Chitosan with 45% deacetylation (CS45) grafted poly (acrylic acid) (CS45-g-PAA) was synthesized and characterized as an adsorbent of paraquat. CS45-g-PAA copolymers were prepared using H2O2(aq) as an initiator and NH4OH(aq) as a promoter. The highest grafting percentage of 44.2% was obtained using the traditional kinetic method. However, a maximum grafting percentage of 52.6% was calculated for the central composite design (CCD). Experimental results based on the reaction conditions that were predicted from the CCD are consistent with theoretical calculations. The grafted copolymer was characterized by FTIR, BET, and SEM. A representative CS45-g-PAA copolymer was hydrolyzed to a salt type (CS45-g-PANa) and used in the adsorption of paraquat. The adsorption equilibrium data correlate more closely with the Langmuir isotherm than with the Freundlich equation. The maximum adsorption capacity of CS45-g-PANa is 396.7 mg/g-adsorbent. This value clearly exceeds that of Fuller's earth and the activated carbon which is the most commonly used binding agent for paraquat. POLYM. ENG. SCI., 2013. (c) 2012 Society of Plastics Engineers