| At present,research on microplastic surface systems mainly focuses on the aquatic environment.Due to the more complex soil environment,the research on the surface systems characteristics of microplastics in soil is lagging behind.Therefore,this study focused on the formation and adsorption characteristics of soil microplastic surface systems.On the basis of exploring the soil particles load ability of different microplastic surfaces and the adsorption characteristics of related surface systems,we further studied the effects of microplastics on the stability and adsorption characteristics of earthworm casts,which is a natural-like aggregates.Polyester microfibers(average length:0.631mm,average diameter:5μm)were used in this study.Firstly,the physical surface system of microplastics(C-PMF,C-HPMF,S-PMF,S-HPMF)was formed by loading soil clay and silt particles on primary polyester microfibers(PMF)and thermal aged polyester microfibers(HPMF),and the water stability,hygroscopicity and adsorption characteristics of methylene blue were measured.Secondly,the organic matter OM(Vicia villosa Roth Var.,V;Populus nigra,P;goat manure,GM;0%and 3%soil dry weight)and PMF(0%,1%soil dry weight)were mixed with the soil as the bait of earthworm.Culture of Esenia foetida to produce earthworm casts(EWCs,EWCs-V,EWCs-P,EWCs-GM,EWCs-PMF,EWCs-V-PMF,EWCs-P-PMF,EWCs-GM-PMF).The distribution characteristics of polyester microfiber in earthworm casts and its influence on the water stability and adsorption characteristics of earthworm casts were determined.The main results are as follows:(1)The surface properties of polyester microfiber and the properties of surface substances affected the loading capacity.The loading capacity of HPMF to soil particles was different from that of PMF because the surface became rough due to 4h high temperature(230℃)treatment.Under the condition of low concentration of soil particles(15g/L),PMF(0.22 and 0.24g/L)had significantly higher clay and silt load than HPMF(0.07 and 0.09g/L).When soil particle concentration was 150 and 300g/L,HPMF(1.9 and 4.13g/g)had a significantly higher load on silt than PMF(1.27 and2.11g/g),but there was no significant difference in the load on clay particles.(2)The stability of the physical surface system of polyester microfiber was mainly determined by the properties of soil particles loaded on the surface,while the surface roughness of polyester microfiber has little influence.The water stability(MWD)of C-PMF and C-HPMF(6.29 mm and 5.82mm)was significantly higher than that of S-PMF and S-HPMF(5.04 mm and 5.06mm).HPMF did not significantly improve the water stability of the physical surface system of polyester microfiber.(3)A large number of polyester microfibers with small length were enriched in earthworm casts.The distribution trend of PMF in earthworm casts treated by natural organic compounds was consistent with 0%OM,mainly distributed in the range of0-0.5mm(70-80%),while the primary PMF used in this study was mainly distributed in the range of 0-1mm(80%).Under the condition of 3%OM(104.73,81.80 and143.43),the number of polyester microfibers in single earthworm casts was significantly higher than 0%OM(42.82),and the number of polyester microfibers in3%GM(143.43)was significantly higher than that in 3%V and P(104.73 and 81.80).The total length of polyester microfiber in the earthworm casts increased only slightly.The total length of polyester microfiber in the earthworm casts treated by 3%P(25.73mm)was not significantly different from that of 0%OM(16.70mm).The addition of 3%V and GM(35.56 mm and 49.88mm)significantly increased the total length of polyester microfiber in the earthworm casts,but there was no significant difference between the two treatments.(4)Polyester microfiber mainly affected the water stability of earthworm casts to a certain extent by affecting natural organic matter.At 0%PMF added level,the water stability of earthworm casts(MWD)in 3%GM(19.56mm)treatment was significantly lower than that in 0%OM(28.76mm)treatment,and there was no significant difference among other treatments.At 1%PMF added level,the water stability of earthworm casts treated by 3%V,P and GM(15.91,15.74 and 20.37mm)was significantly lower than that treated by 0%OM(39.84mm).In addition,in 3%V treatment,the addition of 1%PMF(15.91mm)significantly reduced the water stability of earthworm casts compared with 0%PMF(27.67mm).In 0%OM,3%V and P treatment,the addition of PMF did not affect the water stability of earthworm casts.(5)The adsorption capacity of the polyester microfiber surface systems is mainly determined by the properties of the substances loaded.The maximum hygroscopicity of C-PMF and C-HPMF(20.11%and 19.01%)was significantly higher than that of S-PMF and S-HPMF(1.39%and 1.11%).Among them,HPMF significantly increased the maximum hygroscopicity capacity of clay polyester microfiber surface system,but had no significant effect on the silt polyester microfiber surface system.In addition,the methylene blue maximum adsorption capacities of C-PMF and C-HPMF(27.35,28.73mg/g)were significantly higher than those of S-PMF and S-HPMF(1.57,1.60mg/g),but HPMF did not significantly increase the maximum adsorption capacity.The adsorption capacity of earthworm casts is between polyester microfiber clayed surface systems and silty surface system.The maximum hygroscopicity of earthworm casts is determined by the content of natural organic matter.The maximum hygroscopicity of earthworm casts under 3%V,P and GM(14.46,14.47 and 14.52%)treatments was significantly lower than that under 0%OM(15.45%)treatment,but the PMF had no significant effect.However,the methylene blue maximum adsorption capacity of EWCs-PMF(19.97mg/g)had no significantly different with that of EWCs(19.44mg/g),indicating that the PMF did not affect the methylene blue adsorption ability of earthworm casts.(6)Different polyester microfiber surface systems have different adsorption properties.The adsorption of methylene blue on the polyester microfiber surface system was a spontaneous process(△G<0k J/mol).The adsorption data were more in line with the pseudo-second-order kinetic model(R~2>0.99),indicating that the adsorption process was jointly determined by the polyester microfiber surface system and methylene blue.Among them,the data on the adsorption of methylene blue by the polyester microfiber physical surface systems(C-PMF,C-HPMF,S-PMF,S-HPMF)are more in line with the Langmuir isotherm(R~2>0.9),indicating that its surface properties are relatively uniform,and the adsorption process is energetically favourable(0<R_L<1).However,the data on the adsorption of methylene blue by earthworm casts(EWCs,EWCs-PMF)were more in line with the Freundlich isotherm(R~2>0.99),indicating that the earthworm casts surface properties are not uniform.The earthworm casts adsorption of methylene blue is energetically favourable and is dominated by a physical adsorption process(n>1).This study showed that the stability and adsorption capacity of polyester microfiber clay surface system are significantly higher than that of silty surface system,while the stability and adsorption capacity of earthworm casts are significantly related to the type of natural organic matter added to the bait.Therefore,the surface system of polyester microfiber in soil environment is mainly determined by the properties of the loaded materials. |
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