Improved mineralization by combined advanced oxidation processes

Abstract

Different single treatments and a combined process based on two advanced oxidation processes, catalytic wet peroxide oxidation and photocatalysis, have been tested for the purpose of achieving complete mineralization using phenol as target compound at medium-range concentration (200 mg L⁻¹). The heterogenous catalysts that were used were a home-made activated carbon-supported iron catalyst (FeCN), and the commercial Aeroxide titania P25. An important improvement in the rate and percentage of TOC removal was achieved by combining both catalysts in a hybrid process based on a mixture of FeCN and TiO₂ P25 (50:50 wt.%) in the same photoassisted reactor in ambient conditions. TOC evolution has been modelled for all the treatments for comparative purposes. The hybrid process allowed a highly efficient use of hydrogen peroxide with the almost complete oxidation of phenol to CO₂ and H₂O by using the theoretical stoichiometric amount of H₂O₂. Among the different advantages of this hybrid process is the rapid and effective degradation of the aromatic compounds adsorbed onto the Fe/CN catalyst surface as a consequence of synergistic effect of the two catalysts in the presence of irradiation light in ambient conditions, achieving a higher degree of mineralization of short-chain organic acids that are resistant and refractory to CWPO treatment. Finally, the stability and durability of this catalytic mixture (FeCN + titania P25) in hybrid mode have been examined through four consecutive cycles. A constant organic matter removal was observed during the last three consecutive cycles in which 90% of total organic carbon conversion was achieved.

Introduction

The increasingly stringent wastewater regulations [1] and the increase in water recycling demand the implementation of more efficient technologies for the abatement of pollutants from aqueous effluents. Wastewater from many industrial processes often contain organic pollutants that are toxic and not amenable to direct biological treatment so they must be submitted to treatments capable of breaking down that kind of pollutants.

Therefore, there is a growing interest in cost-effective technologies that combine industrial progress and environmental protection. In the last two decades the emergence of advanced oxidation processes (AOPs) proved to be as promising solution due to their high potential for ultimate destruction of many recalcitrant pollutants [2], [3].

These processes involve the formation of highly reactive free radical species, in particular hydroxyl radicals, far more powerful than the commonly used oxidants, like molecular oxygen and ozone [4]. The versatility of AOPs is favoured by the fact that they offer different possible ways for hydroxyl radical generation, which comply with the specific treatment requirements [5]. Nevertheless, depending on the targeted effluent quality, treatment by advanced oxidation may imply high operation cost due to the high doses of reagents required for mineralization.

The use of sequential or simultaneous AOP hybrid configurations can improve the mineralization of refractory compounds [6], reducing the final operation cost. Among the AOPs, those based on hydrogen peroxide, like Fenton, heterogeneous Fenton or Catalytic Wet Peroxide Oxidation (CWPO), photo-Fenton, TiO₂–H₂O₂–UV, solar-driven Fenton process, etc. that involve catalysis are receiving an increasing interest [7], [8], [9]. Therefore, combinations of these processes can provide high efficiencies on a simple design basis since they operate under ambient or mild conditions.

The aim of this work is focused on the study of the combination of two AOPs based on heterogeneous catalysis using phenol as target pollutant. The selected AOPs were CWPO based on an iron-activated carbon catalyst [10] and heterogeneous photocatalysis with H₂O₂ and TiO₂. In the case of CWPO with an activated carbon-supported Fe catalyst, the iron active sites located on the catalyst surface promote the generation of OH. In heterogeneous photocatalysis a wide-band gap semiconductor, like titania, is irradiated with light and the excited electron-hole pairs produced can be applied in wastewater treatment processes in the presence of oxygen or hydrogen peroxide to degrade organic pollutants through OH generation [11]. When applied individually, these processes show important drawbacks that are derived from rapid catalyst deactivation in the case of CWPO [12], or the lower capacity of TiO₂ photocatalysis for treating wastewater with a relatively high pollutant concentration [13].

Several authors have reported the beneficial effect of adding activated carbon (AC) in photocatalytic oxidation with TiO₂. The adsorption capacity of AC enhances the efficiency of the photocatalyst [14], [15], [16], [17], [18]. On the other hand, photo-Fenton processes have demonstrated to accelerate the photo-oxidation reaction rate [13], [19], [20]. Some recent publications show the use of composites with iron species or TiO₂ and carbonaceous materials as promising photocatalysts for breaking down organic pollutants [21], [22], [23].