This study aimed to evaluate the effect of duck feather keratin modified with sodium metabisulphite on the physicochemical properties and the adsorption equilibrium of methylene blue (MB). The duck feather powder was modified with a 0.325 M Na2S2O5 solution, and batch adsorption was carried out at pH 8 for 110 minutes using measured initial MB concentrations equivalent to nominal levels of 25 to 300 mg L-1. Each adsorption condition was carried out in three independent batches. SEM-EDX, FTIR and BET analyses showed that the modification resulted in a rougher matrix, changes in morphological composition and a shift in the amide band of the functional groups, as well as an increase in surface area from 0.1437 to 0.2770 m2 g-1 and pore volume from 0.0045 to 0.0088 cm3 g-1. Across the entire concentration range, the average equilibrium capacity increased by 9.3-58.9% following modification. Non-linear model calculations showed that the unmodified data could be similarly explained by both the Freundlich and Langmuir equations, with the Freundlich equation yielding a slightly lower RMSE, whilst the modified data were better described by the Langmuir equation. The estimated Langmuir capacity increased from 13.250 ± 0.612 mg g-1 to 20.177 ± 0.751 mg g-1, equivalent to an increase of 52.3%, whilst K_L increased from 0.140 ± 0.032 to 1.656 ± 0.315 L mg-1, with R2 = 0.963 and RMSE = 1.333 mg g-1. These results indicate a relationship between sulphitolytic modification, changes in site accessibility and surface chemistry, as well as enhanced MB removal. The characterisation results and equilibrium models remain complementary evidence and do not independently establish the adsorption mechanism.