Polystyrene Nanoplastics-Enhanced Contaminant Transport: Role ofIrreversible Adsorption in Glassy Polymeric DomainJin Liu,†,‡ Yini Ma,‡ Dongqiang Zhu,§ Tianjiao Xia,† Yu Qi,† Yao Yao,‡ Xiaoran Guo,‡ Rong Ji,*,‡
and Wei Chen*,†
†College of Environmental Science and Engineering, Ministry of Education Key Laboratory of Pollution Processes and EnvironmentalCriteria, Tianjin Key Laboratory of Environmental Remediation and Pollution Control, Nankai University, Tianjin 300350, P. R.China‡State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, P.R. China§College of Urban and Environmental Sciences, Peking University, Beijing 100871, P. R. China
*S Supporting Information
ABSTRACT: Nanoplastics (NPs) are becoming an emergingpollutant of global concern. A potential risk is that NPs may serveas carriers to increase the spreading of coexisting contaminants. Inthis study, we examined the effects of polystyrene nanoplastics(PSNPs, 100 nm), used as a model NP, on the transport of fiveorganic contaminants of different polarity in saturated soil. Thepresence of low concentrations of PSNPs significantly enhancedthe transport of nonpolar (pyrene) and weakly polar (2,2′,4,4′-tetrabromodiphenyl ether) compounds, but had essentially noeffects on the transport of three polar compounds (bisphenol A,bisphenol F, and 4-nonylphenol). The strikingly different effects ofNPs on the transport of nonpolar/weakly polar versus polarcontaminants could not be explained with different adsorptionaffinities, but was consistent with the polarity-dependent extents ofdesorption hysteresis. Notably, desorption hysteresis was only observed for nonpolar/weakly polar contaminants, likely becausenonpolar compounds tended to adsorb in the inner matrices of glassy polymeric structure of polystyrene (resulting in physicalentrapment of adsorbates), whereas polar compounds favored surface adsorption. This hypothesis was verified with supplementaladsorption and desorption experiments of pyrene and 4-nonylphenol using a dense, glassy polystyrene polymer and a flexible,rubbery polyethylene polymer. Overall, the findings of this study underscore the potentially significant environmental implicationof NPs as contaminant carriers.
■ INTRODUCTION
The occurrence of microplastics (MPs) and nanoplastics (NPs)in the environment is becoming an increasing concern, as largequantities of these materials have been detected in environ-mental media ranging from surface waters and sediments tobeach sands and deep-sea waters all over the world.1−4 MPs areoperationally defined as plastic particles smaller than 5 mm.5,6
In the environment MPs can further break down to form NPs(with sizes less than 1 μm or 100 nm,7,8 depending on differentclassifications) through prolonged mechanical abrasion, UVradiation, and microbial activity.8,9 Moreover, NPs may also beintroduced to the natural environment from use of consumerproducts.10 It has been shown that MPs and NPs can affect themetabolism, growth, mortality, and reproduction of aquaticorganisms,11,12 in similar ways as many engineered nanoma-terials.13−15 Additionally, accumulation and persistence of MPsand NPs may eventually cause these materials to reach thelevels that can affect the functioning and biodiversity of soil.16,17
Owing to their high surface-to-volume ratio and high surfacehydrophobicity, MPs and NPs have strong adsorption affinitiesfor a range of environmental contaminants, in particular, highlyhydrophobic organic chemicals such as polychlorinatedbiphenyls, polycyclic aromatic hydrocarbons, polybrominateddiphenyl ethers, and perfluorinated surfactants.18−23 Thus,there is a growing concern on an “indirect” effect of MPs andNPs, that is, these materials may serve as carriers to enhancethe bioaccumulation of contaminants in living organ-isms,11,24−27 and may also result in the so-called “TrojanHorse” effects.28−30 Similarly, MPs and NPs may serve ascarriers for environmental contaminants in soil, facilitating thespreading of contaminants.31,32 Between MPs and NPs, the
Received: October 10, 2017Revised: January 19, 2018Accepted: February 8, 2018Published: February 8, 2018
latter will likely have stronger effects on contaminant transport.Specifically, NPs are much smaller in size, which gives them notonly greater adsorption affinities for contaminants (e.g., due tothe larger surface areas),18,27 but also high colloidal stability andmobility.33
