Fenton-driven oxidation of contaminant-spent granular activated carbon (GAC): GAC selection and implications.

Raw materials, activation methods, and post-activation treatment used in manufacturing granular activated carbon (GAC) results in a spectrum of physicochemical characteristics that potentially impact the adsorption oxidation treatment process. A comprehensive study is lacking that assesses the effect of GAC characteristics on adsorption oxidation treatment of contaminant spent-GAC. Consequently, it is inherently assumed the treatment process is GAC-independent. Here, GACs (n = 31) were characterized and used in the hydrogen peroxide (H2O2)-based adsorption oxidation treatment of 2-chlorophenol (2CP)-spent GAC. The GACs exhibited a range in surface area, pore volume distribution, metals content, surface functionality, and H2O2 reaction. Chloride recovery, the treatment metric for 2CP oxidation, indicated a wide range in oxidation (0-49.2%) where bituminous- and wood-based GAC performed best. A selected subset of GACs (n = 12), amended with iron, methyl tert-butyl ether (MTBE), and H2O2, exhibited a range in oxidative treatment (1.1-57.9%). Correlations were established between GAC surface functionality, H2O2 reactivity, adsorption, and MTBE oxidation indicating multiple parameters play a collective and compounding role. The order of GACs successfully used in the treatment process is bituminous-based coal > wood > coconut > peat. Results showed adsorption oxidation treatment is GAC-dependent, and therefore, GAC selection is a key factor in the success of this technology.

Iron optimization for Fenton-driven oxidation of MTBE-spent granular activated carbon.

Fenton-driven chemical oxidation of methyl tert-butyl ether (MTBE)-spent granular activated carbon (GAC) was accomplished through the addition of iron (Fe) and hydrogen peroxide (H2O2) (15.9 g/L; pH 3). The Fe concentration in GAC was incrementally varied (1020-25 660 mg/kg) by the addition of increasing concentrations of Fe solution (FeSO4-7H2O). MTBE degradation in Fe-amended GAC increased by an order of magnitude over Fe-unamended GAC and H2O2 reaction was predominantly (99%) attributed to GAC-bound Fe within the porous structure of the GAC. Imaging and microanalysis of GAC particles indicated limited penetration of Fe into GAC. The optimal Fe concentration was 6710 mg/kg (1020 mg/kg background; 5690 mg/kg amended Fe) and resulted in the greatest MTBE removal and maximum Fe loading oxidation efficiency (MTBE oxidized (microg)/ Fe loaded to GAC (mg/Kg)). At lower Fe concentrations, the H2O2 reaction was Fe limited. At higher Fe concentrations, the H2O2 reaction was not entirely Fe limited, and reductions in GAC surface area, GAC pore volume, MTBE adsorption, and Fe loading oxidation efficiency were measured. Results are consistent with nonuniform distribution of Fe, pore blockage in H2O2 transport, unavailable Fe, and limitations in H2O2 diffusive transport, and emphasize the importance of optimal Fe loading.

Fenton-driven chemical regeneration of MTBE-spent GAC.

Methyl tert-butyl ether (MTBE)-spent granular activated carbon (GAC) was chemically regenerated utilizing the Fenton mechanism. Two successive GAC regeneration cycles were performed involving iterative adsorption and oxidation processes: MTBE was adsorbed to the GAC, oxidized, re-adsorbed, oxidized, and finally re-adsorbed. Oxidant solutions comprised of hydrogen peroxide (H2O2) (1.7-2.0%) and FeSO4 x 7H2O (3 g/L) (pH 2.5), were recirculated through the GAC column (30% bed expansion). The regeneration efficiency after two full cycles of treatment was calculated to be 91%. The cost of H2O2 was 0.59 dollars/kg GAC (0.27 dollars/lb) per regeneration cycle. There was no loss of sorptive capacity. Small reductions in carbon surface area and pore volume were measured. The lack of carbon deterioration under aggressive oxidative conditions was attributed to the oxidation of the target contaminants relative to the oxidation of carbon surfaces. The reaction byproducts from MTBE oxidation, tertiary butanol and acetone, were also degraded and did not accumulate significantly on the GAC. Excessive accumulation of Fe on the GAC and consequent interference with MTBE sorption and carbon regeneration was controlled by monitoring and adjusting Fe in the oxidative solution.