Binding of As3+ and As5+ to Fe(III) Oxyhydroxide Clusters and the Influence of Aluminum Substitution: A Molecular Perspective

Fe(III) oxides and oxyhydroxides play a very important role in contaminant cycling and mobility in the environment through numerous sorption mechanisms owing to their nanoparticulate nature. Generally coprecipitated from mixtures of metal ions in natural environments, Fe(III) oxyhydroxides are often doped by various impurity metal ions to a certain degree. These dopant/impurity ions then play a crucial role in the geochemical cycling of toxic contaminants like arsenic via modified adsorption energetics on Fe(III) oxyhydroxide nanoparticles. Aluminum (Al) commonly coexists with ferric salts and minerals in nature and affects the arsenic (As) binding abilities of Fe(III) oxyhydroxides. We use electronic structure studies to model the As binding potential of Al-doped Fe(III) oxyhydroxide clusters, using a bottom-up molecular approach to understand their role in As fixation. We start from small Al-doped Fe(III) oxyhydroxide clusters, like dimers and trimers, and gradually study larger clusters including the delta-Fe-13 Keggin cluster, evaluating their As binding potential with respect to pure undoped Fe(III) oxyhydroxide clusters at each step. The calculated reaction free energies clearly show that Al doping into Fe(III) oxyhydroxide clusters reduces their As3+ binding potential, whereas the As5+ binding is not affected much due to Al doping.

Automated summary

Fe(III) oxides and oxyhydroxides, due to their nanoparticulate nature, play a crucial role in contaminant cycling and mobility in the environment through various sorption mechanisms. Doping of impurity ions, such as aluminum (Al), into Fe(III) oxyhydroxide nanoparticles modifies their adsorption energetics, affecting the binding potential of toxic contaminants like arsenic (As). Our electronic structure studies demonstrate that Al doping reduces the As binding potential of Fe(III) oxyhydroxide clusters for As3+, while it has minimal effect on As5+ binding.