Advanced oxidation processes

Advanced oxidation processes (AOPs, e.g. heterogeneous and homogeneous photocatalysis, electrochemical oxidation, ozonation, ultrasound irradiation, Fenton and alike reactions, and many more) have been investigated for the treatment of emerging pollutants over the past 20 years (Klavarioti et al., 2009). In particular, the occurrence of persistence micro-pollutants in various water matrices, such as pharmaceuticals and personal care products, raises serious environmental concerns since these xenobiotics can re-enter the water cycle, i.e. escaping intact from the conventional wastewater treatment plants and finally ending up in surface and ground waters (Verlicchi et al., 2012).AOPs can effectively degrade organic pollutants typically found in environmental matrices (secondary treated effluents, surface waters, ground waters) at concentrations ranging from the ng/L to low mg/L level. This said, the specific treatment cost (i.e. the cost per unit mass of removed pollutant and/or per unit volume of effluent), as well as the environmental footprint are generally high; both are usually related to the treatment performance, which, in turn, depends on the specific treatment conditions and the quality of the water matrix.

This study focuses on the photocatalytic degradation of a pharmaceutic compound essentially employed for the treatment of hypertension using TiO2 as catalyst and UV-A irradiation. An efficient elimination yield of 99% was obtained after 10 minutes of irradiation at initial pollutant concentration of 5 mg/L and 1.2 g/L of catalyst. A higher mineralization yield of 87% was reached in 2h of reaction. Results showed that under all studied process conditions the target molecule was degraded according to a pseudo first order kinetics. Obtained data clearly demonstrate the potential application of the investigated process for the remediation of polluted water.

The application of non-thermal plasmas in wastewater and air purification received a lot of attention, but their potential application in drinking water treatment has scarcely been investigated. Classified as Advanced Oxidation Processes, plasmas ignited in water or at the air-water interface generate a vast range of reactive species capable of removing water contaminants.
The efficiency to degrade cylindrospermopsin (CYN, cyanobacterial toxin) was compared for six different plasma sources. A spark discharge showed the most energy-efficient degradation, followed by the other investigated systems, which showed similar trends.
Two approaches were selected for further in-depth study of the degradation efficiency and underlying mechanisms. For a follow-up detailed study, a corona-like and a dielectric barrier discharge were selected based on the CYN degradation efficiency, usability of the reactors and plasma-chemistry. For the corona-like plasma, the degradation efficiency increased with increasing voltage and solution pH. After 15 min of plasma treatment at pH ≥ 7.5, degradation of CYN even progressed without further plasma application. The pH-dependency was not observed for the dielectric barrier discharge (DBD), whose degradation efficiency increased with decreasing operating voltage. The corona-like plasma promotes degradation primarily via OH, whereas the DBD produces mainly O3 and NOx.

Cavitation based advanced oxidation processes (Cav-AOPs), are a promising alternative to currently used wastewater treatment technologies. Amplified interest in this “hot” topic results in increased number of research on several aspects relating to formation of cavitation phenomena and its utilization for wastewater treatment as well as hybrid processes based of application of external oxidants effectively converted to radical species in cavitation conditions. The paper discusses a state of the art of cavitation based AOPs, as well as presents recent developments in this field of our research group. The principles of cavitation combined with AOPs will be presented followed by evaluation of their effectiveness in oxidation of organic contaminants and comparison of hydrodynamic and acoustic cavitation processes used for same type of pollutants. An examples of degraded particular pollutants will include chlorinated and nitro derivatives as well as other emerging environmental pollutants. Applications for disinfection of water will be also addressed. The paper will present also results of studies on degradation of pharmaceuticals as well as pre-treatment of real post-oxidative effluents formed during bitumen production as an examples of possible implementation of cavitation based processes in real industrial scenario.

Ozone is commonly used in advanced oxidation processes (AOPs) in combinations with hydrogen peroxide (H2O2) and UV radiation (UV). Hydrodynamic cavitation (HC) has been experimentally proven to result in effects, typical of AOPs. Combinations of AOPs with O3, H2O2 and UV, and HC (with cavitation numbers less than 0.2, generated by various orifice plates and nozzles, with number of passes up to 12) were experimentally assessed on model water, containing organic matter. Various synthetic organic micropollutants (iohexol, diatrizoic acid and metaldehyde with concentrations of 10 μg L–1) were selected as the target compounds. At dosages of O3, H2O2 and UV above 2 mg L–1, 4 mg L–1 and 450 mJ cm–2, respectively, herein applied HC had no beneficial effect on target pollutants removal. At lower dosages of the aforementioned oxidants, HC was able to improve the removal rate of the target pollutants by as much as 15 %. Moreover, in terms of electrical energy consumption, the hybrid process with HC was found to be as efficient per order (90 %) of removal (EEO, kWh m–3 order–1).

The present study reported the decolorization by UV/H2O2 of three dyes and their binary and ternary mixtures in water, simulating colored effluents of the textile industry. The effect of the initial concentration of H2O2, the initial concentration of dyes and the reaction kinetics in the decolorization of textile dyes in water was studied. The efficiency of decolorization of three treatment processes was compared (UV, H2O2 and UV/H2O2). Moreover, after decolorization of the simulated waters by UV/H2O2 the effluents were then treated by adsorption using a commercial aluminosilicate and a silica gel (waste from industry). The phytotoxicity of the effluents was determined before and after adsorption treatment using the Raphanus sativus bioassay.

Formation of toxic by-products, such as chlorinated intermediates, is one of the major drawbacks of advanced oxidation processes for saline wastewater treatment. Here a comparative analysis of electrochemical oxidation and photocatalytic degradation of 4-Ethylphenol, a non-chlorinated starting model compound of the group of alkylphenols, is presented. Main intermediates have been identified and quantified for brackish [0.03 mol*L-1] and sea water [0.6 mol*L-1] salt concentrations representative for the salt levels in various industrial effluents. Our comparison indicates that photocatalytic treatment (using TiO2 photocatalysts) might be favorable over electrochemical treatments with Pt or BDD anodes due to the minor role of chlorination and the limited formation of chlorinated compounds in photocatalysis. Finally, using photoelectrochemical degradation an external surface recombination mechanism of photogenerated charge carriers will be presented explaining the absence of chlorinated compounds during photocatalytic wastewater treatment.