Ion-exchange behavior of carboxylate groups in fibrous cellulose oxidized by the TEMPO-mediated system

Abstract

When native cellulose is treated by catalytic oxidation with 2,2,6,6-tetramethylpiperidine-1-oxy radical (TEMPO)/NaBr/NaClO under aqueous conditions, significant amounts of carboxylate and aldehyde groups can be introduced on surfaces of cellulose I crystallites and into disordered regions without any changes in crystallinity of cellulose I or in the original fibrous morphology. In this study, behavior of ion exchange of carboxylate groups in the TEMPO-oxidized fibrous cellulose prepared from cotton linters was compared with that of fibrous carboxymethyl cellulose (F-CMC) with almost the same carboxylate content as that of the TEMPO-oxidized cellulose. These fibrous celluloses were soaked in various metal salt solutions, and metal ion contents in the celluloses were determined by X-ray fluorescence analysis after washing with water and drying of the celluloses. In all metal salts examined in the ion-exchange experiments, the TEMPO-oxidized cellulose had metal ion contents higher than those of F-CMC. Especially, lead, calcium and silver ions were greatly introduced into the TEMPO-oxidized cellulose with the metal ion/carboxylate molar ratio of about 1:1. The adsorption selectivity of metal ions on the TEMPO-oxidized cellulose was also studied using aqueous solutions containing multiple metal salts for soaking, and the following selectivity order was obtained: Pb 2+>La 3+>Al 3+>Cu 2+>Ba 2+>Ni 2+>Co 2+>Cd 2+, Sr 2+, Mn 2+, Ca 2+>Mg 2+ Moreover, swelling ability of the TEMPO-oxidized celluloses having various metal carboxylate groups in water and wet tensile strength of handsheets prepared thereof were studied in terms of kinds of metal ion introduced. Degree of dissociation of the metal carboxylate groups in these celluloses is likely to affect the results of the above characteristics of the TEMPO-oxidized celluloses.