<p>This work examines the adsorption behaviour of Fe₄, Fe₃Ni, Fe₂Ni₂, FeNi₃ and Ni₄ clusters on graphene, including binding energy, charge transfer, band gap, electron density and density of states (DOS) plots. We further employ Density Functional Theory (DFT) to study the adsorption capacity of diverse surfaces like pristine graphene, mono-vacant graphene, nitrogen doped pyridine resembling graphene and Fe-Ni mixed metal clusters decorated graphene surfaces towards toxic gas sulfur dioxide (SO₂). The interaction between the adsorbent and the adsorbate occurs through the parallel and perpendicular modes of adsorption. The adsorption of toxic gas SO₂ on the Fe-Ni mixed metal cluster decorated graphene is the novelty of the work. It has been noted that the physisorption of SO₂ towards pristine graphene (PG) change to chemisorption after the modification of PG with Fe-Ni metal cluster. The FeNi₃ cluster-decorated graphene causes SO₂ to dissociate in the Fe-mode, following adsorption with a high adsorption energy value of -3.579&#xa0;eV. The FeNi₃-PG is found best performing adsorbent surface towards the adsorption of toxic SO₂ gas. The adsorption energy of SO₂ over the modified graphene substrate varies from − 2.118&#xa0;eV to -3.579&#xa0;eV. The density of state, partial density of state, electron density difference and electrostatic potential surface plots are also used to examine the adsorption investigation.</p>