A Double-spike Method Study Of Cr Stable Isotopic Analysis And Its Monitoring Application In Cr-contaminated Water

As one of VIA elements and transition metals,chromium occurs at a wide variety of rocks, minerals,fluids and organics,showing an active feature during ore- and rock-forming, hydrothermal and biological processes.Chromium isotope is classified as non-traditional stable isotope,and has been applied in various fields such as early evolution of solar system and planets, impact events of extraterrestrial materials,hydrothermal deposit origins and mineralized material resources,Cr-contamination monitoring for surface environments.In contrast with traditional stable isotope systems,Cr isotope analysis is not available for most laboratories.However,partly benefited from improved TIMS and MC-ICP-MS,analytical techniques for Cr stable isotope have been developed successively.The documented works have not only deepened our understanding on the natural fractionation mechanism of Cr isotope,but also have promoted the researches in the fields of cosmochemistry,environmental pollution and ore deposits,which shows a significant potential in the future.A Cr isotopic method for environmental monitoring of Cr-contamination has been applied since 2000.This method was established in China during 2005 to 2007 in the State Key Laboratory of Geological Processes and Mineral Resources supported by National Natural Science Foundation of China,and its primary application in Cr-contaminated groundwater was also reported.To the mass discrimination correction caused by mass spectrometer measuring, this primary study used an external correction method.For analysis of isotopes with non-constant composition,the double spike method possesses highest accuracy in mass-discrimination correction,which is critical to recognize small degree variations among samples.Based on documented pre-works,this study aims to establish a chemical protocol for the double-spike method using the facility of the State Key Laboratory;coupled with specially designed chemical digestions for a variety of samples,this double-spike method is applied for solid samples,e.g. black shale and magnetite minerals.The final part of this study discusses the correlation between the hydrochemistry and Cr isotope of Cr-polluted water samples collected from a inorganic chemical factory in Huangshi City.This study consists of the following works:1.Analytical technique of double spike method for Cr isotope(1) Spike preparation Single spikes of ⁵⁰Cr and ⁵⁴Cr was firstly prepared through dissolving metal powders of high abundant ⁵⁰Cr and ⁵⁴Cr,respectively,and their Cr isotopic compositions were preciously measured.The double-spike mixture then was made through mixing the prepared single-spike solutions of ⁵⁰Cr and ⁵⁴Cr according to a proportion 1:1.For a hydrological application,the double-spike mixture was further oxidized to turn all Cr ions into form of Cr(â…¥).(2) Chemical separation of Cr from liquid samplesBased on the previous protocol,a procedure suitable for double-spike method was adapted, which is available for the correction of mass discriminations resulted both from the chemical separation and instrumental measurement during ionization.To achieve this goal,it is notable that the ⁵⁰Cr-⁵⁴Cr double-spike must be added and homogenized with sample solution before the loading of spiked mixture into ion exchange column.In addition,an optimized proportion for ⁵⁰Cr,⁵²Cr and ⁵⁴Cr of 1:1:1 in the mixture is required.(3) Offline treatment of analysis dataBased on the principle of mass discrimination correction using double-spike method,a calculation program was made for offline treatment of analysis data using a circular iterative technique.The program was designed using C Programming Language.Besides,a calculation program for the Cr isotopic composition of the mixed double-spike solution was also finished.To testify the programming and the Cr-separation chemistry,the liquid standard SRM979 was analyzed using the prepared double-spike tracer.The good agreement between the results and the standard's certificated values well proves the reliabilities both the compositional estimates of double-spike,chemical procedure and the off-line treatment.2.Chemical protocol of digestion and Cr-separation for solid geological samples(1) Sample digestionDivers chemical digestions were set up for distinct solid geological samples.To avoid of potential isotopic fractionation due to the existence of different facies with variable valence of Cr, the digested solution must be heated to dryness and then dissolved using 2N nitric acid to insure all Cr ions in the solution being in form of Cr(â…¢).The final solution was mixed with the ⁵⁰Cr-⁵⁴Cr double spike liquid according to the optimum proportion.(2) Cr separation and purificationThe homogenized mixture solution was oxidized using(NH₄)₂S₂O₈ resulting a Cr ion transformation from Cr(â…¢) to Cr(â…¥).Before loading the solution into the anion exchange column,the pH value of the anion resin is adjusted to a weak acid nature.The collected Cr-contained portion has variable contents of SO₄^2-,which may interfere the Cr isotopic measurement,it is purified by re-loading the collected solution twice into the recycled column.Rather than a 0.2N H₂SO₃ used by previous works,2N HNO₃ is used as a reducing agent during this study.Along with a test of SRM989,it shows that this reducer woks as well as H₂SO₃ or even better.Given that the HNO₃ is a basic chemical reagent and easy of purification,thus this replacement is also beneficial to the Cr isotopic analysis for low-Cr content samples.The adopted chemical protocol is used for some typical solid geological samples,including black shale and magnetite minerals.It shows that black shales possessδ⁵³Cr values less than zero, while those of magnetite and iron-oxide minerals sample larger than zero.This result likely indicates that rocks formed under oxidizing conditions tend to be enriched in heavy Cr isotopes, whereas those under reducing conditions enriched in lighter isotopes.3.Correlation between Cr isotope and hydrochemistry of Cr-contaminated waterA discussion is made on the correlation between Cr isotope and hydrochemistry of Cr-contaminated waters collected from an inorganic chemical factory in Huangshi City.It reveals that highly Cr-contaminated water possesses a high SO₄-(Na+K) contents,which is also characterized by high degree of mineralization and lowerδ⁵³Cr value.In contrasting,lowly Cr-contaminated water is a high HCO₃-(Na+K) position with lower degree of mineralization and higherδ⁵³Cr value.To surface waters without evident Cr pollution,they display a HCO₃-Ca composition,along with low degrees of mineralization andδ⁵³Cr values close to zero.