Water Research 137(2018)362374Contents listsavailableatScienceDirectKESTERCHWater Researchjournalhomepage:www.elsevier.com/locate/watresELSEVIERReviewMicroplastics in freshwater systems: A review on occurrence真environmentaleffects,andmethodsformicroplasticsdetectionChodhiorJingyi Li, Huihui Liu, J. Paul ChenDepartmentofCivilandEvironmentalEngineering,NationalUniversityofSingapore0KentRidgeCrescent,Singapore,119260,SingaporARTICLEEINFOABSTRACTArticle history:The continuous increase in synthetic plastic production and poor management in plastic waste have ledReceived 6 November 2017to a tremendous increase in the dumping into our aqueous environment. Consequently.microplasticsAccepted22December2017commonly defined as sizes less than 5 mm are produced and stay in both seawater and freshwaterAvailable online 28 December 2017environment.Thepresence of microplastics as a new type of emerging contaminant has become a greatissue of concerns from public and government authorities. The sources of microplastics to freshwaterKeywords:systems are many with the largest portion from wastewater treatment plants. The abundance ofAnalytical methodsmicroplastics varies with the location, from above 1 million pieces per cubic meter to less than 1 piece inFreshwaters100 cubic meters.MicroplasticsMicroplastics can cause several harmful physical effects on humans and living organisms through suchOccurrencemechanisms as entanglement and ingestion. The microplastics can act as carriers of various toxins suchas additives from industrial production processes and persistent contaminants by the sorption in waters.Those toxins may causegreat healthproblems to humans.Afew studies on thefishes demonstrated thatthe microplastics and the associated toxins are bio-accumulated and cause such problems as intestinaldamage and change in metabolic profiles.In studies of microplastics, fresh water is first sampled by the nets with typical mesh size of 330 μm forcollection of microplastics. After the volume reducing process, the samples will then go through thepurification process including density separation by such inorganic salts as sodium chloride and diges-tion process by oxidizing agents or enzymes. The sequence of these two processes (namely purificationand digestion) is dependent on the sample type. The purified samples can be studied by severalanalytical methods. The commonly used methods for the qualification studies are FTIR spectroscopy.Raman spectroscopy,pyrolysis-Gc/MS, and liquid chromatography.A tagging method can be used in thequantification study. Our literature study finds that there is still no universal accepted quantification andqualification tools of microplastics in fresh waters.More work isanticipated so as to obtain accurateinformation on microplastics in freshwater, which can then be used for the better assessment of theenvironmental risk.@ 2017 Elsevier Ltd. All rights reserved.Contents3631.Introduction3642.Microplastics in freshwater systems..3642.1.Occurrence....3652.2.Environmental impacts3.Microplastics sampling methods ..3673674.Sample extraction and purification3685.Microplastics identification and quantification.+.6.371Challenges and recommendations·Corresponding author.E-mail addresses: paulchen@nus.edu.sg, jpaulchen@gatech.edu (J. Paul Chen).https://doi.org/10.1016/j.watres.2017.12.0560043-1354/0 2017 Elsevier Ltd, All rights reserved
Review Microplastics in freshwater systems: A review on occurrence, environmental effects, and methods for microplastics detection Jingyi Li, Huihui Liu, J. Paul Chen* Department of Civil and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore, 119260, Singapore article info Article history: Received 6 November 2017 Accepted 22 December 2017 Available online 28 December 2017 Keywords: Analytical methods Freshwaters Microplastics Occurrence abstract The continuous increase in synthetic plastic production and poor management in plastic waste have led to a tremendous increase in the dumping into our aqueous environment. Consequently, microplastics commonly defined as sizes less than 5 mm are produced and stay in both seawater and freshwater environment. The presence of microplastics as a new type of emerging contaminant has become a great issue of concerns from public and government authorities. The sources of microplastics to freshwater systems are many with the largest portion from wastewater treatment plants. The abundance of microplastics varies with the location, from above 1 million pieces per cubic meter to less than 1 piece in 100 cubic meters. Microplastics can cause several harmful physical effects on humans and living organisms through such mechanisms as entanglement and ingestion. The microplastics can act as carriers of various toxins such as additives from industrial production processes and persistent contaminants by the sorption in waters. Those toxins may cause great health problems to humans. A few studies on the fishes demonstrated that the microplastics and the associated toxins are bio-accumulated and cause such problems as intestinal damage and change in metabolic profiles. In studies of microplastics, fresh water is first sampled by the nets with typical mesh size of 330 mm for collection of microplastics. After the volume reducing process, the samples will then go through the purification process including density separation by such inorganic salts as sodium chloride and digestion process by oxidizing agents or enzymes. The sequence of these two processes (namely purification and digestion) is dependent on the sample type. The purified samples can be studied by several analytical methods. The commonly used methods for the qualification studies are FTIR spectroscopy, Raman spectroscopy, pyrolysis-GC/MS, and liquid chromatography. A tagging method can be used in the quantification study. Our literature study finds that there is still no universal accepted quantification and qualification tools of microplastics in fresh waters. More work is anticipated so as to obtain accurate information on microplastics in freshwater, which can then be used for the better assessment of the environmental risk. © 2017 Elsevier Ltd. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363 2. Microplastics in freshwater systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364 2.1. Occurrence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364 2.2. Environmental impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365 3. Microplastics sampling methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367 4. Sample extraction and purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367 5. Microplastics identification and quantification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368 6. Challenges and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371 * Corresponding author. E-mail addresses: paulchen@nus.edu.sg, jpaulchen@gatech.edu (J. Paul Chen). Contents lists available at ScienceDirect Water Research journal homepage: www.elsevier.com/locate/watres https://doi.org/10.1016/j.watres.2017.12.056 0043-1354/© 2017 Elsevier Ltd. All rights reserved. Water Research 137 (2018) 362e374
