| Organotin compounds have been widely used as pesticides,antifouling paints and other biocides and have various other commercial uses.The fate of these compounds constitutes one of today's public environmental concerns since some organotin compounds,especially triphenyltin(TPT) and tributyltin(TBT) are high toxic to aquatic life even at very low concentrations.Many studies have focused on the transportation,transformation,removal processes and toxic effects of organotin compounds.However,few microscopic researches have investigated the kinetics and mechanisms of related chemical processes.The current study introduces the nanosecond laser flash photolysis - transient absorption spectra technique into the research on the photochemical reaction mechanisms of triphenyltin and tributyltin with hydroxyl radical in the aqueous solution.Based on the results of laser flash photolysis study,the direct and indirect photolysis oftriphenyltin and tributyltin under 365nm and 253.7nm UV lamp were also studied,and the significance of these reactions for the removal of organotin compounds in the natural environment was discussed.In the laser flash photolysis study,Hydroxyl radical was generated by the photolysis of nitrous acid using 355nm laser light.On the basis of the investigation results of the reaction of hydroxyl radical with nitrous acid and CI,the reactions in the TPTCI-HNO2 and TBTCI-HNO2 aqueous solution after irradiated by 355nm nanosecond laser light were studied respectively,and the following points could be drawn:(1) Triphenyltin cation(TPT+) reacts with hydroxyl radical to form TPT+·OH adduct and the rate constant is(7.2±1.4)×109L mol-1 s-1.The absorption of TPT+·OH adduct lies below 350nm,with a peak absorption at approximately 310nm.(2) TPT+·OH adduct decays with a first-order rate constant of(1.15±0.15)×105 s-1 in the N2 saturated aqueous solution.When O2 exists,TPT+·OH adduct can react with O2 at a rate constant of(6.0±2.9)×107 L mol-1 s-1.(3) Tributyltin cation(TBT+) can also react with hydroxyl radical to form TBT+·OH adduct and the rate constant is(1.02±0.07)×1010L mol-1 s-1.The absorption of TBT+.OH adduct lies below 320nm,and the absorption peak which should be below 250nm could not be determined in this study.(4) TBT+.OH adduct decays at a first-order rate constant of(3.50±0.32)×105 s-1 in the N2 saturated aqueous solution.When O2 exists,TBT+·OH adduct can react with O2 very fast at a rate constant of(6.4±1.3)×108 L mol-1 s-1.Further more,on the basis of the above results and the photolysis experiment under 365nm and 253.7nm UV lamp,the following conclusions could be drawn:(1) Direct photolysis of organotin compounds is of little significance due to their negligible absorption above 290nm,although the mean bond dissociation energy of Sn-C bond is low.Hence,the commonly stated "triphenyltin species are rapidly dealkylated by UV irradiation" is not well founded.(2) The indirect photolysis of TPT may play an important role in the removal process of TPT used as pesticide,since most bacteria lack the ability to biodegrade TPT, and heat,acid and alkali under natural environment conditions could not cleave the Sn-C bond of TPT,either.When the aqueous phase contains HNO2/NO2-,TPT can be transformed to nitrobenzene,which is also an important pollutant.As for the TPT entering the oceanic environments,indirect photolysis of TPT is of little significance since the concentration of reactive radicals is low.(3) For TBT compounds,neither direct nor indirect photolysis plays an important role in the removal process of TBT in natural environments since TBT compounds are mostly found in the oceanic environments with low concentration of reactive radicals.However,indirect photolysis may be very important for the removal of BTs entering the atmosphere.(4) The experimental results also implied that UV/H2O2 and similar methods may be efficient to treat organotin polluted wastewater.
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