Sorption And Transport Of Antibiotics On Micro/Nano-plastics In Aquatic Environment

Along with the widespread use of plastics around the world,the problem of plastics pollution in the environment has become more and more serious.Microplastics and nanoplastics,also called micro/nano-plastics,usually degraded from plastics or released from various application of them,and their environmental behaviors as well as toxicities towards organism in waters have aroused global concern.Microplastics(MPs)can interact with numerous organic pollutants due to their high surface areas and hydrophobicity.However,few studies focused on the interaction mechanism between organic contaminants and nanoplastics(NPs)with smaller size and more surface functionalization modification.Due to the increasing production and emissions of antibiotics,they have been recognized as one of the contaminants of emerging concerns.Besides,antibiotics are known to pose a potential threat for their negative impacts towards humans and animals for inducing antibiotic resistant genes.Since antibiotics and NPs coexist ubiquitously in waters,interactions will inevitably occur upon contact between them,which will influence the transport,fate and the risks of antibiotics.Therefore,studies on the sorption and transport of antibiotics on NPs are of great significance for evaluating their environmental fate and ecological effect.Here,the sorption behaviors of two fluoroquinolones(FQs),including norfloxacin(NOR)and levofloxacin(LEV)on polystyrene NPs(nano-PS)and carboxyl-functionalized polystyrene NPs(nano-PS-COOH)were investigated.Combined with FTIR of NPs before and after sorption,the properties of NPs and FQs,the sorption mechanisms were discussed.Besides,the effects of pH,salinity and dissolved organic matter(DOM)on the sorption behaviors of FQs onto NPs were explored,then the possible mechanisms were proposed.Moreover,the transport and retention behaviors of NPs alone and co-transport with NOR of NPs under different salinity conditions in porous media were studied,then the transport mechanisms of NPs were proposed.Based on the sorption behaviors between NOR and NPs,the co-transport of the two pollutants was further analyzed to understand the effect of co-exist contaminants on NOR or NPs transport alone.The main conclusions are as below:(1)The two FQs sorption isotherms were nonlinear and well fitted by Langmuir model.The sorption capacities of NOR and LEV on nano-PS-COOH were more than 54.9%and 69.6%higher than that of nano-PS,and their physical interactions,including polar interaction,electrostatic interaction and hydrogen bonding may be the dominant mechanisms.(2)Moreover,the increase of pH firstly increased the sorption of two FQs on NPs and then decreased because NOR and LEV had a reverse charge at different pH values.Salinity inhibited the sorption process,which may because salinity could neutralize the surface charge of NPs to decrease the electrostatic attraction with FQs,facilitate the aggregation and produce salting-in effects of FQs.Besides,DOM such as humic acid inhibited the FQs sorption process on NPs could be ascribed to the much stronger interaction between HA and FQs compared with NPs.(3)With the salinity increased from 0.035‰ to 35‰,the mass percentage recovered from the effluent(Meff)of Nile-red-dyed PS(NR-PS)in quartz sand was decreased from 87.9%to 1.1%,which may because salinity could induce aggregation of NR-PS and decrease the electrostatic repulsion between NR-PS and sand.Under lower salinity(0.035-3.5‰),the vertical migration distance of NR-PS was 0-3 cm at the inlet of column,while 0-1 cm under 35‰ salinity.Under 0.035‰ or 1.75‰ salinity condition,the Meff NOR was increased by 9-12.6%when co-transported with NR-PS,but had unobvious changes under 3.5‰ salinity.The Meff of NR-PS was deceased by 20.9%(0.035‰ NaCl),25.5%(1.75‰ NaCl))and 5.4%(3.5‰ NaCl))when co-transported with NOR.The reduction of mobility of NR-PS in sand may due to the inhibition of electrostatic repulsion between NR-PS and NOR by sorption.