New methodological approach for deep penetrating geochemistry and environmental studies part 1: on-site soil extraction of trace and rare earth elements

Overview & Synopsis

The use of on-site soil extractions for direct quantification provides a fast and efficient method for environmental monitoring and mineral prospecting. In this study, an on-site extraction protocol was developed and tested on northern boreal soils using partial extractants: 0.01 M calcium chloride (CaCl2) and 0.43 M nitric acid (HNO3). CaCl2 is commonly used as an extractant for mobile and directly available elements, while 0.43 M HNO3 is used to extract also the reactive pool with the weakly bound elements from soil particle surfaces. These two extractants were chosen because they have (i) no specific constraints for electrochemical or spectrometric analysis for element quantification and (ii) are more environmentally friendly than the ones often used for mineral exploration; e.g., the aqua regia (AR) or mobile metal ions like Ionic leachTM (IL, ALS). Extractions with HNO3 offered the same patterns as those obtained with AR but with better contrast for anomalies, while the CaCl2 results were very similar to those obtained with IL. Nevertheless, neither HNO3 nor CaCl2 was effective in mobilizing Au, W, or Sn due to their low concentrations. Our study demonstrated that on-site soil extractions with HNO3 and CaCl2 were potentially good candidates for the detection of REEs, such as Ce and La, and some trace elements, especially Cd, for both extractions, Co, Pb and U with HNO3, and Mn and Ni with CaCl2. This work highlights the potential of on-site extraction using partial extractants. This is a key step prior to the quantification of elements by on-site voltametric techniques.