Effects of different frequencies and other variables on the salicylic acid and terephthalic acid dosimeters applied to the advanced Fenton process and activated carbon cloth
Advanced Oxidative Processes (AOPs) depend on the production of hydroxyl radicals for the destruction of organic pollutants. The effectiveness of six high ultrasound frequencies (378 kHz, 583 kHz, 862 kHz, 995 kHz, 1144 kHz, and 1175 kHz) and one low ultrasound frequency (20 kHz) for the production of hydroxyl radicals was determined by use of the salicylic acid (SA) and terephthalic acid (TA) dosimeters. Using SA, the ability of the frequencies to produce hydroxyl radicals in the presence of the oxidant hydrogen peroxide was assessed. Additionally, the use of selected ultrasonic frequencies was studied in combination with hydrogen peroxide for the degradation of the model pollutant, acid orange 7 (AO7) in the presence of iron powder or activated carbon cloth (ACC). Frequencies 378 kHz and 583 kHz were judged to produce the most hydroxyl radicals when using SA and 583 kHz was identified as producing the most using TA. Hydroxyl radical attack on SA resulted in two products (2,5-dihydroxybenzoic acid (2,5-DHB) and 2,3-dihydroxybenzoic acid (2,3-DHB)), generally, in a 1:1 ratio; adding hydrogen peroxide led to increased production of hydroxylated products but altered the ratio of 2,5-DHB:2,3-DHB to 1:1.9 (20 kHz) and 1:1.2 (high frequencies). It was found that 2,5-DHB and 2,3-DHB are themselves susceptible to hydroxyl radical attack at all the tested frequencies. Using 20 kHz, 86.7 % of the decrease in SA concentration was accounted for by hydroxylated products; using 583 kHz, 862 kHz, 995 kHz 1144 kHz and 1175 kHz it was 49.9 % and using 378 kHz it was only 38.1 %. It was found that even after sonication was stopped there was a continued reaction of hydroxyl radicals with SA over 4 days in samples with and without added hydrogen peroxide. In samples with no added hydrogen peroxide, 2,5-DHB concentrations decreased substantially over 24 h, attributed to hydrogen radicals and/or hydroperoxyl radicals. Using AO7, 583 kHz was compared to 20 kHz in conjunction with iron and hydrogen peroxide. It was 20 kHz that was the most successful frequency for the degradation of AO7 with only 13.1 % remaining after 24 h (2 h of ultrasound treatment and 22 h stirring), 583 kHz resulted in 18.8 % remaining. ACC, hydrogen peroxide and 583 kHz was also investigated for degrading AO7 and resulted in 80.6 % removal of AO7 after 24 h (2 h of ultrasound treatment and 22 h stirring). Overall, the most effective method for the degradation of AO7 was still iron, hydrogen peroxide and 20 kHz.
High frequency ultrasound, 20 kHz, Salicylic acid dosimeter, Terephthalic acid dosimeter, Calorimetry, Hydroxyl radicals, Azo dye, Acid Orange 7, Advanced Fenton Process (AFP), Activated carbon cloth (ACC), Hydrogen peroxide, 2,5-Dihydroxy benzoic acid (2,5-DHB), 2,3-Dihydroxy benzoic acid (2,3-DHB)
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