363J.Lietal. /WaterResearch137(2018)362-374371Conclusions7.Acknowledgements.371References.3711.Introductionhorizontalperspective,microplastics were reportedly found intropical areas (Ng and Obbard,2006;Norand Obbard,2014);theyPlastic products have such outstanding features as light-weight,were even seen in thepolar waters of Antarctica and Arctic(Barnesbeing durable and versatile, and production with low costet al.,2010: Bergmann et al.,2015).When one looks at vertical(Hammer et al.,2012; Ivleva et al.,2017).However,plasticdebrisdistribution,microplasticsexistinbenthiczoneofwater bodies,hasraisedglobalconcernsoveritswidedistributionandassociatedwatercolumns,surfacewatersandbeaches.Somereportshaveenvironmental consequences.The annual global production ofshown the concentrations in surface water vary from 10-5to105plastic product in 2016 alone was around 322 million tonnespieces/m3(LiebezeitandDubaish,2012:Desforges etal.,2014:Friasetal.,2014:Limaetal,2014;Auta etal.,2017)and 40to400pieces/(Europe,2016).Anestimationofupto10%ofplasticfragmentswould end up in marine environment as per suggested by Cole et al.Lin sediments (Zurcher,2009; Browne etal.,2011:Antunes et al.2013:Frias et al.,2014;Norand Obbard,2014).Mostrecently.(2o11),duetoextensiveusageandincreasingproductioninplasticproduct, and poor management (Rochman, 2015) While dimin-China Central Television website (cctv.com) reported a group ofishing aesthetic value of water environment, plastic debris is likelyChinese scientists have discovered the presence of microplastics into pose threats to public health and cause biodiversity lossAntarcticwaters(China.org.cn,2018).Furthermore,the distribution(Thompsonetal.2009:Gall andThompson,2015).shows clear geographical variations (Fossi et al.,2012;CollignonMicroplastics are widely defined as synthetic polymers with anetal.,2014:deLuciaetal,2014:Desforgesetal,2014)upper size limit of 5mm andwithout specifiedlower limitThefactors affectingthedistribution include suchlarge-scaleforces as currents driven by wind and geostrophic circulation(Thompson etal.,2009).Theycanbe categorized intoprimarymicroplasticsandsecondarymicroplastics.(Law et al.,2010),turbulence and oceanographic effects (BallentThe definition of primary microplastics is the microplastics,et al, 2012; Turra et al, 2014).As key factors, the inherent prop-which are originally manufactured to have a size less than 5 mmerties of microplastics such as density.shape and size of micro-and mainly found in textiles, medicines and such personal careplastics can affecttransportation and distribution patterns (Eerkes-productsasfacialandbodyscrubs(Coleetal.,2011:Browne2015)Medranoetal,2015)The aforementioned factors are more likely to play importantTheseprimary microplastics canbetransportedby rivers,dischargefrom water treatment plants, wind and surface run-off into eitherroles in a large freshwater environment like riverine systems;fresh water and seawater environments (Gall and Thompson,however, they become limited on smaller isolated fresh water2015)systems, where natural factors and long water residence timeSecondarymicroplastics arederived from fragmentation ordominantlyaffectquantity of microplastics(Freeetal.,2014)largeplasticdebrisduetosuchprocessesasphoto-degradation,Hence,microplasticsin open and dynamicfresh waters wouldphysical, chemical and biological interactions (Thompson et al.,eventually end up in marine environment, while microplastics in2009:Galganietal.,2013).Theoriginsofsecondarymicroplasticsisolatedandstaticwatersbodieswouldremainandaccumulateininclude fishing nets, industrial resin pellets, household items andthe waters.other discarded plastic debris (Eerkes-Medrano et al, 2015).Fresh waters may accumulate numerous microplastic particlesNotably.it was found that the majority of microplastics are sec-andfibers:however,lesseffortshavebeenmadetomonitortheondarymicroplastics(Eriksenetal.,2013)andthattheabundancemicroplasticsinfreshwatersthanthoseinseawaters.Suchfreshin waters would increase along with the increase in input of plasticwaters can be the sources (like waste water plants),transferringdebrisfrom different origins, leadingto continuous transformationmedia (like rivers) and sink (like isolated lakes) of microplastics,ofsecondarymicroplastics(Coleetal,2011).Whenmicroplasticswhichmaydifferfromthoseinseawatersbecauselargevariationsare exposed in the environment, there is a higher possibility ofin quantity can be expected (Klein et al.,2018),Meanwhile, thebreak-down of microplastics to nanoplastics that may have higherproperties of microplastics can be quite heterogeneous.Fonenvironmental risks due to the nature of nano-sizes.instance,microplastics in sewage are heavilycontaminated byMicroplastics can originate from both land- and ocean-basedorganic contents and exist as relatively large pieces; on the othersources (Hammer et al.,2o12).The ocean-based sources,due tohand in clean fresh waters are nearly free of organic contents andcommercial fishing, vessels and other activities in marine envi-hardly seen by naked eyes (Orb, 2017). In addition, some lakes orronment,onlycontribute20%of totalplasticdebris inmarinerivers with fresh water are close to the areas with high population.where higher microplastics abundance was detected (Eriksen et al.environment(Andrady.2011)Themicroplasticsfromterrestrialsourcescontributetheremaining80%.Terrestrialsourcesinclude2013).Anothersignificantcharacteristicofmicroplasticsstudiesindifferent origins that mainly are personal care products, air-blastingfreshwater systems is that the sample sizes are small. However,process,improperlydisposedplasticsandleachatesfromlandfilllargesamplingareasarenecessarytoadequatelyreducethelarge(Coleetal,2011),Onceterrestrialmicroplasticsarereleasedintovariationsdueto spatial andtemporal changes(Ryanetal,2009)the natural water systems, most of them would be transported toAs a result, we found that there was an urgent need to review theoceansbyrivers,whiletheremainingwouldresideinfreshwatercurrentresearchworkandmethodologies onmicroplasticsinfreshenvironment, including such isolated water systems as remotewaters in order that appropriate sampling, quantification andmountainlakes (Browneetal.,2010;Freeetal.,2014)identification approaches can be developed for the study in fresh-Microplasticsareofgreatpublicconcernsfortheubiquitouswater samples.presence and persistence in the aquatic environment. The globalThe objective of this review paper is to reveal the currentpresence of microplasticshas beenfound in recentyears.Fromknowledge about microplastics in fresh waters for a better
