| Selenium is an essential trace element for human and animals. In recent years, selenium has attracted the attention of scientists worldwide.Selenium deficiency or excess will affect the human health.In the environment, the mobilization, transformation, bioavailability and toxicity of selenium are mostly being controlled by its chemical speciation. Chemical analysis for element forms is the frontier in the field of analytical chemistry. Generally, the distribution of selenium species in surface environment (rocks, soils, sediments, waters and plants), it plays an important role in environmental and ecological research. Although Selenium concentration in rocks represents the major source to those in soils and plants, little geochemical studies have been done Concerning the Se-rich rocks. In recent years, China Geological Survey Bureau launched a multi-objective regional ecological and geochemical survey, and as a result, large selenium-rich areas were discovered in some provinces and cities. The selenium resources in these regions were evaluated and developed, however,there is a still lack of proper speciation methods and corresponding reference material for Selenium. Therefore rock reference material and developing a sequential extraction method of Selenium is important for studying geochemical behavior of Selenium and exploitating Se resources. The Se-rich rocks in Enshi and Ziyang County were selected for this study. By summarizing the status of development and morphological analysis of selenium speciation combined with the geological and geochemical characteristics of Se-rich rock, we developed fiveSe-rich rock reference materials and standards selenium speciation method. This is the first time to develop reference material for Se-enriched rocks. The Selenium speciation analysis method not only provides technical support for routine analytical laboratory testing work, but also provides an accurate and reliable basis for evaluation of technical data quality assurance for geological survey.This research is divided into the following six aspects:1. A five-step sequential extraction procedure was designed on the previous studies by Kulp(, Zhu Jianming and Zhang zhong. This procedure includes:Water soluble Se, exchangeable Se, organic bound Se, sulfide/selenide associated Se and residual Se. We improved this procedure in order to make it suitable for Se-rich carbonaceous rocks. The extracant,shaking time and liquid-to-solid ratio used in the schemes from rock matrix were discussed, and the determination conditions,linear range,precision and the recovery of standard addition were evaluated. The results showed that the inspection limit and precision(CV %:<10) of the used instrument and the recovery of standard addition(85.1%～114.55%) were good enough for the determination.2. Enshi province is the most typical Se-rich area in China. In 2011, the international human and animal trace element nutrition society, officially considered the city a "World selenium". On the other hand,Ziyang County, Located in Daba region, is the second most selenium-rich area in China after Enshi city. Ziyang became the world famous selenium region because of its special geological conditions. According to the standard material properties and applications, we selected two suitable areas(i.e.,Enshi and Ziyang) for sample collection points. Five representative reference materials samples were collected from Enshi city and Ziyang county. These series of samples reference material have applicability, representativeness.3. By analyzing the composition of selenium-rich black shales from Ziyang County, we found that the Se concentrations in the rocks collected from Gaoqiao and Shuangan towns vary greatly, whereas samples from Wamiao town are quite homogeneous. The major chemical composition of the rock samples comprises SiO2; and the dominant minerals present in the rock samples include:quartz, illite, pyrite, dolomite, albite and calcite. There is a positive linear correlation between selenium content and sulfur content and Selenium content and carbon content; generally, the rock samples in the study areas are enriched in As,Cd,Pb,P and V. The main oxides present in selenium-rich black shales from Enshi city are SiO2, followed by A12O3, CaO and Fe2O3 The rock samples are composed of 72.74% quartz, followed by illite and albite. and they are generally enriched in As,Cd,Cr,Pb,N,V and T1. The average concentration of these elements reach 5.76 mg/kg,10.62mg/kg,228.71 mg/kg,89.52 mg/kg,43.29 mg/kg,684.42 mg/kg and 588.97mg/kg. The highest enriched elements are T1 and Cd, followed by V, whereas, the enrichment factor for other elements are less than ten times.4. According to the "standard" (JJG1006-1994) program, the sample F test to test the homogeneity of the sample were conducted in this study. The results of homogeneity experiments and data statistics showed good homogeneity,which could meet the requirements of National Standard samples. Also, the results show that the five standard substance candidates are uniform and it is possible to continue with the next step which involves testing the values through laboratory collaborative research. The results show that the uncertainty generated from the inhomogeneity and the standard deviation of analytical methods are in a same size, Therefore, in the final calculation of the total uncertainty of the reference material, the candidate substances uniformity factors must be taken into account.5. No statistically significant error was found in the data. All measured data are up to the requirements of measurement. So far, the results showed that the most characteristic values of the candidate samples can remain stable in10-month period. Experimental data indicate that the stability of the reference material is relatively stable and meet the needs of the actual research.6. Standard requirements for reference materials, such as uniformity and stability of qualified inspection have meet the National metrology authentication, and it has a rich experience of analysis and tests at laboratories, all this combined value of setting of five standard substance. We summarize the fixed value providing by seven laboratories, technical decision according to the method of error, laboratory data unqualified rejection detection, the technique to determine the qualified laboratory data using Grubbs test, to determine whether there is a large systematic error detection data provided by laboratory by standard deviation or average deviation difference detection data. The reproducibility of data between different laboratories is good, which basically reached the requirements of characterization. |
PDF Full Text Request