Advanced Oxidation Processes (AOPs): solutions for the degradation of emerging contaminants in liquid phases

Wastewater is the main source of micropollutants and although treated in urban wastewater treatment plants (WWTPs), it can continue to contain recalcitrant substances, potentially dangerous for the health of living organisms and for the environment.It is therefore necessary to implement appropriate actions to limit its diffusion. Advanced oxidation processes (AOPs), which use hydroxyl radicals for the remediation of organic contaminants in wastewater, are highly effective innovative methods to accelerate the remediation process. AOPs can amplify their action if combined with ozone (O3) and if subjected to mono and polychromatic irradiation. The main objective of the PhD thesis was precisely to apply heterogeneous and homogeneous photolysis and photocatalysis systems in the liquid phase, for the removal of levofloxacin (LFX), a widely used antibiotic belonging to the quinolone family. Comparison of degradation kinetics, chromatographic detection of degradation products and evaluation of toxicity were some of the steps of the work. In detail, this thesis consists of six chapters. Chapter 1 provides an accurate bibliographic analysis, with the aim of giving an overview of the numerous fields of investigation connected with the topic of this thesis: a description of wastewater characteristics and of the Italian legislative decree 152/06 which regulates discharges into the environment, the organization of wastewater treatment plants, the problem of emerging contaminants, including antibiotics and in particular the class of fluoroquinolones, taken into consideration as case study of this thesis, the advanced oxidation processes (AOPs), and, finally, a brief review on the toxicity tests used in this work. Chapter 2 contains the first experiment of this thesis, which consists in the degradation of levofloxacin in distilled water, through the processes of photolysis and heterogeneous photocatalysis with titanium dioxide (TiO2). The first purpose of this chapter was to evaluate the efficiency of heterogeneous photocatalysis with TiO2, a process that in the literature is taken as a "reference" model, a very effective technique. Experimental results obtained demonstrate that this process is effective to remove levofloxacin and its by-products in almost 4 hours, and seems to follow a second-order kinetic. However, this process cannot be applied on a large scale due to of the high costs of treatment for the recovery and separation of the photocatalyst from the solution. Chapter 3 provides an evaluation of the efficiency of TiO2 as photocatalyst immobilized on the surface of a borosilicate tube for the degradation of levofloxacin through a "continuous" irradiation system, different from the "static" system used in chapter 2. In particular, not only a kinetic study was carried out, but also some toxicity tests, including the Vibrio fischeri bioluminescence inhibition test and the phytotoxicity tests on Lepidium sativum and Solanum lycopersicum, in order to better understand the potential applicability of this system. More specifically, in the case of phytotoxicity tests, the last sample of each photodegradation test carried out with the "continuous" system (with and without the TiO2-coated tube) was tested; in the case of the Vibrio fischeri assays, the most representative samples were tested, i.e. those that seemed to present the greatest number of transformation products, on the basis of the chromatograms obtained by liquid chromatography analysis. From experimental results, both photolysis and photocatalysis follow a very similar trend and degradations seem to be modelled with a first-order kinetic. The toxicity tests on Vibrio fischeri showed a low toxicity of the starting solution containing only levofloxacin. All the solutions subjected to photolysis were found to be toxic, while the solutions subjected to photocatalysis highly toxic. This unexpected result has been attributed to the probable detachment of titanium dioxide nanoparticles from the surface of the tube, whose toxicity is also being confirmed by the European Food Safety Authority (EFSA). Phytotoxicity tests confirmed these results showing an inhibition of seed germination, root elongation and growth index in both the plants tested. Chapter 4 is reported in this thesis as an under review article submitted to “Environmental Science and Pollution Research (ESPR)”. In this scientific work, levofloxacin solution was treated using hydrogen peroxide (H2O2), peroxymonosulfate (PMS) and peroxidisulfate (PDS), as oxidizing agents, which respectively provide only OH• radicals, both OH•/SO4•– radical, and SO4•– radicals. The efficiency of the three oxidizing agents was tested in different pH conditions, and the most efficient treatment (simulated radiation/PDS) was applied (and optimized) to remove levofloxacin from a simulated wastewater (SWW). In this case the main transformation products (TPs) were identified by liquid chromatography coupled with mass spectrometry, the degradation pathway was suggested and toxicity tests on Escherichia coli (LMG2092), a Gram-negative bacterium, and Micrococcus flavus (DSM1790), a Gram-positive bacterium, were performed. Experimental results demonstrated that simulated irradiation/H2O2 treatment showed less impact on LFX reduction than the combined AOPs of simulated irradiation/PMS and simulated irradiation/PDS. In contrast, PMS and PDS were able to degrade levofloxacin completely. In particular, the PMS resulted the best in phosphate buffer because it has been able to completely transform LFX into LFX N-oxide in 30 seconds, through a non-radicals mechanism. However, except for phosphate buffer, simulated irradiation/PDS system showed the best performance achieving a complete degradation of LFX after 10 minutes of irradiation in all mediums investigated. This system was successfully applied in simulated wastewater (SWW) by using three different concentrations of PDS to optimize the process, and in all cases the degradation followed a first-order kinetic. Selected samples obtained from the photocatalytic treatment of LFX in SWW with the highest concentration tested of PDS were tested on Escherichia coli and Micrococcus flavus. Both the cultures of Gram-positive and Gram-negative bacteria were not affected after the effective degradation of levofloxacin by the sulphate radical based AOP. Chapter 5 provides a further insight into the potential eco-friendliness of sulphate radicals treatment for the degradation of levofloxacin. In particular, some of the samples obtained from the treatment of levofloxacin with peroxidisulfate in distilled water were subjected to the MTT (3- (4,5-dimethylthiazol-2-yl) -2,5-diphenyltetrazolium) bioassay on human epithelial-like lung cancer cell line A549. These experimental tests were carried out in Greece, at the University of Ioannina. Experimental results demonstrated that there was no toxicity effect on the cells viability. Finally, chapter 6 provides a brief description of the internship period carried out at Hydros S.r.l. (Tito, Potenza). During this experience, I participated in the design of fumes and wastewater treatment systems, and in the optimization of the parameters of the biological process of nitrification/denitrification/oxidation present within a real wastewater treatment plant. In conclusion, I can say that this PhD thesis demonstrates that AOPs may be an alternatve eco-friendly treatment for the removal of contaminants from wastewater effluents. The experimental results tends to demonstrate that solar advanced oxidation processes has the potential to open new feasible remediation strategies for WWTPs effluent tertiary treatment before wastewater reuse in irrigation for instance. However, most investigations are done at lab-scale. For a practical view and commercial uses, much more work is necessary to switch from batch work to a large scale to find out the efficiency and ecotoxicity of the processes.