Search

Catalyzed H 2 O 2 propagations (CHP) is characterized by the most robust reactivity of any of the in situ chemical oxidation (ISCO) technologies because it generates the strong oxidant hydroxyl radical along with nucleophiles + reductants, such as superoxide radical. The most common groundwater contaminants, trichloroethylene (TCE) and perchloroethylene (PCE), were used as model contaminants in evaluating the effect of hydrogen peroxide (H 2 O 2 ) dosage on contaminant destruction kinetics. Both TCE and PCE degradation rates increased with H 2 O 2 dosages up to 0.1 M, and then decreased with higher H 2 O 2 dosages. Parallel reactions conducted with the addition of the hydroxyl radical scavenger 2-propanol and the hydroxyl radical-specific probe nitrobenzene confirmed that hydroxyl radical is primarily responsible for TCE and PCE degradation; however, 5-20% of their degradation was attributed to a non-hydroxyl radical mechanism. Reactions conducted with the superoxide probe tetranitromethane showed that superoxide generation rates increased with increasing H 2 O 2 doses. These results were confirmed by electron spin resonance spectroscopy. Therefore, the non-hydroxyl radical pathway for TCE and PCE degradation at H 2 O 2 ≥0.5 M was likely via nucleophilic attack by superoxide. The results of this research demonstrate that contaminants present in the aqueous phase that are reactive with hydroxyl radical require only low doses of H 2 O 2 (≤0.1 M), but subsurface systems contaminated with species not reactive with hydroxyl radical (e.g., carbon tetrachloride) require H 2 O 2 concentrations ≥0.5 M., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Sodium persulfate has become the most popular oxidant source for the in situ chemical oxidation (ISCO) treatment of organic contaminants in the subsurface. The most common persulfate activators, iron chelates and base, are often ineffective in initiating the generation of reactive oxygen species in field applications. In this study, glucose was investigated as a persulfate activator in systems containing varying concentrations of sodium hydroxide using nitrobenzene as a hydroxyl radical probe and hexachloroethane as a reductant + nucleophile probe. Glucose activation of persulfate increased as a function of sodium hydroxide addition, but was still effective at circumneutral pH regimes. Use of central composite rotatable experimental designs showed that hydroxyl radical and reductant + nucleophile generation rates increased as a function of persulfate at near-neutral pH regimes. Glucose activation of persulfate has the advantages over other activation pathways of more options and flexibility for effective process chemistry and of minimizing or eliminating the mass of sodium hydroxide added to the subsurface. The results of this research can be applied in the field by first evaluating glucose activation compared to base and iron chelate activation of persulfate in laboratory treatability studies., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

The displacement of sorbed metals during the treatment of soils and groundwater with modified Fenton's reagent was investigated using five common metal contaminants (cadmium, copper, lead, nickel, and zinc) and kaolinite as a model sorbent. Modified Fenton's conditions included three H(2)O(2) concentrations (0.9, 1.8, 2.7 M) and two catalysts: soluble iron (III) at pH 3 and iron (III)-NTA chelate at pH 6. Iron (III)-catalyzed Fenton's reactions released significant amounts of zinc, cadmium, and copper. Modified Fenton's reactions catalyzed by iron (III)-NTA released zinc, cadmium, copper, and lead, and resulted in greater amounts of metals release than the iron (III)-catalyzed reactions. Metals release may have been mediated by transient oxygen species, such as superoxide, generated by propagation reactions, which become dominant at the relatively high hydrogen peroxide concentrations used. Metals release from kaolinite was undetectable when sufficiently low hydrogen peroxide concentrations were maintained to minimize propagation reactions. These results indicate that using dilute concentrations of hydrogen peroxide for Fenton's ISCO may minimize potential metals mobility when treating contaminated soils and groundwater containing a mixture of organic and metal contaminants.

Hydrogen peroxide (H2O2) catalyzed by soluble iron or naturally occurring soil minerals, (i.e., modified Fenton's reagent) was investigated as a basis for mineralizing sorbed and NAPL-phase benzo[a]pyrene (BaP), a hydrophobic and toxic polycyclic aromatic hydrocarbon, in two soils of different complexity. 14C-Benzo[a]pyrene was added to silica sand and a silt loam soil, and mineralization was investigated using three-level central composite rotatable experimental designs. The effects of H2O2 concentration, slurry volume, and iron(II) amendment were investigated in the silica sand systems. In a Palouse loess silt loam soil, the variables included H2O2 concentration, slurry volume, and pH, with H2O2 catalyzed by naturally occurring iron oxyhydroxides. Regression equations generated from the data were used to develop three-dimensional response surfaces describing BaP mineralization. Based on the recovery of 14C-CO2, 70% BaP mineralization was achieved in the sand within 24 h using 15 M H2O2 and an iron(II) concentration of 6.6 mM with a slurry volume of 0.3 x the field capacity of the sand. For the silt loam soil, 85% mineralization of BaP was observed using 15 M H2O2, no iron amendment, and a slurry volume of 20 x the soil field capacity. The balance of the radiolabeled carbon remained as unreacted BaP in the soil fraction. Gas-purge measurements over 5 d confirmed negligible desorption under nontreatment conditions. However, oxidation reactions were complete within 24 h and promoted up to 85% BaP mineralization, documenting that the natural rate of desorption/dissolution did not control the rate of oxidation and mineralization of the BaP. The results show that catalyzed H2O2 has the ability to rapidly mineralize sorbed/NAPL-phase BaP and that partitioning, which is often the rate-limiting factor in soil remediation, does not appear to limit the rate of vigorous Fenton-like treatment.

Contaminant degradation, stoichiometry, and role of hydroxyl radicals (OH*) in four Fenton's systems were investigated using trichloroethylene (TCE) as a model contaminant. A standard Fenton's system, a modified soluble iron system with a pulse input of hydrogen peroxide, and two modified mineral-catalyzed systems (pH 3 and 7) were studied. In the standard Fenton's system, which had the most efficient reaction stoichiometry, 78% of the TCE was degraded; however, chloride analysis indicated that no more than two of the three chlorines were displaced per TCE molecule degraded. Although the modified soluble iron system was characterized by 91% TCE degradation, chloride analysis also indicated that no more than two of the chlorines were lost from the TCE. In the goethite system of pH 3, > 99% of the TCE was degraded. Near-complete release of chloride suggested that the TCE may have been mineralized. Only 22% degradation of TCE was achieved in the pH 7 goethite system. and there was minimal release of chloride. The mineral-catalyzed reactions exhibited the least efficient reaction stoichiometry of the four systems. Experiments using hydroxyl radical scavengers showed that the standard Fenton's system degraded TCE entirely by hydroxyl radical mechanisms, while approximately 10-15% of the degradation achieved in the modified soluble iron and goethite-catalyzed systems at pH 3 was mediated by non-hydroxyl radical mechanisms. In the goethite system at pH 7, only non-hydroxyl radical mechanisms were found. The goethite-catalyzed system at pH 3 effectively degraded the parent compound and may have the potential to mineralize contaminants when used for in situ soil and groundwater remediation and ex situ waste stream treatment in packed-bed reactors.