7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371 1. Introduction Plastic products have such outstanding features as light-weight, being durable and versatile, and production with low cost (Hammer et al., 2012; Ivleva et al., 2017). However, plastic debris has raised global concerns over its wide distribution and associated environmental consequences. The annual global production of plastic product in 2016 alone was around 322 million tonnes (Europe, 2016). An estimation of up to 10% of plastic fragments would end up in marine environment as per suggested by Cole et al. (2011), due to extensive usage and increasing production in plastic product, and poor management (Rochman, 2015). While diminishing aesthetic value of water environment, plastic debris is likely to pose threats to public health and cause biodiversity loss (Thompson et al., 2009; Gall and Thompson, 2015). Microplastics are widely defined as synthetic polymers with an upper size limit of 5 mm and without specified lower limit (Thompson et al., 2009). They can be categorized into primary microplastics and secondary microplastics. The definition of primary microplastics is the microplastics, which are originally manufactured to have a size less than 5 mm and mainly found in textiles, medicines and such personal care products as facial and body scrubs (Cole et al., 2011; Browne, 2015). These primary microplastics can be transported by rivers, discharge from water treatment plants, wind and surface run-off into either fresh water and seawater environments (Gall and Thompson, 2015). Secondary microplastics are derived from fragmentation or large plastic debris due to such processes as photo-degradation, physical, chemical and biological interactions (Thompson et al., 2009; Galgani et al., 2013). The origins of secondary microplastics include fishing nets, industrial resin pellets, household items and other discarded plastic debris (Eerkes-Medrano et al., 2015). Notably, it was found that the majority of microplastics are secondary microplastics (Eriksen et al., 2013) and that the abundance in waters would increase along with the increase in input of plastic debris from different origins, leading to continuous transformation of secondary microplastics (Cole et al., 2011). When microplastics are exposed in the environment, there is a higher possibility of break-down of microplastics to nanoplastics that may have higher environmental risks due to the nature of nano-sizes. Microplastics can originate from both land- and ocean-based sources (Hammer et al., 2012). The ocean-based sources, due to commercial fishing, vessels and other activities in marine environment, only contribute 20% of total plastic debris in marine environment (Andrady, 2011). The microplastics from terrestrial sources contribute the remaining 80%. Terrestrial sources include different origins that mainly are personal care products, air-blasting process, improperly disposed plastics and leachates from landfill (Cole et al., 2011). Once terrestrial microplastics are released into the natural water systems, most of them would be transported to oceans by rivers, while the remaining would reside in fresh water environment, including such isolated water systems as remote mountain lakes (Browne et al., 2010; Free et al., 2014). Microplastics are of great public concerns for the ubiquitous presence and persistence in the aquatic environment. The global presence of microplastics has been found in recent years. From horizontal perspective, microplastics were reportedly found in tropical areas (Ng and Obbard, 2006; Nor and Obbard, 2014); they were even seen in the polar waters of Antarctica and Arctic (Barnes et al., 2010; Bergmann et al., 2015). When one looks at vertical distribution, microplastics exist in benthic zone of water bodies, water columns, surface waters and beaches. Some reports have shown the concentrations in surface water vary from 105 to 105 pieces/m3 (Liebezeit and Dubaish, 2012; Desforges et al., 2014; Frias et al., 2014; Lima et al., 2014; Auta et al., 2017) and 40 to 400 pieces/ L in sediments (Zurcher, 2009; Browne et al., 2011; Antunes et al., 2013; Frias et al., 2014; Nor and Obbard, 2014). Most recently, China Central Television website (cctv.com) reported a group of Chinese scientists have discovered the presence of microplastics in Antarctic waters (China.org.cn, 2018). Furthermore, the distribution shows clear geographical variations (Fossi et al., 2012; Collignon et al., 2014; de Lucia et al., 2014; Desforges et al., 2014). The factors affecting the distribution include such large-scale forces as currents driven by wind and geostrophic circulation (Law et al., 2010), turbulence and oceanographic effects (Ballent et al., 2012; Turra et al., 2014). As key factors, the inherent properties of microplastics such as density, shape and size of microplastics can affect transportation and distribution patterns (EerkesMedrano et al., 2015). The aforementioned factors are more likely to play important roles in a large freshwater environment like riverine systems; however, they become limited on smaller isolated fresh water systems, where natural factors and long water residence time dominantly affect quantity of microplastics (Free et al., 2014). Hence, microplastics in open and dynamic fresh waters would eventually end up in marine environment, while microplastics in isolated and static waters bodies would remain and accumulate in the waters. Fresh waters may accumulate numerous microplastic particles and fibers; however, less efforts have been made to monitor the microplastics in fresh waters than those in seawaters. Such fresh waters can be the sources (like waste water plants), transferring media (like rivers) and sink (like isolated lakes) of microplastics, which may differ from those in seawaters because large variations in quantity can be expected (Klein et al., 2018). Meanwhile, the properties of microplastics can be quite heterogeneous. For instance, microplastics in sewage are heavily contaminated by organic contents and exist as relatively large pieces; on the other hand in clean fresh waters are nearly free of organic contents and hardly seen by naked eyes (Orb, 2017). In addition, some lakes or rivers with fresh water are close to the areas with high population, where higher microplastics abundance was detected (Eriksen et al., 2013). Another significant characteristic of microplastics studies in freshwater systems is that the sample sizes are small. However, large sampling areas are necessary to adequately reduce the large variations due to spatial and temporal changes (Ryan et al., 2009). As a result, we found that there was an urgent need to review the current research work and methodologies on microplastics in fresh waters in order that appropriate sampling, quantification and identification approaches can be developed for the study in freshwater samples. The objective of this review paper is to reveal the current knowledge about microplastics in fresh waters for a better J. Li et al. / Water Research 137 (2018) 362e374 363
