Adsorption of anionic surfactant by activated carbon: Effect of surface chemistry, ionic strength, and hydrophobicity

This work investigates the interrelationship between the adsorbent surface chemistry and relative hydrophobic nature and the solution ionic strength and solution hydrophobicity during dilute anionic surfactant solution adsorption. The adsorbents were coconut- and coal-based, steam-activated carbons and wood-based, acid-activated carbons. The adsorptives were dodecanoic acid and octanoic acid dissolved in water or strong caustic solution. The activated carbons (ACs) were immersed in the strong caustic solution at different temperatures for various lengths of time prior to adsorption measurements. The oxygen content and surface functional group chemistry of each AC sample was analyzed using elemental analysis and X-ray photoelectron spectroscopy. The equilibrium solution concentration of each surfactant was determined using gas chromatography-flame ionization detection techniques. The amount of surfactant adsorbed by each AC indicates that an inverse linear relationship exists between the amount of surfactant adsorbed and the adsorbent oxygen content. Although the steam-activated carbons offer lower pore volumes than the acid-activated carbons, they exhibit the higher adsorption capacity. The high solution chemistry pH promotes adsorbent surface oxidation and anionic surfactant dissociation, leading to an interfacial repulsion force and decreased C-12- and C-8-acid adsorption. Overall, this study demonstrates that both the surface chemistry and the solution properties should be considered for an adsorption analysis since each has a significant influence on the adsorption process. (C) 2001 Academic Press.