| Several types of granular activated carbons (GACs) were subjected to batch and flow-through tests to examine the influence of oxidation on their sorption potential. Based on pore structure, elemental composition, and point of zero charge (pHPZC), the variation in sorption affinity for selected heavy metals, methylbenzotriazole (MeBT) and carboxybenzotriazole (CBT), alone and in combination, was observed. The sorption capacities of commercial GACs, specifically designed to retain phenyl-conjugated organics (Calgon MRX-P and Filtrasorb F400), were significantly increased by oxidation with nitric acid. The final pH of metal-containing solutions used to challenge GACs, were equilibrated near the pHPZC of these oxidized GACs (c.a. pH 3) impregnated with benzotriazoles (BTs), which enhanced their retention capacity for some semi-precious transition metals.During column breakthrough experiments observing the removal of four model radionuclides and 14 transition metals in acidic media, GAC columns loaded with CBT-impregnated carbon demonstrated enhanced metal retention ability over carbon alone, with the best relative retention capacity for Cu, Pb, Ag, and U ions. Hard and soft acid/base behavior, pHPZC of the GACs, metal:ligand complexation, and triazole co-precipitation were collectively attributed to the observed differences in metal affinity and associated immobilization potential.MRX-P and F400 carbons challenged with hard water (800 mg/L as CaCO 3), were compared in both oxidized and non-oxidized forms for their metal removal potential with and without associated benzotriazoles. GAC removal efficiency of Ag increased in response to hardness, whereas the immobilization capacity of Cu and Pb cations declined. The highest metal capacities were expressed by oxidized GACs, pre-adsorbed with CBT, regardless of water quality conditions these GAC were also the best performers for removing silver alone, and in combination.Oxidized MRX-P carbons impregnated with BTs were tested for their ability to maintain metal removal efficiency during 20 cycles of loading, subsequent acid stripping, and reloading at pH 3.5. Cu, Pb and Ag retained in columns were stripped by strong acids, and effluents were analyzed for metal and BTs content. Columns packed with oxidized GACs saturated with BTs performed through 20 cycles without significant reduction in metal removal capacity. CBT-impregnated oxidized GAC showed the best capacity for Ag and Cu ions compared to its MeBT counterpart. No more than 5% of the total triazole derivatives loaded on the column were washed out over 20 loading and stripping cycles and approached asymptotic levels near their analytical detection limit (c.a. 1ppm by HPLC).CBT-impregnated oxidized GAC was chosen to conduct pilot-scale column study to compare its copper sorption efficiency before and after regeneration by treating industrial acidic wastewater obtained from the SAE Circuits Inc. Compared side-by-side with commercial chelating resin, Purolite S-930, currently used at this facility to treat copper-containing wastewaters, BT/GAC combinations demonstrated a higher metal volumetric removal capacity than its ion exchange counterpart, and proved to be economically feasible from a materials perspective. |
