Effects Of Microplastics On Solute Transport In Soils With Different Textures

Microplastics are plastic particles with particle size less than 5 mm,which are widely present in the soil environment.The presence of microplastics disrupts the original equilibrium of the soil system and changes the physical and chemical properties of the soil,as well as affects the soil hydrology and material transport processes.Therefore,in this study,vertical solute replacement experiments were conducted to investigate the effects of microplastic particle size and concentration on the anion and cation transport processes in different soil textures,using the CXTFIT2.1 program nested in STANMOD software to fit the anion and cation breakthrough curves and invert the relevant parameters to analyze the effects of microplastic particle size and concentration on the fitted parameters.The effect of microplastic particle size and concentration on the fitted parameters was analyzed.The main conclusions are as follows:（1）The degree of influence of microplastics on Cl^-breakthrough process tends to increase with the increase of particle size.The effect of smaller particle size（48-58μm and 100-150μm）microplastics on the Cl^-transport process was not obvious,while larger particle size（600-1000μm and 1-2 mm）microplastics could both promote and inhibit the process depending on the added concentration.The effect of microplastic particle size on Cl^-transport process is mainly related to particle size and soil particle size.If the particle size of microplastic is much smaller than or equal to the soil,it will not have a significant effect on the solute transport process in a certain concentration range,and if the particle size of microplastic is larger than the soil,it will have an inhibitory or facilitative effect at different added concentrations.（2）Soil texture is the main factor controlling solute transport,and the concentration of microplastics has different effects on the solute transport process in different soil textures.Both anions and cations have faster transport processes in aeolian sandy soil than loess soil,which is mainly influenced by soil texture;in aeolian sandy soil,the effect of lower concentration of microplastics on the breakthrough curve is less obvious,while when the concentration of microplastics is greater than 1%,the promotion effect of microplastics on solute transport gradually appears,and the higher the concentration,the more obvious the promotion effect;in loess soil,when the added concentration of microplastics is 0.5%and 2%,the effect of microplastics on solute transport is more obvious.The effect of microplastics on the solute transport process was not obvious when the added concentration of microplastics was 0.5%and 2%,respectively,while the effect on the solute transport process was more obvious when the added concentration of microplastics was 1%and 5%.（3）After adding microplastics,the Two-Region model（TR）can fit the Cl^-breakthrough curve better than the equilibrium CDE model,and the fitting results obtained by both models are similar.The breakthrough of Cl^-can be considered as a physical equilibrium process.In the aeolian sandy soil column,48-58μm microplastics had less influence on each fitted value.When 100-150μm microplastics were added at 2%,the dispersionλdecreased by 17%and became 0.83 times of the original one.Large size of 600-1000μm and 1-2 mm microplastics could increaseλto 1.02-4.93 and 1.21-2.07 times of the distant one,respectively.In the loess soil column,the degree ofλincrease was 48-58μm>1-2 mm>600-1000μm>100-150μm in descending order.（4）The Two-Site model（TS）fitted the Ca²⁺breakthrough curve well and the breakthrough of Ca²⁺can be considered as a non-equilibrium process.1%and 2%microplastic addition concentrations did not significantly affect the parameters of Ca²⁺in the aeolian sandy soil.The addition of microplastics resulted in a smaller retardation factor R in the Ca²⁺transport process.Among the added concentrations set in the experiment,the 2%added concentration resulted in the smallest R value,which was 0.85-0.92 times of the original one.However,there was no significant correlation between R and added particle size in each added particle size.The main reasons for the decrease of the retardation factor R:microplastics increased the average pore flow rate of the sandy soil;microplastics replaced the original soil,which reduced the adsorption sites.The presence of microplastics in loess soil can change R to 0.91-1.87 times of the original one.48-58μm microplastics have a small effect on R,and at 0.5%,they have no significant effect on the parameters.100-150μm microplastics can increase R significantly at lower additions（0.5%and 1%）.600-1000μm microplastics can increase R at all levels.R increases while 1-2 mm microplastics are divided by the addition concentration of 1%,less than 1%R will become smaller and more than 1%R will increase.