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Rate constants of dichloride radical anion reactions with molecules of environmental interest in aqueous solution: a review.

ABSTRACT: Natural waters, water droplets in the air at coastal regions and wastewaters usually contain chloride ions (Cl^-) in relatively high concentrations in the milimolar range. In the reactions of highly oxidizing radicals (e.g., ^•OH, ^•NO₃, or SO₄^•-) in the nature or during wastewater treatment in advanced oxidation processes the chloride ions easily transform to chlorine containing radicals, such as Cl^•, Cl₂^•-, and ClO^•. This transformation basically affects the degradation of organic molecules. In this review about 400 rate constants of the dichloride radical anion (Cl₂^•-) with about 300 organic molecules is discussed together with the reaction mechanisms. The reactions with phenols, anilines, sulfur compounds (with sulfur atom in lower oxidation state), and molecules with conjugated electron systems are suggested to take place with electron transfer mechanism. The rate constant is high (10⁷-10⁹ M^-1 s^-1) when the reduction potential the one-electron oxidized species/molecule couple is well below that of the Cl₂^•-/2Cl^- couple.

SUBMITTER: Wojnarovits L

PROVIDER: S-EPMC8354983 | biostudies-literature |

REPOSITORIES: biostudies-literature

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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.

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Rate constants of dichloride radical anion reactions with molecules of environmental interest in aqueous solution: a review.

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