Effects of Fe competition on REE binding to humic acid: Origin of REE pattern variability in organic waters

Competitive mechanisms between rare earth elements (REE) and iron (Fe) for humic acid (HA) binding were investigated by coupling laboratory experiments and modeling calculations using PHREEQC/Model VI. This study aims, firstly, at determining the effect of Fe on REE–HA binding, in order to explain the REE pattern variability observed in natural organic-rich waters. Secondly, it has previously been shown that light and heavy REE (L- and HREE) speciation with HA molecules differ with pH. Therefore, REE–HA complexation patterns have been used as a probe of Fe–HA binding mechanisms. At pH 3, i.e. pH conditions at which Fe3 + binds to HA, Fe is shown to be a strong competitor for heavy REE (HREE), suggesting that Fe3 + has a marked affinity for the few strong HA multidentate sites. At pH 6, i.e. under pH conditions at which hydrolyzed Fe species bind to HA, Fe appears to compete equally for every REE, thereby indicating that Fe has the same relative affinity for carboxylic and phenolic HA sites as LREE and HREE, respectively. Fractionation of REE in organic-rich natural waters depends mainly on the coupling of two factors: (i) the total dissolved metal concentration (i.e. the HA metal loading) and (ii) the competition between REE and major cations (i.e. Fe and Al). The pH, which regulates the speciation of these competitive metals, is, therefore, indirectly the main controlling factor of REE fractionation in organic-rich waters.