SURFACE CHARACTERIZATION OF A POLYACRYLONITRILE BASED ACTIVATED CARBON AND THE EFFECT OF pH ON ITS ADSORPTION FROM AQUEOUS PHENOL AND 2,3,4-TRICHLOROPHENOL SOLUTION

Abstract

Granular highly nanoporous activated carbon was prepared from polyacrylonitrile (PAN). The BET surface area was found to be 544 m²/g with V_{\rm tot} = 0.278 cm³/g, and V_{\rm micro}= 0.266 cm³/g. Aqueous treatment reduced the surface area to 364 m²/g. This carbon contains O and N related surface functional groups. Surface groups containing these hetero atoms are responsible for the acid/base character of this carbon in aqueous solutions. The microporous pore network produces a wide hysteresis loop, observed when the granular carbon was studied by continuous titration. This loop, which was found to diminish, but not to disappear completely when the particle size was reduced, is attributed to irreversible hydrolysis of surface esters and/or lactones. The surface concentration of the functional groups titrated by the equilibrium Boehm method is 449.2 \mu equiv/g, of which 112.9 \mu equiv/g have acidic and 336.3 \mu equiv/g have basic character. Within the acidic species three subgroups were distinguished corresponding to carboxylic, lactonic and phenolic surface groups. PAN based carbon was investigated as a means of removing weak aromatic acids, phenol and 2,3,4-trichlorophenol from aqueous solutions. Both the adsorption capacity and the overall interaction parameter, K (both derived from a fit to the Langmuir equation) depend on the adsorbed species and on the pH. The former is a consequence of the different water solubilities of the solute molecules, while the latter stems from the pH sensitivity of both the surface functional groups and these weak acids. The effect of pH is more marked for 2,3,4-trichlorophenol.