The Analysis Technique For Cr Isotope And Its Application In Water Contamination Research

Chromium is a common pollutant, and Cr isotopic system shows strong sensitivity to the environment changing. The evidence has shown that Cr isotope fractionation occur during the oxidation-deoxidize process. So the Cr isotope can be used to indicate the reduced quantity of Cr (â…¥) in the water and to estimate the contaminated source. So the Cr isotope technique has become a new method for quantificational research on water contamination in the last few years. Cr isotope analysis technique was developed overseas in 2000 and has been applied these years. No work has been done in the field in China. The dissertation has designed the relevant experiment project based on the literature, and developed the process for Cr separation from natural water matrix and mass spectrometric analysis by a series of experiments. All the work has been done in the isotope lad supplied by "State Key Laboratory of Geological Processes and Mineral Resources". The developed process is used to analysis 11 water samples around a chemical plant in Huangshi HuBei province, primary application is studied according to the analysis results which indicated the contamination degree of the water. The main contents in the dissertation include:On the basis of the overseas literatures, the process of Cr isotope chemistry separation and purification are established and optimized. The two-column anion-exchange chromatography method has been developed for the separation of Cr(â…¥) from water samples. The H₂SO₃ is used to reduce the Cr(â…¥) during the process, the relation between the quantity of H₂SO₃ and the recovery of Cr has been determined by systemic conditional experiments. The experiment results have shown that the recovery of Cr come to 100% though the 0.1M H₂SO₃ is reduced by lml, smaller volume of H₂SO₃ can also reduce the background during the process. Both MQ H₂O and 0.1M HC1 have been used to elute Cr in the purification step to give out the difference in the recovery. The result has shown that MQ H₂O just come to 60%～70% while 0.1M HCl come to about 100%. So weak acidic liquor is necessary for eluting the cations which retained in the anion-exchange resin. The same conclusion is very important for the separation of Cr(â…¥) from major elements(Ca,Mg et al). The curve of the eluting process has come out that only 15ml 0.1M HCl can achieve to separate Cr from other metal cations. The author summarize out the operating process of chemical preparation for Cr isotope analyses of the water samples according to the production.Conditional experiments have been carried on the mass spectrometric analysis for Cr isotope on the build of overseas literatures. By studying on the relation between mass spectrometry and the enhancer component, 4ul silica gel(5%)and 1ul H₃BO₃(0.25M) has been determined to obtain the best result. The results of conditional experiments on the mass spectrometric analysis also show that, either the mixture of sample and enhancer or separation of them can give out the same effect. Besides, the single filament technique can bring out measure precision no more than double filament technique; moreover, single filament technique can reduce the expense of filament material. At present, Cr isotope reference standard is NIST SRM979. The correlation of the thermal fractionation with analysis temperature is studied based on the given isotope ratios, the result show that filament current between 2200mA and 2400mA can raise smaller fractionation. The fractionation factorαis determined by the measured ratios of SRM979. The external correction technique is employed to correct for isotope discrimination in mass spectrometry when nature water samples are measured. The technique of Cr isotope determination in thermal-ionization mass spectrometry has been given according to the production.The characteristic of Cr isotopes system for different substance in nature has been summarized according to the literature data. The terrestrial rocks and Cr reagents in industry yieldδ⁵³Cr values close to zero. Theδ⁵³Cr values of the waste water just as plating baths are also close to zero before its release into nature water environment. Theδ⁵³Cr values of Cr(â…¥) will rise gradually after its release and the Cr(â…¥) reduced during the self-decontaminate of the water. The measured groundwaterδ⁵³Cr values range from 1.1 to 5.8‰due to the discrimination among different waters in pollution degree. The fractionation mechanism for Cr isotopes is introduced, the process of oxidation-deoxidize has been known as the mostly causation of Cr isotope fractionation in the water.In the last part of the dissertation, Cr isotopes of the water samples around the chemical factory have been measured by the established analysis technique, the yielded results indicate the water contamination at a certain extent. The analysis data have shown obvious relativity betweenδ⁵³Cr values and water contamination degree, the Cr isotope system characteristic can preferably explain the distribution law of the contaminated water. The relationship between pollution source and theδ⁵³Cr has been studied, based on the space distribution ofδ⁵³Cr values and the local hydrology information in the studying region. All the work gives an example to apply the Cr isotope to define and control the unknown water pollution.In conclusion, the dissertation has been the first one to develop the process of Cr separation from natural water matrix and mass spectrometric, which fill up the gap for Cr isotope research in China. The first batch of Cr isotope data has been obtained and new approach has been offered for water contamination survey.