Previous studies on nanoparticles-facilitated contaminanttransport indicate that the extent of facilitated transport ofcontaminants relies largely on the nature of nanoparticles−contaminant interaction.30,34,35 In particular, at low nano-particle (i.e., “carrier”) concentrations significant facilitatedtransport of organic contaminants requires not only strongadsorption of contaminants to the carriers, but also significantdesorption hysteresis (a collective term referring to both slowdesorption kinetics and thermodynamically irreversible adsorp-tion36−38) of contaminants from the carriers.30,39−41 Desorp-tion hysteresis can either be due to the physical entrapment ofcontaminants in the complex matrices of the carriers, or strongspecific adsorptive interactions that lead to irreversible bindingof contaminants to the carriers.35 To date, little is known abouthow the capability of NPs as contaminant carriers varies as afunction of contaminant and plastics properties. Even thoughlessons can be learned from previous work on othernanoparticles (e.g., engineered carbon nanomaterials),29,30,42,43
it is not always possible to extrapolate the specific effects ofNPs on contaminant transport from findings using othernanoparticles, considering their differences in size, shape,chemical compositions, pore structures, and aggregationproperties in aqueous solutions, to mention a few.The objective of this study was to examine the capabilities of
NPs to enhance the transport of common organic contami-nants in saturated porous media, and to reveal the predominantmechanisms responsible for NPs-facilitated transport. Poly-styrene nanoplastics (PSNPs) were selected as representativemodel NPs, because polystyrene accounts for approximately
90% of the total plastic demand and is widely found in theenvironment.27,44 Additionally, polystyrene particles have beenused as a probe MPs and NPs in many research to investigatethe detrimental effects of MPs and NPs on organisms.11,27,45,46
Five model organic compounds, including pyrene, 2,2′,4,4′-tetrabromodiphenyl ether (BDE47), bisphenol A (BPA),bisphenol F (BPF), and 4-nonylphenol (4-NP) were selectedas the test contaminants to represent organic contaminants ofvaried polarity and hydrophobicity. Additionally, pyrene andBDE47 are persistent organic contaminants, and BPA, BPF,and 4-NP are common endocrine disruptor compounds.47−49
The effects of PSNPs on the transport of the five contaminantswere examined at different PSNPs concentrations, usingcolumn transport experiments. Batch adsorption and desorp-tion experiments of the contaminants were carried out tounderstand the differential effects of PSNPs on the transport ofnonpolar/weakly polar compounds versus that of polarcompounds. Environmental implications are discussed.
■ MATERIALS AND METHODS
Materials. Fluorescent PSNPs, supplied as an aqueoussuspension of polymeric particles (1% solids by weight), werepurchased from Thermo Fisher Scientific Inc. (Fremont, CA).The average particle size, confirmed with scanning electronmicroscopy (SEM) (S-3400 N II, Hitachi, Japan) by measuringmore than 200 individual particles (see Supporting Information(SI) Figure S1), was 80.4 ± 7.9 nm. The Fourier transforminfrared (FTIR) transmission spectra of the PSNPs (SI FigureS2), obtained using a Thermo Nicolet NEXUS 870spectrometer (Thermo Nicolet Corporation, Madison, WI),confirmed that material was free of surface functional groups.Two microsized (50−100 μm) polymers, including apolystyrene and a low-density polyethylene, were obtained
Table 1. Experimental Setups and Breakthrough Results of Column Experiments
aAverage value of last three data points of respective BTCs before flushed with background solutions. C/C0_PSNPs and C/C0_cont. represent the ratioof effluent PSNPs and contaminant conentrations to their initial total concentrations in the influent.
from J&K Chemical (Beijing, China) and Sigma−Aldrich (St.Louis, MO), respectively.
14C-labeled pyrene (2.18 GBq/mmol) was purchased fromAmerican Radiolabeled Chemicals (St. Louis, MO). 14C-labeledBDE47 (2.44 GBq/mmol), BPA (0.74 GBq/mmol), BPF (2.82GBq/mmol) and 4-NP (2.78 GBq/mmol) were synthesizedusing 14C-labeled phenol as a precursor. The physicochemicalcharacteristics of the compounds are given in SI Table S1.Nonlabeled pyrene, BDE47, BPA, BPF, and 4-NP (all withpurity >99%) were purchased from Sigma−Aldrich (St. Louis,MO).Lula soil was collected from a ranch near Lula, OK, U.S.A.
