Neodymium (Nd) is a rare earth element (REE) that is essential for high-tech products in energy generation and storage, transportation, communication, and computing. Adsorption of metals, including REEs, to iron oxides plays a critical role in controlling their mobility in the environment and is strongly influenced by water chemistry.  The mobility of Nd in the subsurface influences its occurrence in ore deposits. In this study, batch experiments of Nd adsorption to goethite were conducted with varying water chemistry parameters, including pH (from 3.0 to 8.0), background salt (0.01 M NaCl, 0.001 M NaCl, and 0.01 M CaCl<sub>2</sub>), and dissolved inorganic carbon (equilibrium with air and carbonate-free). The adsorption of Nd was also investigated in the presence of 100 µM citric acid, which is a common organic ligand in natural soil and water systems. The macroscopic equilibrium adsorption was interpreted within the reaction-based equilibrium framework of surface complexation modeling (SCM). A set of surface complexation reactions were identified together with their equilibrium constants by optimizing the fit of the model output to the experimental data. The choice of surface complexation reactions was constrained and guided by extended X-ray absorption fine structure spectroscopy results. Adsorption of Nd was strongly affected by pH with an increase from essentially no adsorption to almost complete adsorption as the pH increased from 3.0 to 8.0. Citrate increased the adsorption below pH 5.5, but it inhibited the adsorption above pH 5.5. The effects of citrate may be caused by its modification of the goethite surface charge and by its complexation of Nd. Background salts and dissolved inorganic carbon had negligible effects on Nd adsorption. For adsorption in the absence of citric acid, a model with only one bidentate inner-sphere surface reaction could successfully simulate adsorption over a broad range of conditions. The observed effect of citrate was more precisely modeled by including two additional ternary Nd-citrate surface complexes. This study contributes to a more comprehensive database of Nd adsorption to mineral surfaces, and the SCM provides predictive capabilities for Nd transport in the presence of goethite under different environmental conditions.

Surface complexation modeling of neodymium adsorption to goethite

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