Energetics of water adsorption and immersion on carbons: Part 2. Adsorption on non-modified, oxidised, and modified via ionic exchange carbons from polyfurfuryl alcohol

Water adsorption isotherms and the corresponding enthalpies of adsorption were measured on the series of microporous activated carbons obtained from polyfurfuryl alcohol. The initial carbon (A), containing a negligibly small amount of surface oxides, was oxidised using conc. nitric acid (the obtained adsorbent is called carbon B), and the active hydrogen of carbon B was exchanged (via ionic exchange process) with Na+ (two carbons C and D were obtained, respectively) and with Cr3+ (carbon E). The isotherms and the enthalpies of water adsorption were described using DS, Barton and DʹArcy and Watt adsorption isotherm equations as well as the corresponding enthalpy formulas, developed in our previous paper. The applicability of the above models is discussed and it is shown that the DʹArcy and Watt isotherm equation describes the obtained adsorption and enthalpy of adsorption data with the best fit for almost all the systems studied. This leads to the conclusion that it is necessary to take chemisorption of water into account, not only for oxidised carbons but also for carbons modified via the ionic-exchange process. The validity of this conclusion seems to be confirmed by the calculated hydration numbers of surface cations that are similar to those obtained for water adsorption on Dowex Resins. For the initial carbon, which does not contain oxygen on the surface, the Bimodal DS model leads to the most effective results among the models studied.