Relationships between adsorption mechanisms and pore structure for adsorption of natural organic matter by virgin and reactivated granular activated carbons during water treatment

Temporal changes in the pore structure of granular activated carbons (GACs) were investigated for two reactivated and one virgin GAC during water treatment. One reactivated GAC had experienced five cycles of treatment and reactivation (5×Reac), whereas the other had experienced twelve cycles (12×Reac). For the virgin GAC that became loaded with natural organic matter (NOM), nearly all pore volume loss occurred in pores <40 Å, with little change in pores >40 Å. For the 5×Reac, the greatest loss of pore volume occurred in the 100-500 Å pores. The 12×Reac GAC experienced some loss of pore volume in the 100-500 Å pores. For each GAC, NOM adsorption initially caused pore volume loss in pores that were only slightly wider than the NOM itself. Subsequent to this initial stage, NOM was mostly retained in the 20-40 Å pores for the virgin GAC and in the 100-500 Å pores for the 5×Reac GAC. This was followed by gradual migration of the NOM into smaller pores. A lull in NOM loading revealed distinctions in how NOM migrated in the virgin and reactivated GACs. This was characterized by more gradual changes in pore volume distribution for the reactivated GAC than for the virgin GAC. This work is original in that it offers evidence that organic sorbate may desorb to an extent that can be discerned by pore volume distributions when the total organic carbon influent levels drop below previous levels. The work also offers impact on the GAC field in that it carefully appraises relative sorption performance of a virgin, 5×Reac, and 12×Reac GAC during a concurrent loading interval. The results show that micropores, mesopores, and positive surface charge are important for sorbing NOM.