Simultaneous production and functionalization of cellulose nanofibrils (CNFs) for heavy metal ion removal is an economical and promising solution to expedite their use in water treatment. In this work, carboxymethylated CNFs (CMCNFs) with a carboxylate content up to 2.7 mmol/g are prepared by a combination of carboxymethylation and homogenization, which show diameters of 3.40-3.53 nm and lengths of 1210.6-383.3 nm. The effect of experimental conditions (including pH, carboxylate content, contact time, initial Cu2+ concentration) on the removal performance of CMCNFs for Cu2+ is investigated in detail. Adsorption performances of CMCNFs present a record high equilibrium Cu2+ removal capacity of 115.3 mg/g at pH 5.0. Additionally, the underlying mechanism for the removal of Cu2+ ions was uncovered by coupling the fitting results based on pseudo-second-order kinetic and Langmuir isotherm models with various characterizations such as scanning electron microscopy, energy dispersive spectroscopy (EDS), EDS mapping, X-ray photoelectron spectroscopy, atomic force microscopy, and powder X-ray diffraction. Finally, the potential application of CMCNF-2.7 with high carboxylate content in converting copper-contaminated water into drinking water was demonstrated. CMCNFs provide a new selection for the design of novel nanocellulose-based materials for water treatments.