Mechanism of lead pollution detection in soil using terahertz spectrum

Heavy metals in soil are dangerous contaminants that limit the development of modern agriculture. The terahertz time-domain spectroscopy technique has shown its potential for rapid detection of heavy metals. However, the related detection mechanism has not been revealed. We combined terahertz technology with path analysis to observe and identify lead pollution in soil at different pH levels (5.5, 7.0, 8.5), and to analyze the related mechanism. First, solutions with pH values of 5.5, 7.0, and 8.5 were configured to prepare the soil samples with lead contents of 300, 600, 900, and 1200 mg/kg. The pressed-slice method with a pressure of 3.5 t and 200 mg in weight was selected to produce the soil tablets at each level. The terahertz technique was used to provide the terahertz absorption spectra. Then, the distribution of each chemical binding state of lead in soil samples under different pH conditions was observed. Finally, a path analysis method was used to explore the influence of each chemical binding state on the terahertz spectrum. The results showed that the bound state of Fe–Mn oxide lead and exchangeable lead had the greatest influence on the terahertz spectral curve, promoting and inhibiting the terahertz absorption, respectively, whereas other bound states presented an auxiliary role. This research investigates the detection mechanism of the influence of different bound lead in the soil on the terahertz absorption spectrum and provides a theoretical basis for the development of a rapid detection method using terahertz technology.