Hydrofluoric Acid‐Free Digestion of Geological Samples for the Quantification of Rare Earth Elements by Inductively Coupled Plasma‐Optical Emission Spectrometry

• Published in: Geostandards and geoanalytical research 2019-12, Vol.43 (4), p.689-699
• Main Authors: Bezerra de Oliveira, Andrey Linhares, Afonso, Júlio Carlos, Silva, Lilian, Alcover Neto, Arnaldo, Castro Carneiro, Manuel, Dias da Silva, Lílian Irene, Couto Monteiro, Maria Inês
• Format: Article
• Language: English
• Subjects: Acids; condensed phosphoric acid; Digestion; Earth; Emissions; Environmental studies; Fluorides; Geological samples; hydrofluoric acid‐free digestion; ICP‐OES; Phosphoric acid; Plasma; Rare earth elements; Scientific imaging; Spectrometry; Total dissolved solids; Trace elements; Zircon
• ISSN: 1639-4488; 1751-908X
• DOI: 10.1111/ggr.12286

Rare earth elements (REEs) are very important to technological development as well as to geochemical and environmental studies. In this work, hydrofluoric acid (HF) was replaced by condensed phosphoric acid (CPA) in the digestion of geological samples, and the quantification of REEs was performed by inductively coupled plasma‐optical emission spectrometry (ICP‐OES). Six international reference materials (RMs), named DC86318, CGL 111, CGL 124, CGL 126, OKA‐2 and COQ‐1 and three Brazilian ore samples, named Araxá, Catalão and Pitinga were analysed. Only zircon and xenotime, which are potential REE‐bearing minerals, were not completely dissolved. Nevertheless, no REE associated with zircon was detected. The investigated digestion method presented many advantages: It was relatively fast (3 h), avoided fluoride precipitation, it was less hazardous because handling diluted H3PO4 is safer than HF, NH4F or NH4HF2 aqueous solutions, it preserved the quartz fittings of the measurement equipment and the final solution contained lower levels of total dissolved solids than those produced by the fusion method. Key points Hydrofluoric acid was replaced by condensed phosphoric acid in the digestion of geological samples. Six international RMs and three Brazilian ore samples, which contained different potential REE bearing minerals, were analyzed. The digestion method was effective for the dissolution of most of the potential REE bearing minerals.