364J.Lietal. / Water Research137(2018)362374understandingofmicroplasticscontaminationandpotentialrisks.abundanceinfreshwatersystemsvariedgreatlyfromalmostnoneSummaries of sampling methods and comparisons of differenttoseveralmillionpiecespercubicmeter.Thissignificantdifferencequantificationandidentificationapproachesarepresented.Severalresultsfromsuchkeyfactorsassamplinglocations,humanactiv-key challenges are discussed and suggestions are provided forities,inherent natural conditions and sampling approaches(Eerkes-Medrano et al., 2015).further research work.Many terrestrial sources contribute the microplastics. Amongthem, wastewater treatment is one of the dominant sources of2. Microplastics in freshwater systemsmicroplastics（MagnussonandNoren,2014;Talvitie,2014;EstahbanatiandFahrenfeld,2016:Murphyetal.,2016;2.1.OccurrenceDyachenkoetal.2017:Mintenigetal.,2017).Table2lists thekeyresults from the microplastics studies on several wastewaterMostofeffortsontheresearchofmicroplasticshavebeenplacedtreatmentplants(WWTPs)ThoughWWTPscanremoveupto95%onseawaterenvironment.Lessthan4%ofmicroplastics-relatedofmicroplastics(Talvitie,2014;Talvitieetal.,2017)andtertiarystudiesarereportedlyassociatedwithfreshwaters(Lambertandtreatmentcanhavea90%removalefficiencyoffineparticlesofsizeWagner,2018)Thelimitedinformation,however,revealedthatlarger than 10μm (Wardrop et al,2016),there is substantialtheabundance ofmicroplastics in freshwatersis comparableto thatamount of microplastics being discharged into natural waters viaof marine environment (Peng et al, 2017) and the distribution isWWTPs.highly heterogeneous (Klein et al.,2018).Table 1 summarizes someRochman et al. (2015) used the published data by Magnussonof therelevant studies on themicroplasticsabundance in fresh-andNoren(2014)andMartinandEizhvertina(2014)toestimatewater matrices. The mean/averaged values of microplasticsTable1Studies on microplastics contamination in natural fresh water systems.CollectionDepthSeparationMeanMaximumLocationCollectionPurificationIdentificationReferenceCut-off sizeabundancesubstrateabundance(μm)Austrian Danube,Stationary conical 5000.5m-Visualization-0.317p/m3141.7 p/m2(Lechner et al.Austriadriftnets2014)3008.93×10°p/3.9×10°p/(Manietal.Rhine river-Enzyme + H202 FTIRSieves Manta netkm?km22015)1×10° p/m2 1.87 ×105-FTIR0.7 μm glassDutch river delta andBulk water 2 L--(Leslie et al.Amsterdam canalsfiltersp/ma2017)800.130 p/m2106 p/m3Great ParisPlankton net-Visualization1.6 μm filter(Dris etal,2015)0.35m0.35 p/m2Great ParisManta Trawl33000.3m Visualization1.6 μm filter0.45 p/m3(Dris et al, 2015)300Lake Geneva Manta Trawl-.Visualization4.81×10°p/ (Alencastro,一km?2012)-1.36 ×107Three Gorges Dam,Trawl112-FTIR1.6mm8.47 ×10°p/(Zhang et al.stainless sieve km?p/km2China2015)1m4.70×10p/1.26×104Three Gorges Dam,Teflon pumpand 4830%H202Visualization + Raman 0.45 μm glass(Di and Wang.map/ma2017)Chinastainless steelmicrofiberfiltersieve1m4.14 ×10°p/ 1.02×104Yangtze EstuaryTeflon pump and 3230% H202Visualization1.2 μm(Zhao et.al, stainless steelcellulosem3p/m32014)sievenitrate fltersTeflon pump and 500.2m30% H202FTIR8.93×10°p/ (Wang etalLakes, Wuhan, China0.45 μm glassm3 stainless steelmicrofiber2017)sievefilter3330.3 m30% H202FTIR + SEM-EDS100μm6.8 × 10° p/ (Su et al., 2016)Taihu lake, China Plankton netkm2polycarbonatefilter-Bulk water 5 L-30% H2025μm2.58 ×104(Su et al., 2016)Taihu lake, ChinaFTIR + SEM-EDS-polycarbonatep/m3filter-2.03 ×10° p/ 4.44 ×10433330% H202(Free et al.Lake Hovsgol, Mongolia Manta trawlVisualizationTyler sieveskm?P/km22014)333-Lake Winnipeg. Canada Manta trawl30% H202SEM-EDS250 μm sieve1.93×10°p/7.48 ×10°p/ (Anderson et al.km2km22017)-Los Angeles river, SanHand net, Manta800,500, 333 VisualizationTyler sieves1.29×10°p/(Moore etal.4Gabriel river, Coyote trawlm32011)Creek3330.24.2 p/m332 p/m329 Great Lakes Neuston net30% H202+FebVisualization125 μm sieve(Baidwin etal)tributaries, USA0.35m2016)3332M HCISEM-EDS4.30 ×10° p/ 4.66 ×105(Eriksen et al.,Laurentian Great Lakes, Manta trawl-Tyler sievesUSAkm2p/km?2013)153-30% H202+FebRaritan River, USAPlankton netVisualizationSieves一(Estahbanati andFahrenfeld,2016)3.1×10~4-0.19 p/m2Goiana Estuary, BrazilConical plankton 300-Visualization45 μm mesh(Lima et al.)-2.6 × 103net2014)p/m3Data werestandardizedfor consistencyb Wet peroxide oxidation
understanding of microplastics contamination and potential risks. Summaries of sampling methods and comparisons of different quantification and identification approaches are presented. Several key challenges are discussed and suggestions are provided for further research work. 2. Microplastics in freshwater systems 2.1. Occurrence Most of efforts on the research of microplastics have been placed on seawater environment. Less than 4% of microplastics-related studies are reportedly associated with freshwaters (Lambert and Wagner, 2018). The limited information, however, revealed that the abundance of microplastics in freshwaters is comparable to that of marine environment (Peng et al., 2017) and the distribution is highly heterogeneous (Klein et al., 2018). Table 1 summarizes some of the relevant studies on the microplastics abundance in freshwater matrices. The mean/averaged values of microplastics abundance in fresh water systems varied greatly from almost none to several million pieces per cubic meter. This significant difference results from such key factors as sampling locations, human activities, inherent natural conditions and sampling approaches (Eerkes-Medrano et al., 2015). Many terrestrial sources contribute the microplastics. Among them, wastewater treatment is one of the dominant sources of microplastics (Magnusson and Noren, 2014; Talvitie, 2014; Estahbanati and Fahrenfeld, 2016; Murphy et al., 2016; Dyachenko et al., 2017; Mintenig et al., 2017). Table 2 lists the key results from the microplastics studies on several wastewater treatment plants (WWTPs). Though WWTPs can remove up to 95% of microplastics (Talvitie, 2014; Talvitie et al., 2017) and tertiary treatment can have a 90% removal efficiency of fine particles of size larger than 10 mm (Wardrop et al., 2016), there is substantial amount of microplastics being discharged into natural waters via WWTPs. Rochman et al. (2015) used the published data by Magnusson and Noren (2014) and Martin and Eizhvertina (2014) to estimate Table 1 Studies on microplastics contamination in natural fresh water systems. Location Collection Collection Cut-off size (mm) Depth Purification Identification Separation substrate Mean abundancea Maximum abundancea Reference Austrian Danube, Austria Stationary conical driftnets 500 0.5 m e Visualization e 0.317p/m3 141.7 p/m3 (Lechner et al., 2014) Rhine river Manta net 300 e Enzyme þ H2O2 FTIR Sieves 8.93 105 p/ km2 3.9 106 p/ km2 (Mani et al., 2015) Dutch river delta and Amsterdam canals Bulk water 2 L e ee FTIR 0.7 mm glass filters 1 105 p/m3 1.87 105 p/m3 (Leslie et al., 2017) Great Paris Plankton net 80 0.1 e0.35 m e Visualization 1.6 mm filter 30 p/m3 106 p/m3 (Dris et al., 2015) Great Paris Manta Trawl 330 0e0.3 m e Visualization 1.6 mm filter 0.35 p/m3 0.45 p/m3 (Dris et al., 2015) Lake Geneva Manta Trawl 300 e e Visualization e 4.81 104 p/ km2 e (Alencastro, 2012) Three Gorges Dam, China Trawl 112 e e FTIR 1.6 mm stainless sieve 8.47 106 p/ km2 1.36 107 p/km2 (Zhang et al., 2015) Three Gorges Dam, China Teflon pump and stainless steel sieve 48 1 m 30% H2O2 Visualization þ Raman 0.45 mm glass microfiber filter 4.70 103 p/ m3 1.26 104 p/m3 (Di and Wang, 