Introduction

With the rapid development of industrialization and urbanization, soil environmental issues have attracted more and more attention. Soil heavy metal pollution, especially soil metal pollution in industrial areas, has gradually become a hot spot in environmental scientific research. The heavy metals in soil pollution mainly refer to heavy metals such as mercury, cadmium, lead, chromium and various metals with obvious toxicity, such as arsenic. They also refer to general heavy metals with certain toxicity, such as zinc, copper, cobalt, nickel and tin. The pollution of soil by heavy metals is a short-term irreversible process. In the soil-plant system, heavy metal pollution enters agricultural products through the food chain, affecting the quality and safety of agricultural products and endangering human health. Therefore, it is necessary to monitor heavy metal pollution in the soil.

According to the latest survey data from the Ministry of Land and Resources, more than 10% of China's arable land has been polluted by heavy metals. Most of them are concentrated in economically developed areas. The China International Environment and Development Cooperation Committee announced that the Ministry of Environmental Protection has conducted a sample survey of harmful heavy metals in the soil of 300,000 hectares of basic farmland in China. The study found that 36,000 hectares of soil exceeded the standard for heavy metals, exceeding the standard by 12.1%.

In recent years, heavy metal pollution accidents have occurred frequently. These heavy metal pollution incidents not only brought huge economic losses to the society, but also threatened people's healthy lives. They have also attracted the attention of relevant national environmental protection departments. At the same time, it is particularly urgent and important to deepen the prevention and control of heavy metals and deepen the monitoring of heavy metals.

At the same time, on December 14, 2015, the Ministry of Environmental Protection issued "Environmental Standards for Soils and Sediments HJ 780-2015", "Measurement of Inorganic Elements in Soils and Sediments by Wavelength Dispersive X-ray Fluorescence Spectrometry". This standard was implemented on February 1, 2016. The standard specifies the mass fraction range of 25 inorganic oxides and 7 oxides. In response to this standard, ESI launched the EDX9000B fluorescence spectrometer, which can quickly and accurately analyze the metal elements in the soil. The detection limits of lead and arsenic are slightly higher than the standard requirements; Fe, Co, Ni, Cu, Zn, Ga, Rb, Y, Ba, Sr, Br, Th and K, Ca, Ti, V, Cr, Mn The detection limit and quantification limit of Zr element fully meet the requirements of HJ 780-2015 environmental protection standard.

The basic principle of XRF spectrometer:

X-rays have the unique ability to ionize or "excite" the elements present in the material. When ionized elements return to a relaxed or stable state, they emit fluorescent X-rays whose energy level is a "characteristic" of the specific element that emits these X-rays.

The ESI XRF analyzer takes advantage of this phenomenon by sending ionized X-rays into the sample, measuring the energy level of the returned fluorescent X-rays (the "feature" of the element), and counting these X-rays to determine the relative concentration of each individual.

Energy dispersive X-ray spectrometer:Compass200; an electronic balance (precision 0.0001g); an automatic laminating machine (pressure not less than 40T); a blast drying furnace; a vibration mill; a non-metallic sample sieve (200 Item)

Reagents: boric acid powder (analytical grade); soil national standard materials; soil samples

1.2 Sample collection, storage and pretreatment

The collection and maintenance of soil samples are carried out in accordance with HJ/T166. Air drying or sample drying should be carried out in accordance with the relevant regulations of HJ/T166. After the sample was ground, it was passed through a 200-mesh sieve and dried at 105°C for use.

Use an electronic balance to weigh 5.00 g sieved (200 mesh) soil standard or sample and 12.00 g boric acid powder (strip material) with a weighing error of ±0.05 g. Then it is put into a tablet press to form a sheet, the pressure is 30T (pressure range is 20~30T), and the holding time is 30s.

1.3 Calibration establishment and sample analysis

Set appropriate measurement conditions, use Compass200 to scan national standard materials (referred to as standard products) GSS-1~GSS-15 to establish a linear working curve of the content and intensity of the elements in the soil standard. Then, the unknown sample is measured.

2. Measurement and data analysis

2.1 Detection limit of metals in soil

Compass 200 is equipped with three sets of filters, which can set the best test conditions according to the characteristics of the elements of interest in the soil. Use the national soil standard GSS-1-GSS-15 calibration instrument to establish an environmental soil working curve. Under the environmental soil working curve, high-purity SiO2 was used as the blank substrate and tested 11 times continuously. According to the detection limit formula: 3 times the standard deviation of the blank matrix divided by the sensitivity of the instrument.

1. Co is seriously interfered by Fe element Kb, and the extremely low detection limit is determined by the formula.

2. Since this model is not evacuated (or filled with helium), light elements such as Na2O, MgO, Al2O3 and SiO2 are not used. However, La, Ce and Hf are not affected by the scattering background. P, S and Cl are higher in the national soil standards. There is no suitable low standard. The main reason is that the background is greatly affected. Do not consider large, irregular spectrum.

3. The elements that meet the national standards are K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Rb, Y, Ba, Sr, Br, Th and Zr. Arsenic and lead