Adsorption of surfactants on carbon black and paper fiber in the presence of calcium ions

Deinking is an important step in recycling of waste paper and flotation is commonly used in this process. By studying the interaction between added surfactant and the solid surfaces of ink pigment and pulp, the fundamental mechanism of flotation deinking can be better understood. In this work, the adsorption of two anionic surfactants (sodium dodecyl sulfate, SDS and sodium octanoate, C8) on both a model ink (hydrophobic carbon black) and a model fiber (hydrophilic office paper) was studied. The effect of pH on the SDS adsorption and the co-adsorption of calcium and surfactant on both surfaces also were investigated. The SDS adsorbs on carbon black as a tail-down monolayer (hemimicelle) while on paper fiber as a head-down, head-out bilayer (admicelle). The C8 forms admicelles on both carbon black and paper fiber indicating the stronger interaction of the carboxylate group with the carbon surface than the surfactant sulfate group, causing the C8 to adsorb at higher levels than SDS on carbon black. This helps explain why soaps are used widely as the surfactant in flotation deinking operations. Calcium causes surfactant adsorption to increase on carbon black as it adsorbs between negatively charge surface sites and the anionic head group of the surfactant (bridging) especially at low surfactant levels while not enhancing surfactant adsorption on paper fiber, explaining its activation effect in deinking processes. At high surfactant loadings, increasing surfactant concentration can cause calcium adsorption to decrease (calcium exclusion effect), probably due to covering up of negative adsorption sites on the surface.

This figure shows adsorption isotherms of sodium dodecyl sulfate (SDS) and sodium octanoate (C8) on carbon black at various initial calcium ion concentrations. As calcium ion concentration increases, the adsorption of SDS or C8 increases. The surfactant adsorption increase is more pronounced at low surfactant concentrations and for C8 as opposed to SDS. We have postulated that calcium adsorbs on negative surface sites and acts as a bridge between the surface and negatively charged surfactant head groups.Figure optionsDownload as PowerPoint slideHighlights
► The carboxylate group of C8 has a stronger interaction with both studied surfaces than the sulfate group of SDS.
► Ca can increase surfactant adsorption due to calcium adsorption and co-adsorption (calcium bridging effect).
► The results help explain why carboxylated surfactants with Ca at high pH are effective flotation agents.