This thesis investigated the impact of aluminum (Al) substitution in ferrihydrite (FH) on both its chemical and physical properties. Al was coprecipitated with FH under controlled hydrolysis conditions to yield various % mol substitutions. Sulfate adsorption was measured across pHs to examine any changes in surface reactivity. The samples’ morphology, specific surface areas (SSAs), crystallinity, and phase transformation upon heating were evaluated and parameterized to allow conclusions on the role of Al on the physical properties. Sulfate sorption diminished across pHs for all Al saturation levels. X-ray Diffraction revealed goethite (GT) presence was negatively influenced by Al. The SSAs of samples increased non-linearly with increasing % mol Al, indicating a decreasing particle size with more Al content. Transmission Electron Microscopy micrographs showed the FH nanoparticles transformed to acicular/blocky laths of GT crystals and lenticular/platy hematite (HM) crystals with occasional grainy appearance at both room temperature and upon active heating. The phase transformation alongside the derived aspect ratios (length/width) of the GT crystals were retarded by the Al substitution.