The adsorption behavior of the sulphate ($SO{_4}^{2-}$) on the external surface of (5,0), (8,0), and (10,0) zigzag AlNNTs was studied by using density functional calculations. Adsorption energies in the nanotubes are about -8.59, -8.04, -8.60 eV with a charge transfer of 0.59, 0.48, 0.56|e| from the sulphate ion to the nanotubes, respectively. The adsorption energies indicated that sulphate ion can be absorbed strongly on the nanotubes. Therefore, these nanotubes can be used for adsorption of sulphate ion from the environmental systems. It was found that diameter of the AlNNTs has slight role in the adsorption of sulphate ion. The electronic properties of the nanotubes showed notable changes upon the adsorption process.