Microplastics Distribution Characteristics In Soils And The Combined Ecological Risk Of Microplastics And Cadmium

Microplastics(MPs),which are defined as small plastic debris(particle size within0.1?m?5 mm),as a new type of environmental contaminant,have recently been found widely distributed in soils.There have been mounting concerns about the ecological environment risk induced by MPs,due to it characteristics of small size,broadly input way,easily been transferred,hard to be removed and the ability to adsorb/desorb other type of pollutants.Soil as an important part of ecosystem,hence,determining the fate of microplastics,as well as the extent of contamination,in soils environments is essential for realistic assessments of their ecotoxicity and ecosystem effects.Compared with large plastics,MPs exhibit large specific areas,which makes MPs can interact with surrounding pollutants and may be a vector for these substances,such as heavy metal ions,consequently,pose more complex ecological risks.However,there is scant of available data on the degree of MPs pollution in soils,which leads to the lack of actual value of soil MPs in the later ecotoxicity test.Investigating the adsorption/desorption mechanism of heavy metal ions onto MPs and learning the synergetic ecotoxicity of MPs and heavy metals on soil animals,can provide theoretical data for further study.Yet,systematic studies on the adsorption/desorption mechanism of heavy metal ions onto MPs,and most of MPs related ecotoxicity studies are focused on aquatic organisms.Therefore,first we took riparian soils then woodland soils,vegetable plot soils and vacant land soils from suburb,to investigate the extent of MPs contamination.Following that,we analysed the adsorption mechanism of metal ions onto MPs and their desorption phenomenon and studied the combined toxic effects of MPs and cadmium on earthworm.Main results of these studies are as follow:(1)we showed that microplastics contamination is omnipresent in numerous riparian soil samples collected along the Yangtze River,with average MPs abundance value of 3877.4±2356.6 p/kg.Subsoils(10?15 cm)showed higher levels of MPs(4005.1±2472.8 p/kg)than topsoils(10?15 cm,3748.5±2301.2 p/kg).While polyamide(32%)was the most commonly found polymer in the samples,and small MPs particles(<200?m)accounted for approximately 70%of the MPs detected in subsoils.In terms of shape,microfragments were the most common type of MPs particle,accounting for 34%of total MPs,followed by microfibers(30.3%).Furthermore,MPs contamination was found to be positively correlated with both the population of the study area and precipitation,yet negatively correlated with the elevation of sampling site.(2)results showed that the abundance of MPs of typical suburban soils ranged from(2.2±0.6)×10~4–(6.9±0.9)×10~5 p/kg,and 81.7%of particles measured ranged from 10 to 100?m.In woodland,the abundance of MPs((4.1±0.8)×10~5 p/kg)was significantly higher than that in vegetable plots((1.6±0.4)×10~5 p/kg)and vacant land((1.2±0.5)×10~5 p/kg).The most prevalent form of MP particles was microfragment(53.4%),followed by microfiber(15.2%).Polyethylene,polypropylene,and polystyrene were commonly found at each sampling site,with polyethylene as the dominant form(36.1%).Moreover,MPs particles in the soil environment contained different levels of heavy metals,including Cd,Cr,Pb,Ag,Cu,Sb,Hg,Fe,and Mn.The concentration of heavy metals in MPs,particularly Cd,Pb,Mn,and Hg,was closely related to the extent of heavy metal contamination in the soil environment,inferring that MPs in the soil presumably have the ability of adsorbing heavy metals.(3)the adsorption of cadmium(Cd(?))onto five different types of MPs were compared to examine the relationship between the surface characteristics and the adsorption properties of MPs.Our results showed that polyamide had the highest Cd(?)adsorption capability with a value of 1.70±0.04 mg/g,followed by polyvinyl chloride(1.04±0.03 mg/g),polystyrene(0.76±0.02 mg/g),acrylonitrile butadiene styrene(0.65±0.02 mg/g)and polyethylene terephthalate(0.25±0.01 mg/g).The specific surface area and total pore volume were closely correlated with the adsorption capacity of the MPs,and the?-?interaction,electrostatic interaction and oxygen-containing functional groups played crucial roles in the adsorption of Cd(?)onto the MPs.The sorption capabilities of Cd(?)onto the MPs first increased and then decreased with increasing solution p H from 2.0 to 9.0.In addition,the adsorption capacities were suppressed with the presence of lead ions(20–80 mg/L),while the coexistence of phenanthrene had a minor impact.Interestingly,the presence of humic acid promoted the desorption of Cd(?) from the MPs both in the synthetic earthworm gut and in the sediment system.A higher desorption rate was observed in the simulated gut environment,suggesting that metal-contaminated MPs would pose higher ecological risks to macroinvertebrates.(4)we analyzed the adverse effects of exposure to MPs and co-exposure to MPs and Cd(?)on the earthworm Eisenia foetida(E.foetida).Results showed that exposure to MPs only or to a combination of MPs+Cd decreased growth rate and increased the mortality(>300 mg/kg)after exposure for 42 d,and co-exposure to MPs+Cd(?)(>3000 mg/kg)posing higher negative influence on the growth of E.foetida.MPs exposure can induce oxidative damages,the contents of LPO and GSH increased,and the presence of Cd(?)increased the negative effects of MPs.Furthermore,the MPs particles can be retained within E.foetida,with values of 4.3–67.2 particles/g earthworm and can increase the accumulation of Cd in earthworm from 9.7%to 161.3%.Collectively,the results of this study demonstrate that combined exposure to MPs and Cd(?)poses higher negative effects on E.foetida.The combined exposure of MPs and Cd had a higher negative impact on the earthworm,indicating that MPs may increase the negative impact of heavy metal ions in the soil environment and interaction of MPs and Cd(?) made MPs have more complex ecological risk.