| Due to the fact that the chemical structure of micro-pollutants in drinking water are stable,they are difficult to degrade in common drinking water process.These pollutants seriously affect the water quality and drinking water safety.Sulfate radical(SO4-·)-based advanced oxidation processes are effective in degrading organic contaminants in drinking water.UV photolysis of peroxydisulfate(UV/PDS),as a green SO4-· generation method,has a promising prospect in engineering application.However,it is insufficient to study on the mechanism of micro-pollutants degradation in UV/PDS process.This research seeks to investigate the effects of water matrix on atrazine(ATZ),2,4,6-trichloroanisole(TCA),and triclosan(TCS)degradation by UV/PDS process.Kinetic parameters were developed in an effort to model the contributions of various reactive species in UV/PDS process under different water quality conditions.Degradation efficiencies and oxidation pathways of these model pollutants were compared between the conventional hydroxyl radical(HO·)-based AOPs(i.e.,UV photolysis of hydrogen peroxide(UV/H2O2))and UV/PDS process.The formation of two primary products(i.e.,deethylatrazine(DEA)and deisopropylatrazine(DIA))along with the degradation of ATZ in UV/PDS and UV/H2O2 processes was investigated in the absence vs presence of various water matrixes,and then the characteristics of secondary reactive radicals were evaluated by monitoring the changes of the DEA to DIA ratio.ATZ,TCA,and TCS could be degraded effectively using UV/PDS process.The removal efficiency increased with increasing the dosage of PDS,while the presence of NOM had significantly scavenge effect on the degradation of those three compounds.Carbonate/bicarbonate(HCO3-/CO32-)played a minor role in the degradation of TCS,but it had significantly scavenge effects on the removal of ATZ and TCA.The observed pseudo-first-order rate constant for TCA degradation was slightly impacted by the presence of Cl-.But a significant inhibitory effect was observed for ATZ and TCS removal when Cl-concentration was present.Over the p H range of 4-9,the removal efficiencies of ATZ and TCA by UV/PDS increased with p H decreasing When p H=9,the efficiency was higher than that when p H=5~8 for TCS removal.A simple steady-state kinetic model was developed based on the initial rates of compounds destruction,which could well describe the apparent pseudo-first-order rate constants.Modeling results shows the steady-state concentrations of SO4-· and HO· increased with increasing PDS dosage.Degradation rate of ATZ and TCA decreased with p H increasing from 4.0 to 9.0,which could be explained by the lower radical scavenging effect of dihydrogen phosphate than hydrogen phosphate under acidic conditions.The steady-state concentration of SO4-· was much higher than that of HO·,suggesting that the former played a major role for compounds destruction.UV played an important role in the TCS degradation,which was slightly impacted the ATZ and TCA degradation.NOM significantly decreased the oxidation rates of target compounds due to its radical scavenging effects and UV absorption with the former one being dominant.The steady-state concentrations of SO4-· and HO· decreased with the increase of CO32-/HCO3-and Cl-dosage.The generated CO3-· could react with ATZ and TCS effectively.ATZ and TCA could degraded by reactive chloride species.UV/PDS process was more effective in the degradation of these three organic micro-pollutants than the UV/H2O2 under the same conditions,mainly ascribed to the fact that the molar extinction coefficients at 254 nm and the quantum efficiency of photolysis for PDS were higher than those for H2O2.NOM and HCO3-/CO32-showed the scavenging effect on both SO4-· and HO·,and the inhibition was much more significant for the latter.At neutral condition,Cl-reacted with SO4-· at a higher second-order rate constant than HO·,thus resulting in a relatively stronger scavenging effect on SO4-·.Oxidation products of these three compounds in UV/PDS and UV/H2O2 processes were identified.Finally the degradation pathways were tentatively proposed.Results showed that same oxidation products were produced from the degradation of ATZ and TCA in these two processes,suggesting that ele ctron transfer and hydrogen abstraction played a primary role.Extra hydroxylation products were detected in the TCS degradation in UV/H2O2 process compared with UV/PDS process because of a stronger hydroxylation for organic compounds oxidation by HO·.SO4-·displayed a more distinctive prevalence to the ethyl function of ATZ than HO·,leading to the higher ratio of DEA/DIA in UV/PDS system than that in UV/H2O2 system in pure water.The yields of DEA and DIA were not impacted by NOM or HCO3-/CO32-.Howbeit,the increase of DIA yield as well as the decrease of DEA yield were interestingly observed in the presence of Cl-or increasing solution p H,which was attributed to the promotion of Cl-and OH-at moderate concentration(mM range)for the conversion of SO4-· into HO·.Differing from UV/PDS system,all these factors did not change DEA and DIA yields.Moreover,it was confirmed that RCs had a greater selectivity but a relatively lower reactivity on attacking the ethyl function than that of SO4-·.These findings were also strengthened by monitoring the degradation of ATZ as well as the formation of DEA and DIA in three natural waters. |
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