Some kinetic and synergistic considerations on the oxidation of the azo compound Ponceau 4R by solar-mediated heterogeneous photocatalytic ozonation

A series of advanced oxidation processes were applied to degrade a typical pollutant of industrial effluents known as Ponceau 4R. For this, a magnetic Fenton catalyst was prepared by co-precipitation of Fe2+ and Fe3+ ions over activated carbon. This catalyst was characterized by XRD, FTIR, VSM and applied in oxidation processes involving the use of ozone, hydrogen peroxide and solar light. Experiments were carried out at laboratory scale in a semi-continuous reactor loaded with a solution of 50 mg L–1 of Ponceau 4R in different water matrices (deionized water, domestic wastewater and seawater) and bubbled with air or an ozone–air mixture. When used, the initial concentration of H2O2 was 3.3 × 10–2 mol L–1 and that of the catalyst was 500 mg L–1. Results show that Ponceau 4R does not photolyze under solar radiation, it is not directly oxidized by H2O2 (in the dark) but it can be partially or entirely degraded through photo-Fenton and ozone-based oxidation processes. Taking into account the initial depletion rate of azo groups concentration and colour removal yield, the efficiency of the tested systems follows the order: hydrogen peroxide photolysis < photo–Fenton process < peroxone < ozonation < catalytic ozonation ≈ photocatalytic ozonation. Ponceau 4R was also completely removed from real domestic wastewater and seawater samples, which were doped with this compound and subjected to photocatalytic ozonation treatment, in less than 60 min of reaction time, when the matrices pH was previously adjusted to 3. However, when applied at the natural pH of these water matrices (8 and 8.7, respectively), the pollutant conversion was 80%.