Reflectance spectroscopy study of Cd contamination in the sediments of the Changjiang River, China

Sediments of the Changjiang River have been found in recent studies to be enriched in cadmium (Cd). The possibility and mechanisms for evaluating total Cd concentration and its binding form using reflectance spectroscopy within the visible-near-infrared (VNIR) region (400-2500 nm) have been investigated. Bottom sediments (69 samples) in the lower reaches of the river were collected for chemical analyses and spectral measurements. Total Cd concentration in the sediments was found to be exponentially related to the spectral proxies for organic matter (spectral reflectance at 400-530 nm), clay minerals (first derivative (FD) values at the shoulders related to absorption bands near 1400, 1900, and 2200 nm), and Fe oxides (FD values at 560-760 nm). The results indicated that the spectrally featureless Cd was mostly bound to these spectrally active materials, which made it possible for Cd concentration to be determined from reflectance spectra. This conclusion was also confirmed by the results of chemically sequential extraction of Cd. This study has demonstrated the usage and theoretical basis of reflectance spectroscopy, which is a rapid and inexpensive analytical method, for evaluating contamination by heavy metals and their binding forms in sediments.