Irreversible adsorption of phenol on carbon surfaces

This study was designed to investigate the mechanisms of irreversible adsorption of phenol on activated carbon. In contrast to most studies conducted to date, irreversibility due to surface chemistry and adsorbent microporosity was evaluated independently, with the aid of model adsorbents. First, an attempt was made at characterizing phenol adsorption on graphite, a nonporous model carbon adsorbent. The results indicated that irreversible adsorption and oxidative coupling occurred simultaneously at high pH, but only irreversible adsorption was detected significantly at low pH, demonstrating that irreversible adsorption and oxidative coupling on graphite are independent processes.; The adsorption and recovery of model phenols (including oxidative coupling products) was investigated, to test the hypothesis of strong adsorption of oxidative coupling products as a cause for irreversible adsorption on graphite. The results showed that dimers have more affinity for the surface than phenol, but are recoverable from the surface of graphite by methanol extraction, with efficiencies close to phenol. This demonstrated that strong interactions of dimers to the surface cannot explain irreversible adsorption.; Chemisorption was investigated as a mechanism for irreversible adsorption of phenol on graphite. Rate studies demonstrated the rate-related nature of irreversible adsorption. Positive and variable isosteric heats of adsorption combined with low irreversibility on graphite preloaded with phenol, demonstrated that irreversible adsorption was caused by chemisorption to specific sites on the surface.; Irreversible adsorption was further studied on activated carbon (TOG from Calgon) and activated fibers (ACFs from Kynol). Irreversible uptake presented two components in the porous adsorbents. One was surface related and similar to that found for graphite. The other component was caused by trapping in micropores, which resulted from the combination of microporosity and oxidative coupling; the effect was more intense for activated carbon at high pH. Regeneration of benzene from activated carbon exposed to phenol demonstrated the pore trapping effect.