Fenton oxidation of synthetic food dyes by Fe-embedded coffee biochar catalysts prepared at different pyrolysis temperatures: A mechanism study

Abstract

• Fe-embedded coffee biochar catalysts were synthesized at high pyrolysis temperatures. • Improving catalytic properties was most pronounced for Fe@HCBC prepared at 700 °C. • Fe@HCBC700/H 2 O 2 system showed the highest degradation of synthetic food dyes. • ESR study demonstrated a significant role of •OH in degradation of synthetic food dyes. • Heterogeneous Fenton-like reaction governed the degradation of synthetic food dyes. Fe-embedded biochar catalysts originating from waste coffee grounds (Fe@HCBCs) were developed at different pyrolysis temperatures (500, 600, and 700 °C) for the Fenton oxidation of synthetic food dyes: amaranth (AM) and sunset yellow (SY). Increasing the pyrolysis temperature significantly improved the physicochemical properties of the Fe@HCBCs (i.e., surface area, total pore volume, and Fe content), which led to the highest AM and SY removal in the Fe@HCBC700/H 2 O 2 system. The Fe@HCBC700/H 2 O 2 system showed 19.0 and 14.6 mg(dyes)/g(catalysts)·h of catalytic reaction rate for AM and SY, respectively. The results of the scavenging experiment and electron spin resonance revealed that hydroxyl radicals (•OH) were the dominant reactive oxygen species for the oxidation of AM and SY by the Fe@HCBCs/H 2 O 2 system. The heterogeneous Fenton-like reaction between the Fe content of the Fe@HCBC surfaces and H 2 O 2 was the main contributor to the oxidation of AM and SY compared with the homogeneous Fenton reaction caused by aqueous Fe with H 2 O 2 . The effects of catalyst dosage (0.1–0.5 g/L), H 2 O 2 concentration (0.0–12.5 mM), pH (3.0–9.0), and solution temperature (15–35 °C) on the oxidation of AM and SY by the Fe@HCBCs/H 2 O 2 systems at pH 3.0 were also investigated. The opposite result was obtained in a suspension with pH > 5.0; that is, the removal efficiency gradually increased as the pyrolysis temperature decreased (500 °C > 600 °C > 700 °C) because of the abundance of O-containing functional groups on the surface of the Fe@HCBC500. These observations imply that pyrolysis temperatures might have a strong impact on the catalytic degradation of synthetic food dyes using the Fe@HCBCs/H 2 O 2 systems.