Competitive binding of REE to humic acid and manganese oxide: Impact of reaction kinetics on development of cerium anomaly and REE adsorption

Abstract

The competitive binding of rare earth elements (REE) to purified humic acid (HA) and MnO 2 was studied experimentally using various HA/MnO 2 ratios over a range of pH (3 to 8). MnO 2, humic acid and REE solutions were simultaneously mixed to investigate the kinetics of the competitive reactions. Aqueous REE–HA complex is the dominant species whatever the experiment time, pH and HA/MnO 2 ratio. The value of the distribution coefficients between MnO 2 and solution (log K d Ree/Mno 2 ) increases with the HA/MnO 2 ratio, indicating that part of the REE–HA complexes are adsorbed onto MnO 2. The development of a Ce anomaly appears strongly limited in comparison with inorganic experimental conditions. Throughout the experimental run time, for HA/MnO 2 ratios of less than 0.4, MnO 2 acts as a competitor leading to a partial dissociation of the REE–HA complex. The majority of the dissociated REE is readsorbed onto the MnO 2 surface. The readsorption of REE is expressed by an increased Ce anomaly on the log K d Ree/Mno 2 pattern as well as a change in shape of the coefficient distribution of REE between soluble HA and solution pattern (log K d Ree/HA decrease for the heavy rare earth elements — HREE). Thus, REE are not only bound to MnO 2 as a REE–HA complex, but also as REE(III). Moreover, the competition between HA and MnO 2 for REE binding is shown to be higher at low pH (< 6) and low DOC/Mn ratio. This study partially confirms previous work that demonstrated the control of REE adsorption by organic matter, while shedding more light on the impact of pH as well as complexation reaction competition on long-term REE partitioning between solid surface and organic solutions. The latter point is important as regards to REE speciation under conditions typical of rock and/or mineral alteration.