Acid modification of sheep wool changed the surface charge from negative to positive one as demonstrated by means of zero charge point determination. Positive surface facilitated adsorption of nitrate anions. Amount of nitrates in solution within 20–100 mg.dm−3was determined applying a home-developed UV spectrometry method. A little more effective modifier was 0.01 molar hydrochloric acid than 0.1 molar one. In the case of citric acid concentration, the effect was opposite. Wool modified by HCl showed higher nitrate adsorption (up to 5 mg.g−1) compared to the sample modified by citric acid presenting a larger difference due to the modifier concentration (up to 1.7 mg.g−1of nitrate and 3, resp.). Testing eight adsorption isotherm models indicated inhomogeneity of the adsorbed layer from chemical as well as physical aspect. Freundlich, Temkin and Halsey models fitted best. The study demonstrated that biopolymer modified using a suitable acid has a potential to remove nitrates from aquatic environment.