Study On The Transport Mechanism Of Aging Microplastics In Porous Media

Microplastics,as a highly mobile persistent pollutant,have attracted extensive attention of scholars because of their small size,large abundance and strong hydrophobicity.So far,most studies have focused on the source,distribution and potential toxicity of microplastics in the environment,while few studies have focused on the migration and deposition release behavior of microplastics in soil,and most studies have focused on the migration behavior of unmodified original microplastics in saturated porous media.When microplastics enter the natural environment,they will undergo different aging processes,such as ultraviolet light illumination,thermal radiation and chemical oxidation,etc.These aging processes will change the physical and chemical characteristics of microplastics and affect their migration process in the soil environment.Therefore,in this text,the aging method of hydrogen peroxide was used to prepare polystyrene aging microplastics,and the physicochemical properties of the original microplastic and aged microplastic were characterized by the scanning electron microscopy,Fourier transform infrared spectroscopy,nano particle size and zeta potentiometer.Then,taking the original and aged microplastics as the research object,the migration rules of the two kinds of microplastics were investigated under three typical environmental factors including ionic strength(IS=10,25 m M Na⁺;1,10m M Ca²⁺),cation type(Na⁺,Ca²⁺)and humic acid（5,15 mg/L）in saturated or unsaturated states.Combined with the two-point dynamic retention model and DLVO theory,the migration mechanism of aged microplastics was explored under different soil compound factors.The main conclusions are as follows:（1）Preparation and characterization of aging microplasticsIn this experiment,polystyrene microplastics were used as raw materials to prepare the aged microplastics by hydrogen peroxide aging method.The original microplastics and aged microplastics were characterized by scanning electron microscope,Fourier transform infrared spectroscopy,nano particle size and zeta potentiometer.Compared with the original microplastics,the surface roughness of the aged microplastics was higher,the abundance of oxygen-containing functional groups was enhanced,and the aged microplastics had a more negative zeta potential.The results of particle size distribution showed that the hydrodynamic diameter of the original and aged microplastics increased respectively to 3329 nm and 2225 nm in the presence of humic acid（15 mg/L）and IS(10 m M Ca²⁺),while the diameter of the two kinds of microplastics were about 800 nm under other experimental conditions.（2）Transport mechanism of aging microplastics in saturated porous mediaIn the saturated porous media filled column,the outflow rate of the two kinds of microplastics decreased but the retention rate increased with the increase of IS.Compared with the original microplastics,the IS has a lightly inhibition on the migration ability of aged microplastics,since the surface of aged microplastics presents more negative zeta potential,for which the hydromechanical diameter was lower than that of the original microplastics.With the strain effect of filling medium reduced,the migration ability of aging microplastics was enhancing.The penetration curves of microplastics were obviously diverse with different cation types.Under the same IS,the outflow rate of univalent cation（Na⁺）were obviously higher than that of divalent cation(Ca²⁺).In addition,the outflow rates of the two migrations increased with the increase of humic acid concentration in Na⁺solution,while the opposite results were observed in Ca²⁺solution.For example,when the IS=10 m M and the concentration of humic acid is 0,5 and 15 mg/L,the migration rates of aged microplastics in Na⁺solution was 66.78%,72.36%and 78.18%,respectively.In Ca²⁺solution,the migration rates of aged microplastics were 50.43%,29.68%and3.43%,respectively.The vertical distribution results showed that most of the microplastics were distributed at the top of the packed column（0-4 cm）at a high divalent IS(10 m M Ca²⁺),especially at a humic acid concentration of 15 mg/L.In addition,the microplastics aggregated at the entrance of the packed column and inhibited their further migration in the column.The curve fitting analysis of the migration of the aged and the original microplastic was studied by two dynamic reserve model in saturated porous packed column.With the loss of the IS and the increase of the concentration of humic acid,the power retention coefficient of enrollments in 2（k₂）of two microplastics were significantly reduced and weakened the irreversible detention of the microplastic in the filling medium under the condition of Na⁺solution.However,in the condition of Ca²⁺solution,the retention of microplastics in the filling medium is mainly controlled by the irreversible adhesion of k₂,and the microplastics are easy to be blocked and trapped in quartz sand.In addition,the retention coefficient of kinetic site 1（k₁）and kinetic site 2（k₂）of aged microplastics were both lower than those of the original