2017) Yangtze Estuary Teflon pump and stainless steel sieve 32 1 m 30% H2O2 Visualization 1.2 mm cellulose nitrate filters 4.14 103 p/ m3 1.02 104 p/m3 (Zhao et al., 2014) Lakes, Wuhan, China Teflon pump and stainless steel sieve 50 0.2 m 30% H2O2 FTIR 0.45 mm glass microfiber filter e 8.93 103 p/ m3 (Wang et al., 2017) Taihu lake, China Plankton net 333 0.3 m 30% H2O2 FTIR þ SEM-EDS 100 mm polycarbonate filter e 6.8 106 p/ km2 (Su et al., 2016) Taihu lake, China Bulk water 5 L e e 30% H2O2 FTIR þ SEM-EDS 5 mm polycarbonate filter e 2.58 104 p/m3 (Su et al., 2016) Lake Hovsgol, Mongolia Manta trawl 333 e 30% H2O2 Visualization Tyler sieves 2.03 104 p/ km2 4.44 104 p/km2 (Free et al., 2014) Lake Winnipeg, Canada Manta trawl 333 e 30% H2O2 SEM-EDS 250 mm sieve 1.93 105 p/ km2 7.48 105 p/ km2 (Anderson et al., 2017) Los Angeles river, San Gabriel river, Coyote Creek Hand net, Manta trawl 800, 500, 333 e e Visualization Tyler sieves e 1.29 104 p/ m3 (Moore et al., 2011) 29 Great Lakes tributaries, USA Neuston net 333 0.2 e0.35 m 30% H2O2þFeb Visualization 125 mm sieve 4.2 p/m3 32 p/m3 (Baldwin et al., 2016) Laurentian Great Lakes, USA Manta trawl 333 e 2 M HCl SEM-EDS Tyler sieves 4.30 104 p/ km2 4.66 105 p/km2 (Eriksen et al., 2013) Raritan River, USA Plankton net 153 e 30% H2O2þFeb Visualization Sieves e e (Estahbanati and Fahrenfeld, 2016) Goiana Estuary, Brazil Conical plankton net 300 e e Visualization 45 mm mesh 3.1 104 -2.6 103 p/m3 0.19 p/m3 (Lima et al., 2014) a Data were standardized for consistency. b Wet peroxide oxidation 364 J. Li et al. / Water Research 137 (2018) 362e374
365J. Li et al. / Water Research 137 (2018) 362374Table 2Microplastics detected in waste water treatment plants.LocationCollectionCollection Cut-PurificationIdentification Mean abundanceMaximumReferenceabundanceoff size (μm)Efluents of 17 WWTPs, USAExtraction pump + Tyler 355, 12530 %H202Visualization 50 p/m3195 p/m3(Mason et al,2016)sievesEast Bay Municipal Utility355, 12530%H202FTIRSieves-169p/m3(Dyachenko et al.,District's wWTP, USA2017)-FTIR-Influent:(Talvitie et al.Viikinmaki WWTP, FinlandPumping through filters300, 100, 209 ×10° p/m32017)or metallic beakerEffluent:3.5 × 103 p/m365-FTIRInfluent: 1.57 × 10° p/m2WWTP, River Clyde, GlasgowSteel buckets(Murphy et al.,2016)Effluent:250 p/m2Effluents of 12 WWTPs inA custom made pumping 10Enzymes + H202 FTIR9 ×10° p/m2(Mintenig et al.)2017)Lower Saxony. GermanydeviceVisualization Influent: 1.6× 10° p/m3b300, 100, 20-WWTP, St. PetersburgA specific filter device(Talvitie, 2014)Effluent:7×103p/m3b300-FTIRLangeviksverket, Lysekil.Ruttner samplerInfluent: 1.5 × 10° p/m*:(Magnusson andSwedenEfluent: 8.25 p/m3Noren,2014)-7 wWTPs, Netherland-FTIRGlass jars (2 L)Influent: 7.3×10*p/m3Influent:(Leslie et al,Efluent: 5.2 ×10° p/m5.66 × 10°p/2017)maEffluent:9.1 × 10° p/m2--Ljubljana, SloveniaEffluent: 13.9 mg/m’ PE(Kaicikova et al.,2017)(estimated)Data was standardizedforconsistencyb Data represents the number of synthetic particles.the amount of microbeads discharged into the waterways in theThough to what extent of physical effects would affect organismsUSA They came to a conservative conclusion that 8 billionremain uncertain,entanglement effect that is often associated withmicrobeads (pieces) released from the municipal WWTPs per day.comparativelylargeanimalsisvisiblewhenwecompareitwithMason et al. (2016) studied the effluent samples of 17 WWTPs andingestion. Entanglement could cause severe impacts on aquaticpredicted that the average discharge of microbeads from USspecies; they can even be fatal by the means of drowning, suffo-municipal WWTPs was 13 billion pieces per day, similar to thatcating, strangulating or starving (Allsopp et al., 2006). The vulner-from Rochman et al. (2015). Given the fact that high volume ofable species includesea turtles,mammals,seabirds,andtreated and untreated wastewater is released globally and only 60%crustaceans(Gilardietal,2010).Whentheseanimalsdrown inof municipal wastewater is treated (Mateo-Sagastaetal,2015).aghost nets, they may suffer suffocation and starvation; when theirhuge amount of microplastics would enter the environment via thepredators appear, they are bound to die (Derraik, 2002).However, there is no report on entanglement incidence indischarges from WWTPs. In addition, other sources such as surfacerun-off, atmospheric fallout (Driset al,2016)and direct wastefreshwaterbodies.Nevertheless,theoccurrenceofentanglementindisposal contributethe increasein the microplasticsflow intomarineorganismshasprovidedaclearindicationtothesituationinaqueous environment.freshwaterenvironment.In September 2017,Orb Media,a nonprofit journalism organi-Ingestion does not directly impose fatal effects on organisms,zation, published a report that claims the presence of microplasticseven thoughit has well been observed.Thechroniceffects howeverin drinking water. This cross-border research tested 159 drinkingbecome akey issue (Wright etal.,2013a).Ryan(1987)conducted awater samples from five continents found out that 83% of themsurvey on the potential effects of plastic ingestion on domesticwere contaminated with tiny plastic debris (Orb, 2017).Sincechicks, in order to simulate the biological behavior of ingestedmicroplastics can directly enter human bodies, it may be a long-plastics on seabirds. It was found that there was a positive rela-term exposure if people drink microplastics-containing water.tionship between reduced food consumption andfeed of plastics.AThese findings would trigger public concerns over the safety ofresearch team suggested that a negative correlation between thedrinking water and food.fitness of seabirds and theingested plasticdebris (Spear et al.1995).The negative correlation was also observed on the fish(Lusher et al.,2014).The pathways of microplastics entering the2.2.Environmentalimpactstract include direct and indirect ingestion. Fish mainly ingestsmicroplastics via predation activities; the accumulation effects canThe concerns over microplastics are about the potential harmsbe observed in higher trophic levels, such as seabirds, seals and seathat can impose on organisms and humans. The environmentallions(McMahonetal.,1999;Eriksson and Burton,2003:Romeoimpactscanbecataloguedtophysical,chemical andbiologicalet al.,2015).Theconcentration factorofmicroplasticsfrom sur-impacts,as describedbelow.Thefindingsontheimpactsaremainlyrounding waters to seals was reported to be as high as 160 timesbased on marine environment, but can be used for fresh water(ErikssonandBurton,2003;Wrightetal.,2013b;Eerkes-Medranoenvironment.et al.,2015).Physical impacts mainly include entanglement and ingestionIt was found that entanglementhappenedmorefrequently thanbased on the work on the microplastics in sweater. The studyingestion.55% of marine organisms incidences are associated withconductedbyLaist(1997)hasshownover200marinespeciesentanglement; ingestion contributes to 31% of incidences (Gall andsuffered from the entanglement and ingestion of plastic debris
