| Plastic mulching film is widely used in agricultural production because of its advantages in heat preservation,moisture retention,inhibition of weed growth,promotion of faster crop development and increase in crop yield.Currently,plastic mulching film is mainly made of polyethylene（PE）,which is difficult to degrade in the environment.In addition,the plastic mulching film is thin and difficult to recycle,resulting in the residue of plastic mulching film in farmland soil increasingly serious.The residual plastic mulching film can fragment in farmland soil to form small plastic debris,known as microplastics（<5 mm）.As a type of emerging pollutants,microplastics are generally considered to have the potential adverse effects by themselves.Meanwhile,there is growing concern over microplastics as carriers for co-existing organic pollutants and may cause new combined pollution.However,previous studies mainly focused on the aqueous environment,and it is still unclear about the effects of farmland residual microplastics on the environmental fate and bioavailability of co-existing pollutants in the soil environment.In this study,unaged PE mulching film-derived microplastics（PE MPs）and field-aged PE mulching film-derived microplastics（PE-aged MPs）which prepared from unused PE mulching film and residual PE mulching film as raw materials,respectively,were selected as study objects.Atrazine was selected as a typical organic pollutant in soil to investigate the adsorption-desorption behavior of atrazine on soil,PE MPs and PE-aged MPs by batch equilibrium adsorption test and simulated earthworm digestive fluid desorption test,the effects of microplastics on the adsorption-desorption behavior of atrazine in soil,and the effects of microplastics on the bioaccumulation of atrazine in earthworms.This study will help to understand the interaction effects and mechanisms of residual mulching film-derived microplastics and pesticide combined pollution,and provide scientific support for improving the ecological risk assessment of microplastics in soil.The main results of this study are as follows:（1）The aging process in the field caused cracks,pits and granular bumps,and generated new oxygen-containing functional groups on the surface of PE mulching film,and led to a decrease in the crystallinity of the PE mulching film.（2）The adsorption of atrazine on soil,PEMPs and PE-aged MPs all reached equilibrium at 24 h.The adsorption kinetic data were well fitted by pseudo-second-order model（R~2>0.990）,indicating that the adsorption of atrazine on soil,PE MPs and PE-aged MPs is a complex process,which is influenced by a combination of mechanisms including external surface diffusion,intraparticle diffusion and the interaction between the adsorbate and the active sites.The adsorption isotherms of atrazine on soil,PE MPs and PE-aged MPs could be well fitted by both Langmuir model（R~2>990）and Freundlich model（R~2>0.987）,and the fitting results were better for Langmuir model,indicating that the interactions between atrazine on soil,PE MPs and PE-aged MPs were mono-or multi-layer adsorption processes on heterogeneous surfaces,and the monolayer coverage dominated the adsorption process.The theoretical maximum adsorption capacity of atrazine on soil,PE MPs and PE-aged MPs followed the order of PE-aged MPs(450±23 mg·kg-1)>PE MPs(398±16 mg·kg-1)>soil(367±10 mg·kg-1),indicating that the sorption capacity of microplastics for atrazine was higher than that of soil,and the field aging process significantly increased the adsorption capacity of microplastics for atrazine.The desorption capacity and desorption rate of atrazine by soil,PE MPs and PE-aged MPs in simulated earthworm digestive fluid were significantly higher than those in Ca Cl2 solution,and the desorption capacity and desorption rate of atrazine by soil,PE MPs and PE-aged MPs in different background solutions followed the order of PE-aged MPs>PE MPs>soil.（3）The adsorption capacity and adsorption rate of atrazine by soil added with 1%,5%and10%of PE MPs or PE-aged MPs were significantly higher than those by soil added without microplastics.And,the desorption capacity and desorption rate of atrazine by soil added with5%and 10%of PE MPs or 0.1%,1%,5%and 10%of PE-aged MPs were significantly higher than those by soil added without microplastics.The adsorption capacity,adsorption rate,desorption capacity,and desorption rate of atrazine by soil added with microplastics increased significantly with the additive dose of microplastics.In addition,the adsorption capacity,adsorption rate,desorption capacity,and desorption rate of atrazine by soil added with PE-aged MPs were mostly higher than those by soil added with PE MPs.（4）PEMPs and PE-aged MPs used in this study could be ingested by two ecologically different earthworms Eisenia fetida and Metaphire guillelmi with M.guillelmi（an anecic species）ingesting more microplastics than E.fetida（an epigeic species）.The concentration of atrazine in earthworms in different treatment groups increased firstly and then decreased with increasing of exposure time.The concentration of atrazine in earthworms in different treatment groups was significantly different at the early stage of bioaccumulation,and the time when the maximum concentration of atrazine in earthworms reached was varied among different treatment groups.However,there was no significant difference for the concentration of atrazine among different treatment groups at the late stage of bioaccumulation.For the maximum concentrations of atrazine in earthworms,the concentration of atrazine in earthworms in the PE MPs treatment group was slightly higher than that in the control group,while the concentration of atrazine in earthworms in the PE-aged MPs treatment group was significantly higher than in the PE MPs treatment group and the control group,indicating that the addition of PE MPs and PE-aged MPs increased the bioaccumulation of atrazine in earthworms especially for PE-aged MPs.In addition,the concentration of atrazine in M.guillelmi was significantly higher than that in E.fetida. |
