Towards Building a Unified Adsorption Model for Goethite Based on Variable Crystal Face Contributions: III. Carbonate Adsorption

Goethite, a ubiquitous Fe(III) oxyhydroxide mineral, typically occurs in very small particle sizes whose interfacial properties critically influence the fate and transport of ionic species in natural systems. The surface site density of synthetic goethite increases with particle size, resulting in enhanced adsorption capacity per unit area. In the first two parts of this study, we modeled the adsorption of protons, As(V), Pb(II), Zn(II), and phosphate on goethite as a function of particle size, adsorbate concentration, pH, and ionic strength, using unified parameters within the extended CD-MUSIC framework. Here, we extend this work to characterize the interfacial behavior of carbonate in goethite suspensions, using a comprehensive dataset generated previously under both closed and open CO2 system conditions. Carbonate oxyanions, prevalent in geochemical environments, exhibit competitive and complexation interactions with other ions and mineral surfaces. Although a bidentate bridging surface carbonate complex has been successful in previous modeling efforts on goethite, we found that the size of the carbonate moiety makes it impossible to bind in this manner to the available surface sites and we propose a novel complex configuration that is consistent with interatomic distances and infrared evidence. Unified affinity constants and charge distribution parameters for this complex were derived, providing further validation of the extended CD-MUSIC model for describing relevant goethite/aqueous interfacial reactions.