the amount of microbeads discharged into the waterways in the USA. They came to a conservative conclusion that 8 billion microbeads (pieces) released from the municipal WWTPs per day. Mason et al. (2016) studied the effluent samples of 17 WWTPs and predicted that the average discharge of microbeads from US municipal WWTPs was 13 billion pieces per day, similar to that from Rochman et al. (2015). Given the fact that high volume of treated and untreated wastewater is released globally and only 60% of municipal wastewater is treated (Mateo-Sagasta et al., 2015), a huge amount of microplastics would enter the environment via the discharges from WWTPs. In addition, other sources such as surface run-off, atmospheric fallout (Dris et al., 2016) and direct waste disposal contribute the increase in the microplastics flow into aqueous environment. In September 2017, Orb Media, a nonprofit journalism organization, published a report that claims the presence of microplastics in drinking water. This cross-border research tested 159 drinking water samples from five continents found out that 83% of them were contaminated with tiny plastic debris (Orb, 2017). Since microplastics can directly enter human bodies, it may be a longterm exposure if people drink microplastics-containing water. These findings would trigger public concerns over the safety of drinking water and food. 2.2. Environmental impacts The concerns over microplastics are about the potential harms that can impose on organisms and humans. The environmental impacts can be catalogued to physical, chemical and biological impacts, as described below. The findings on the impacts are mainly based on marine environment, but can be used for fresh water environment. Physical impacts mainly include entanglement and ingestion based on the work on the microplastics in sweater. The study conducted by Laist (1997) has shown over 200 marine species suffered from the entanglement and ingestion of plastic debris. Though to what extent of physical effects would affect organisms remain uncertain, entanglement effect that is often associated with comparatively large animals is visible when we compare it with ingestion. Entanglement could cause severe impacts on aquatic species; they can even be fatal by the means of drowning, suffocating, strangulating or starving (Allsopp et al., 2006). The vulnerable species include sea turtles, mammals, seabirds, and crustaceans (Gilardi et al., 2010). When these animals drown in ghost nets, they may suffer suffocation and starvation; when their predators appear, they are bound to die (Derraik, 2002). However, there is no report on entanglement incidence in freshwater bodies. Nevertheless, the occurrence of entanglement in marine organisms has provided a clear indication to the situation in freshwater environment. Ingestion does not directly impose fatal effects on organisms, even though it has well been observed. The chronic effects however become a key issue (Wright et al., 2013a). Ryan (1987) conducted a survey on the potential effects of plastic ingestion on domestic chicks, in order to simulate the biological behavior of ingested plastics on seabirds. It was found that there was a positive relationship between reduced food consumption and feed of plastics. A research team suggested that a negative correlation between the fitness of seabirds and the ingested plastic debris (Spear et al., 1995). The negative correlation was also observed on the fish (Lusher et al., 2014). The pathways of microplastics entering the tract include direct and indirect ingestion. Fish mainly ingests microplastics via predation activities; the accumulation effects can be observed in higher trophic levels, such as seabirds, seals and sea lions (McMahon et al., 1999; Eriksson and Burton, 2003; Romeo et al., 2015). The concentration factor of microplastics from surrounding waters to seals was reported to be as high as 160 times (Eriksson and Burton, 2003; Wright et al., 2013b; Eerkes-Medrano et al., 2015). It was found that entanglement happened more frequently than ingestion. 55% of marine organisms incidences are associated with entanglement; ingestion contributes to 31% of incidences (Gall and Table 2 Microplastics detected in waste water treatment plants. Location Collection Collection Cutoff size (mm) Purification Identification Mean abundancea Maximum abundancea Reference Effluents of 17 WWTPs, USA Extraction pump þ Tyler sieves 355, 125 30 %H2O2 Visualization 50 p/m3 195 p/m3 (Mason et al., 2016) East Bay Municipal Utility District's WWTP, USA Sieves 355, 125 30 %H2O2 FTIR e 169 p/m3 (Dyachenko et al., 2017) Viikinm€ aki WWTP, Finland Pumping through filters or metallic beaker 300, 100, 20 e FTIR e Influent: 9 105 p/m3 Effluent: 3.5 103 p/m3 (Talvitie et al., 2017) WWTP, River Clyde, Glasgow Steel buckets 65 e FTIR Influent: 1.57 104 p/m3 Effluent: 250 p/m3 e (Murphy et al., 2016) Effluents of 12 WWTPs in Lower Saxony, Germany A custom made pumping device 10 Enzymes þ H2O2 FTIR e 9 103 p/m3 (Mintenig et al., 2017) WWTP, St. Petersburg A specific filter device 300, 100, 20 e Visualization Influent: 1.6 105 p/m3 b Effluent:7 103 p/m3 b e (Talvitie, 2014) Långeviksverket, Lysekil, Sweden Ruttner sampler 300 e FTIR Influent: 1.5 104 p/m3 ; Effluent: 8.25 p/m3 e (Magnusson and Noren, 2014 ) 7 WWTPs, Netherland Glass jars (2 L) e e FTIR Influent: 7.3 104 p/m3 Effluent: 5.2 104 p/m3 Influent: 5.66 105 p/ m3 Effluent: 9.1 104 p/m3 (Leslie et al., 2017) Ljubljana, Slovenia e ee Effluent: 13.9 mg/m3 PE (estimated) e (Kalcíkov a et al., 2017) a Data was standardized for consistency. b Data represents the number of synthetic particles. J. Li et al. / Water Research 137 (2018) 362e374 365
366J. Li et al. / Water Research 137 (2018)362-374Thompson, 2015). Other solid matters contribute the rest (14%).microplastics can be released when microplastics are ingested andEntanglement ofmicroplasticsmainlyhappensto compara-stayinsideofhumanbody.wherepH isratherlow,temperatureistively large marine organism. On the other hand, ingestion ofcomparatively high and digestive liquids are present. A few studiesmicroplastics can befound throughout almost all the trophiclevels,confirmedthatthedesorptionrateof sorbedcontaminants in or-such as zooplankton taxa (Cole et al, 2013), marine lugwormganisms was accelerated, substantially faster than that in seawater(Wright et al..2013a).mussel (von Moos et al.,2012),oyster(Teuten etal.,2007:Bakir etal,2014).(Sussarellu et al., 2016).fish (Rochman et al.,2013). sea turtlesHowever, very limited information is available about the real(Bugoni et al, 2001),dolphins (Denuncio et al, 2011), whalessorption behavior for water-borne pollutants in fresh water on the(Walkerand Coe,1989)and seabirds (Derraik,2002)microplastics. The toxicity of microplastics in fresh waters is notThe chemical and biological impacts play key roles. Afterwell understood.ingestion, microplastics cause toxicity effects to humans and livingAfewlab-scalestudieshaveprovidedsomeimplicationsontheorganisms through several pathways and mechanisms.The poly-potential biological hazards from microplastics. A research con-meric compounds used in production of plastics, the additives suchductedbyMaetal.