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

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 (i) they have no specific constraints for electrochemical or spectrometric analysis for element quantification and (ii) they are more environmentally friendly than the ones often used for mineral exploration, e.g. the aqua regia (AR) or mobile metal ions such as Ionic Leach™ (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.