Magnetic MgFeO@BC Derived from Rice Husk as Peroxymonosulfate Activator for Sulfamethoxazole Degradation: Performance and Reaction Mechanism

Abstract

Heterogeneous Mg-Fe oxide/biochar (MgFeO@BC) nanocomposites were synthesized by a co-precipitation method and used as biochar-based catalysts to activate peroxymonosulfate (PMS) for sulfamethoxazole (SMX) removal. The optimal conditions for SMX degradation were examined as follows: pH 7.0, MgFeO@BC of 0.4 g/L, PMS concentration of 0.6 mM and SMX concentration of 10.0 mg/L at 25 ℃. In the MgFeO@BC/PMS system, the removal efficiency of SMX was 99.0% in water within 40 min under optimal conditions. In the MgFeO@BC/PMS system, the removal efficiencies of tetracycline (TC), cephalexin (CEX), ciprofloxacin (CIP), 4-chloro-3-methyl phenol (CMP) and SMX within 40 min are 95.3%, 98.4%, 98.2%, 97.5% and 99.0%, respectively. The radical quenching experiments and electron spin resonance (ESR) analysis suggested that both non-radical pathway and radical pathway advanced SMX degradation. SMX was oxidized by sulfate radicals (SO₄^•-), hydroxyl radicals (•OH) and singlet oxygen (¹O₂), and SO₄^•- acted as the main active species. MgFeO@BC exhibits a higher current density, and therefore, a higher electron migration rate and redox capacity. Due to the large number of available binding sites on the surface of MgFeO@BC and the low amount of ion leaching during the catalytic reaction, the system has good anti-interference ability and stability. Finally, the intermediates of SMX were detected.

Keywords: biochar-based catalysts; nonradical pathway; peroxymonosulfate; sulfate radicals.

Figure 1

SEM images of BC (a), Fe₃O₄@BC (b), MgO@BC (c) and MgFeO@BC (d).

Figure 2

HRTEM images (a,b), SAED (c), EDX (d) and C, Fe, Mg, N, O element distribution (e–g).

Figure 3

XRD patterns (a), FT-IR spectra (b), Raman spectra (c), room-temperature magnetization curve (d), nitrogen adsorption–desorption isotherms (e) and pore size distribution (f) of catalysts.

Figure 4

The adsorption efficiencies of SMX in various reaction systems. Reaction conditions: [SMX]₀ = 10.0 mg/L, [catalyst]₀ = 0.4 g/L, initial pH = 7.0 and T = 25 ℃.

Figure 5

SMX degradation efficiencies (a), and k_obs in different systems (b). Reaction conditions: [SMX]₀ = 10.0 mg/L, [catalyst]₀ = 0.4 g/L, [PMS]₀ = 0.6 mM, initial pH = 7.0 and T = 25 ℃.

Figure 6

Effects of different parameters on SMX removal in MgFeO@BC/PMS system: (a) catalyst dosage, (b) PMS concentration, (c) solution pH, (d) SMX concentration. (Conditions: pH = 7.0; [SMX]₀ = 10.0 mg/L; [catalyst] = 0.4 g/L; [PMS] = 0.6 mM; T = 25 ℃.)

Figure 7

Zeta potentials of MgFeO@BC catalyst.

Figure 8

SMX degradation efficiency under different scavengers (a–d). (Conditions: [SMX]₀ = 10.0 mg/L; [MgFeO@BC]₀ = 0.4 g/L; [PMS]₀ = 0.6 mM; pH = 7.0; T = 25 ℃.)

Figure 9

ESR signals of (a) TEMP-¹O₂, and (b) DMPO-X. (Conditions: [SMX]₀ = 10.0 mg/L; [MgFeO@BC]₀ = 0.4 g/L; [PMS]₀ = 0.6 mM; pH = 7.0; T = 25 ℃; [TEMP] = [DMPO] = 10.0 mM.)

Figure 10

XPS spectra of full-range survey (a), C 1s (b), Fe 2p (c) for MgFeO@BC; CV curves of catalysts (d).

Figure 11

Possible pathways for degradation of SMX in MgFeO@BC/PMS system.

Figure 12

Proposed mechanism of SMX degradation in MgFeO@BC/PMS system.

Figure 13

Effects of Cl⁻ (a), HCO₃⁻ (b), NO₃⁻ (c) and HA (d) on SMX degradation. (Conditions: [SMX]₀ = 10.0 mg/L; [MgFeO@BC]₀ = 0.4 g/L; [PMS]₀ = 0.6 mM; pH = 7.0; T = 25 ℃.)

Figure 14

The degradation of SMX in different aqueous substrates (a), the removal of various pollutants in MgFeO@BC/PMS system (b), reusability of MgFeO@BC after 5 consecutive cycles (c), and the concentration of leached ions (d). (Conditions: [substrate]₀ = 10.0 mg/L; [MgFeO@BC]₀ = 0.4 g/L; [PMS]₀ = 0.6 mM; pH = 7.0; T = 25 ℃.)