Amy L. Teel

Mechanism of base activation of persulfate.

Base is the most commonly used activator of persulfate for the treatment of contaminated groundwater by in situ chemical oxidation (ISCO). A mechanism for the base activation of persulfate is proposed involving the base-catalyzed hydrolysis of persulfate to hydroperoxide anion and sulfate followed by the reduction of another persulfate molecule by hydroperoxide. Reduction by hydroperoxide decomposes persulfate into sulfate radical and sulfate anion, and hydroperoxide is oxidized to superoxide. The base-catalyzed hydrolysis of persulfate was supported by kinetic analyses of persulfate decomposition at various base:persulfate molar ratios and an increased rate of persulfate decomposition in D(2)O vs H(2)O. Stoichiometric analyses confirmed that hydroperoxide reacts with persulfate in a 1:1 molar ratio. Addition of hydroperoxide to basic persulfate systems resulted in rapid decomposition of the hydroperoxide and persulfate and decomposition of the superoxide probe hexachloroethane. The presence of superoxide was confirmed with scavenging by Cu(II). Electron spin resonance spectroscopy confirmed the generation of sulfate radical, hydroxyl radical, and superoxide. The results of this research are consistent with the widespread reactivity reported for base-activated persulfate when it is used for ISCO.

Comparison of mineral and soluble iron Fenton's catalysts for the treatment of trichloroethylene.

Contaminant degradation, stoichiometry, and role of hydroxyl radicals (OH*) in four Fentons systems were investigated using trichloroethylene (TCE) as a model contaminant. A standard Fentons 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 Fentons 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 Fentons 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.

Hydrogen peroxide decomposition in model subsurface systems.

Rates of hydrogen peroxide decomposition, hydroxyl radical production, and oxygen evolution were investigated in silica sand-goethite slurries using unstabilized and stabilized hydrogen peroxide formulations. The goethite-catalyzed decomposition of unstabilized hydrogen peroxide formulations resulted in more rapid hydrogen peroxide loss and oxygen evolution relative to systems containing a highly stabilized hydrogen peroxide formulation. Systems at neutral pH and those containing higher goethite concentrations were characterized by higher rates of hydrogen peroxide decomposition and by more oxygen evolution. The stabilized hydrogen peroxide formulation showed greater hydroxyl radical production relative to the unstabilized formulations. Furthermore, hydroxyl radical production rates were greater at neutral pH than at the acidic pH regimes. The results suggest that when stabilized hydrogen peroxide is injected into the subsurface during in situ bioremediation, naturally occurring minerals such as goethite may initiate Fenton-like reactions. While these reactions may prove to be toxic to microorganisms, they have the potential to chemically oxidize contaminants in soils and groundwater.

Mechanism of persulfate activation by phenols.

The activation of persulfate by phenols was investigated to further the understanding of persulfate chemistry for in situ chemical oxidation (ISCO). Phenol (pKa = 10.0) activated persulfate at pH 12 but not at pH 8, suggesting activation occurred only via the phenoxide form. Evaluation of the phenoxide activation mechanism was complicated by the concurrent activation of persulfate by hydroperoxide anion, which is generated by the base catalyzed hydrolysis of persulfate. Therefore, phenoxide activation was investigated using pentachlorophenoxide at pH 8.3, midway between the pKa of pentachlorophenol (pKa = 4.8) and that of hydrogen peroxide (pKa = 11.8). Of the two possible mechanisms for phenoxide activation of persulfate (reduction or nucleophilic attack) the results were consistent with reduction of persulfate by phenoxide with oxidation of the phenoxide. The concentration of phenoxide required for maximum persulfate activation was low (1 mM). The results of this research document that phenoxides activate persulfate via reduction; phenolic moieties ubiquitous to soil organic matter in the subsurface may have a significant role in the activation of persulfate during its injection into the subsurface for ISCO. Furthermore, the results provide the foundation for activation of persulfate by other organic anions without the toxicity of phenols, such as keto acids.

Mineralization of a sorbed polycyclic aromatic hydrocarbon in two soils using catalyzed hydrogen peroxide.

Hydrogen peroxide (H2O2) catalyzed by soluble iron or naturally occurring soil minerals, (i.e., modified Fentons 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.

Degradation of carbon tetrachloride by modified Fenton’s reagent

The degradation of tetrachloromethane (carbon tetrachloride-CT) by modified Fentons reagent (catalyzed hydrogen peroxide) was investigated using a range of hydrogen peroxide concentrations and 1 mM iron(III) catalyst. The documented reactive species in modified Fentons reactions, hydroxyl radical (OH*), is not reactive with CT, yet CT degradation was observed in the Fentons reactions and was confirmed by chloride generation. Because CT is not reactive with OH*, a reductive mechanism which may involve superoxide radical anion is proposed for CT degradation in modified Fentons systems. Scavenging of reductants by excess chloroform prevented CT degradation, confirming a reductive mechanism. Similar to CT, three other oxidized aliphatic compounds, hexachloroethane, bromotrichloromethane, and tetranitromethane, were also degraded by modified Fentons reagent. The results show that modified Fentons reactions act through a reductive mechanism to degrade compounds that are not reactive with OH*, which broadens the scope of this process for hazardous waste treatment and remediation.