microplastics,indicating that the migration effect of aged microplastics was better.And fewer microplastics were trapped in the quartz sand column,which was consistent with the experimental results of packed column migration.With the decrease of IS and the increase of humic acid concentration,DLVO analysis showed that the secondary energy minimum well has a small value and limited influence on colloidal particle agglomeration.The energy barrier enhanced between the two microplastics and the filling medium,resulting in the difficult deposition of microplastics on the surface of the filling medium.In addition,the energy barrier between aged microplastics and quartz sand was significantly higher than that of the original microplastics,which further proves that aged microplastics have a higher outflow rate and lower retention rate than the original microplastics.（3）Transport mechanism of aging microplastics in unsaturated porous mediaIn the unsaturated porous media filled column,the increase of IS inhibited the migration of the original or aged microplastics,and the aged microplastics had a higher outflow rate than the original microplastics.For example,when there was no humic acid and IS was 10 and 25 m M Na⁺,the outflow rate of aged（original）microplastics was67.81%（56.20%）and 55.44%（36.21%）,respectively.The carbonyl group which formed on the surface of microplastics in the aging process enhanced the polarity of microplastics,and it increased the electrone gativity,decreased the hydrophobicity of microplastics and enhanced the stability of microplastics,resulting to improve the migration ability of aged microplastics in porous media.The inhibition effect of Ca²⁺solution was more obvious than that of Na⁺solution,and the outflow rate of microplastics was reduced,which were consistent with the research results in saturated porous media filled column.When the IS was 0 m M,the outflow rate of aged microplastics increased gradually with the increase of humic acid concentration,but the increase was not significant.When the humic acid concentration was 0,5 or 15 mg/L,the outflow rate of aged microplastics was 92.02%,94.47%and95.25%,respectively.In addition,the inhibition effect of IS on migration of aged microplastics was lower than that of original microplastics under the same chemical conditions,which was related to the hydrodynamic diameter of aged microplastics.Under the condition of IS of 10 m M Ca²⁺and humic acid concentration of 15 mg/L,the mobility of the microplastics was the lowest,and the mobility of the original and aged microplastics was 1.42%and 11.59%,respectively,which could be verified by zeta potential characterization results.Because in this solution condition,the zeta potential of quartz sand,original and aging microplastics was-10.46 m V,-4.37 m V and-8.04 m V,respectively,indicating that microplastics were unstable.At the same time,the result exhitied that the electrostatic repulsion between microplastics was significantly decreased and the filling medium was reduced,which promotes the deposition of microplastics.The two-point dynamic retention model had better fitting analysis effect on migration curve of microplastics in unsaturated porous media filled column.Under most experimental conditions,with the decrease of IS and the increase of humic acid concentration,k₁ gradually decreased,and k₁ was significantly greater than k₂.The k₂value of the aged microplastics was always less than the k₂ value of the original microplastics in unsaturated quartz sand.DLVO analysis showed that the repulsive force at the interface between microplastics and air decreases with the increase of ionic strength.Under the condition of the same ionic strength(IS=10 m M Na⁺/Ca²⁺),the repulsive force of the double electric layer in Ca²⁺solution IS lower than that in Na+solution,indicating that microplastics are easy to form larger aggregates and deposit in porous media,and difficult to be separated by external forces.In conclusion,IS,cation type and humic acid have significant effects on the migration of aging microplastics in porous media.Under various chemical solution conditions,the aged microplastics had a higher outflow rate than the original microplastics.Compared with the original microplastics,the oxygen-containing functional groups on the surface of the aged microplastics increased,while the hydrodynamic diameter,zeta potential and hydrophobicity decreased,thus improving the migration ability of the aged microplastics in porous media.Compared with the migration results of the original and aged microplastics in the saturated porous media filled column,which were not different in that of the unsaturated porous media filled column.Therefore,with the decrease of water content,the strain generated by the air-water-solid interface（that is the effect of capillary condensation on the increase of colloidal deposition）is not the main mechanism affecting the migration of microplastics.This study provides an important scientific basis for further understanding the aging characteristics and environmental behavior of microplastics in soil and the migration mechanism of aging microplastics in soil environment.