Research on heavy metal ions detection in soil with terahertz time-domain spectroscopy

Nowadays, heavy metals pollution is becoming a serious problem in agriculture. This paper reports a preliminary work on a feasibility study of applying terahertz (THz) technology for heavy metal ions detection in soil. This study was first conducted at Oklahoma State University, and then carried out at China Agricultural University and Capital Normal University. Pure soil was collected in an experimental field, which contains nearly no heavy metal ions from standard detection; in the experiment, heavy metal ions were mainly Pb2+, Cr3+, Zn2+ and Cu2+ from chemical compounds. Based on the National Standard for Heavy Metals Pollution, a set of soil samples with different polluted levels were prepared in the lab. The metal ions concentration levels were selected as 50ppm, 300ppm and 700ppm. Each soil sample was pre-processed by collecting, weighing, mixing, drying, grinding and labeling before measurements. The thickness of soil samples was selected as 1.5mm and 3mm. The absorption spectra for the soil samples with different heavy metal ions were collected using THz time-domain spectroscopy (THz-TDS) equipments separately at Oklahoma State University and Capital Normal University. The test results showed that soil samples with Pb2+, Cr3+, Zn2+ and Cu2+ had different absorption characteristics within the bandwidth of 0.1-1.1THz. A narrow bandwidth only up to 1.1THz was got because the soil particles showed much absorption and scattering properties to the THz spectroscopy. Different soil samples with different concentration of heavy metal ions also showed much difference and it could be used to predict the heavy metal concentration in the future. The results from the preliminary study show a potential of THz technology applied for heavy metal ions detection in agricultural fields environment. However, since the high scattering features of samples and high cost of equipments, the measurement methods and practical issues needs to be further investigated and improved to make the THz technology a feasible tool for soil heavy metal ions detection.