Competition between Humic Acid and Carbonates for Rare Earth Elements Complexation

Olivier Pourret (1) , Mélanie Davranche (1) , Gérard Gruau (1) , Aline N. Dia (1)

1 GR - Géosciences Rennes

The competitive binding of rare earth elements (REE) to humic acid (HA) and carbonates was studied experimentally at various pH and alkalinity values by combining Ultrafiltration and Inductively Coupled Plasma Mass Spectrometry techniques. The results show that the REE species occur as binary humate or carbonate complexes but not as ternary REEcarbonate-humate as previously proposed. The results also reveal the strong pH and alkalinity dependence of the competition as well as the existence of a systematic fractionation across the REE series. Specifically, carbonate complexation is at a maximum at pH 10 and increase with increasing alkalinity and with the atomic number of the REE (LuCO3 >> LaCO3). Modelling of the data using Model VI and recently published stability constants for complexation of REE by humic acid well reproduced the experimental data, confirming the ability of Model VI to accurately determine REE speciation in natural waters. This modelling also confirms the reliability of recently published stability constants. This work shed more light not only on the competition between carbonates and HA for REE complexation but also on the reliability of WHAM 6 and Model VI for calculating the speciation of REE with organic matter in alkaline organic rich-water.

Keywords: ultrafiltration; Rare earth elements; humic acid; carbonates; binding; speciation

Domains: Geochemistry

Olivier Pourret, Mélanie Davranche, Gérard Gruau, Aline N. Dia. Competition between Humic Acid and Carbonates for Rare Earth Elements Complexation. Journal of Colloid and Interface Science, 2007, 305 (1), pp.25-31. ⟨10.1016/j.jcis.2006.09.020⟩. ⟨insu-00154696⟩