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Sulfate Radical Scavenging by Mineral Surfaces in Persulfate-Driven Oxidation Systems: Reaction Rate Constants and Implications.

ABSTRACT: Activated persulfate (PS) is a common method used to generate sulfate radicals (SO₄^•-), a powerful oxidant capable of degrading a broad array of environmental contaminants. The reaction of SO₄^•- with nontarget species (i.e., scavenging) contributes significantly to treatment inefficiency. Radical scavenging in this manner has been quantified for nontarget chemical species in the aqueous phase but has never been quantified for solid phase media. Kinetic analysis and laboratory methods were developed to quantify the SO₄^•- scavenging rate constant (k_?S) for alumina, a naturally occurring mineral in soil and aquifer materials. SO₄^•- were generated in UV and thermally activated persulfate (UV-APS, T-APS) batch systems, and the loss of rhodamine B (RhB) served as an indicator of SO₄^•- activity. k_?S for alumina was 2.42 × 10⁴ and 2.03 × 10⁴ m^-2 s^-1 for UV-APS and T-APS oxidative treatment systems, respectively. At [alumina] >5 g L^-1, the reaction of SO₄^•- with solid phase media increased over the aqueous phase reactions with RhB and aqueous scavengers. SO₄^•- scavenging by solid surfaces was orders of magnitude greater than the reaction with the target compound and scavengers in the aqueous phase, underscoring the significant role of solid surfaces in scavenging SO₄^•-.

SUBMITTER: Crincoli KR

PROVIDER: S-EPMC7534697 | biostudies-literature | 2020 Feb

REPOSITORIES: biostudies-literature

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Sulfate Radical Scavenging by Mineral Surfaces in Persulfate-Driven Oxidation Systems: Reaction Rate Constants and Implications.

Crincoli Klara Rusevova KR Green Constance C Huling Scott G SG

Environmental science & technology 20200122 3

Activated persulfate (PS) is a common method used to generate sulfate radicals (SO4•-), a powerful oxidant capable of degrading a broad array of environmental contaminants. The reaction of SO4•- with nontarget species (i.e., scavenging) contributes significantly to treatment inefficiency. Radical scavenging in this manner has been quantified for nontarget chemical species in the aqueous phase but has never been quantified for solid phase media. Kinetic ...[more]

PMID: 31967801

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Project description:Advanced oxidation treatment processes used in various applications to treat contaminated soil, water, and groundwater involve powerful radical intermediates, including hydroxyl radicals (•OH). Inefficiency in •OH-driven treatment systems involves scavenging reactions where •OH react with non-target species in the aqueous and solid phases. Here, •OH were generated in iron (Fe)- and UV-activated hydrogen peroxide (Fe-AHP, UV-AHP) systems where the loss of rhodamine B served as a quantitative metric for •OH activity. Kinetic analysis methods were developed to estimate the specific •OH surface scavenging rate constant (k≡S). In the Fe-AHP system, k≡S for silica (2.85 × 106 1/m2 × s) and alumina (3.92 × 106 1/m2 × s) were similar. In the UV-AHP system, estimates of k≡S for silica (4.50 × 106 1/m2 × s) and alumina (7.45 × 106 1/m2 × s) were higher. k≡S for montmorillonite (MMT) in the UV-AHP system was ≤4.22 × 105 1/m2 × s. Overall, k≡S,silica ∼ k≡S, alumina > k≡S,MMT indicating k≡S is mineral specific. Radical scavenging was dominated by surface scavenging at 10-50 g/L silica, alumina, or MMT, in both Fe-AHP and UV-AHP systems. The experimentally-derived surface •OH scavenging rate constants were extended to in-situ chemical oxidation (ISCO) treatment conditions to contrast •OH reaction rates with contaminant and aqueous phase reactants found in aquifer systems. •OH reaction was dominated by solid surfaces comprised of silica, alumina, and montmorillonite minerals relative to •OH reaction with trichloroethylene, the target compound, and H2O2, a well-documented radical scavenger. These results indicate that solid mineral surfaces play a key role in limiting the degradation rate of contaminants found in soil and groundwater, and the overall treatment efficiency in ISCO systems. The aggressive •OH scavenging measured was partially attributed to the relative abundance of scavenging sites on mineral surfaces.

Project description:Cytochrome P450 enzymes are heme based monoxygenases that catalyse a range of oxygen atom transfer reactions with various substrates, including aliphatic and aromatic hydroxylation as well as epoxidation reactions. The active species is short-lived and difficult to trap and characterize experimentally, moreover, it reacts in a regioselective manner with substrates leading to aliphatic hydroxylation and epoxidation products, but the origin of this regioselectivity is poorly understood. We have synthesized a model complex and studied it with low-pressure Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry (MS). A novel approach was devised using the reaction of [FeIII(TPFPP)]+ (TPFPP = meso-tetrakis(pentafluorophenyl)porphinato dianion) with iodosylbenzene as a terminal oxidant which leads to the production of ions corresponding to [FeIV(O)(TPFPP+?)]+. This species was isolated in the gas-phase and studied in its reactivity with a variety of olefins. Product patterns and rate constants under Ideal Gas conditions were determined by FT-ICR MS. All substrates react with [FeIV(O)(TPFPP+?)]+ by a more or less efficient oxygen atom transfer process. In addition, substrates with low ionization energies react by a charge-transfer channel, which enabled us to determine the electron affinity of [FeIV(O)(TPFPP+?)]+ for the first time. Interestingly, no hydrogen atom abstraction pathways are observed for the reaction of [FeIV(O)(TPFPP+?)]+ with prototypical olefins such as propene, cyclohexene and cyclohexadiene and also no kinetic isotope effect in the reaction rate is found, which suggests that the competition between epoxidation and hydroxylation - in the gas-phase - is in favour of substrate epoxidation. This notion further implies that P450 enzymes will need to adapt their substrate binding pocket, in order to enable favourable aliphatic hydroxylation over double bond epoxidation pathways. The MS studies yield a large test-set of experimental reaction rates of iron(iv)-oxo porphyrin cation radical complexes, so far unprecedented in the gas-phase, providing a benchmark for calibration studies using computational techniques. Preliminary computational results presented here confirm the observed trends excellently and rationalize the reactivities within the framework of thermochemical considerations and valence bond schemes.

Dataset Information

Sulfate Radical Scavenging by Mineral Surfaces in Persulfate-Driven Oxidation Systems: Reaction Rate Constants and Implications.

Publications

Sulfate Radical Scavenging by Mineral Surfaces in Persulfate-Driven Oxidation Systems: Reaction Rate Constants and Implications.

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