Advanced oxidation technology represented by hydroxyl radicals has great potential to remove residual antibiotics. In this study, we systematically compared the metronidazole (MTZ) degradation behavior and mechanism in the UV and UV/H₂O₂ systems at pH 3.00 condition. The results show that the initial reaction rates were 0.147 and 1.47 µM min⁻¹ in the UV and UV/H₂O₂ systems, respectively. The main reason for the slow direct photolysis of MTZ is the relatively low molar absorption coefficient (2645.44 M⁻¹ cm⁻¹) and quantum yield (5.9 × 10⁻³ mol Einstein⁻¹). Then, we measured $k_{\mathrm{MTZ},\mathrm{OH}_{ }^{•}}$ as 2.79 (±0.12) × 10⁹ M⁻¹ s⁻¹ by competitive kinetics, and calculated $k_{\mathrm{MTZ},\mathrm{OH}_{ }^{•}}$ and ${\left[\mathrm{OH}_{ }^{•}\right]}_{\mathrm{SS}}$ as 2.43 (±0.11) × 10⁹ M⁻¹ s⁻¹ and 2.36 × 10⁻¹³ M by establishing a kinetic model based on the steady-state hypothesis in our UV/H₂O₂ system. The contribution of direct photolysis and ^•OH to the MTZ degradation was 9.9% and 90.1%. ^•OH plays a major role in the MTZ degradation, and ^•OH was the main active material in the UV/H₂O₂ system. This result was also confirmed by MTZ degradation and radicals’ identification experiments. MTZ degradation increases with H₂O₂ dosage, but excessive H₂O₂ had the opposite effect. A complex matrix has influence on MTZ degradation. Organic matter could inhibit the degradation of MTZ, and the quenching of the radical was the main reason. ${\mathrm{NO}}_3^{-}$ promoted the MTZ degradation, while ${\mathrm{SO}}_4^{2-}$ and Cl⁻ had no effect. These results are of fundamental and practical importance in understanding the MTZ degradation, and to help select preferred processes for the optimal removal of antibiotics in natural water bodies, such as rivers, lakes, and groundwater

At present, the application of phytoremediation technology in the ecological remediation of heavy metal tailings is receiving more and more attention. In this study, the physiological and biochemical response and tolerance mechanism of woody plant Nerium indicum to Pb and Zn under different proportions of inorganic modifier calcium carbonate (C1: 5%, C2: 10%, C3: 15%) and organic modifier mushroom residue (M1: 10%, M2: 20%, M3: 30%) was compared. The results showed that the pH value has a trend of C group > M group > CK group and organic matter has a trend of M group > CK group > C group. Phosphatase activity and catalase activity has a trend of M group > C group > CK group, but catalase was more vulnerable to the calcium carbonate concentration. Both modifiers can promote the transformation of Pb, Zn, Cu, and Cd in tailings to more stable organic bound and residual states. However, the stabilization effect of mushroom residue is better, and its stability is Pb, Zn > Cd, Cu. Both modifiers can increase the biomass of Nerium indicum and the modification effect of mushroom residue is better than calcium carbonate. Pb/Zn content and accumulation in Nerium indicum organs showed root > stem > leaf in all groups. Compared with the CK group, the enrichment coefficient of Pb/Zn in C1 and M1 groups decreased, while the translocation factor of Pb/Zn in C1 and M1 groups increased. With the increase in modifier concentration, the enrichment coefficient increases about 1.75~52.94%, but the translocation factor decreases rapidly (20.01~64.46%). Clearly, both the calcium carbonate and mushroom residue amendment could promote the growth ability of Nerium indicum in lead–zinc tailings and strengthen the phytoremediation potential.