In-Situ Regeneration of Granular Activated Carbon (GAC) Using Fenton's Reagents

Abstract

Fenton-dependent recovery of carbon initially saturated with one of several chlorinated aliphatic contaminants was studied in batch and continuous-flow reactors. A specialty carbon, URV-MOD 1 (Calgon) was employed to minimize non-productive H2O2 demand - that which does not yield hydroxyl or superoxide radicals.Enhancement of PCE degradation kinetics by ferric iron addition is limited by iron solubility, even at relatively low pH. Quinone addition increased the pseudo-first-order rate constant for PCE loss temporarily. Only copper addition sustainably enhanced the specific rate of PCE loss. For copper-to-iron molar ratios of 0.25 to 5, the pseudo-first-order rate constant for PCE transformation was increased by a factor of 3.5. It is apparent that the effect of copper addition on Fenton-dependent reaction rates is complex, and involves a shift in chemical mechanism, as indicated by the differing slopes in the Arrhenius plot (with and without copper).A mathematical model was developed to evaluate the effect of operational parameters ([Fe(III)]T:[H2O2]o ratio and pH) on degradation kinetics and optimize the PCE degradation process in homogeneous reaction mixtures. The model simulated experimental degradation of the organic target in a homogeneous Fenton-reaction system. The model requires further refinement to simulate Fenton's systems in which ions in solution (such as sulfate and chloride) play significant roles.In continuous-flow reactors, Fenton's reagents were cycled through spent GAC in columns to degrade one of seven chlorinated compounds tested. The contaminant with the weakest adsorption characteristics, methylene chloride, was 99% lost from the carbon surface during a 14-hour regeneration period. At the field site, the GAC was saturated with gases containing TCE and PCE from a soil vapor extraction (SVE) system. In the field, up to 95% of the sorbed TCE was removed from GAC during regeneration periods of 50-60 hours. Recovery of PCE-loaded GAC was significantly slower. Column experiments show that there is minimal loss of carbon adsorption capacity during Fenton treatment and that the rate of GAC regeneration is compound specific. Scoping-level cost estimates indicated that field use of Fenton regeneration is not cost effective without optimization and/or iron surface amendments, except in the case of the most soluble VOCs.