Phosphite Migration At The Sediment-water Interface And Its Photochemical Transformation In Northern Lake Taihu

Recent evidences have convinced the presence of reduced phosphorus(P)species,which are extensively involved in the biogeochemical cycle of P.Accordingly,the environmental behavior of reduced P species has received extensive interest.Phosphite is an important intermediate of the P cycle.Its migration and transformation are important parts of the P cycle.The earlier studies of our group have shown that phosphite concentration in Lake Taihu water was lower in summer and autumn than in spring and winter,lower in bottom than in overlying water and sedimentary phosphite concentration was lower in deeper layer.These results suggested that there may be migration and photo-transformation of phosphite in lake water.In this work,the flux at the sediment-water interface of northern Lake Taihu and photochemical transformation in lake water of phosphite have been studied,aiming to provide reference for the biogeochemical cycle of P.The main results are as follows:1.P-containing ions(Phosphite,phosphate,glyphosate and aminomethylphosphonic acid(AMPA))in natural water were determined by two-dimensional ion chromatography system coupled with capillary ion chromatography(2D-CIC).Samples were injected into the first dimensional analytical column(AS11-HC)for preliminary separation.According to the different retention times in the first dimension,target ions were switched to the second dimensional capillary column(MAX-100)for further separation.The results showed that the detection limits(S/N=3)were 0.18 nmol L-1 for phosphite,0.073 nmol L-1 for phosphate,0.15 nmol L-1 for glyphosate and 2.6 nmol L-1 for AMPA.The relative standard deviation(RSD,n=6)of peak area ranged from 2.7%to 4.6%.The recoveries ranged from 80.1%to 118.4%.It demonstrated that coupled capillary system could afford the simple detection of P-containing ions in natural water with complex matrix.2.Intact sediment core simulation was employed to measure the flux of phosphite at the sediment-water interface in northern Lake Taihu.Phosphite fluxes(?mol m-2 d-1)ranged from-38.21±1.14 to 7.10±2.18,with an annual average of-4.72±10.40.On the whole,phosphite migrated from water into sediment and the sediment is primarily a sink.The average yearly migration of phosphite in Lake Taihu from water to sediment was estimated to be(4.04±8.88)×106 mol y-1.The highest seasonal negative phosphite fluxes(prnol m-2 d-1)occurred in winter(-10.44±18.63),followed by summer(-8.04±5.61)and spring(-2.61±4.17).In autumn,phosphite flux was 2.20±4.07.Higher annual average negative fluxes of phosphite(?mol m-2 d-1)appeared in site ZSB(-12.70±17.96),where the highest contents of total soluble P and phisphite in water,IPs and TPs in sediment could be observed.Phosphite fluxes showed significant negative correlations with the original phosphite concentration in water(r=-0.840,p<0.01),organic matter(r=-0.720,p<0.01)and phosphate bound to Ca(Ca-Ps)(r?-0.632,p<0.05)in sediment.These results indicate that microbiological processes and P species bound to Ca may play an important role in the P redox cycle.No significant correlations between phosphite fluxes and dissolved oxygen or oxidation-reduction potential were observed.3.The loss of phosphite in Taihu Lake water(THW)under sunlight including direct photolysis,indirect photolysis and effect of microorganisms was evaluated.Under sunlight,no direct photolysis was observed.The indirect photolysis and effect of microorganism play important roles in the loss of phosphite.The reaction follows the pseudo-first-order reaction kinetics.The kinetics of phosphite in STE-F-THW(Sterilized;Filtered;Taihu Water)was 0.0102 d-1,and the half-life was 68.0 d.In original THW,they were 0.0324 d-1 and 21.4 d,indicating that suspended particles and suspended microorganisms in THW may play important roles in the loss of phosphite.NO3-is widely existed ions with photoactivity,and Taihu usually bears N pollution.The loss of phosphite in THW with high concentration of NO3-under sunlight was studied.The results showed that the reaction rate was accelerated with 1 mmol L-NO3-added,as a result that all the half-lives decreased to 15.0-18.0 d.The loss of phosphite was promoted with high NO3-pollution.4.The photooxidation process of 10 ?mol L-1 phosphite in nitrate solutions was studied under simulated irradiation.The results demonstrated that the photooxidation of phosphite followed the pseudo-first-order kinetics and the photooxidation rate increased rapidly with increasing initial NO3-concentration and decreasing pH values.The addition of Fe3+ or Fe2+with photoactivity accelerated the photooxidation,and the addition of Mn2+ or Cd2+inhibited the photooxidation.Br-,NO2-and HCO3-in environmental concentrations decreased the photoreaction,and HCO3-showed the strongest inhibition.The addition of Suwannee River Humic Acid(SRHA)or Suwannee River Fulvic Acid(SRFA)strongly inhibited the photooxidation process,meanwhile the SRHA presented stronger inhibition than SRFA with the same carbon concentration.The increased amount of phosphate coincided with the decreased amount of phosphite,which indicated that the photooxidation product of phosphite is phosphate.ROS(reactive oxygen species)were found to play an important role in the phoshite photooxidation reaction by using isopropanol and sodium azide(NaN3)as ROS quenchers.That DMPO-OH product was observed in electron spin resonance(ESR)experiments and both quenchers decreased the intensity of DMPO-OH,indicated that OH was the main oxidant produced in this system.5.The photooxidation of phosphite in aqueous solution in the presence of ferric and oxalate ions under a Xe lamp was investigated.The photooxidation of phosphite followed pseudo-first-order reaction kinetics.The kinetics constant of 100 ?mol L-1 phosphite was 0.0039 min-1 at pH 3 and Fe(III)/Oxalate 10.0/100.0 ?mol L-1.The photooxidation was dependent upon the pH values,phosphite/ferric/oxalate concentration,and light intensity.The decrease of phosphite coincided with the increase of phosphate.The addition of 2-proponal,NaN3 or furfuryl alcohol inhibited the photooxidation of phosphite.