Oxidative decomposition and mineralization of caffeine by advanced oxidation processes: The effect of hybridization

Abstract

The study consists of a detailed investigation of the degradability of the emerging water contaminant-caffeine by homogeneous and heterogeneous Advanced Oxidation Processes (AOP's), estimation of a synergy index for each hybrid operation thereof, and proposing the most plausible reaction mechanisms that are consistent with the experimental data. It also encompasses evaluation of the effect of the water matrix represented by carbonate species and humic acids, as strong scavengers of hydroxyl radicals. The results showed that single AOP's such as sonolysis (577 kHz) and photolysis with H₂O₂ provided complete caffeine elimination, but they were insufficient for the mineralization of the compound. Hybrid AOP's were considerably more effective, particularly when operated at a heterogeneous mode using commercial TiO₂. The most effective hybrid process was UV-H₂O₂/TiO₂, which provided more than 75% TOC decay at the minimum test doses of the reagent and catalyst. While the addition of ultrasound to the process significantly increased the rate of caffeine decomposition, it reduced the overall degradation of the compound to 64% in terms of TOC decay. The antagonistic effect was attributed to the formation of excess H₂O₂, and the presence of cavity clouds and/or high density layers that inhibited the transmission of UV light. The effect of natural water ingredients was found to reduce the reaction rates, signifying the major contribution of hydroxyl radicals to the destruction of caffeine. The proposed reaction mechanisms based on OH radical attack and the calculated energy barriers were in good agreement with the experimentally detected reaction byproducts.

Keywords: AOP’s; Cavitation; Energy barrier; Hybrid; OH; Synergy index.

Fig. 1

The drop in pH during sonolysis of caffeine (a), and the variations in the the apparent reaction rate constant within 15 and 30-min intervals by the initial solution pH (b). The buffering agent was sodium phosphate (0.1 M; pH 7.0), pH adjustment was made with NaOH (0.1 M), the initial values of pH and concentration were 4.0 and 5 mg L⁻¹, respectively.

Fig. 2

The pseudo-1st order rate of caffeine decay and the corresponding rate constants by 60-min sonolysis of the compound under air and argon atmospheres (“control” refers to sonolysis without gas injection). Initial conditions were C₀ = 5 mg L⁻¹, pH₀ = 4.0, gas flow rate = 1.5 L min⁻¹.

Fig. 3

Comparison of single processes for the rate of caffeine decay during 1-h reaction at pH 4, H₂O₂ = 10 mg L⁻¹ and C₀ = 5 mg L⁻¹. No gas injection was applied during any of the processes, but all samples were air-saturated prior to the experiments.

Fig. 4

The effect of pH on the rate of caffeine oxidation (C₀ = 5 mg L⁻¹) by 30-min sonolysis in the presence of UV-irradiation. The data referred to as “Control” belong to singly applied US at pH 4. The rate constants of the hybrid process were 0.065 min⁻¹ and 0.051 min⁻¹, at pH 4 and 7, respectively.

Fig. 5

The effect of H₂O₂ dose on the rate of caffeine (C₀ = 5 mg L⁻¹) oxidation by 30-min operation of US/UV. Numbers in parenthesis represent the estimated apparent reaction rate constants at the applied H₂O₂ dose.

Fig. 6

Photo- and sonocatalytic decay of caffeine (C₀ = 5 mg L⁻¹) and its oxidation byproducts by 30-min sonolysis and photolysis of the compound in the presence of 0.1 g L⁻¹ TiO₂ at pH 4. The solid lines are the fit of Eq. (9) to the data, with estimated reaction rate constants of 0.19 and 0. 0.05 min⁻¹ for UV/TiO₂ and US/TiO₂, respectively.

Fig. 7

The impact of TiO₂ concentration on the rate of caffeine decay and the degree of C-mineralization by 15 and 60-min photocatalysis, respectively in the presence of ultrasound. Initial conditions were pH = 4, C₀ = 5 mg L⁻¹.

Fig. 8

The adsorption equilibrium of caffeine (5 mg L⁻¹) during 60-min shaking in the presence of four different concentrations of TiO₂.

Fig. 9

The rate of H₂O₂ production during 30-min sonolysis, sono-photolysis, sono-catalysis and sonophotocatalysis of ultrapure water.

Fig. 10

The reaction pathways and Gibbs free energies (kcal mol⁻¹) of caffeine decomposition in acidic water (pH 4), leading to the formation of P1 (a) and P2 (b).

Fig. 11

•OH-mediated reaction pathways and Gibbs free energies (kcal mol⁻¹) for the oxidative degradation of caffeine at pH 4.0 to produce P3, P4 (a), P3₂, P3₃, P4₂, P5 and P6 (b).