A multilayer adsorption of perfluorohexanesulfonic and perfluorobutanesulfonic acids on bio-based polyurethane/chitosan foam: Advanced interpretation of the adsorption mechanism

Abstract

This work presents an application of a multilayer model derived from statistical physics to the adsorption experimental data of two industrial pollutants, namely perfluorohexanesulfonic (PFHxS) and Perfluorobutanesulfonic (PFBS) acids on a bio–based polyurethane /chitosan(PU/CH) adsorbent foam, at T = 298–328 K and pH 4. This model shows that the number of PFHxS and PFBS layers formed on adsorbent surface increases from 1.61 to 3.80, and from 1.56 to 2.26 respectively by increasing the temperature.This means that the adsorbate molecules are arranged on a variable number of layers. For instance, at 328 K, it was deduced that around 80 % of PFHxS acid was taken up by the PU/CH composite, forming a total of four layers, whereas the remaining 20 % contributed to the formation of three layers. The estimation and analysis of the number of molecules bound per active site is useful to understand the adsorption mechanism. PFHxS and PFBS adsorb on PU/CH in non-parallel configuration, through a multimolecular docking characterized by more than one molecule per active site. The PU/CH performances are investigated in terms of the maximum adsorption capacity of both the tested industrial pollutants, retrieving similar values for both PFHxS and PFBS. Two adsorption energies are calculated showing that physical forces are contributing to remove these pollutants. Interstingly, thermodynamic calculations are also introduced and interpreted to understand the adsorption mechanism at macroscopic level. These theoretical scientific evidences provide a plausible explanation of the PFHxS and PFBS adsorption mechanisms and contribute to understand this interfacial process at the molecular level.