Performance of first derivative UV/Visible spectra for kinetic and isothermal study of simultaneous adsorption of o-nitrophenol and p-nitrophenol onto Al2O3 and HDTMA+/Al2O3 composite

Adsorption of ortho-nitrophenol (o-NP) and para-nitrophenol (p-NP) from single and binary aqueous solutions onto Al2O3 and its composite, hexadecyltrimethylammonium bromide (HDTMA+/Al2O3), were studied in batch experiments at T = 25 °C and at pH = 6. In order to overcome the problem of overlapping absorbance bands of o-NP and p-NP, a first-order derivative UV/Visible spectrophotometric method (FODS) was applied for the simultaneous determination of each adsorbate in their residual binary solutions. Experimental adsorption data of o-NP and p-NP in single and binary solutions were found to follow adequately a pseudo-second order kinetic model. Based on the values of R2, χ2 and Δq(%), the Freundlich model showed better fitting to the equilibrium isotherms data, followed by Redlich-Peterson and Langmuir models, suggesting the existence of heterogeneous sites on the adsorbent surfaces. In all the experiments, HDTMA+/Al2O3 showed a superior adsorption capacity for o-NP and p-NP than Al2O3, and the adsorption of o-NP on both adsorbents was favored. However, adsorption capacity of each isomer in binary mixtures were found lower than that in a single component, reflecting a competitive adsorption mechanism, involving mainly the hydrogen bonding between NO2 and OH groups of o-NP/p-NP and the functional groups of the adsorbents, which affected their selectivity.