| As a new organic pollutant,the environmental behavior and effects of Dechlorane Plus(DP)have been paid much attention.The pollution status of DP in soil of priority areas,such as e-waste disposal points,flame retardant production plants and urban industrial areas,is becoming more and more serious.Soil-circle is an important component of natural environment,and the migration and transformation process of organic pollutants in soil-plant system is closely related to the safety of agricultural products and human health.However,few studies on the environmental behavior and ecological effects of DP in soil have been made,and its migration and transformation process in soil-plant system are not clear.Therefore,taking typical paddy soils in Nanjing,Jiangsu Province as an example,this research established the analysis methods of different fractions of DP in soil,and studied their fraction transformation under the aging effect.The migration and transformation of DP in the soil-rice system was systematically studied by laboratory simulation of soil-plant system,and the fraction transformation and dissipation mechanism of DP in the rhizosphere were revealed.The main research results are as follows:(1)On the basis of previous studies,different fractions of organic pollutants in soil were defined,that is the labile,stable-adsorbed,and bound-residue fractions,respectively.Afterwards,DP was taken as an example to establish and verify the extraction methods of its fractions.The three fractions were extracted by Tenax-TA,organic solvents,and alkaline hydrolysis.(2)The established method for extracting the fractions of DP was used to study the transformation of DP fractions in the soil under the effect of"aging effect"after 260days.The stereoselective enrichment of two isomers of DP in labile and bound-residue fractions was observed.Furthermore,first-order kinetics model was used to quantitatively analyze the effects of long-term aging on the fraction transformation of DP in soil.(3)A rhizobox system was designed to observe the absorption and accumulation of DP in plants and the translocation of DP in plant tissues through a 150-day rice cultivation experiment.The results showed that the absorption rate of DP and the absorption amount were relatively low.The translocation factors that demonstrate the capacity of translocation of DP from roots to shoots were calculated at low levels,indicating that the translocation of DP in rice was restricted.Some DP molecules could enter the symplast pathway through the lipid bilayer after passive diffusion.After breaking through the Casparian strip,DP enters the vascular system to transit to the upper part of plant roots.The fraction transformation of DP in rhizosphere soil and its relationship with dissipation of DP in soil were investigated.Comparing to the initial concentrations,total DP in the root compartment were decreased by 55.4%,47.6%and23.8%,respectively in the S1,S2,and S3 soils after 150 days of rice cultivation,indicating that plant growth significantly promoted the dissipation of DP in the soil near roots.Through the correlation analysis of microbial carbon(MBC)and dissipation amount of DP in soil of each compartment.It is concluded that microbial degradation in rhizosphere is the main cause of DP dissipation in rhizosphere soil.(4)This study established a mathematical model based on plant absorption,transport and fraction transformation to predict the total purification amount of DP in plants and analyze the contribution of each dissipation process(plant absorption,transfer in soil and bound-residue formation)to the total dissipation amount of DP under rhizosphere effect.The results showed that rhizosphere effect has"activation effect"on bioavailability of DP and can change the process of bound-residue formation of DP.The concentration of DP in plant roots was positively correlated with the labile concentrations of DP in the rhizosphere(R~2=0.852-0.961).There were spatiotemporal variations in the fractions of DP in rhizosphere soil,and the correlation analysis of dissolved organic carbon(DOC)and labile fraction in soil showed that DOC had a promoting effect on bioavailability of DP.Finally,the model results showed that the contribution to total DP dissipation was as follows:microbial degradation accounted for 8.33-54.14%,bound-residue formation accounted for 3.64-16.43%,and plant absorption accounted for 0.54-3.85%.With all these factors,the half-life of DP in soil was shortened to 105 days.The main innovations of this study are as follows:1)a three-step sequential extraction method was established to study fractions transformation of DP in aging soils.The results showed that DP fractions changed during aging and were persistent in soil.2)a model for description of fractions transformation of DP was established to simulate the transformation process between DP fractions during a 260-day aging period.In addition,Sobol global sensitivity analysis(SGSA)was conducted on the model to study the sensitiveness factors affecting the transformation between DP fractions.3)a rhizobox system was designed and constructed to study the dissipation of DP in rhizosphere soil,the accumulation and depuration of DP in plants,and the transformation of each fraction of DP in rhizosphere soil.4)a multi-phase transport coupling model based on plant absorption,transport and fractions transformation was established,and the contribution of microbial degradation,plant absorption and purification,diffusion and transport in soil to the total dissipation of DP was analyzed and quantified.The environmental behavior of DP in soil illustrated in this study can help to understand the fate of organic pollutants in soil-plant systems and provide basic information for DP risk assessment and further land management in current remediation hotspots. |
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