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

Abstract

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.

Validation of microwave acid digestion, diffusive gradients in thin-film preconcentration and inductively coupled plasma optical emission spectrometry methodology for the determination of REEs in natural zeolites

“…LODs in the range of 0.07–29 μg L −1 were attained for REEs measured by ICP-OES in geological samples digested with condensed phosphoric acid. 46 Li et al 27 obtained improved LODs in the range of 0.0548 (Tb)–0.541 (La) μg L −1 for REE determination in fruit samples by ICP-OES, after preconcentration with N , N , N ′, N ′-tetraoctyl diglycolamide (TODGA). Compared to ICP-OES, lower LODs have been reported in several other studies using ICP-MS as the detection technique.…”

Atomic spectrometry update – a review of advances in environmental analysis

“…For the quantification of REEs in geological samples by ICP-AES, a digestion method using condensed phosphoric acid instead of HF was re-evaluated. 247 Results for six international RMs and three Brazilian ore samples indicated that although many potential REE-bearing minerals were dissolved by this method, zircon and xenotime remained in the undigested residue so the method should be used with caution depending on the sample mineralogy. Rondan et al 248 developed a digestion method for the ultra-trace determination of Se and Te in coal by ICP-MS using microwave-induced combustion under O 2 at 20 bar without the use of HF.…”

A general strategy for the voltammetric trace determination of tellurium in geochemical and environmental matrices after arsenic coprecipitation and critical assessment of digestion schemes

“…The boiling mixture of HClO 4 with H 3 PO 4 has been recommended as a reliable reagent especially for the analysis of sulfides, because no losses of sulfur occur (Hoyle and Diehl 1971). H 3 PO 4 , either on its own or in conjunction with HClO 4 , has recently been discussed as an alternative to HF for the determination of rare earth elements (REE) in a variety of matrices (Hannaker and Hou 1984;Bezerra de Oliveira et al 2019).…”