Enhancing the Adsorption Performance of Chitosan to Phenol Derivatives through Hydrophobic Modification: Kinetics, Equilibrium, and Mechanisms

Abstract

Adsorption is effective methods to remove trace organic pollutants in water, and the development of cheap and efficient adsorption materials is essential to the real application of this technique. In this study, chitosan, a natural aminopolysaccharide, was grafted with palmityl chloride using an acylation procedure to improve its hydrophobicity and adsorption capacity to hydrophobic organic contaminants. A hydrophobically-modified chitosan (HMC2) was characterized using Fourier transform infrared spectroscopy (FTIR), and then was tested to adsorb phenolic pollutants from water. The batch adsorption experiments were employed to investigate the adsorption equilibrium and kinetics of 4-nonylphenol(4-NP), α-naphthol(Nap), 4-chlorophenol(4-CP) and phenol(Phe) on HMC2 and chitosan. The effect of temperature, salt concentration, and pH on adsorption was studied for the understanding of adsorption processes. Moreover, a linear free energy relationship approach and FTIR technique were used to identify the predominant adsorption mechanisms. The obtained results showed that the modified chitosan had higher adsorption capacity to 4-NP, Nap, 4-CP and Phe than the unmodified one. The adsorption kinetics of 4-NP, Nap and 4-CP on chitosan and HMC2 were found to be well fitted by the first-order kinetic model. The adsorption isotherms of 4-NP, Nap, 4-CP and Phe on HMC2 conformed to the linear or Freundlich models. The pH range from 5.6-8.5 appeared to have non-significant effect on the adsorption capacity of HMC2. However, the increasing salt concentration was found to favor the adsorption. The adsorption enthalpy changes (ΔH0) for 4-NP, Nap and CP on HMC2 varied from -3.82 to -0.44 kJ/mol, and the adsorption entropy changes (ΔS0) changed from 21.7 to 85.9 J/(mol·K). When the logarithms of the distribution constant (logKd) were correlated with the logarithms of the octanol-water partition coefficient (logKow), a good linear relationship was observed:logKd=1.451·logKow - 4.893 (r2=0.999). Thus, the hydrophobic interaction was proposed as a predominant mechanism for adsorption of nonionized 4-NP, Nap, 4-CP, and Phe on HMC2, which was further confirmed by the FTIR data. This study may provide guidance for tailoring high-performance chitosan to selectively remove organic pollutants in water.