Microbial And Photodegradation Of Several S-triazine Herbicides In Soil

Morden agriculture is largely depending on the use of pesticides.While the globally consumed amount of pesticides is climbing year by year,it brings about huge residues of pesticides in the natural environment.Some persistent pesticides remained in the environment cause detrimental effects on both plants and animals.S-triazine herbicides have been widely used for decades,accounting for 2.3%of the global consumption of pesticides.Due to its persistence in environment,this type of pesticide is likely leaching from soil to understreams or surface waters,because it is frequently detected in environments,animals and even humans.S-triazine has been proven as carcinogenic and endocrine disrupting substances.Although it was banned by the European Union in 2004,it is still manufactured and applied in our country.In this paper,the degradation behavior of propazine in natural soil was investigated.The degradation of propazine was rapid,but was largely associated with the s-triazine-degrading bacterias in soil.The dynamic microbial degradation behaviors of propazine were analyzed,including the microbial biomass,community structure and diversity,soil enzyme,s-triazine catabolic genes,and degradation products.In addition,novel propazine-degrading bacteria consortia Agr-B were isolated by screening which used propazine as sole nitrogen and carbon resource.The bacteria consortia were identified by 16S rDNAs.We identified the metabolites of propazine with Agr-B in the different soil samples.We further investigated the effects of different environmental factors on the biodegradation behavior and monitored the degradation dynamics of the pesticide.Furthermore,the photodegradation behavior of prometryn on soil surface was identified.The effects of different factors on photodegradation in soil and the photodegradation products as well as the photodegradation pathways were demonstrated.Detailed descriptions are shown as follows:(1)The degradation rate of propazine was found to be low at first,and became fast in the following two days.The degradation of propazine was less affected by soil adsorption,but its degradation behavior was followed by the first-order kinetics.The microbes in soil were found to paly a major role in propazine degradation.Monitoring the dynamic changes of SMBN and SMBC during the process showed that SMBN and SMBC in the treatment group were significantly different from that of the control.Both began to increase at the 5 d and reached the peak at 11 d and 15 d,respectively.Meanwhile,the activities of CAT,DHA,PO and POD in the soil changed dynamically along with the degradation of the herbicide.PCR-DGGE analysis revealed the differences in the microbial community structure between the treatment and the control at the different time intervals.For the treatment group,the microbial diversity decreased along with the degradation period.There were 40 DGGE gel bands sequenced and their homology comparatively was anslyzed.Transcripts of the s-triazine catabolic genes,trzN,atzB and atzC in the soil were determined by semi-quantitative RT-PCR and well confirmed by qRT-PCR.The expression level of the genes was corresponding to the degradation period of the pesticide.Five metabolites of propazine degradation were characterized using UPLC-QTOF-MS/MS.The dimeric product 4,4',6,6'-4-Isopropylamino-2,2'-bi-1,3,5-triazine(DIP)was evidenced for the generation of reactive oxygen species during microbiological degradation of propazine.DIP and N,N'-Diisopropyl-6-methoxy-1,3,5-triazine-2,4-diamine(MEP)were also the products during the process of microbial degradation of s-triazine,which was reported here for the first time.Finally,a complete degradation pathway of propazine in soil was proposed.(2)Several soil samples were collected from different environments.The samples were inoculated in the MSM with 60 mg L-1 propazine.Following 8 weeks of acclimation,two types of effective propazine-degrading bacteria consortias Agr-B and Agr-C were isolated.The degradation rate of propazinein was close to 100%within 6 days.The propazine-degrading bacteria were isolated and purified from the consortias,but no single strains with good degradation ability were obtained.The total DNA in Agr-B was extracted for 16S rDNA.Sequencing results showed that the OTUs included in Agr-B belonged to Methylophilus and Paracoccus,and a few belonged to Arthrobacter.The degradation kinetics of propazine with Agr-B was assessed.The model best fitted for the growth curve of Agr-B was the log-logistic model with four parameters.The logistic model fitted in well with the degradation curve of propazine.Finally,the metabolite of propazine degraded by Agr-B was characterized by UPLC-QTOF-MS/MS,with only one metabolite hydroxy-propazine availabe.