Polyaniline–iron(II) nitrate was prepared by the polymerization of aniline hydrochloride with Fe(NO₃)₂. The as-prepared materials were characterized for surface area and pore volume and were used to remove the reactive orange 16 (RO16) dye from an aqueous solution. Batch studies were conducted as a function of pH (2–12), adsorbent amount (10–100 mg), initial RO16 concentration (100–300 mg L⁻¹), contact time (10–240 min), and temperature (303–323 K). RO16 was removed at high speed, and equilibrium was achieved in 80 min. Langmuir (six linear forms, i.e., L-I–VI) and other isotherm models were explored for their applicability. With the maximum adsorption capacity of 508.7267 mg g⁻¹ and a pH of 4 at 313 K, the adsorption isotherm could be adequately characterised using the Langmuir (L-V) model. The kinetics of the adsorption process were investigated by fitting experimental data to pseudo-second order (PSO) (type-I–VI) and other kinetic models, with the findings indicating that the adsorption closely matched the PSO-I model. For isotherm models, twelve linear error functions were investigated. The absorption process was spontaneous, endothermic, and feasible according to the thermodynamics study (ΔG° = −8.8888 kJ mol⁻¹, ΔH° = 3.1940 kJ mol⁻¹, and ΔS° = 39.8749 J mol⁻¹ K⁻¹). The phototoxicity studies revealed that the untreated dye was highly toxic compared to the treated dye. It was also shown that the material could be recycled substantially, with an RO16 value of 82.8%. The findings also indicated that the PANI/Fe(NO₃)₂ material was sufficient for RO16 dye adsorption in both model and real water samples.