Effect of calcium in field-spent GACs on pore development during regeneration

Thermal regenerations were conducted on a granular activated carbon (GAC) that had become exhausted (spent) in a water treatment plant where it had been in service nearly four years. This GAC contained 2 percent calcium. With this field-loaded calcium intact, proper regeneration restored the spent GAC to the same pore structure and surface area that was exhibited by several virgin GACs. When calcium appeared inside the spent GAC, it always caused micropores to be converted to small mesopores during thermal oxidation. In contrast, if the calcium was leached out of the spent GAC with acid, thermal oxidation increased only the micropore volume. Several regeneration variables were tested, including pyrolysis temperature (650-950°C), oxidation temperature (650-950°C), oxidant gas (steam and CO₂), oxidant flow rate, and oxidant extent. All regenerated products of the spent GAC that had been taken to the same targeted apparent density displayed the same surface area and pore structure except those regenerated at low temperatures (650-750°C) with both steam and CO₂ together: these produced carbons with lower volumes of small pores and lower surface areas.