Aluminum (Al) is one of the most abundant metals in soils and rocks with no biological function and exhibits a pronounced toxicity to plants particularly under acidic conditions. Dissolved Al can further strongly affect the microstructure of clay minerals, pore spaces and binding strength between particles, modifying water stability and expansion. The mobility of Al in soil environment is largely governed by sorption to Fe oxides such as goethite and ferrihydrite, which exhibit large specific surface areas and high surface reactivity suggesting marked binding capacity for Al. Therefore, it is important to improve the understanding of sorption and immobilization of Al to these common Fe oxides. To elucidate the molecular mechanisms of Al sorption, this study combined batch sorption experiments, extended X-ray absorption fine structure spectroscopy (EXAFS) and surface complexation modelling. To quantitatively describe the experimental sorption edge data, the charge distribution multisite ion complexation (CD-MUSIC) model was applied using Visual MINTEQ.

The experiments revealed that sorption of Al on goethite and ferrihydrite takes place over a wide pH range including acidic and alkaline conditions. Sorption increased with rising pH. In the circumneutral pH range the formation of Al(OH)₃ can occur. At alkaline pH marked sorption capability for anionic [Al(OH)₄]^- was observed. Kinetic experiments at pH 4 and 5 in the concentration range up to 3 mM showed rapid Al sorption at pH 5 with equilibrium near conditions reached within a few hours, while at pH 4 the adjustment took several days. Ferrihydrite demonstrated higher sorption than goethite with a sorption capacity (per gram) about twice that of gothite at pH 5 as estimated by the Langmuir model.

Spectra obtained by XAS were modelled using an Al-O and Al-Fe single-scattering paths, as well as different multi-scattering paths. Also, the implementation of Al-Al paths was tested. The preliminary shell-fit results suggest inner-sphere surface complexation with Al bound to the Fe oxides surfaces by different binding modes, possibly including edge-sharing as well as corner-sharing surface complexes. Based on the molecular level information from EXAFS, the sorption edge data of Al on ferrihydrite and goethite were fitted. The experimental sorption edges could be satisfactorily reproduced with optimized constants.

Overall, this study provides molecular-level insights into the mechanisms controlling Al binding to two common Fe oxides. The combination of spectroscopic analysis and surface complexation modeling improves predictive capabilities for Al mobility and availability in acidic soil/rock environments and thereby contributes to a better understanding of Al toxicity for plants and microorganisms and its microstructural impact.