Origin of the outstanding performance of Zn[sbnd]Al and Mg[sbnd]Fe layered double hydroxides in the adsorption of 2-nitrophenol: A statistical physics assessment

Abstract

This paper describes the adsorption of toxic pollutant 2-nitrophenol on ZnAl and MgFe layered double hydroxides. Mechanistic interpretation was provided via a statistical physics assessment. Experimental and theoretical results indicated that both adsorbents exhibit high adsorption capacities at saturation. In particular, the 2-nitrophenol adsorption capacities were 556, 629, 695 and 710 mg/g for ZnAl-LDH and 362, 603, 648 and 650 mg/g for MgFe-LDH in the temperature range of 298–328 K. Zn–Al-LDH showed an outstanding performance to remove 2-nitrophenol compared to other adsorbents of the literature having adsorption capacities below 100 mg/g. Adsorption mechanisms were analyzed via the calculation of the bonded number of 2-nitrophenol molecules by the adsorbent functional group, the adsorption energy and the density of these functional groups. It was demonstrated that this organic compound was adsorbed via total non-horizontal (i.e., total inclined position) and mixed (horizontal and non-horizontal) orientations depending on adsorption temperatures and tested adsorbent. Calculated adsorption energies indicated an endothermic removal mechanism involving physical interactions. Modeling analysis concluded that the steric parameters ‘bonded number of adsorbate molecules per adsorbent (parameter n)’ and ‘density of functional groups (parameter Dm)’ were the relevant parameters to control the 2-nitrophenol adsorption mechanism.