Carbon dioxide-switched removal of nitrate ions from water by cellulose nanocrystal-grafted and free multi-responsive block copolymers

Temperature- and carbon dioxide (CO2)-responsive block copolymers of dimethylaminoethyl methacrylate (DMAEMA) and N-isopropylacrylamide (NIPAAm) (PDMAEMA-b-PNIPAAm) with different block lengths were synthesized by atom transfer radical polymerization (ATRP). In addition, cellulose nanocrystals (CNCs) were grafted with PDMAEMA-b-PNIPAAm chains through surface initiated-ATRP. After studying self-assembly and responsivity of the free PDMAEMA-b-PNIPAAm to different stimuli, the free and CNC-grafted PDMAEMA-b-PNIPAAm chains were finally used as CO2-switched absorbents of nitrate ions from aqueous solutions. Temperature responsivity investigations showed different critical solution temperatures (CST) for the copolymers depending on the length of blocks. CO2-responsivity of the copolymers is accompanied by reduction of pH upon the purging CO2 and protonation of the tertiary amine groups of PDMAEMA block as (O-2-responsive functional- ities. Micellar and vesicular morphologies were estimated for the free copolymers with different ratios of hydrophilic/hydrophobic blocks. CO2 -switched removal of nitrate ions was carried out at temperature below and above the CST of the copolymers (40 and 60 degrees C, respectively). The free copolymers with higher length of PDMAEMA block shows the highest absorption capacity of nitrate ions (490 mg/g) through the electrostatic interactions in the presence of CO2 at temperatures below the CST. N-2-switched ion desorption, temperature responsivity, and filtration ability of the CNC-grafted copolymers provide the easy regeneration process. (C) 2020 Elsevier B.V. All rights reserved.