Surface chemistry of nanoporous carbon and the effect of pH on adsorption from aqueous phenol and 2,3,4-trichlorophenol solutions

Granular nanoporous activated carbon prepared from polyacrylonitrile (PAN) was investigated as a means of removing weak aromatic acids from aqueous solutions. This carbon is highly nanoporous, the BET surface area being 544 m2/g with Vtot=0.278 cm3/g, and Vmicro=0.266 cm3/g. Aqueous treatment reduced the surface area to 364 m2/g. ar nanoporous carbon prepared from PAN contains O and N related surface functional groups. The surface concentration of both oxygen and nitrogen atoms was found by XPS to be 5.3%. Surface groups containing these hetero atoms are responsible for the acid–base character of this carbon in aqueous solutions. The pHPZC of the carbon is 8.4. 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 μeq./g, of which 112.9 μeq./g have acidic and 336.3 μeq./g have basic character. Within the acidic species three subgroups, namely the regions pKa<6.37, 6.37<pKa<10.25 and 10.25<pKa<15.74. id–base behavior influences the adsorption performance of the granular carbon in aqueous solutions of weak aromatic acids, in this case phenol and 2,3,4-trichlorophenol. 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 K values show a sequence pH=3<pH=11 < unbuffered in the case of phenol, and pH=11<pH=3 < unbuffered, when trichlorophenol was used as a solute. The different K values indicate different adsorption mechanisms. The effect of pH is more marked for 2,3,4-trichlorophenol.