Comparison of UVC and UVC/persulfate processes for tetracycline removal in water

The comparative study on the elimination of tetracycline (TC) utilizing UVC and UVC/PS processes was systematically conducted. The TC degradation via UVC irradiation and UVC/PS treatment well fitted pseudo first-order kinetics. Hydroxyl radicals (center dot OH) and sulfate radicals (SO4-center dot) were identified as the primary reactive species in UVC/PS process. Based on competition kinetic method, the second-order reaction rate constants of TC reacting with center dot OH and SO4-center dot were calculated as 4.6 x 10(9) and 2.2 x 10(9) M-1 s(-1), respectively. The contributions of UVC photolysis, center dot OH and SO4-center dot to TC degradation were calculated as 5.91%, 61.97% and 28.11% at pH 7.0, respectively. The TC degradation in UVC and UVC/PS process was highly pH-dependent and enhanced with the increase of solution pH. Water matrix constituents played the multiple roles in TC degradation, including radical sensitizer, radical quencher and light absorber. Natural organic matters, CO32- and HCO3- enhanced the degradation during UVC photolysis, whereas the opposite phenomenon occurred in UVC/PS process. Cl- presented the inhibitory effect, while TC degradation can be accelerated by NO3- both in UVC and UVC/PS processes. The TC degradation in UVC/PS process can be classified into the hydroxylation, demethylation, decarbonylation, photocyclinzation, dehydration and cleavage of C-N bond. After UVC and UVC/PS pretreatment, the decreased formation of dichloroacetic acid and trichloroacetic acid and the increased production of trichloromethane have been found. The TC removal was evaluated in terms of electrical energy per order, and its value can be considerably reduced via the combination of UVC and PS. Except for ultrapure water, the removal of TC from real waters by UVC/PS treatment could also be desirable.