Nitrate () is a common pollutant in groundwater. Nitrate nitrogen isotope provides a direct means for identifying pollution sources and studying nitrogen transformation mechanism. The traditional nitrogen isotope analysis methods are all physical and chemical methods to convert samples into gas N2 which can be determined by mass spectrometry, while the denitrifying bacteria method is to convert samples into gas N2O which can be determined by mass spectrometry by using naturally occurring denitrifying bacteria without N2O reductase. The advantage of denitrifying bacteria method is that it can be carried out at normal temperature and pressure due to the action of enzyme. Because of the specificity of the enzyme, it can only be converted into N2O from complex solution, which is not affected by organic nitrogen, thus eliminating the tedious sample pretreatment and greatly shortening the analysis time. This method can analyze the isotopic composition of nitrate nitrogen with the concentration as low as μg/L, and the required sample size is small (n μ l ~ n mL), because the background value of atmospheric N2O greatly reduces the detection limit of low mass spectrometry. The specificity of low-pass enzyme reaction is improved; By transforming the ions to be measured into gases, interfering substances can be greatly purified. In addition, denitrifying bacteria method can also be used to test nitrate oxygen isotope, but isotope fractionation and exchange must be corrected. The experimental process of denitrifying bacteria method includes the following key steps: the selection and cultivation of bacteria, the transformation of samples into N2O, the extraction and purification of N2O, isotope test and the correction of isotope test results.
On the basis of predecessors' work, the project has been technically improved according to laboratory conditions, mainly including: special bacterial culture bottle, bacterial culture temperature, special N2 purging device, sample pretreatment, appropriate sample amount, isotope mass spectrometer with preconcentration device to purify samples, δ 15N test, etc. A simple, efficient and applicable denitrifying bacteria method for nitrate nitrogen isotope analysis in groundwater and sediments was established. This technology can be used to prepare samples in general microbiology laboratory and send them to commercial isotope laboratory for nitrogen isotope mass spectrometry detection, which has popularization significance. The sample size of 6 ~ 20μ g is suitable for the analysis system. Δ δ15n analysis of international isotope standards and field samples has good repeatability and accuracy better than 0.5‰.
Second, the scope of application and application examples
This technique can be used to test nitrate nitrogen isotopes in vadose zone sediments underlying groundwater and soil, as well as surface water and soil. This technique has been applied to the determination of nitrate nitrogen isotopes in thick vadose zone and groundwater in sewage irrigation area of Shijiazhuang suburb, showing its superiority. It can also be used to detect thick vadose zone sediment samples with small sample size, especially sand samples with low nitrate content. According to the results of nitrogen isotope analysis, the source of nitrate pollution in groundwater in sewage irrigation area is determined, and the prevention measures are put forward. This technology was rated as one of the top ten work progress in our hospital in 2008. It was introduced at the symposium on hydrogeochemistry and isotope technology held in Harbin in 2009, and related papers were published in Progress of Earth Science, Nuclear Technology, Journal of Earth Science-China Geo University and Environmental Geoscience, which attracted wide attention.
Third, promote the transformation mode.
Conference exchange, technical consultation, technical service, etc.
Technical support unit: Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences.
Contacts: Shen and Zhang Cuiyun
Mailing address: No.268, Zhonghua North Street, Shijiazhuang, Hebei Province
Postal code: 05006 1
Tel: 03 1 1-67598657
E-mail :zshenghb@vip.sina.com.cn Shen Jianmei 1965@sina.com.