Insights into the potential applications of permanganate/peroxymonosulfate systems: enhancement via amorphous MnO2, effects of water matrices, and optimization using response surface methodology

Abstract

In this study, we developed a novel self-catalytic oxidation system involving peroxymonosulfate (PMS) and permanganate (KMnO₄), named as CUPP, to efficiently mineralize sulfamethoxazole (SMX) in groundwater. It was found that amorphous MnO₂ derived from the in situ reduction of KMnO₄ can directly adsorb HSO₅^-, a complex hydroxyl group, mediate the internal disproportionation reaction of HSO₅^- with the manganese complex, and effectively activate PMS, thereby promoting the oxidation of SMX and its degradation intermediates through sulfonate radiation. Furthermore, by using electron spin resonance (EPR), HPLC/MS full scan, and response surface methodology, the coexistence of HO˙, SO₄^-˙, O₂^-˙, ¹O₂, and active chlorine (Cl₂, HOCl) in the CUPP system was confirmed. A total of 24 intermediate products were detected, and four possible degradation pathways were identified for SMX. In addition, it was found that the CUPP system has a strong impact resistance to pH variations, groundwater anions, and natural organic matter stress. Undoubtedly, the CUPP system presents an innovative approach for the degradation of various emerging organic pollutants in groundwater.

Fig. 1. Degradation of SMX in different treatment systems.

Fig. 2. (a) Indirect identification of radical active species. (b) Percentage degradation of SMX at 45 minutes of reaction time. (c) Effects of l-tryptophan on SMX degradation in the CUPP system. (d) SMX degradation rate after 45 min reaction. Note: labels at the top of the error bars indicate the level of statistical significance.

Fig. 3. (a) EPR detection mapping of HO˙ and SO₄⁻˙ (DMPO capture); (b) EPR profiling of O₂⁻˙ (DMPO capture). (c) EPR assays of ¹O₂ (TEMP capture). (d) Measurement of the residual chlorine concentration.

Fig. 4. (a) SEM electron microscopy of AMO. (b) Reaction solution production of AMO. (c) EDS spectrum.

Fig. 5. Diagram of SMX degradation pathways (A–D).

Fig. 6. Effects of oxone and KMnO₄ initial concentrations on SMX degradation (reaction conditions: [SMX]₀ = 5 mg L⁻¹, [oxone]₀ = 2–10 mM, [KMnO₄]₀ = 0.4–3.2 mM, pH_ini = 7.0 (10 mM phosphate buffer), and 25 °C; degradation rate was measured at 30 min). Note: labels at the top of the error bars indicate the level of statistical significance.

Fig. 7. (a) Effect of changing the oxone concentration on the SMX degradation in the PMS-alone system and CUPP system; (b) effect of changing the KMnO₄ concentration on the SMX degradation in the PM-alone system and CUPP system. Note: labels at the top of the error bars indicate the level of statistical significance.

Fig. 8. Effects of pH on SMX degradation rate (reaction conditions: [SMX]₀ = 5 mg L⁻¹, [oxone]₀ = 6 mM, [KMnO₄]₀ = 2 mM, pH_ini = 3.6–9.0 (buffer), and 25 °C. Degradation rate was measured at 30 min).

Fig. 9. (a–c) Degradation trend of SMX in the CUPP system under different HA, Cl⁻, and HCO₃⁻ addition levels (embedded second-order kinetic fitting curve); (d–f) effects of HA, Cl⁻, and HCO₃⁻ additions on the k_obs degradation of SMX by CUPP. Note: labels at the top of the error bars indicate the level of statistical significance.

Fig. 10. The interaction response surface between the PMS concentration, PM concentration, and pH: (a–c) contour line; (d–f) 3D surface diagram.