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Ultrasound/chlorine sono-hybrid-advanced oxidation process: Impact of dissolved organic matter and mineral constituents.

ABSTRACT: In this work, after exploring the first report on the synergism of combining ultrasound (US: 600 kHz) and chlorine toward the degradation of Allura Red AC (ARAC) textile dye, as a contaminant model, the impact of various mineral water constituents (Cl^-, SO₄^2-, NO₃^-, HCO₃^- and NO₂^-) and natural organic matter, i.e., humic acid (HA), on the performance of the US/chlorine sono-hybrid process was assessed for the first time. Additionally, the process effectiveness was evaluated in a real natural mineral water (NMW) of a known composition. Firstly, it was found that the combination of ultrasound and chlorine (0.25 mM) at pH 5.5 in cylindrical standing wave ultrasonic reactor (f = 600 kHz and P_e = 120 W, equivalent to P_A ∼ 2.3 atm) enhanced in a drastic manner the degradation rate of ARAC; the removal rate being 320% much higher than the arithmetic sum of the two separated processes. The source of the synergistic effect was attributed to the effective implication of reactive chlorine species (RCS: Cl, ClO and Cl₂^-) in the degradation process. Radical probe technique using nitrobenzene (NB) as a specific quencher of the acoustically generated hydroxyl radical confirmed the dominant implication of RCS in the overall degradation rate of ARAC by US/chlorine system. Overall, the presence of humic acid and mineral anions decreased the efficiency of the sono-hybrid process; however, the inhibition degrees depend on the type and the concentration of the selected additives. The reaction of these additives with the generated RCS is presumably the reason for the finding results. The inhibiting effect of Cl^-, SO₄^2-, NO₃^- and NO₂^- was more pronounced in US/chlorine process as compared to US alone, whereas the inverse scenario was remarked for the effect of HA. These outcomes were associated to the difference in the reactivity of HA and mineral anions toward RCS and OH oxidizing species, in addition to the more selective character of RCS than hydroxyl radical. The displacement of the reaction zone with increasing the additive concentration may also be another influencing factor that favors competition reactions, which subsequently reduce the available reactive species in the reacting medium. The NMW exerted reductions of 43% and 10% in the process efficiency at pH 5.5 and 8, respectively, thereby confirming the RCS-quenching mechanism by the water matrix constituents. Hence, this work provided a precise understanding of the overall mechanism of chlorine activation by ultrasound to promote organic compounds degradation in water.

SUBMITTER: Hamdaoui O

PROVIDER: S-EPMC8783144 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:The atmospheric oxidation of organics occurs primarily via reaction cycles involving gas phase radical species, catalysed by nitric oxide (NO), which result in the production of secondary pollutants such as ozone. For these oxidation cycles to occur, they must be initialized by a primary radical, i.e., a radical formed from non-radical precursors. Once formed, these primary radicals can result in the oxidation of organic compounds to produce peroxy radicals that, providing sufficient NO is present, can re-generate "secondary" radicals which can go on to oxidize further organics. Thus, one primary radical can result in the catalytic oxidation of multiple organics. Although the photolysis of ozone in the presence of water vapor to form two hydroxyl (OH) radicals is accepted as the dominant tropospheric primary radical source, multiple other primary radical sources exist and can dominate in certain environments. The chemical reactivity of different radicals to organic and inorganic compounds can be very different, however, and how these differences in radical chemistry impact atmospheric organic oxidation under different atmospheric conditions has not been previously demonstrated. In this work, we use a series of model simulations to investigate the impact of the chemical reactivity of the primary radical on the effectiveness in initializing organic oxidation and thus the production of the secondary pollutant ozone. We compare the chemistries of the OH and atomic chlorine (Cl) radicals and their effectiveness at initializing organic oxidation under different nitrogen oxide and organic concentrations. The OH radical is the dominant tropospheric radical, with both primary and secondary sources. In contrast, Cl has primary sources that show significant spatial heterogeneity throughout the troposphere but is not typically regenerated in catalytic cycles. Both primary OH and Cl can initiate organic oxidation, but this work shows that the relative effectiveness with which they oxidize organics and produce ozone depends on their balance of propagation vs termination reactions which is in turn determined by the chemical environment in which they are produced. In particular, our work shows that in high NOx radical-limited environments, like those found in many urban areas, Cl will be more efficient at oxidizing organics than OH.

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Ultrasound/chlorine sono-hybrid-advanced oxidation process: Impact of dissolved organic matter and mineral constituents.

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