| The phosphonate compounds contain various herbicides,flame retardants,which can enter into the water body by different routes.When excessive phosphonate compounds are introduced in water,and after digestion(ultraviolet photodegradation,photocatalytic degradation,microbial degradation,etc.),excessive phosphate is produced,which can lead to eutrophication of water bodies.Source control of contaminants is the key to pollution prevention.Because oxygen isotopes in phosphates retain information on phosphorus sources and is therefore widely used in the analysis of phosphorus sources.However,the mechanism of phosphate oxygen isotope fractionation of phosphonates under different degradation pathways is unclear,hindering the use of phosphate oxygen isotope as a tracer for source analysis.Therefore,the typical phosphonate compounds glyphosate and methylphosphonate were selected as the research objects.The mechanism of phosphate oxygen isotope fractionation in ultraviolet degradation,ultraviolet/TiO2 photocatalytic degradation and microbial degradation was systematically studied.The model of phosphate oxygen isotope under different degradation pathways was constructed,which provided a theoretical basis for the use of phosphate oxygen isotope to trace the source of phosphonates.The main findings are as follows:The UV degradation reaction of methylphosphonate conforms to the first-order reaction kinetics model.The degradation rate is affected by light intensity,pH and initial concentration:1200 W>400 W,pH 10>pH 13>pH 2>pH 7,2.5 mM>5 mM>10 mM.Free radical quenching experiments show that OH plays a major role in the degradation of methylphosphonate,and the contribution rate can reach 87.12%(pH=2).With the increase of pH,the phosphate was enriched by 16O,pH 13 was 2.87‰-3.6‰ smaller than ?18OP of pH 2,and the concentration of reactant had no significant effect on ?18Op.Studies using ?18 Ow labeled water showed that 17%and 14%of the oxygen in glyphosate and methylphosphonate were derived from water,and 8%and 11%of oxygen were derived from oxygen,thereby phosphate ?18Op model of UV degradation was constructed:?18Op=0.750rig?18Op+0.17?18Ow+0.08?18Oo2(glyphosate);?18Op=0.75 Orig?18Op+0.142?18Ow+0.108?18Oo2(methylphosphonate).The isotopic kinetic fractionation value was calculated by this model to be-16.39±3.01 ‰.The photocatalytic degradation rate of glyphosate and methylphosphonate was affected by TiO2 concentration,pH and initial concentration.The UV/TiO2 degradation of glyphosate and methylphosphonate is 16O enrichment with increasing pH.The pH 13 of glyphosate is 2.97‰ smaller than ?18OP of pH 2,and the pH of methylphosphonate is 3.31 ‰ smaller than ?18OP of pH 2;the TiO2 concentration and the initial concentration of the substrate had no significant effect on ?18Op.The ?18Ow labeled water study showed that the linear fitting slopes of glyphosate and methylphosphonate ?18Op and ?18Ow were 0.254 and 0.251,respectively,indicating 25%of the generated exogenous oxygen in PO4 was from water.Phosphate oxygen isotope model for UV/TiO2 photocatalytic degradation of glyphosate and methylphosphonate was constructed:?18Op=0.75 Orig?18Op +0.25 ?180w The ?18O of glyphosate was calculated to be 5.98 ±0.28 ‰,the isotope kinetic fractionation value is-6.97± 1.71 ‰.Using KH2PO4 as phosphorus source and ?18Ow,labeled water to culture Escherichia coli WM81 and Pseudomonas stutzeri WM581,it was found that the?18Ow of KH2PO4 gradually reached the oxygen isotope equilibrium fractionation within 300 and 400 h,respectively.The degradation of methylphosphonate by Escherichia coli WM8 1 and glyphosate by Pseudomonas stutzeri WM58 1 reached the maximum degradation rate at 42 and 30 h,respectively.Using ?18Ow labeled water,it was found that kinetic fractionation occurred instead of equilibrium fractionation in the phosphate-bound water,and the kinetic fractionation value was-5.50 ± 2.86 ‰ The microbial degradation of glyphosate and methylphosphonate occurred simultaneously with the break of C-P bond and the P-O bond.Phosphate ?18Op model was established for the microbial degradation of glyphosate and methylphosphonate:&18Op =0.402 ?18Ow +0.598 Orig?18Op(glyphosate);?18Op=0.345?18Ow+0.655Orig?18Op(methylphosphonate).Using the constructed phosphate oxygen isotope model to predict the digestion process of glyphosate and methylphosphonate in the environment:the phosphate oxygen isotope ?18Op of glyphosate in rivers is-0.2-5.11‰.which is statistical different with other known phosphorus sources(p?0.05);?18Op degradation of phosphate in the ocean at 0-30 0C is 2.86-10.84‰ lower than the equilibrium fraction,and the lower the seawater temperature,the greater the difference.In this paper,the effects of different environmental factors on the phosphate oxygen isotope fractionation of different degradation pathways of phosphonates were studied.The phosphate oxygen isotope model under different degradation pathways was constructed.This paper provide a theoretical basis for application of phosphate oxygen isotope to trace phosphonate pollutants and study cycling process. |
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