Persulfate activation by subsurface minerals.

Persulfate dynamics in the presence of subsurface minerals was investigated as a basis for understanding persulfate activation for in situ chemical oxidation (ISCO). The mineral-mediated decomposition of persulfate and generation of oxidants and reductants was investigated with four iron and manganese oxides and two clay minerals at both low pH (<7) and high pH (>12). The manganese oxide birnessite was the most effective initiator of persulfate for degrading the oxidant probe nitrobenzene, indicating that oxidants are generated at both low and high pH regimes. The iron oxide goethite was the most effective mineral for degrading the reductant probe hexachloroethane. A natural soil and two soil fractions were used to confirm persulfate activation by synthetic minerals. The soil and soil fractions did not effectively promote the generation of oxidants or reductants. However, soil organic matter was found to promote reductant generation at high pH. The results of this research demonstrate that synthetic iron and manganese oxides can activate persulfate to generate reductants and oxidants; however, iron and manganese oxides in the natural soil studied do not show the same reactivity, most likely due to the lower masses of the metal oxides in the soil relative to the masses studied in isolated mineral systems.

A foundation for the risk-based treatment of gasoline-contaminated soils using modified Fenton's reactions

The relative oxidation of representative aromatic and aliphatic hydrocarbons found in gasoline was evaluated to provide the foundation for risk-based treatment of petroleum-contaminated soils and groundwater using modified Fentons reagent (catalyzed hydrogen peroxide). Aromatic components of gasoline are considered more hazardous than the aliphatic fractions due to their higher mobility in the subsurface and their higher acute and chronic toxicities. Benzene, toluene, and mixed xylenes (BTX) were selected as aromatic compounds representative of unleaded gasoline, while nonane, decane, and dodecane (NDD) were used as model aliphatic compounds. The effects of hydrogen peroxide (H(2)O(2)) concentration, iron catalyst concentration, and pH on the degree of treatment of the model compounds were investigated using central composite rotatable experimental designs. Oxidation of the aromatic compounds required less iron and less H(2)O(2) than did oxidation of the aliphatic compounds, while proceeding more effectively at near-neutral pH. Greater than 95% of the BTX was treated at near-neutral pH using 2. 5% H(2)O(2) and 12.5 mM iron (III), while only 37% nonane, 7% decane, and 1% dodecane oxidation was achieved under the same conditions. The results show that the more toxic and mobile aromatic fraction was more effectively oxidized using less H(2)O(2) and more economical conditions, including near-neutral pH, compared to the aliphatic fraction. A process design based on treating only the aromatic fraction of petroleum may provide significantly lower costs when using modified Fentons reagent for the treatment of contaminated soils and groundwater.

Persulfate activation by naturally occurring trace minerals

The potential for 13 naturally occurring minerals to mediate the decomposition of persulfate and generate a range of reactive oxygen species was investigated to provide fundamental information on activation mechanisms when persulfate is used for in situ chemical oxidation (ISCO). Only four of the minerals (cobaltite, ilmenite, pyrite, and siderite) promoted the decomposition of persulfate more rapidly than persulfate-deionized water control systems. The other nine minerals decomposed persulfate at the same rate or more slowly than the control systems. Mineral-mediated persulfate activation was conducted with the addition of one of three probe compounds to detect the generation of reactive oxygen species: anisole (sulfate+hydroxyl radical), nitrobenzene (hydroxyl radical), and hexachloroethane (reductants and nucleophiles). The reduced mineral pyrite promoted rapid generation of sulfate+hydroxyl radical. However, the remainder of the minerals provided minimal potential for the generation of reactive oxygen species. The results of this research demonstrate that the majority of naturally occurring trace minerals do not activate persulfate to generate reactive oxygen species, and other mechanisms of activation are necessary to promote contaminant destruction in the subsurface during persulfate ISCO.

pH and temperature effects on the hydrolysis of three β-lactam antibiotics: ampicillin, cefalotin and cefoxitin.

An understanding of antibiotic hydrolysis rates is important for predicting their environmental persistence. Hydrolysis rates and Arrhenius constants were determined as a function of pH and temperature for three common β-lactam antibiotics, ampicillin, cefalotin, and cefoxitin. Antibiotic hydrolysis rates at pH4-9 at 25 °C, 50 °C, and 60 °C were quantified, and degradation products were identified. The three antibiotics hydrolyzed under ambient conditions (pH7 and 25 °C); half-lives ranged from 5.3 to 27 d. Base-catalyzed hydrolysis rates were significantly greater than acid-catalyzed and neutral pH hydrolysis rates. Hydrolysis rates increased 2.5- to 3.9-fold for a 10 °C increase in temperature. Based on the degradation product masses found, the likely functional groups that underwent hydrolysis were lactam, ester, carbamate, and amide moieties. Many of the proposed products resulting from the hydrolysis of ampicillin, cefalotin, and cefoxitin likely have reduced antimicrobial activity because many products contained a hydrated lactam ring. The results of this research demonstrate that β-lactam antibiotics hydrolyze under ambient pH and temperature conditions. Degradation of β-lactam antibiotics will likely occur over several weeks in most surface waters and over several days in more alkaline systems.