Adsorption of yellowing inhibitors on mechanical pulp

The retention kinetics and equilibrium partition of two benzotriazole yellowing inhibitors were measured on mechanical pulp. Desorption experiments were also carried out. The variables included the inhibitor type and concentration, salt concentration and shear. Only the cationic inhibitor was retained on fibres. The absorption of the inhibitor proceeds by ion-exchange with the counter-ions of the carboxylic groups of the pulp, accompanied by a specific adsorption on the fibre surface. The Donnan equilibrium dominates at low ionic strength whereas specific adsorption dominates at high ionic strength. The rate-determining step is the mass transfer of the inhibitor from the solution to the surface of the fibres. Internal mass transfer (pores) plays a negligible role.