The Research On Soil Heavy Metal Pollution Detection Based On Dielectric Bilinear Theory

Heavy metal pollution in the soil endangers human health,disrupts ecological balance,and threatens food security.However,real-time on-site detection of heavy metals has always been a technical problem at present,and an accurate,economical and practical on-site detection method is urgently needed.This paper builds a new bilinear dielectric calculation model based on the dielectric two-wire theory and explores the adaptability of the model to the measurement of soil dielectric properties.On this basis,soil dielectric spectroscopy measurements were carried out on loess,loessial soil and sandy soil under 6 different pollution concentrations of heavy metals(Cu²⁺,Cr⁶⁺),4 different volumetric water contents and 7 different temperatures.The effects of different heavy metal(Cu²⁺,Cr⁶⁺)pollution concentrations on soil dielectric properties under different physical conditions（soil texture,temperature,frequency,and volumetric water content）were investigated,and the heavy metal(Cu²⁺,Cr⁶⁺)pollution concentrations were analyzed.The relationship with the soil dielectric constant（complex dielectric constant,apparent dielectric constant）and the influence of soil texture,temperature,frequency and volumetric water content.For the heavy metals copper(Cu²⁺)and chromium(Cr⁶⁺)in soil,empirical formulas for the detection of pollution concentration in frequency domain dielectric method were constructed,respectively.Regression correlation analysis and verification sample verification applicability analysis were conducted for the two empirical formulas.This article draws the following main conclusions:（1）The new bilinear dielectric calculation model based on the dielectric bilinear theory has a good applicability to the measurement of dielectric spectroscopy of mixed materials based on the14.28mm self-made calibration probe.（2）The model has good adaptability to the measurement of soil dielectric properties.Based on the 14.28mm self-made calibration probe,905MHz～1903MHz is the optimal frequency band for measuring the moisture content of the soil frequency domain dielectric method with the real part of the complex dielectric and the apparent dielectric constant,and 1502.88MHz is the optimal frequency point.Compared with the Topp formula,Roth formula,and Malicki formula,the empirical formula of soil volumetric water content calculated in this paper has better correlation with the empirical calculation value of the soil measurement and the measured moisture content of the drying method.Its R²=0.8907 and RMSE=0.034cm³/cm³,RPD=3.026;The correlation between the calculated value of the empirical formula for the determination of the volumetric water content of the real part of the soil and the moisture content measured by the drying method is R²=0.9126,RMSE=0.0294 cm³/cm³,and RPD=3.343.（3）The bilinear dielectric calculation model has a good effect on the measurement of dielectric properties of soil heavy metals(Cu²⁺,Cr⁶⁺)pollution.In the frequency range from 1MHz to 700MHz,the imaginary part of the complex permittivity of the soil can more sensitively show the influence of heavy metals(Cu²⁺,Cr⁶⁺)on the dielectric properties of soil;In the frequency range from 100MHz to700MHz,the relationship between the imaginary value of soil complex dielectric constant and the concentration of heavy metal pollution is stable with frequency,and the imaginary value of soil complex dielectric constant under different pollution concentrations of heavy metals(Cu²⁺,Cr⁶⁺)is significantly distinguished.The quantitative analysis of the correlation between the imaginary value of the complex permittivity of the soil and the different pollution concentrations of heavy metals(Cu²⁺,Cr⁶⁺)is affected by the combined effects of temperature,frequency and volumetric water content.It is found that based on the 14.28mm homemade calibration probe,300.42 MHz～500.04MHz frequency band is the ideal frequency band for the detection of soil heavy metal(Cu²⁺,Cr⁶⁺)pollution concentration by frequency domain dielectric method,and 300.42 MHz is the best frequency point.Regression correlation analysis of empirical formula found,the two empirical formulas have a good linear correlation between the calculated values of the heavy metal copper(Cu²⁺)and heavy metal chromium(Cr⁶⁺)pollution concentrations in the soil and the measured values.Their R² is greater than 0.92,the RMSE is less than 35.0mg/kg,and the RPD Greater than 4.8.Analyze the data of verification samples,the correlation between the calculated values of the heavy metal pollution concentrations and the measured values of the two empirical formulas,the determination coefficient R² is greater than 0.91,the RMSE is less than 36.0mg/kg,and the RPD is greater than 4.5.The data results show that the empirical formula for frequency domain dielectric method detection of soil heavy metal copper(Cu²⁺)pollution concentration and the empirical formula for frequency domain dielectric method detection of soil heavy metal chromium(Cr⁶⁺)pollution concentration based on bilinear dielectric calculation model are respectively The detection accuracy of the concentration of heavy metals copper(Cu²⁺)and chromium(Cr⁶⁺)in the soil is more accurate,and the applicability is better.