Fate And Transformation Of Triclosan Coexisted With Graphene In Soil And Plant Based On ¹⁴C Labeling Techniques

Pharmaceuticals and personal care products(PPCPs)are widely used in daily life and frequently detected in the environment.Due to harm to the environment and human beings,they are widely concerned in the field of environmental science.With the development of nanotechnology,the massive use of carbon-based nanomaterials inevitably leads to their release into environment.It is a common phenomenon for carbon-based nanomaterials and organic pollutants to coexist in the environment.As a representational carbon-based nanomaterial,graphene has been widely reported in the field of environmental remediation of organic pollutants such as PPCPs.However,the mechanism of graphene's influence on the environmental behavior of organic pollutants is not yet clear.The broad-spectrum bactericide triclosan is a representative PPCPs,which has attracted worldwide attention.It is of great environmental significance and scientific value to study its environmental behavior in the presence of graphene.In this study,radionuclide tracer technique and modern instrument analysis technology were used to study the pollution evolution,formation,and transformation characteristics of¹⁴C-triclosan in soil or soil-plant systems with/without graphene,and the effect mechanism of graphene on ¹⁴C-triclosan was explored.The main results are as follows:(1)The pollution evolution,degradation and transformation of ¹⁴C-triclosan in soil and its mechanismAt 112 d,the extractable residue(ER)of ¹⁴C-triclosan in red sandy clay soil(S1)was significantly higher than that in yellow loamy soil(S2)and coastal saline soil(S3)(p<0.05),and the bound residue(BR)showed a trend of S3(40.8%)>S2(29.1%)>S1(11.2%),while the mineralization showed the trend of S2(63.4%)>S3(39.2%)>S1(11.1%).The half-life of ¹⁴C-triclosan in soils was 187.8 d(S1),21.2 d(S2)and 27.2 d(S3),and the degradation products were methyl triclosan(M1)and dimer dechlorination(M2)in the three soils,but the dynamic of degradation products was related to the soil type,the percentage of methyl triclosan to the applied amount was1.10–11.0%(S1),2.83–8.84%(S2),3.92–6.64%(S3).M2 had not been reported on degradation studies of triclosan in soil,accounting for 1.12–1.45%(S1),0.72–1.93%(S2),and 0.62–1.20%(S3).During the whole incubation process,the abundance of bacteria in the three soils was S2>S3>S1.Among them,the relative abundances of Burkholderia-Caballeronia-Paraburkholderia,Pseudomonas,Sphingomonas,and Bacillus all increased,and they may be involved in triclosan degradation.Pseudomonas and Sphingomonas were the major degradation bacteriaa.(2)The behavior and fate of ¹⁴C-triclosan in soil under the coexistence of grapheneThe adsorption of ¹⁴C-triclosan in soil under the coexistence of graphene oxide(GO),reduced graphene oxide(RGO),multilayer graphene oxide(MGO),multilayer graphene(MG)and graphene sheet(GS)was studied.RGO was the most significant effect on the adsorption of ¹⁴C-triclosan in soil,and the adsorption capacity of triclosan was positively correlated with the graphene concentration.Under the coexistence of RGO and triclosan,the ER of triclosan in S1 decreased with RGO concentration.In S2 and S3,the trend of ER was the same as that of S1 at0–21 d,but then showed the opposite trend.At high levels(500 mg·kg^-1 soil),RGO can significantly promote the formation of BR;compared with the control,the BR of ¹⁴C-triclosan and its metabolites increased by 13.3%(S1),16.3%(S2)and 22.1%(S3),respectively.At the same time,RGO inhibited the mineralization of ¹⁴C-triclosan and its metabolites in soil compared with control,which were decreased by 2.8%(S1),30.6%(S2)and 31.1%(S3),respectively.The reduction trend of the ¹⁴C-triclosan accorded with the first-order kinetic equation.In the treatment of RGO 500 mg·kg^-1,the half-life of triclosan was shortened by 14 d in S1 and extended by 2.2 d(S2)and 13.8 d(S3),respectively.Adding RGO or not did not change the type of degradation products,but delayed the formation time of the products.RGO(500 mg·kg^-1)in S1 promoted the reduction of triclosan,while RGO(500 mg·kg^-1)in S2 and S3 promoted the reduction of triclosan first and then inhibited.RGO did not affect the abundance of bacteria in S1,but reduced the abundance of bacteria in S2 and S3.In addition,RGO(500 mg·kg^-1)significantly affected the bacterial community composition at 84 d–112 d.In general,RGO inhibited the increase of the possible triclosan-degrading bacteria.It was speculated that RGO may affect the environmental behavior of triclosan in the soil by affecting the diversity of bacterial communities.(3)The uptake and accumulation of ¹⁴C-triclosan in plants under the co-existence of grapheneIn the soil-peanut/radish plant system without RGO,only less than 1.70%of the¹⁴C-triclosan was uptaken by peanut and radish plant,and most of the radioactive components remained in the soil,and the highest concentration was detected in the root,suggesting that triclosan and its metabolites were difficult to transloacate from roots to leaves and stems.The soil-radish plant system was selected to study the mechanism of graphene on the fate of ¹⁴C-triclosan.Overall,RGO inhibited the reduction of ¹⁴C-triclosan in the soil.At 30 days,¹⁴C-triclosan decreased with the increase of RGO dosage and showed the opposite trend after 30 days.RGO did not change the degradation products of triclosan in the soil,but it inhibited its degradation rate in the soil and delayed the formation of its dimer dechlorination.RGO(>250 mg·kg^-1)can significantly inhibit the uptake of triclosan by radish plants,and uptake was negatively correlated with RGO concentration.RGO inhibited the accumulation of ¹⁴C-triclosan in various parts of radish plants,especially roots.Meanwhile,RGO improved the translocation capacity of triclosan from roots to leaves and stems.By analyzing the extractable residue composition of triclosan in radish plants under this system,the three metabolites are sulfated triclosan,sulfated glucose and triclosan conjugate and dechlorinated triclosan.The addition of RGO will affect the composition of the metabolites of triclosan in radish plants.The results showed that graphene can significantly change the pollution evolution,formation,and metabolic transformation of triclosan in soil,and can promote the formation of bound residue between triclosan and soil matrix.The study also confirmed that graphene can change triclosan formation and reduction in the soil-radish plant system,and can also reduce the accumulation and bioavailability of triclosan by the radish plant.The research results provide more information for scientific evaluation of the environmental safety of triclosan under the co-existence of graphene and provide reference techniques for similar future research.