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In surface and near-surface weathering environments, the mobilization and partial loss of palladium (Pd) under oxidizing and weakly acidic conditions has been attributed to aqueous chloride complexation. However, prior work has also observed that a portion of Pd is retained by iron (oxyhydr)oxides in the weathering zone. The effect chloride has on the relative amount of Pd mobilization versus retention by iron (oxyhydr)oxides is currently unclear. We studied the effect of chloride complexation on Pd(II) adsorption to two iron (oxyhydr)oxides, hematite and 2-line ferrihydrite, at pH 4. Increasing chloride concentration suppresses Pd adsorption for both hematite and ferrihydrite, which displaymore » similar binding affinities under the conditions studied. Thermodynamic modeling of aqueous Pd speciation indicates that greater suppression of binding to iron (oxyhydr)oxides should occur than is observed because of the strength of Pd-Cl complexation, implying that additional interactions at the mineral surface are counteracting this effect. While increasing dissolved chloride concentration does not measurably impact mineral surface charging, extended X-ray absorption fine structure (EXAFS) spectra indicate that ternary Pd-Cl surface complexes form on both hematite and ferrihydrite. The number of Cl ligands in the surface species increase at greater chloride concentration. A mixture of bidentate and monodentate surface species are indicated by the EXAFS spectra, although the fitting uncertainties precludes determining whether these vary in relative abundance with chloride concentration. In order to offset the effect of strong aqueous Pd-Cl complexation and align with our EXAFS results, a surface complexation model developed for Pd adsorption to hematite involves a mixture of three ternary surface complexes containing 1, 2, and 3 chloride ligands. Our results show that Pd is mobilized as a chloride complex in platinum group element-rich weathering zones. As a result, porewater chloride concentrations are thus a dominant control on Pd retention by iron (oxyhydr)oxides in these weakly acidic environments.« less

The adsorption of rare earth elements (REEs) to iron oxides can regulate the mobility of REEs in the environment and is heavily influenced by water chemistry. This study utilized batch experiments to examine the adsorption of Nd, Dy, and Yb to goethite under varying pH, electrolyte (type and concentration), and concentrations of dissolved inorganic carbon and citrate. REE adsorption was strongly influenced by pH, with an increase from essentially no adsorption at pH 3.0 to nearly complete adsorption at pH 6.5 and higher. Citrate enhanced the adsorption of REEs at low pH (<5.0), likely by forming goethite-REE-citrate ternary surface complexes.more » However, citrate inhibited the adsorption of REEs at higher pH (>5.0) by forming aqueous REE-citrate complexes. Ionic strength had a small influence on REE adsorption, and the presence of dissolved inorganic carbon had no discernible effect. Equilibrium adsorption was interpreted with a triple-layer surface complexation model (SCM). The selection of surface complexation reactions was guided by extended X-ray absorption fine structure spectra. An SCM with a single bidentate inner-sphere surface complexation reaction for Nd and two inner-sphere surface complexation reactions (one monodentate and one bidentate reaction) for Dy and Yb effectively simulated adsorption across a broad range of conditions in the absence of citrate. Accounting for the effects of citrate on REE adsorption required the addition of up to two ternary REE-citrate-goethite surface complexes. The SCM can enable predictions of REE transport in subsurface environments that have goethite as an important adsorbent mineral. Furthermore, this predictive capability could contribute to identifying potential REE sources and facilitating efficient extraction of REEs.« less