Kinetic and Mechanistic Study of Rhodamine B Degradation by H2O2 and Cu/Al2O3/g-C3N4 Composite

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.

Effect of pH on the Catalytic Degradation of Rhodamine B by Synthesized CDs/g-C3N4/CuxO Composites

“…It can be seen that higher pH inhibits Cu leaching; the maximum concentration at pH 3 was about 60 times that at pH 11. According to the Eh-pH diagram of Cu-H 2 O, ,, it mainly exists in the form of Cu 2+ or Cu + , and Cu 2 O and Cu­(OH) 2 under acidic and alkali conditions, respectively. It was observed that the leaching of copper ions on the catalyst surface was promoted under acidic conditions; it had a certain inhibitory effect on the leaching of ions on the catalyst surface under alkaline conditions, thereby indicating that the surface reaction of the composite was dominant in the reaction.…”

“…Similar to the redox properties of iron, copper can undergo a Fenton-like system with H 2 O 2 to achieve mutual conversion between Cu + and Cu 2+ and produce • OH, as shown in eqs and . , It should be noted that eq is a rate-limiting step and Cu 2+ can be from a copper complex [Cu­(H 2 O) 6 ] + at a neutral pH, which can be used in the Cu 2+ /Cu + /H 2 O 2 Fenton-like system . Cu + /Cu 2+ has limited applicability due to the narrow range of pH and the extreme volatility of Cu + .…”

Sustainable and Environmental Catalysis

“…The specific reaction rate constant of Rh B degradation was calculated as (5.9 ± 0.07) × 10 −9 m•s −1 , and the activation energy was calculated to be 71.0 kJ/mol. The recycling experiment demonstrated its durability for Rh B removal and proved that the degradation reaction was surface dominated, with a negligible leaching of copper species in solution [19].…”

A Comprehensive Review of Graphitic Carbon Nitride (g-C3N4)–Metal Oxide-Based Nanocomposites: Potential for Photocatalysis and Sensing

“…Cu is a conventional metal using for the improvement of photocatalytic performance. Zhou [349]. The Cu immobilized Al 2 O 3 /g-C 3 N 4 also showed promising stability for the treatment of water pollution.…”