(2016)selectedDaphniamagnaasamodelfreshas coper ions used during production of plastics are toxic. Morewaterorganismtostudythetoxicityofmicroplasticstogetherwithimportantly,various toxins in waters that are initially sorbed ontosorbedphenanthrene.Theyconcludedthatnano-sizedPSexhibitmicroplastics may subsequently be desorbed inside of human andhigh toxicity and physical damage to Daphnia magna and toxicityanimal bodies.was enhanced by sorbed phenanthrene.Microplastics are made of polymeric compounds that can causeA recent study demonstrated that the mixture of antimicrobialflorfenicol and microplastics caused higher inhibition level ofcertain health effects.For example,polystyrene (PS),resistant tobiodegradation can accumulate in the stomach of fish (Carpentercholinesterase activity on freshwater bivalve Corbicula flunineaet al.,1972)and can translocate through blood circulation (Chenthan thatofflorfenicol or microplastics(Guilhermino et al.,2018).Itet al.,2006).For oysters after being experienced a two-monthwas found that microplastics caused certain levels of biologicalexposure to Ps microplastics, the decreases in oocyte number,effects on Corbicula flumineaand and their predator Acipenserdiameter and sperm velocity were reported; reproductive disrup-transmontanus(Rochmanetal..2017)A few studies on larval and adult zebrafish showed thattion for marine filter feeders was expected (Sussarellu et al.,2016).Avarietyofadditivesareaddedduringplasticsproduction,tomicroplasticswerefirstingested,becameaccumulated,andimprove physical properties, such as color, flame resistance, andconsequently caused alternations in locomotion, intestinal damage,hardness.They can be low molecular or polymeric,inorganic orand changeinmetabolicprofiles(Luetal,2016:Chen etal.,2017:Sleight et al., 2017: Lei et al., 2018). With regards to other freshorganic substances. The most common additive is plasticizer that isfor improvement of plasticityor viscosity.For example,polyvinylwater organisms, the toxicity studies on marine organisms maychloride(PvC)musthaveplasticizerslikephthalatesandbisphenolprovide suggestive information.Hence,itisof importancetoA in order that thermal and photo-degradation can be minimizedinvestigate the interaction between microplastics and key com-(Hammeretal.,20i2).Otheradditivesincludecolorantsandflamepounds in lab-scale experiments, which will be helpful in the riskretardants. It is anticipated that these chemicals would accumulateassessmentofmicroplasticsin humanbodies throughbioaccumulation process,some ofwhichThe biological effects include the potential to geographicallyare well known as endocrine disrupting compounds. Some studiestransfer microorganisms(Oberbeckmann et al.,2015)Microor-confirmed that such additives as bisphenol A, polybrominatedganisms can quickly colonize the surface ofmicroplastics and bediphenyl ethers, tetrabromobisphenol A and phthalates are presenttransported with the movement of microplastics, as plastics areinhumans (Talsnessetal.,2009)usuallydurableand persistentthan othermedia(Thiel and Gutow,In addition, some compounds with heavy metals such as chro-2005).While this interaction iscommonlyknown and possiblemium, cadmium and lead are often used in production of colorants,consequences are raised, such as the introduction of pathogens to astabilizers and plasticizers(Ernst et al.,2000;Murphy,2001).Theyclean environment,limited literatures are available to reveal thecan be released from plastic debris into water systems and furtherdiverse biofilms communities, and even less for freshwater envi-enter the food chain to cause bioaccumulation of toxins inronment.(McCormick et al.,2014)conducted high-throughputorganismssequencinganalysis on themicroplasticscollected in an urbanMicroplastics can be a vector for water-borne hydrophobicriver in Chicago, llinois. They found out some attached taxa werepollutants (Teuten et al.,2009;Lee etal.,2014:Bakiret al.,2016:plastic decomposing organisms and pathogens: the findings sug-O'Connoretal.,2016;Ziccardi etal,2016).Examplesincludepol-gest that microplastics can transport bacterial assemblages inychlorinated biphenyls (PCBs)(Velzeboer et al.,2014).andfreshwater ecosystems. Their study also emphasized that patho-genic wastewater-associated organisms could be discharged intodichlorodiphenyltrichloroethane(DDT)(Rios etal.,2010).whichare well known for their high toxicity and persistence in thewaterways by means of the microplastics with the attachment ofenvironment. Due to the large specific surface areas and intrinsicthe organisms. A study on microplastics-associated bacteria inhydrophobicity, the potential of hydrophobic chemical adsorptionYangtze Estuaryalso confirmed thepresence ofpotential pathogensontothesurfaceofmicroplasticshascausedgreatconcernsoveronmicroplastics(liangetal,2018)microplastics (Horton et al., 2017)Another biological effect is the change in the plastic physicalPCBs are well known to be carcinogenic, mutagenic, and/orpropertiesbecauseofbiofilmsthatattachontosurfaceofmicro-2012)DDT can lead to adverseteratogenic (Hammer etplastics.Firstly.thedensity ofmicroplasticscanbeincreased (Car)aneurological effectsandimmunodeficiency(Mansourietal.,2017).etal.,2016),enablinglightmicroplasticstosinkinthewatercol-The partition studies show the large partitioning coefficients forumnandbenthiczones.Furthermore,thebiofilmscanaltertheorganic compounds, in the range of 10410° (Andrady, 2011).A fewsurface nature of microplastics and make the surface less hydro-studies suggested that microplastics could sorb high amounts ofphobic (Lobelle and Cunliffe,2011:Zettler et al,2013).The findingsPCBs form the surrounding seawater in coastal areas of USA andmay provide some insights on studies associated with sorption ofJapan (Hammer et al,2012).Microplastics can sorb lubrication oilspersistent organic pollutants. Whether the attached biofilms canand heavy metals (Angiolillo et al.,2015;Hu et al.,2017).enhance or weakentheinteractionsbetweenmicroplasticsandTheaforementionedpollutantsinitiallysorbedonthesurfaceofwater-borne pollutants remains unknown to us
