Quantitative Estimation Of Concentrations Of Dissolved Rare Earth Elements Using Reflectance Spectroscopy

Weathered crust rare earth ore is a new type of rare earth resource in China, which was first discovered in Jiangxi in 1969. Nowadays, unauthorized mining of weathered crust rare earth ores becomes more and more serious as the price of rare earths(REE) is rising, which lead to severe pollution and wastage of REE, Estimation of concentration of dissolved REE in the leaching liquor pools and the rivers nearby plays an important role for solving the pollution and wastage problem. Conventionally, quantitative measurement of the abundance of REE in aqueous solutions is done by laboratory-based analytical techniques, which are time-consuming and costly. Consequently, a simple and economic method is necessary for measuring the content of REE in aqueous media.Spectral reflectance in the visible and near-infrared wavelengths provides a rapid and cost-effective means for characterizing the chemical composition of absorbing materials. Spectral parameters such as depth of absorption bands are widely used to quantitatively estimate the composition of samples from hyperspectral reflectance data in soil, mineralogy as well as vegetation study. However, little research has been conducted on the spectral characteristic of REE and their relation with chemical features. In our study, ten aqueous media samples collected in south Jiangxi and twenty-five aqueous media samples made in laboratary were obtained; with the reflectance spectra and concentrations of REE were measured by reflectance spectroscopy and inductively coupled plasma mass spectrometry(ICP-MS), respectively. Based on the analysis of the spectra of these samples and spectra study of REE by predecessors, we found the spectra of the dissoved REE present the mixture spectral characteristics of pure water and REE oxide. Six diagnostic absorption features of REE were recognized in visible and near-infrared wavelengths, and several little absorption features of REE were also recognized. Reflectance, differential reflectance, absorption depth were analyzed with the concentrations of REE, and the results indicated that absorption depth analysis was coherent with the absorption characteristic of REE spectra, with the r2 value more than 0.97. Then the intensity of each of these six absorption band is linearly correlated with the abundance of total REE, and linear regression models were derived that can be used for estimating the concentration of REE in aqueous media samples. Finally, detection limits of REE quantitative modeling was derived at about 30ppm by laboratory study.Through the correlation analysis of concentrations of fifteen REEs and spectral absoprtopn depth, the REE group(light REE or high REE) can be derived. The outcome of this study can be applied to quantitative estimation of rare earth contamination of stream water systems nearby rare earth ores; and also has implications for quantitative eatimation of rare earth elements in other media such as weathering crusts and rocks.In addition,the study provides theoretical support for REE extration by aerospace hyperspectral remote sensing.