Carboxylated Cellulose Nanocrystal Microbeads for Removal of Organic Dyes from Wastewater: Effects of Kinetics and Diffusion on Binding and Release

Carboxylated cellulose nanocrystals (cCNC) can be used to make cellulose microbeads by spray drying from aqueous suspension. The rescaling of cCNC to micrometer-size particle agglomerates yields a type of biodegradable cellulose microbead with potential for dye removal for water purification. The present study offers insight into small molecule transport properties that are important to design and assess the performance of cCNC microbeads in this context. In our study, cationic methylene blue (MB) dye was used as a model compound to probe uptake kinetics and diffusion. The study goes beyond the framework of pseudo-first-order or pseudo-second-order kinetics in that it explicitly engages the problem of diffusion. We introduce a two-resistance model based on film–pore diffusion that is conditioned by choice of an adsorption isotherm. The numerical study is combined with experiment to give more detailed insight into mass transport of MB in the cCNC microbeads. Langmuir, Temkin, and Freundlich isotherms were investigated. Film–pore diffusion based on Langmuir adsorption suggests how trafficking of MB molecules in the microbeads depends on pH and ionic strength. A “gating” mechanism is proposed to account for how MB uptake, release, or inhibition of binding depends on pH and ionic strength. At neutral pH, the effective diffusion coefficient, Deff, is 7.5 × 10–11 m2/s. Thermodynamic parameters were determined. The process is spontaneous but slightly endothermic with activation energy of about 20 kJ/mol.