Cellulose is an abundant, renewable and highly biodegradable biopolymer with production in Brazil of approximately 20 million tons in 2017. Obtaining nano-cellulose from different sources and using it in a variety of applications has been the subject of a great amount of research, given its differentiated properties and the possibility of modifications, based on reactions with other organic and inorganic substances. Among the potential applications that nanocellulose presents is the water and wastewater treatment, by processes such as membrane separation, ion exchange and adsorption. The main process for obtaining cellulose nanocrystals is the acid hydrolysis of the amorphous region, remaining more crystalline regions. The most applied production procedures for cellulose nanocrystals consist of the reaction of pure cellulosic material with strong acids with controlled temperature, stirring and time. The crystalline nano-cellulose (CNC) was obtained through acid hydrolysis with sulfuric acid (40 percent) and then modified with citric acid (1.2M), CNC-Mod, for further characterization and adsorption test of metal cations. The samples produced were characterized by techniques such as STEM-EDS, AFM, XRD, FTIR and TGA. Cobalt nitrate (Co(NO3)2), Sodium chloride (NaCl) and mercury chloride (HgCl2) solutions were placed in contact with cellulose, CNC and CNC-Mod samples for a maximum of 3 hours, then having the solid and supernatant separated. The solid samples obtained in the adsorption process were then subjected to analyzes such as XRD, STEM-EDS and TGA. While the supernatants were analyzed by ICP-OES and the results indicates the occurrence of adsorption for all samples and metallic cations.