Effect of oxygen functional groups on mercury retention in activated carbons

Two commercial activated carbons (F400 and RWE) were tested for their Hg° retention capacity, in order to elucidate shortcomings that still exist, with respect to the oxygen functional groups. The number and the nature of oxygen containing functional groups were altered by heating under H2 at 1100°C or treatment with HNO3. The activated carbons tested were found suitable for mercury retention and the observed differences are attributed to their diversified characteristics (surface chemistry and pore structure). Acidic activated carbons promptly adsorb Hg°, since it behaves as a Lewis basis. When oxygen groups were almost completely removed, by H2 thermal treatment, the Hg° retention ability became practically zero. Acid treatment increased both the number and the acidic oxygen groups; resulting in increased Hg° retention. The Langmuir isotherm equation was used to describe the adsorption (physisorption and chemisorption) of Hg°, occurring in activated carbons. The fitting curve of the Langmuir equation and the correlation coefficient R2 indicate quite fair linear approximation (R2 = 0.9773). Reflecting the spontaneous nature of the adsorption reaction, the Gibbs free energy change calculated is negative (-23.5 kJ/mol and -30.9 kJ/mol, for the RWE and F400 respectively).