Methylene blue adsorption onto hydrogels made from different Eucalyptus dissolving pulps

Bleached kraft pulp from Eucalyptus benthamii, E. globulus and E. nitens was purified via cold caustic extraction (CCE) with 5% and 10% (w/v) NaOH and activated with dilute acid hydrolysis (samples were coded as CCE5-H and CCE10-H, respectively) to produce dissolving-grade pulp. Hydrogels from CCE5-H and CCE10-H pulps were fabricated through the N-methylmorpholine N-oxide route and tested for methylene blue (MB) adsorption. The morphology, fiber biometry, molecular and supramolecular structure, thermal stability and swelling capacity were also investigated. A higher degree of crystallinity was observed for cellulose in CCE10-H hydrogels. CCE10- H hydrogels also resulted in denser structures with lower swelling than CCE5-H hydrogels. The experimental data for MB adsorption were fitted better to the Langmuir model, which led to estimated maximum adsorption capacities of 36-51 mg/g for CCE5-H hydrogels and 27-33 mg/g for CCE10-H hydrogels. Regarding the Eucalyptus species, hydrogels fabricated from E. nitens exhibited the highest capacities for adsorption of MB. This was attributed to the lowest crystalline fractions and crystallite sizes, and the highest kink indexes and intrinsic viscosities of E. nitens dissolving pulps, which resulted in regenerated hydrogels with highest pore coverage and water absorption capacities, and better accessibility to MB particles.