,the sorbed Zn is dominated by Zn-Al LDH,while at high Si concentration,inner-sphere complexes dominate.In addition,the inner-sphere complexes not bond to?-Al2O3 surface but bond to soluble aluminosilicates that result from Al and Si co precipitation.Zn isotope fractionation behavior corresponds to its sorption mechanism,that is,the inner-sphere sorption leads a remarkable isotope fractionation(0.47‰),while the surface precipitation hardly fractionates zinc isotopes.The reason of this correspondence is Zn molecular-level structure.The shorter the Zn-O bond length,the heavier the enriched zinc isotope,and the greater the magnitude of isotope fractionation.Zn in solution and in Zn-Al LDH is octehedred by six oxygens with Zn-O bond length of 2.06 (?) and Zn in inner-sphere complexes is tetrahedred by four oxygens with Zn-O bond length of 1.97 (?).In Si presence systems,Zn complexation by Si-containing phases in the main mechanism which cause that large isotope fractionation.This work reveales the mechanisms of Zn sorption and Zn isotope fractionation during sorption at ?-Al2O3/solution interface using EXAFS technology,and the relationship between Zn sorption mechanism and its isotope fractionation behavior.The findings obtained in this work provide molecular-lever information for understanding Zn and its isotopes cycling in supergene environment.