Carbonate Adsorption to Ferrihydrite: Competitive Interaction with Phosphate for Use in Soil Systems

Carbonate (CO₃) interacts with Fe-(hydr)oxide nanoparticles, affecting the availability and geochemical cycle of other important oxyanions in nature. Here, we studied the carbonate-phosphate interaction in closed systems with freshly prepared ferrihydrite (Fh), using batch experiments that cover a wide range of pH values, ionic strength, and CO₃ and PO₄ concentrations. The surface speciation of CO₃ has been assessed by interpreting the ion competition with the Charge Distribution (CD) model, using CD coefficients derived from MO/DTF optimized geometries. Adsorption of CO₃ occurs predominately via formation of bidentate inner-sphere complexes, either (≡FeO)₂CO or (≡FeO)₂CO··Na⁺. The latter complex is electrostatically promoted at high pH and in the presence of adsorbed PO₄. Additionally, a minor complex is present at high CO₃ loadings. The CD model, solely parametrized by measuring the pH-dependent PO₄ adsorption as a function of the CO₃ concentration, successfully predicts the CO₃ adsorption to Fh in single-ion systems. The adsorption affinity of CO₃ to Fh is higher than to goethite, particularly at high pH and CO₃ loadings due to the enhanced formation (≡FeO)₂CO··Na⁺. The PO₄ adsorption isotherm in 0.5 M NaHCO₃ can be well described, being relevant for assessing the reactive surface area of the natural oxide fraction with soil extractions and CD modeling. Additionally, we have evaluated the enhanced Fh solubility due to Fe(III)-CO₃ complex formation and resolved a new species (Fe(CO₃)₂(OH)₂ ^3-(aq)), which is dominant in closed systems at high pH. The measured solubility of our Fh agrees with the size-dependent solubility predicted using the surface Gibbs free energy of Fh.