(3)The inoculation fluid was prepared with the Agr-B which was inoculated in three natural soils NJ,JX and YC with different physical and chemical properties for the bioaugmentation experiments.Addition of Agr-B promoted the degradation of propazine in soil NJ and JX.The kinetic analysis showed that the exponential growth model IV fitted it well with the degradation of propazine in soil.The effects of different environmental factors on the bioaugmentation experiment were presented,showing that the increase of inoculation rate facilitated the degradation of propazine.Under the low soil moisture(30%)and waterflooding(>100%)situation,the lag phases in the degradation of propazine were shorter than that of 60%situation,indicating that Agr-B could survive and degrade the propazine under the low-moisture,low-salt and low-oxygen environments.Although addition of DOC prolonged the lag phases of propazine degradation,the degradation rate of propazine was approaching over 95%within 20 days.Addition of urea significantly accelerated the degradation of propazine.But the high concentration of urea inhibited the maximum growth rate of microbes,indicating that the added urea stimulated the availability of propazine to Agr-B,rather than promoting the growth of the consortia.Finally,Agr-B rapidly degraded atrazine and prometryn in the soil,but the degradation rate of atrazine and prometryn was not as fast as propazine.(4)The study of the photodegradation of prometryn in red soil was carried out.The degradation rate of prometryn in soil was up to 98%with 14 days of UV irradiation(6 h per day).From the point of energy saving,the 15 W low pressure mercury lamp treatment showed the best performance for photodegradation.Soil moisture had a great influence on photodegradation in soil,with the degradation rate of humidity being increased when the moisture was between 0-60%.However,the reduced photodegradation rate of prometryn was observed with 90%soil humidity.The analysis of ultraviolet visible diffuse reflectance spectrum at 200-500 nm proved that the absorption of light by the surface of soil was inhibited by higher soil moisture.Similarly,both higher temperature and higher dose of prometryn promoted the prometryn degradation.Addition of TiO2 increased the degradation rate of prometryn in the soil.Nine degraded products of prometryn were identified by UPLC-LTQ OrbiTrapMS/MS.The photodegradation pathways of prometryn in the soil were proposed under darkness,ultraviolet light and ultraviolet light with TiO2,respectively.There was no demethylthiolation of prometryn under the darkness,and the prometryn was degraded into smaller products N,N'-diisopropyl-1,3,5-triazine-2,4-diamine(DSP)and 6-hydroxy-1,3,5-triazine-2,4-diamine(HDDPP),whereas under the UV and photocatalytic exposure,the degradation was mainly through demethylthiolation,N-dealkylation and hydroxylation.In a similar way,addition of TiO2 enhanced the degree of prometryn degradation.(5)The Fe3O4 nano particles were modified by citric acid(CA),sodium citrate(SCA)and trisodium pyrophosphate(TSP),respectively which enhanced the dispersibility of Fe3O4 particles.The Fe3O4@TiO2 particle was synthesized with the titanium precursor(tetraisopropyl titanate)under the ammonia hydrolysis.The modified and unmodified Fe3O4 nano particles and the synthesized Fe3O4@TiO2 particles were characterized by FT-IR.The modifiers were connected to the metal oxide surface with coordinate bond.The XRD,SEM and TEM analysis of the synthesized Fe3O4@TiO2 particles suggested the anatase TiO2 coated on the Fe3O4 particle successfully.The photodegradation of prometryn occurred in the soil with the synthesized Fe3O4@TiO2 particles.Addition of the particles obviously promoted the photodegradation of prometryn in soil.After the five times light experiment,the Fe3O4@TiO2 material could be separated magnetically and recovered from the soil.The average recovery rates were close to 95%,suggesting no any residues of the nanoparticles left in the soil environment.