Determination of Rare Earth Elements in Sepiolite by ICP-MS Using Microwave Digestion

Abstract

BACKGROUND Sepiolite is a fibrous hydrous magnesium rich silicate clay mineral with the content of rare earth elements between 1×10-7 and 1×10-5. There is no national standard method for the determination of rare earth elements in sepiolite. Determination of rare earth elements in rocks commonly uses ICP-MS. Sample pretreatments generally use closed digestion and alkaline fusion, but these two treatments are time consuming and inefficient. OBJECTIVES To find an efficient dissolution method for the determination of 15 rare earth elements in sepiolite. METHODS By comparing three sample pretreatment methods of nitrate hydrofluoric acid hydrogen peroxide, nitric acid-hydrofluoric acid and nitric-acid hydrogen peroxide, the nitric acid-hydrofluoric acid solution system was selected. Microwave digestion was carried out and the hydrofluoric acid was driven off to avoid the formation of insoluble fluorides from the hydrofluoric acid. The content of 15 rare earth elements was determined by ICP-MS. RESULTS Due to the high content of magnesium in sepiolite, two internal standards of 103Rh and 185Re are selected to compensate for the drift of analytical signals and to correct matrix effects. The recoveries of the method range from 91.2% to 110.9%. The detection limits are 0.002-0.011 μg/L, and the precisions are 0.81%-2.79%. CONCLUSIONS The result of this method is in good agreement with that of the closed acid-digestion ICP-MS method, and with a small amount of acid (7 mL), high digestion efficiency (1 h), and a lower detection limit.

Keywords

• sepiolite
• rare earth elements
• microwave digestion
• nitric acid-hydrofluoric acid
• inductively coupled plasma-mass spectrometry