Enhancement of nitrophenol adsorption in the presence of anionic surfactant and the effect of the substituent position

Abstract

Adsorption behavior of phenol, ortho-nitrophenol (ONP), meta-nitrophenol (MNP) and para-nitrophenol (PNP) on a kaolinite-type clay has been investigated by means of spectrophotometric method at 298 K. Adsorbability of the phenolic compounds in aqueous solution decreases in the following order; ONP > phenol > MNP ≅ PNP. Influence of an anionic surfactant, sodium dodecylsulfate (SDS), on adsorption behavior of phenol and nitro-substituted phenols onto kaolinite is studied by determining their adsorption isotherms. Experimental evidence has shown that phenol does not adsorb to the surface of kaolinite in the presence of SDS due to large repulsive, electrostatic barrier between negatively charged phenol and negatively charged adsorptive SDS micelles or hemimmicelles. The adsorption capacity of kaolinite for the aromatic nitro compounds in the SDS system is markedly increased compared with the adsorption onto bare kaolinite surface especially at high-concentration region. Adsorption results were interpreted by comparing isotherms of phenolic compounds and SDS. It is shown that bilayer structure of SDS forms even at low SDS concentrations. In this region, PNP molecules are adsorbed via adsolubilisation whereas penetration of ONP and PNP is difficult because of their structural properties and electrical interactions. However, adsolubilisation phenomenon dominates adsorption process of all nitrophenol compounds at higher concentrations. Data show that adsorption capacities of the adsorbates in the presence of SDS follow the order ONP > MNP > PNP. The differences in the adsorption capacities depending on position of nitro group on phenol ring are correlated with the capacity of SDS. The equilibrium isotherms of phenols have been analyzed using Freundlich and Langmuir equations.