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

The competitive binding of rare earth elements (REE) to purified humic acid (HA) and MnO₂ was studied experimentally using various HA/MnO₂ ratios over a range of pH (3 to 8). MnO₂, 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/MnO₂ ratio. The value of the distribution coefficients between MnO₂ and solution (log K_d^Ree/Mno₂) increases with the HA/MnO₂ ratio, indicating that part of the REE–HA complexes are adsorbed onto MnO₂. The development of a Ce anomaly appears strongly limited in comparison with inorganic experimental conditions. Throughout the experimental run time, for HA/MnO₂ ratios of less than 0.4, MnO₂ acts as a competitor leading to a partial dissociation of the REE–HA complex. The majority of the dissociated REE is readsorbed onto the MnO₂ surface. The readsorption of REE is expressed by an increased Ce anomaly on the log K_d^Ree/Mno₂ 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 MnO₂ as a REE–HA complex, but also as REE(III). Moreover, the competition between HA and MnO₂ 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.