UV degradation of organic pollutants in water

1. Introduction The Directive 2013/39/EU sets a "watch list" of emerging contaminants to be monitored in surface waters. At the present, the list contains diclofenac, 17-beta-estradiolo and 17-alpha-ethinylestradiol, but no environmental quality standards has been suggested for these compounds. The emerging organic contaminants (EOC) have been defined as compounds that have no regulatory standard because only recently begun to be detected and quantified in the environment. These contaminants are often recalcitrant to the conventional wastewater treatment processes, so they are released in the environment where they produce multi-component mixtures whose ecotoxicity is probably higher than the single compound itself.1 The employment of new technologies for water purification, such as advanced oxidation processes (AOPs), would provide a complete decomposition of biorecalcitrant compounds to preventing them from persisting in the environment. Among AOPs, UV and ultrasounds based technologies are very promising in the decomposition of organic pollutants in aqueous solution. In 2007, Kidak investigated the effect of ultrasonic cavitation on the decomposition of phenol, demonstrating that the combination of UV-irradiation and ultrasounds provides considerable degrees of degradation.2 2. Report The previous work developed in our laboratory focused on a UV-TiO2 degradation, performed in a Rayonet photochemical reactor, of a mixture of five EOC: benzophenone-3 (BP3), benzophenone-4 (BP4), carbamazepine (CBZ), diclofenac (DCF) and triton X-100 (TRX). These pollutants are some of the most frequently detected in water, they contain chemical groups of different nature, showing different water affinities. A complete mineralization was achieved after 4 hours of photocatalytic treatment.3 The aims of the present work are to scale-up the photocatalytic process and speed up the degradation of the 5 EOC by combining UV irradiation and ultrasounds. The efficiency of the degradation treatments has been determined by following the disappearance of the parent molecules by HPLC and measuring the Total Organic Carbon (TOC) to assess the degree of mineralization. 2.1 Scale-up of the photocatalytic process A prototype consisting of a tank (10 ÷ 25 L), a recycling pump (0.36 m3/h) and four UV lamps (~254 nm, 36 W each) is under development at CNR-ISOF. The prototype has been designed to test photocatalysts supported on substrates with different shapes. 2.2 UV - ultrasounds combined process A ultrasounds source (Elmasonic P30H) has been equipped by two UV lamps (~254 nm, 9 W each), an air blower and a cooler. Two frequencies, 37 and 80 kHz, are available and power can be supplied in the ranges 30 ÷ 100 W and 36 ÷ 120 W, respectively. A dispersed photocatalyst could be added. Figure 1-A shows the residual amount of the five contaminants treated in the prototype, if compared to the residual amounts obtained in the previous lab-scale UVC-TiO2 process,3 DCF disappears in the same time, but BP3 and TRX need double time. In the prototype, BP4 and CBZ residual amounts after 4 hours are about 60%, whereas in the lab-scale test were rather completely decomposed after 2 hours. However, the electrical energy consumed per gram of contamination removed, that is the sum of the masses removed for each contaminant, is 3.1 kWh/g for the prototype against 21 kWh/g in the lab-scale experiment, demonstrating an increase of energy efficiency after the scale-up. Regarding the treatments with ultrasounds, the degradation rate of the contaminants is faster by using the coupled system UV and ultrasounds than by using ultrasounds or UV alone. Figure 1-B reports the results of the coupled technique. However, the electric energy consumed per gram of contamination removed results abou