Transport Of PPCPs In Saturated Porous Media Containing Microplastics

Pharmaceuticals and personal care products(PPCPs)exist widely in the environment.They have the characteristics of high output and wide range of use.Microplastics are also a new kind of pollutants,they have the features of a wide variety,small particle size,large specific surface area,which lead to strong adsorption in the environment.In this paper,tetracycline and triclosan are selected as the research objects,and five types of pristine and aged microplastics are fully mixed with quartz sand at 5%mass fraction to build up a quartz sand-microplastics porous media.The five types of microplastics are polystyrene(PS)、high-density polyethylene(HDPE)、low-density polyethylene(LDPE)、polypropylene(PP)and polyvinyl chloride(PVC).Based on the saturated soil column migration experiment and the soil physical Hydrus model,the transport of tetracycline and triclosan in saturated porous media(quartz sand-microplastics mixture)were analyzed in different solution pH,different types of microplastics,and aged microplastics.According to the experiment and simulation results,the migration parameters reveal the laws of PPCPs transport in saturated porous media and the process mechanism,which provide a theoretical basis for evaluating the environmental risk of PPCPs in the soil environment under the different conditions of microplastics pollution.The model selects the one-dimensional convection dispersion equation and two-site chemical non-equilibrium adsorption model to fit the experimental results,so as to analyze the migration mechanism of tetracycline and triclosan under different experimental conditions.The research results show that the breakthrough rate of TC under neutral conditions is lower,while TCS has almost no breakthrough under acidic conditions,and the breakthrough rate is higher under neutral and alkaline conditions.This is due to the different dissociation forms of TC and TCS under different pH conditions.TC is a zwitterion under neutral conditions,the presence of hydrophobic force,electrostatic force,hydrogen bond,π-π bond,and other complex effects enhances the adsorption capacity,while the transport of TC is inhibited to the greatest extent.However,TCS mainly exists in the form of molecules under acidic conditions.At this time,the electrostatic repulsion is weaker,so the adsorption affinity is the largest.Different types of microplastics have different effects on migration of TC.Under neutral conditions,TC has the lowest breakthrough rate in HDPE and LDPE systems.The breakthrough rate of TC in the presence of LDPE is higher than that in the HDPE system.Compared with HDPE,the kinetic site of adsorption rate is faster when LDPE exists.The results of the addition of PS,PP and PVC are similar to the PE system.They have a strong adsorption and retention effect on TC under neutral conditions,and weaker under acidic and alkaline conditions.Under neutral conditions,the breakthrough rate of TC in the PP system is the largest,and the PVC system is the smallest.The greater electrostatic repulsion weakens the adsorption of PS to TC,so the breakthrough rate of TC in the PS system is slightly higher than that in the PVC system.After aging,the reaction mechanism becomes more complicated.The retardation effect of two different density PE microplastics on TC migration increases after aging,while the adsorption of TC to PS,PP and PVC weakens after aging.After aged,the surface roughness、BET surface area of the microplastics、the negative Zeta potential increase,and the formation of oxygen-containing functional groups enhances the retardation effect of the composite system on TC transport.However,the aging PS,PP and PVC systems accelerate the adsorption rate of the kinetic adsorption sites,showing the opposite trend to the PE system.Among the types of microplastics considered in this study,PS has the highest adsorption affinity for TCS,which is mainly due to the strong polarity of PS and the presence of π-π bonds to enhance the adsorption affinity.The breakthrough rate of TCS in the PP system is greater than that of the PS,PVC,LDPE system,and the trend is more obvious after aging.Aging PS has a stronger inhibitory effect on the migration of TCS.Because of the increase of the distribution coefficient and the increase of the kinetic adsorption rate,the retention of PS on the migration of TCS is enhanced.Because the present research has not yet discussed the impact of the compound action of PPCPs and microplastics in the environment on migration,this research supplements the research in this direction and provides scientific arguments and theoretical supports for the reasonable prediction of the pollution of microplastics and PPCPs coexisting in the environment.