Aluminium competitive effect on rare earth elements binding to humic acid

International audienceCompetitive mechanisms between rare earth elements (REE) and aluminium for humic acid (HA) binding were investigated by combining laboratory experiments and modeling to evaluate the effect of Al on REE–HA complexation. Results indicates that Al3+ competes more efficiently with heavy REE (HREE) than with light REE (LREE) in acidic (pH = 3) and low REE/HA concentration ratio conditions providing evidence for the Al high affinity for the few HA multidentate sites. Under higher pH – 5 to 6 – and high REE/HA conditions, Al is more competitive for LREE suggesting that Al is bound to HA carboxylic rather than phenolic sites. PHREEQC/Model VI Al–HA binding parameters were optimized to simulate precisely both Al binding to HA and Al competitive effect on REE binding to HA. REE–HA binding pattern is satisfactorily simulated for the whole experimental conditions by the ΔLK1A optimization (i.e. ΔLK1A controls the distribution width of log K around log KMA). The present study provides fundamental knowledge on Al binding mechanisms to HA. Aluminium competitive effect on other cations binding to HA depends clearly on its affinity for carboxylic, phenolic or chelate ligands, which is pH dependent. Under circumneutral pH such as in natural waters, Al should lead to LREE-depleted patterns since Al is expected to be bound to weak HA carboxylic groups. As deduced from the behavior of Al species, other potential competitor cations are expected to have their own competitive effect on REE–HA binding. Therefore, in order to reliably understand and model REE–HA patterns in natural waters, a precise knowledge of the exact behavior of the different REE competitor cations is required. Finally, this study highlights the ability of the REE to be used as a “speciation probe” to precisely describe cation interactions with HA as here evidenced for Al