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Ferrihydrite (Fh) is a poorly crystalline oxyhydroxide that has attracted attention due to its importance in geochemistry, ion mobility, adsorption, and as a precursor for the formation of more crystalline phases such as goethite and hematite. A chemical model for the isomorphic substitution of the Fe sites by Al is proposed. DFT/plane wave calculations have been performed to provide information about the local environment of Al-substitution into Fh. The structural and electronic properties have been investigated in detail. Al prefers to replace the octahedral Fe sites, which has been evidenced by the comparison of the calculated Al K-edge X-ray absorption near-edge structure spectra with the available experimental data. The effect of the increasing ionicity of the system upon Al-substitution to the quadrupolar coupling constant of 27Al NMR and the Mössbauer quadrupole splitting and isomer shift parameters have been investigated. The results indicate that these parameters are affected by the concentration of Fe-substituted sites. Al-substitution increases the band gap dispersion with the formation of Al electronic states in the conduction band. The standard Helmholtz free energy of Al-substitution has been estimated and discussed in detail.