PROGRAMME

Session 4(P) - Advanced Oxidation Processes

Dr. Zacharias Frontistis, Prof. Andreja Žgajnar Gotvajn, Dr. Polycarpos Falaras

Sonolytic and hybrid-sonolytic degradation of 6-hydroxy-methyl uracil, a model compound for the study of cyanotoxin cylindrospermopsin.

C Thursday 2 September 11:15 - 11:18

The occurence of cyanotoxins, produced from cyanobacterial blooms, in the aquatic environment, has been a topic of great importance for scientists and authorities, over the last years. Cyanotoxins can greatly affect humans and fauna [1], therefore, their inclusion in monitoring programmes and in regulatory measures is considered of great significance. The target toxin of this study is cylindrospermopsin (CYN), which is an alkaloid compound, responsible for causing multiple organs’ disorders, while being hepatotoxic and neurotoxic [2]. Several Advanced Oxidation Processes (AOPs), using mainly photocatalytic treatment, have been applied for the degradation of CYN [3]. The use of sonolysis for the oxidative degradation of organic compounds, has shown interesting results and satisfactory efficiency in selected applications [4]. Moreover, coupling sonolysis to catalytic AOP systems, has shown to promote degradation efficiency and lead to improved performance facilitating a broader range of applications [5]. The study of CYN’s degradation presents great scientific and regulatory interest. However, CYN is a highly toxic compound and its analytical standards are rare and expensive, thus their use in extensive degradation research is problematic. The use of a more abundant and low-cost model compound could be a suitable alternative. 6-hydroxy-methyl-uracil (6-HMU) could be used as a model compound for CYN, since it contains a part of CYN’s structure, partly responsible for its toxicity [6]. Furthermore, 6-HMU is a stable, non-toxic and reasonably priced substance. In the present study, 6-HMU was used as model compound for CYN sonolytic and hybrid-sonolytic degradation. The research objectives were (1) to investigate and optimise the parameters affecting sonolysis’ kinetics and efficiency, (2) to evaluate the efficacy of the coupled sono-Fenton process on the degradation of 6-HMU and, finally, (3) to detect and identify the transformation products (TPs) arising from the sonolytic degradation of 6-HMU and to compare them with the TPs of CYN’s degradation. The sonolytic and hybrid-sonolytic experiments were conducted in an ultrasound generator K 80 equipped with Transducer E/805/T and ultrasound bath 5/1575, at different intensities, initial concentrations of 6-HMU, pH values, and water matrices. The degradation of 6-HMU was monitored using HPLC-PDA, while the concentrations of Fenton reagents were measured spectrophotometrically. 6-HMU’s TPs were identified and detected by LC-MS/MS. The degradation kinetics of 6-HMU sonolysis in water were mainly affected by the intensity of sonication, initial pH and initial concentration of target compound. Initial degradation rate (ro) varied from 4,69E-05 to 1,70E-04 mmol L-1s-1 under different intial concentrations of 6-HMU, while at pH 10, the highest ro was observed, equal to 8,17E-05 mmol L-1s-1 . Hybrid sono-Fenton system was extensively studied in terms of Fe(II), Fe(III) and H2O2 initial concentrations individually and in combination. The addition of Fe(II) during water sonolysis of 6-HMU, resulted in faster and more effective degradation. Using only 10mg/L Fe(II) increased ro (2,18E-04 mmol L-1s-1 ) was achieved. Sono/Fe(II)/H2O2 system gave very promising results, showing improved degradation rate and efficiency, with limited consumption of reagents. The ratio Fe(II)/H2O2 that gave the most effective degradation, was 1mg/L Fe(II) / 100mg/L H2O2 (ro = 3,07E-04 mmol L-1s-1). Determination of 6-HMU’s TPs displayed great similarities to CYN’s TPs, confirming the initial research hypothesis that 6-HMU could be used as a model compound for the investigation of CYN’s degradation with different treatment techniques.