Abstract
The classic Fenton reaction, which is driven by iron species, has been widely explored for pollutant degradation, but is strictly limited to acidic conditions. In this work, a copper-based Fenton-like catalyst Cu/Al2O3/g-C3N4 was proposed that achieves high degradation efficiencies for Rhodamine B (Rh B) in a wide range of pH 4.9–11.0. The Cu/Al2O3 composite was first prepared via a hydrothermal method followed by a calcination process. The obtained Cu/Al2O3 composite was subsequently stabilized on graphitic carbon nitride (g-C3N4) by the formation of C−O−Cu bonds. The obtained composites were characterized through FT-IR, XRD, TEM, XPS, and N2 adsorption/desorption isotherms, and the immobilized Cu+ was proven to be active sites. The effects of Cu content, g-C3N4 content, H2O2 concentration, and pH on Rh B degradation were systematically investigated. The effect of the catalyst dose was confirmed with a specific reaction rate constant of (5.9 ± 0.07) × 10−9 m·s−1 and the activation energy was calculated to be 71.0 kJ/mol. In 100 min 96.4% of Rh B (initial concentration 20 mg/L, unadjusted pH (4.9)) was removed in the presence of 1 g/L of catalyst and 10 mM of H2O2 at 25 °C, with an observed reaction rate constant of 6.47 × 10−4 s−1. High degradation rates are achieved at neutral and alkaline conditions and a low copper leaching (0.55 mg/L) was observed even after four reaction cycles. Hydroxyl radical (HO·) was identified as the reactive oxygen species by using isopropanol as a radical scavenger and by ESR analysis. HPLC-MS revealed that the degradation of Rh B on Cu/Al2O3/CN composite involves N-de-ethylation, hydroxylation, de-carboxylation, chromophore cleavage, ring opening, and the mineralization process. Based on the results above, a tentative mechanism for the catalytic performance of the Cu/Al2O3/g-C3N4 composite was proposed. In summary, the characteristics of high degradation rate constants, low ion leaching, and the excellent applicability in neutral and alkaline conditions prove the Cu/Al2O3/g-C3N4 composite to be a superior Fenton-like catalyst compared to many conventional ones.
Highlights
With the rapid development of industry, persistent organic pollutants in water have attracted widespread attention due to their persistence, bioaccumulation, and high toxicity [1,2]
The scavenging effect of HO· radical by H+ becomes stronger at low pH values [50,51], and H2 O2 would become more stable under strong acid conditions to form oxonium [H3 O2 ]+, which inhibits its reaction with active species to generate HO· radical in the presence of a large amount of H+ [48]
The plot of ln([Rhodamine B (Rh B)]/[Rh B]0 ) versus reaction time was linearly fitted with an R2 of 0.9913, which demonstrated that the Rh B decline followed a pseudo first-order kinetics, as confirmed by some photocatalytic and Fenton-like systems [48,54]
Summary
With the rapid development of industry, persistent organic pollutants in water have attracted widespread attention due to their persistence, bioaccumulation, and high toxicity [1,2]. As a typical AOP, Fenton reaction is efficient for HO· production, but still faces some limitations, such as the strict acidic pH range (pH < 4) [12,13], formation of iron sludge [14,15], and high cost for catalyst recycling [16,17]. One common strategy for the preparation of copper-based catalysts is the immobilization of copper species on support materials, for example, the immobilization of Cu+ /Cu2+ , copper oxide, or the copper-organic complex [24] on various matrixes like metal oxides [25,26,27], molecular sieve [28,29], and graphitic carbon nitride (g-C3 N4 ) [30,31].
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