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Facilitated transport membranes obtained by coupling polyvinylamine with highly charged carboxymethylated nanocellulose fibers were studied considering both water sorption and gas permeation experiments. In particular, the effect of the L-arginine as a mobile carrier was investigated to understand possible improvements in CO₂ transport across the membranes. The results show that L-arginine addition decreases the water uptake of the membrane, due to the lower polyvinylamine content, but was able to improve the CO₂ transport. Tests carried on at 35 °C and high relative humidity indeed showed an increase of both CO₂ permeability and selectivity with respect to nitrogen and methane. In particular, the CO₂ permeability increased from 160 to about 340 Barrer when arginine loading was increased from 0 to 45 wt%. In the same conditions, selectivity with respect to nitrogen was more than doubled, increasing from 20 to 45. Minor improvements were instead obtained with respect to methane; CO₂/CH₄ selectivity, indeed, even in presence of the mobile carrier, was limited to about 20.