Thompson, 2015). Other solid matters contribute the rest (14%). Entanglement of microplastics mainly happens to comparatively large marine organism. On the other hand, ingestion of microplastics can be found throughout almost all the trophic levels, such as zooplankton taxa (Cole et al., 2013), marine lugworm (Wright et al., 2013a), mussel (von Moos et al., 2012), oyster (Sussarellu et al., 2016), fish (Rochman et al., 2013), sea turtles (Bugoni et al., 2001), dolphins (Denuncio et al., 2011), whales (Walker and Coe, 1989) and seabirds (Derraik, 2002). The chemical and biological impacts play key roles. After ingestion, microplastics cause toxicity effects to humans and living organisms through several pathways and mechanisms. The polymeric compounds used in production of plastics, the additives such as coper ions used during production of plastics are toxic. More importantly, various toxins in waters that are initially sorbed onto microplastics may subsequently be desorbed inside of human and animal bodies. Microplastics are made of polymeric compounds that can cause certain health effects. For example, polystyrene (PS), resistant to biodegradation can accumulate in the stomach of fish (Carpenter et al., 1972) and can translocate through blood circulation (Chen et al., 2006). For oysters after being experienced a two-month exposure to PS microplastics, the decreases in oocyte number, diameter and sperm velocity were reported; reproductive disruption for marine filter feeders was expected (Sussarellu et al., 2016). A variety of additives are added during plastics production, to improve physical properties, such as color, flame resistance, and hardness. They can be low molecular or polymeric, inorganic or organic substances. The most common additive is plasticizer that is for improvement of plasticity or viscosity. For example, polyvinyl chloride (PVC) must have plasticizers like phthalates and bisphenol A in order that thermal and photo-degradation can be minimized (Hammer et al., 2012). Other additives include colorants and flame retardants. It is anticipated that these chemicals would accumulate in human bodies through bioaccumulation process, some of which are well known as endocrine disrupting compounds. Some studies confirmed that such additives as bisphenol A, polybrominated diphenyl ethers, tetrabromobisphenol A and phthalates are present in humans (Talsness et al., 2009). In addition, some compounds with heavy metals such as chromium, cadmium and lead are often used in production of colorants, stabilizers and plasticizers (Ernst et al., 2000; Murphy, 2001). They can be released from plastic debris into water systems and further enter the food chain to cause bioaccumulation of toxins in organisms. Microplastics can be a vector for water-borne hydrophobic pollutants (Teuten et al., 2009; Lee et al., 2014; Bakir et al., 2016; O'Connor et al., 2016; Ziccardi et al., 2016). Examples include polychlorinated biphenyls (PCBs) (Velzeboer et al., 2014), and dichlorodiphenyltrichloroethane (DDT) (Rios et al., 2010), which are well known for their high toxicity and persistence in the environment. Due to the large specific surface areas and intrinsic hydrophobicity, the potential of hydrophobic chemical adsorption onto the surface of microplastics has caused great concerns over microplastics (Horton et al., 2017). PCBs are well known to be carcinogenic, mutagenic, and/or teratogenic (Hammer et al., 2012). DDT can lead to adverse neurological effects and immunodeficiency (Mansouri et al., 2017). The partition studies show the large partitioning coefficients for organic compounds, in the range of 104 e106 (Andrady, 2011). A few studies suggested that microplastics could sorb high amounts of PCBs form the surrounding seawater in coastal areas of USA and Japan (Hammer et al., 2012). Microplastics can sorb lubrication oils and heavy metals (Angiolillo et al., 2015; Hu et al., 2017). The aforementioned pollutants initially sorbed on the surface of microplastics can be released when microplastics are ingested and stay inside of human body, where pH is rather low, temperature is comparatively high and digestive liquids are present. A few studies confirmed that the desorption rate of sorbed contaminants in organisms was accelerated, substantially faster than that in seawater (Teuten et al., 2007; Bakir et al., 2014). However, very limited information is available about the real sorption behavior for water-borne pollutants in fresh water on the microplastics. The toxicity of microplastics in fresh waters is not well understood. A few lab-scale studies have provided some implications on the potential biological hazards from microplastics. A research conducted by Ma et al. (2016) selected Daphnia magna as a model fresh water organism to study the toxicity of microplastics together with sorbed phenanthrene. They concluded that nano-sized PS exhibit high toxicity and physical damage to Daphnia magna and toxicity was enhanced by sorbed phenanthrene. A recent study demonstrated that the mixture of antimicrobial florfenicol and microplastics caused higher inhibition level of cholinesterase activity on freshwater bivalve Corbicula fluninea than that of florfenicol or microplastics (Guilhermino et al., 2018). It was found that microplastics caused certain levels of biological effects on Corbicula flumineaand and their predator Acipenser transmontanus (Rochman et al., 2017). A few studies on larval and adult zebrafish showed that microplastics were first ingested, became accumulated, and consequently caused alternations in locomotion, intestinal damage, and change in metabolic profiles (Lu et al., 2016; Chen et al., 2017; Sleight et al., 2017; Lei et al., 2018). With regards to other fresh water organisms, the toxicity studies on marine organisms may provide suggestive information. Hence, it is of importance to investigate the interaction between microplastics and key compounds in lab-scale experiments, which will be helpful in the risk assessment of microplastics. The biological effects include the potential to geographically transfer microorganisms (Oberbeckmann et al., 2015). Microorganisms can quickly colonize the surface of microplastics and be transported with the movement of microplastics, as plastics are usually durable and persistent than other media (Thiel and Gutow, 2005). While this interaction is commonly known and possible consequences are raised, such as the introduction of pathogens to a clean environment, limited literatures are available to reveal the diverse biofilms communities, and even less for freshwater environment. (McCormick et al., 2014) conducted high-throughput sequencing analysis on the microplastics collected in an urban river in Chicago, Illinois. They found out some attached taxa were plastic decomposing organisms and pathogens; the findings suggest that microplastics can transport bacterial assemblages in freshwater ecosystems. Their study also emphasized that pathogenic wastewater-associated organisms could be discharged into waterways by means of the microplastics with the attachment of the organisms. A study on microplastics-associated bacteria in Yangtze Estuary also confirmed the presence of potential pathogens on microplastics (Jiang et al., 2018). Another biological effect is the change in the plastic physical properties because of biofilms that attach onto surface of microplastics. Firstly, the density of microplastics can be increased (Carr et al., 2016), enabling light microplastics to sink in the water column and benthic zones. Furthermore, the biofilms can alter the surface nature of microplastics and make the surface less hydrophobic (Lobelle and Cunliffe, 2011; Zettler et al., 2013). The findings may provide some insights on studies associated with sorption of persistent organic pollutants. Whether the attached biofilms can enhance or weaken the interactions between microplastics and water-borne pollutants remains unknown to us. 366 J. Li et al. / Water Research 137 (2018) 362e374