Preparation of Soil Certified Reference Materials for Heavy Metals in Contaminated Sites

In China, the mining district and surrounding areas have been high risk areas for soil pollution by heavy metals. These are also the main areas for environmental monitoring of soil. Unfortunately, soil certified reference materials (CRMs) in which matrix and levels of heavy metals consistent with the actual soil in thse aeras are not available. In this study, based on the pollution characteristics of the heavily polluted soil area and high-risk area, and requirements of practical environmental monitoring, agricultural soil surrounding the mining district of red soil area in southern China was collected and prepared to soil certified reference materials according to GB/T 15000 series rules. The collected material was dried at low temperature, ground, sieved, homogenized and radiation-sterilized. Twenty bottles of samples were randomly chosen in order to test the homogeneity. Results show that standard uncertainty due to between-bottle homogeneity ranges from 0.27% to 2.1% (in Table 2), indicating a good homogeneity. The minimum sampling weight was 0.2 g. X-ray Fluorescence Spectrometry method was used for long-term stability study. Stability was assessed by trend analysis, under the preservation conditions of room temperature without direct sunlight. No instability was observed during 12-month monitoring periods, and standard uncertainty due to long-term stability ranged from 0.06% to 2.1% (in Table 3). Elemental contents were determined by 13 laboratories using two different methods. The elements include Ag, Al, As, Ba, Be, Cd, Co, Cr, Cu, Fe, Hg, Mn, Mo, Ni, Pb, Sb, Se, Ti, Tl, V and Zn. Total uncertainty results from uncertainties in certificated values (u_char), between-bottle homogeneity (u_bb) and long-term stability (u_lts). The expanded relative uncertainty of most elements is smaller than 7%, with the exception of Se. The reference material was approved as the national reference material (GSB 07-3272—2015). The presented CRMs were expected to be useful for method validation or quality control of heavy metal analysis in mining districts and surrounding areas by routine testing laboratories.