The mechanism of H2S removal by biochars probably differs from that of the ACs. As proposed by Hedding and Rao (1976), at the virgin carbon surface, dissociation of hydrogen sulfide occurs in the film of adsorbed water and then hydrogen sulfide ions or HS are oxidized by oxygen radicals to elemental sulfur. On the other hand, when caustic is present, it catalyzes oxidation to elemental sulfur until all base is exhausted (Turk et al., 1992). Therefore, the significant decrease in the adsorption capacity of ACs corresponding to exhaustion is usually caused by the formation of sulfuric acid. Only a small decrease in pH is observed among the biochars, which regain their basic pH after exhaustion (Adib et al., 2000, 1999a,b).