This paper studies the process of Cu(II) ions extraction from aqueous solutions by cellulose-containing sorbents: native and modified by L-arginine flax fiber. We processed the experimental kinetic curves of copper ion sorption by pseudo-first and pseudo-second order models and obtain a high correlation coefficient (0.99) when used a pseudo-second order kinetics model. The results of the treatment of the sorption isotherms according to the Langmuir model indicate the model capability for the description of the process of copper ions sorption by cellulose sorbents. As a result of two-stage chemical modification through the oxidation stage of linen fibre with sodium periodate and subsequent modification with L-arginine, we obtain a new sorbent, which can be used to purify aqueous solutions from heavy metal ions. The paper also presents SEM images and elemental analysis of original and modified flax fibre samples. The infrared spectra confirms the changes occurred in the linen fibre during modification. The obtained sorbent significantly surpasses the native linen fibre in its sorption characteristics, as evidenced by the values of the ultimate sorption capacity.