Ion complexation modelling of ferrihydrite: From fundamentals of metal (hydr)oxide nanoparticles to applications in soils systems

Ferrihydrite (Fh) is the most important iron (hydr)oxide nanoparticle in nature. Due to its large surface reactivity, Fh influences the cycling, availability, and mobility of nutrients and pollutants in soils and water bodies, largely via adsorption processes. Ferrihydrite also forms stable complexes with natural organic matter (NOM), contributing to the long-term stabilization of organic carbon in soils. In the context of surface complexation modeling (SCM), studying the surface reactivity of Fh is important because this nanomaterial is envisioned as a good proxy for describing the reactivity of the natural metal (hydr)oxide fraction in soils. However, despite the recognized importance of Fh as a highly reactive material, many fundamental aspects of its surface reactivity have remained poorly understood. This thesis aimed to gain new insights into the surface reactivity of Fh, focusing on the analysis of ion adsorption mechanisms and on the development of a SCM framework for describing the adsorption of a suite of relevant cat- and anions to Fh in a realistic physical-chemical manner. In this thesis, a special emphasis is given to the adsorption of phosphate (PO4) and its interfacial interactions with other ions (i.e. Ca, Mg, CO3) that are relevant in nature and from the perspective of soil chemical analysis. Phosphate has been chosen as a model oxyanion due to its ubiquity in the environment and its high affinity for binding Fe (hydr)oxide surfaces. Moreover, the adsorption of Ca and Mg to Fh has been also studied in single-ion systems due to the abundance of these ions in the environment and their effect on the adsorption behavior of other ions, such as PO4.

The approaches implemented in this thesis comprise adsorption experiments with freshly-prepared Fh nanoparticles and data interpretation using an advanced SCM (i.e. CD-MUSIC model) as well as molecular orbital (MO) calculations that allow to derive independently the charge distribution (CD) coefficients of the surface complexes considered in the modeling. The consistent treatment of the size dependency of the Fh properties is an essential aspect of this thesis. For this, a novel methodology has been proposed to evaluate the specific surface area (SSA) of Fh nanoparticles in suspension and to derive a coherent set of size-dependent values for the molar mass, mass density, and Stern Layer capacitance(s) of Fh, which are all of major importance for scaling and modeling the collected ion adsorption data. The combination of the above approaches allowed the development of a self-consistent thermodynamic database for describing ion adsorption to Fh. The insights obtained from the model systems, using synthetic Fh suspensions, have been applied to assess the reactive surface area (RSA) of the metal (hydr)oxide fraction in a series of agricultural Dutch top-soils and weathered tropical top-soils. Ferrihydrite is found to be a better proxy than well-crystallized goethite for describing the reactivity of the metal (hydr)oxide fraction in both soil series, despite the contrasting differences in the ratio of crystalline and nanocrystalline metal (hydr)oxides in these two soil series. The reactive metal (hydr)oxide fraction in these soils is dominated by nano-sized particles (~1.5 – 5.0 nm) with a variable specific surface area (SSA ~350 – 1700 m2 g‒1). The interaction between metal (hydr)oxide nanoparticles and NOM has been evaluated in these soil series and a structural model has been formulated for the nanoscale arrangement of the organo-mineral associations in soils. Overall, the results of this thesis are relevant from both a fundamental and a practical perspective. They contribute to increase our understanding of molecular-scale ion adsorption processes occurring at the solid-solution interface of Fh, which is important to develop more accurate predictions of the behavior and availability of ions at the macroscopic level, studied at the laboratory and field scale.