Advanced oxidation processes (AOPs) for the mineralization of azo-dyes from wastewaters: homogeneous vs. heterogeneous photocatalytic processes

Advanced oxidation processes (AOPs) for the mineralization of azo-dyes from wastewaters: homogeneous vs. heterogeneous photocatalytic processes Francesco Conte 1, Cristina Calloni 2, Ilenia Rossetti 3 and Gianguido Ramis 4* 1 Chemical Plants and Industrial Chemistry Group, Dipartimento di Chimica, Università degli Studi di Milano, Milan, Italy 2 Chemical Plants and Industrial Chemistry Group, Dipartimento di Chimica, Università degli Studi di Milano, Milan, Italy 3 Chemical Plants and Industrial Chemistry Group, Dipartimento di Chimica, Università degli Studi di Milano, Milan, Italy 4* DICCA, Università degli Studi di Genova, Genoa, Italy, presenting author, corr. author (gianguidoramis@unige.it) INTRODUCTION Textiles industries huge production determines a strong environmental impact: the yearly total impact per person is estimated to be 1.3 tonnes of raw material and over 100 m3 of water. Ca. 700,000 – 1,000,000 tons/years of dyes are produced and more than 280,000 tons are lost in the effluent and often remain as persistent pollutants. Heterogeneous and homogeneous phototreatments can be efficiently used as Advanced Oxidation Processes to degrade such contaminants and a comparison between different technologies has been carried out on the azo-dye Dystar’s Levafix Brilliant Red E-6BA. EXPERIMENTAL Oas a model molecule for this study because it is characterized by low biodegradability TiO2 P25 was employed as a commercial nanostructured material supplied by Evonik and compared with TiO2 FSP, prepared through a homemade flame spray pyrolysis apparatus. Metallic co-catalysts were deposited over the surface by wet impregnation and then reduced at different temperature (Ag, Au, Pt, Pd, Fe). The catalysts were characterised by XRD, N2 physisorption and DR-UV-VIS analyses. The photo-degradation tests were carried out in different cylinder-type double-wall glass reactors of 300-1000 mL capacity, open to air and equipped with a suitable lamp in case of photocatalyzed processes. The Fenton process was carried out either in light or dark conditions by adding Fe salts and H2O2 and compared with UV/H2O2 and Heterogeneous photodegradation processes with the above described catalysts. The light sources employed were characterized by different emission wavelength and power output: a LED-type lamp (white light, 30 W, 2700 lm), an external UV lamp (200W, maximum emission at 365 nm) and two different low power immersed-UV lamp (125 W, maximum emission at 365 nm); natural sunlight. The average irradiance was measured for the different setups through a photo-radiometer sensitive to the UVA fraction and were 116 W/m2 in case of the external-UV lamp and respectively 60 and 260 W/m2 for the low irradiance (lamp 1) and high irradiance (lamp 2) immersed-UV lamp. The progress of the reaction was monitored by UV-Vis and Total Organic Carbon (TOC). Toxicity tests on the treated solutions were also carried out using the Crustacean Daphnia magna. RESULTS AND DISCUSSION The fastest degradation process was Photo-Fenton with the highest power immersion UV lamp (10 min to 97.4% conversion), strictly followed by the UV/H2O2 process (20 min to 100% conversion). The degradation time increased with the other light sources, i.e. sun and LED, but overall, the total reaction time did not exceed 30 min. On the other hand, when it comes to the heterogeneous process, the titania synthetized via flame pyrolysis takes longer time to reach full conversion. At first glance it seems that the homogeneous treatment outperforms the heterogeneous ones, however, in the first case these results were achieved using a large amount of hydrogen peroxide, about 3.5 equivalents, which of course boost the reaction and lowers the time required to achieve a good conversion of the substrate. Figure 9: Degradation of Levafix dye: comparison. No organic carbon was detected after the treatment, except when employing gold deposited catalyst. Acute toxicity tests revealed that the Levafix Briliant Red resulted in a 48h-LC50 of 117 mg/L. The toxic response was dramatic in case of the solutions treated with UV/H2O2 and (Photo)Fenton processed, since a higher mortality of Daphnia magna specimens was observed due to the residual amounts of hydrogen peroxide. Overall, the best toxicity results were achieved in case of the samples treated with titania photocatalyst (P25), since the toxicity of the solutions was lower than the ones containing the bare dye. CONCLUSION Photo-degradation of Levafix Brilliant Red was compared for different AOPs. The homogeneous treatments were more efficient in the degradation of the selected dye, but the main advantages of employing solid nanostructured photocatalyst is that it is not compulsory to add hydrogen peroxide (low toxicity) and the catalyst can be recovered easily.