Sulfate
radical (SO4
•–)- and
hydroxyl radical (HO•)-based advanced oxidation
processes (AOPs) are effective for the removal of organic pollutants
in water treatment. This study compared the interactions of SO4
•– and HO• for
the transformation of phenol in UV/peroxydisulfate (PDS) and UV/H2O2 with the presence of NO2
–, which is widely present in aquatic environments and transforms
SO4
•– and HO• to •NO2. By using laser flash photolysis,
the products of phenol reacting with SO4
•– and HO• were demonstrated to be phenoxy radical
and phenol-HO-adduct radical, respectively. This result, along with
density functional theory (DFT) calculations, indicate that the predominant
reaction mechanisms of phenol with SO4
•– and HO• with phenol are electron transfer and
addition, respectively. The different mechanisms induced the much
higher formation of nitrophenols by SO4
•– than HO• in the presence of NO2
– through the fast combination of phenoxy radicals and •NO2. The conversion yields of phenol to
nitrophenols (including 2-nitrophenol and 4-nitrophenol), were 47.5%
by SO4
•– versus 5.3% by HO• at the experimental conditions. Increasing PDS/H2O2 dosages from 0.2 to 1 mM resulted in a 61.9%
increase of nitrophenol conversion yield in UV/PDS/NO2
– but a 35.4% decrease of that in UV/H2O2/NO2
–. In addition, the significant
formation of phenoxy radicals by SO4
•– also induced many nitrated polymers in UV/PDS/NO2
–, while those induced in UV/H2O2/NO2
– were negligible. The significant
formation of nitrophenols and nitrated polymers increased the mutagenicity
by 860.5% when the removal rate of phenol was 98% by UV/PDS/NO2
–. This is the first study to demonstrate
the different mechanisms of phenol transformation by SO4
•– and HO• in the presence
of NO2
–.
Human rotavirus Wa and porcine rotavirus OSU solutions were irradiated with simulated solar UV and visible light in the presence of different photosensitizers dissolved in buffered solutions. For human rotavirus, the exogenous effects were greater than the endogenous effects under irradiation with full spectrum and UVA and visible light at 25 °C. For porcine rotavirus, the exogenous effects with UVA and visible light irradiation were only observed at high temperatures, >40 °C. The results from dark experiments conducted at different temperatures suggest that porcine rotavirus has higher thermostability than human rotavirus. Concentrations of 3'-MAP excited triplet states of 1.8 fM and above resulted in significant human rotavirus inactivation. The measured excited triplet state concentrations of ≤0.45 fM produced by UVA and visible light irradiation of natural dissolved organic matter solutions were likely not directly responsible for rotavirus inactivation. Instead, the linear correlation for human rotavirus inactivation rate constant (kobs) with the phenol degradation rate constant (kexp) found in both 1 mM NaHCO3 and 1 mM phosphate-buffered solutions suggested that OH radical was a major reactive species for the exogenous inactivation of rotaviruses. Linear correlations between rotavirus kobs and specific UV254 nm absorbance of two river-dissolved organic matter and two effluent organic matter isolates indicated that organic matter aromaticity may help predict formation of radicals responsible for rotavirus inactivation. The results from this study also suggested that the differences in rotavirus strains should be considered when predicting solar inactivation of rotavirus in sunlit surface waters.
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