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

Water adsorption isotherms and the corresponding enthalpies of adsorption w ere measured on the series of microporous activated carbons obtained from p olyfurfuryl alcohol. The initial carbon (A), containing a negligibly small amount of surface oxides, was oxidised using cone. nitric acid (the obtaine d adsorbent is called carbon B), and the active hydrogen of carbon B was ex changed (via ionic exchange process) with Na+ (two carbons C and D were obt ained, respectively) and with Cr3+ (carbon E). The isotherms and the enthal pies of water adsorption were described using DS, Barton and D'Arcy and Wat t adsorption isotherm equations as well as the corresponding enthalpy formu las, developed in our previous paper. The applicability of the above models is discussed and it is shown that the D'Arcy and Watt isotherm equation de scribes the obtained adsorption and enthalpy of adsorption data with the be st fit for almost all the systems studied. This leads to the conclusion tha t it is necessary to take chemisorption of water into account, not only for oxidised carbons but also for carbons modified via the ionic-exchange proc ess. The validity of this conclusion seems to be confirmed by the calculate d hydration numbers of surface cations that are similar to those obtained f or 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 effe ctive results among the models studied. (C) 2001 Elsevier Science B.V. All rights reserved.