Study On The Effect Of Typical Surfactants On The Transport Characteristics Of Microplastics In Paddy Soil

Microplastics possess strong hydrophobicity,small size,and large specific surface area.Once they enter the soil,they can alter the soil structure and physicochemical properties,posing risks to crop production and ultimately impacting human health.Although there have been numerous studies on the distribution and hazards of microplastics in the environment,research on their transport in agricultural soils remains limited.Therefore,this study focuses on paddy soil as the filling medium and employs spherical polystyrene microplastic particles as the research subject.The main influencing factors investigated include anionic and cationic surfactants（sodium dodecyl benzene sulfonate（SDBS）and cetyltrimethylammonium bromide（CTAB）,respectively）to analyze the effects of surfactant concentration（0,50,200 mg/L）,ion strength（IS=0,1,5,10 m M）,cation type(Na⁺,Ca²⁺),moisture content（saturated,unsaturated）,and flow rate（0.5,1.0,1.5 m L/min）on the transport characteristics of microplastics in typical soil environments.The study aims to investigate the transport,transport,and deposition-release processes of microplastics under saturated and unsaturated porous media conditions.Additionally,a two-site kinetic retention model and the Derjaguin-Landau-Verwey-Overbeek（DLVO）theory are employed to explore the transport mechanisms of microplastics under the combined effects of various soil factors.The main conclusions of this paper are as follows:（1）Characterization of microplastics and soil propertiesX-ray fluorescence spectroscopy（XRF）results indicate that the soil contains a high concentration of Fe/Al oxides,which may enhance the retention of microplastics in the soil.The addition of SDBS can reduce the soil’s zeta potential and colloidal particle size,indicating that SDBS can improve the dispersion of soil colloids.On the other hand,the addition of CTAB can increase the soil’s zeta potential and colloidal particle size,suggesting that CTAB can promote the aggregation of soil colloids.The characterization results of microplastics show that SDBS can increase the interfacial contact angle and dynamic particle size of microplastics.However,when the concentration of SDBS increases from 0 mg/L to 200 mg/L,the zeta potential of microplastics decreases from-34.3 m V to-59.2 m V.After the addition of CTAB,the zeta potential of microplastics gradually transitions from negative values to positive values,and the particle size of microplastics significantly increases.（2）Influence of surfactants on the transport mechanism of microplastics under saturated conditionsAt IS=0 m M,the addition of 50 mg/L SDBS or CTAB can promote the transport of microplastics in saturated soil columns.When the concentration of SDBS increases to 200 mg/L,the enhancing effect on microplastic transport is further strengthened.However,when the CTAB concentration reaches 200 mg/L,it inhibits the transport of microplastics.At low ion strength（1 m M）,the addition of surfactants to Ca²⁺promotes the transport of microplastics,while in the presence of Na⁺,surfactants inhibit the transport of microplastics.At higher ion strengths（5,10 m M）,the addition of surfactants promotes the transport of microplastics in Na⁺solutions,while microplastics in Ca²⁺solutions show almost no transport.During the transition from saturated to unsaturated conditions,the addition of surfactants promotes the transport of microplastics in saturated soil,but inhibits their transport in the unsaturated stage.In the CTAB system,a tailing effect is observed during the transport of microplastics in the unsaturated stage.The fitting results of the two-site kinetic retention equation show that the addition of SDBS increases the retention coefficient（k₁）of the reversible kinetic site 1 in the soil and reduces the retention coefficient（k₂）of the irreversible kinetic site 2.The reversible retention sites in the soil decrease with increasing ion strength after the addition of surfactants,resulting in a decrease in the reversible retention of microplastics in the soil.After the addition of CTAB,when IS（Na⁺）increased,k₂increased,but k₁ was greater than k₂,indicating that the addition of CTAB could increase the irreversible retention site of microplastics in soil,indicating that the addition of CTAB would lead to the desorption of some deposited microplastics.DLVO calculation results indicate that the addition of SDBS increases the repulsive barrier(Φ_max)between microplastics and soil,while the addition of CTAB reduces the repulsive barrier.（3）Influence of surfactants on the transport mechanism of microplastics under unsaturated conditionsAs the ion strength and cation valence increase,the transport ability of microplastics gradually decreases.The addition of SDBS promotes the transport of microplastics,and the enhancing effect is stronger in unsaturated soil columns compared to saturated conditions.The addition of CTAB inhibits the transport of microplastics,and the inhibitory effect increases with increasing concentration.At IS=1m M,SDBS promotes the transport of microplastics,while CTAB significantly inhibits their transport,with increased inhibition at a CTAB concentration of 200 mg/L.At higher ion strengths,both surfactants significantly inhibit the transport of microplastics.Comparative experiments at different flow rates demonstrate that an increase in the flow rate of pore water in the soil column promotes the transport of microplastics.The addition of SDBS only promotes the transport of microplastics at a flow rate of 1.5m L/min.In the CTAB system,the most significant promotion of microplastic transport occurs at a flow rate of 1.5 m L/min and IS=1 m M（Na⁺）.During a continuous increase in flow rate（0.5-1.0-1.5 m L/min,1.0-1.5 m L/min）,reversibly retained microplastics in the soil are flushed out,leading to a rapid decrease after the transport peak.The fitting results of the two-site kinetic retention equation show that the addition of 50 mg/L SDBS simultaneously increases the values of k₁ and k₂ with an increase in IS(Ca²⁺).The addition of CTAB increases the value of k₂ with an increase in IS,thereby enhancing the irreversible retention of microplastics in the soil.At IS=1 m M,CTAB increases the value of k₁ and the first-order detachment rate coefficient(k_1d).DLVO calculation results indicate that compared to saturated soil columns,the retained areas in the pore space,strain force,and air-water interface of unsaturated soil columns increase,while the water-soil interface decreases,making the soil more prone to retaining microplastics.