Efficient microwave degradation of humic acids in water using persulfate and activated carbon

Humic acids are complex mixtures of organic molecules of different sizes, molecular weights and functional groups such as phenols, carboxyls, quinones and amino acids. Humic acids occur ubiquitously in media where organic matter is decomposing, such as waters, soils, sediments and organic wastes. The presence of humic acids in untreated water inhibits the oxidation of target pollutants through competing reactions and generates toxic disinfection by-products during the classical disinfection method of chlorination. There is therefore a need for alternative methods that remove humic acids. Here, degradation of humic acids at 10 mg/L was tested using 1000 W microwave irradiation assisted by 0.25–1.2 mM persulfate, with granular activated carbon at 10–50 g/L, at pH 3.0–12.0. In 90s, the highest removal of humic acids, of 75%, and humic acids mineralization, of 41%, was obtained using 1000 W microwave, 50 g/L granular activated carbon and 0.5 mM persulfate at pH 8.0. Under the same conditions without persulfate, removal was only 42% and mineralization 24%. Removal was lower than 7% using either persulfate or microwave alone. High removal with microwave, persulfate and granular activated carbon may be explained by enhanced generation of SO₄^·− and ^·OH radicals and also by better trapping/encapsulation/binding of humic acids in the granular activated carbon matrix.