Adsorption characteristics of surfactants on secondary wood fiber surface

Surfactant residues in recycled wood fiber from the deinking process can have impact on the subsequent bleaching and papermaking processes by increasing bleaching agents and disturbing papermaking wet-end chemistry. In this paper, the fundamental mechanism of surfactants adsorption characteristics on secondary fiber surface was studied. The adsorption isotherms and kinetics of an anionic surfactant, sodium dodecyl benzene sulfonate (SDBS) and a non-ionic surfactant, Triton X-100 (TX-100) on secondary fiber surface with or without the presence of electrolytes (Mg²⁺ ions) were studied by using spectrophotometric methods. Results showed that the adsorption isotherm of SDBS could be subdivided into four regions, and the adsorption of SDBS was increased with the presence of Mg²⁺ ions. While for TX-100, the adsorption isotherm showed typically Langmuir-type adsorption, and its adsorption was decreased with the presence of Mg²⁺ ions. Kinetic analysis indicated that the adsorption processes both fit pseudo-second-order model well. The adsorption rate of both surfactants was affected by the initial surfactant concentration, electrolyte and temperature. The activation parameters confirmed that the adsorption processes of both surfactants were typically diffusion-controlled and endothermic at a temperature range commonly used for current flotation deinking processes. SDBS and TX-100 mainly adsorbed to the hydrophobic sites of secondary fibers surface, which decreased the fiber hydrophobicity and reduced the fiber loss in the flotation deinking process. The results suggested that the concentration of Mg²⁺ ions should be maintained at a low level when SDBS was used in the flotation process.