This thesis characterizes the physicochemical properties of pure (Fh) and aluminum-doped ferrihydrites (AlFh) upon aging and puts forth a hypothesis involving the lattice position occupied by Al within Fh. We used flow and drop solution microcalorimetry, X-ray diffraction, adsorption spectroscopy and magnetic circular dichroism spectroscopy to carry a complete characterization. Aging of pure Fh showed evidence for phase transformation into varying amounts of hematite (Ht) and goethite (Gt) with the former increasing over time. The presence of Al retarded all transformation. Aged AlFhs showed a larger heat of exchange (Qexch) compared to undoped Fhs and increased with age. Results also showed increased magnetism, contraction of the lattice c-axis, heats of phosphate adsorption, and pH decrease with Al incorporation. We posit that Al substitution occurs on the surface of Fh in an octahedrally-coordinated Fe site
