ORIGINAL ARTICLE Removal of Hexavalent Chromium by Cross-Linking Chitosan and N,N’-Methylene Bis-Acrylamide Mohamed N. M. Ismael 1 & Ahmed El Nemr 1 & El Sayed H. El Ashry 2 & Hamida Abdel Hamid 2 Received: 10 March 2020 /Accepted: 30 June 2020/ # Springer Nature Switzerland AG 2020 Abstract The present work aims to make a novel chemical modification of chitosan to improve its ability in removing hexavalent chromium from its aqueous solution. Therefore, the use of N,N / -methylene bis-acrylamide as a cross linker for chitosan has been studied using simple method via nucleophilic addition, in the presence of FeCl 3 . 6H 2 O as a Lewis acid catalyst. The obtained product (CMBA) was tested as an adsorbent for removal of Cr 6+ ions from its aqueous solution. The CMBA was characterized using instrumental tech- niques such as FTIR, SEM, EDX, and TGA analyses. The FTIR analysis showed the presence of a peak at 3297–3355 cm -1 due to the stretching vibration mode of –OH and – NH 2 groups, and a peak at 1745 cm -1 due to the C=O stretching in carboxylic group. The equilibrium data were studied using Langmuir, Freundlich and Tempkin isotherm models. The maximum monolayer adsorption capacity (Q m ) calculated from Langmuir isotherm was 149 mg/g, which showed a 3-fold greater uptake for CMBA over non- modified chitosan. The pseudo-second-order kinetic model was the best model representing the adsorption data with correlation coefficient close to one, indicating that chemisorption is dominating the adsorption process. The intraparticle diffusion study proved that the adsorption process was performed under the control of more than one mechanisms. The present study proved that the novel cross-linked chitosan CMBA is a helpful material for the adsorption of Cr 6+ ions from its water solution. Keywords N,N / -methylene bis-acrylamide . Chitosan . Removal of Cr 6+ ions . FeCl 3 Environmental Processes https://doi.org/10.1007/s40710-020-00447-2 * Ahmed El Nemr ahmedmoustafaelnemr@yahoo.com; ahmed.m.elnemr@gmail.com 1 Marine Pollution Department, Environmental Division, National Institute of Oceanography and Fisheries, Kayet Bey, El-Anfoushy, Alexandria, Egypt 2 Department of Chemistry, Faculty of Science, Alexandria University, Alexandria, Egypt