Degradation of thymol by VUV/UV/NaClO and UV/NaClO processes:Degradation mechanism and water quality affecting factors
VUV/UV/NaClO and UV/NaClO processes were employed to degrade thymol(Tml)in aqueous solution.Effects of pH,CO32-,NO3-and humic acid(HA)on Tml removal were comparatively studied.The degradation products were identified and a possible degradation pathway was proposed.Meanwhile,the alteration of acute toxicity during VUV/UV/NaClO process was investigated,and the most toxic by-products were identified by toxicity assessment and ECOSAR software simulation.The results show that the pseudo first-order kinetic constants(kobs)for Tml removal decreased firstly and then increased with increasing pH in both UV/NaClO and VUV/UV/NaClO processes,and the minimum values were 0.1836 and 0.5547 min-1 at pH=7,respectively.The presence of CO32-favors Tml degradation,while low concentration of NO3-demonstrates a slightly inhibiting effect but high concentration shows the opposite.HA has an inhibiting effect on the degradation of Tml.The impact of water quality on Tml degradation was more obvious during VUV/UV/NaClO process,in which kobs was about 1.05～3.57 times of that of the UV/NaClO process.Tml in the VUV/UV/NaClO process mainly showed radical substitution,addition and oxidative decomposition reactions,and chlorinated,quinone and aldehyde intermediates were generated.The acute toxicity of the VUV/UV/NaClO process treated solution had an increase-decrease-increase profile.The increase of acute toxicity in the later stage may be due to the generation of small-molecule organic compounds such as vinyl propionate and glutaraldehyde.
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