Aspects of the geochemistry of rare earth elements in coal: an experimental approach

Many coals worldwide are slightly enriched in heavy rare earth elements (HREE) relative to light rare earth elements (LREE) as compared with chondrites and shales. A factor contributing to this fractionation could be the ability of the HREE to form complexes with organic compounds which are more stable than those of the LREE. In order to test this hypothesis, experiments on the sorption of several Light and heavy rare earth elements (La, Ce, Sm, Eu, Gd, Ho, Er, Lu) on xylain and humic acids were performed. The sorption on xylain was found to be pH and time dependent. Maximum quantity was sorbed in the range of pH 3 to 5. Sorption equilibrium was reached in 5 to 7 days. The shape of the sorption isotherms for all REE studied were similar. The maximum content sorbed for each REE was about 0.2 mEq/0.5 g xylain. When all REE were present in equal concentration in the initial solution, the quantity sorbed was also 0.2 mEqv/0.5 g xylain. Nearly the same quantity of Na, K, Ca and Mg were displaced from the sorbent. These results suggest a cation-exchange mechanism in which Na, K, Ca and Mg bound to -COOH and -OH groups were replaced by the REE cations. The capacity of humic acids leached from xylain was found to be about 0.05 mEq REE on 0.250 mg humic acids (HA). The individual REE were sorbed in nearly equal quantities. (C) 1999 Elsevier Science B.V. All rights reserved.