The soil contained 45% sand, 36% silt, and 19% clay. Thefractional organic carbon ( fOC) value of the soil was 0.37%.The particle size distribution of the soil (SI Figure S3) wasmeasured using a laser diffraction particle size analyzer(Mastersizer 3000, Malvern, U.K.). The uniformity of the soilwas 0.65, and the coefficient of uniformity was 8.50.Column Transport Experiments. Lula soil was dry-
packed into Omnifit borosilicate glass columns (10 cm ×0.66 cm, Bio-Chem Valve Inc., Boonton, NJ) with 10-μmstainless-steel screens (Valco Instruments Inc., Houston, TX)on both ends. Each column contained approximately 3.5 g ofsoil (dry-weight) with an average length of approximately 7.0cm. The columns were operated in an upward direction usingsyringe pumps (KD Scientific, Holliston, MA). Once packed,the column was flushed at a flow rate of 3 mL/h with at least100 mL deionized water followed by 180 mL backgroundelectrolyte solution (0.5 mM NaCl). The porosity and deadvolume were determined by inverse-fitting the breakthroughcurves (BTCs) of KBr (used as a conservative tracer).The experimental protocols of the column experiments are
summarized in Table 1 and SI Table S2. To prepare theinfluents, the as-purchased PSNPs suspension was firstultrasonicated at 100 W (Vibra-Cell VCX800, Sonics &Material, Newtown, CT) for 5 min and then diluted with abackground electrolyte of 0.5 mM NaCl in amber glass vials togive the working PSNPs concentrations of 5.0−20.3 mg/L.Immediately after adding the PSNPs suspension, a stocksolution of an organic contaminant in methanol was added toeach vial to give a contaminant concentration of approximately10 μg/L. The volume percentage of methanol was kept below0.1% (v/v) to minimize cosolvent effects. The vials were sealedwith Teflon-lined screw caps and equilibrated by tumbling end-over-end at 3 rpm. The concentrations of dissolved and PSNPs-adsorbed contaminants in the influents were determined usinga negligible depletion solid-phase microextraction approach(see SI for detailed procedures). The transmission electronmicroscopy (TEM) images (JEM-2100, JEOL, Tokyo, Japan)of the working suspensions showed that PSNPs were welldispersed (SI Figure S4) and were stable during the course ofcolumn experiments, as indicated by the essentially overlappingdynamic light scattering (DLS) (ZetaSizer Nano ZS, MalvernInstruments, Worcestershire, U.K.) profiles over 14 d (SIFigure S5). The average hydrodynamic diameters of PSNPs indifferent influents were characterized with DLS, and the ζpotential values was measured using a ZetaSizer Nano ZSsystem (Malvern Instruments, Worcestershire, U.K.); thesedata are summarized in Table S2.In a typical column experiment, the influent was pumped
into the column from a 100 mL glass syringe (SGE AnalyticalScience, Victoria, Australia). After 60 pore volumes (PV), abackground electrolyte of 0.5 mM NaCl was used to flush the
columns, until contaminant concentration in the effluent wasbelow the detection limit. Effluent samples were collected every2−3 PV. Each collected sample was split into two aliquots tomeasure the concentrations of PSNPs and contaminants. Theconcentrations of PSNPs were determined using a fluorescencespectrometer (FluoroMax-4, Horiba Scientific, Edison, NJ) (SIFigure S6), based on a pre-established calibration curve ofPSNPs (SI Figure S7). (Adsorption of the five contaminantshad no effects on the fluorescence intensity of PSNPs; SI FigureS8). The contaminants were quantified by determiningradioactivity using a liquid scintillation counter (LS6500,Beckman Coulter, Fullerton, CA) (the detailed proceduresare given in the SI). In the experiments of pyrene and BDE47,selected effluent samples were taken to verify that thecontaminants in the effluents were mainly associated with thePSNPs, using a previously developed method.41 In theexperiments of BPA and BPF, the contaminants in the influentand effluent of randomly selected samples were analyzed usingreversed-phase high-performance liquid chromatography (Agi-lent HPLC Series 1100, Agilent Technologies, Germany) witha radio flow detector to confirm that no degradation of BPAand BPF occurred in the soil column (representativechromatograms are shown in SI Figure S9).
Batch Adsorption and Desorption Experiments.Adsorption experiments to PSNPs were carried out using abatch adsorption approach.50 Aliquots of 5, 10, or 20 mg/LPSNPs suspension in 0.5 mM NaCl were added to a series of20 mL amber glass vials. Then, a certain amount of acontaminant stock solution was added to each of the vials. Thevials were sealed with Teflon-lined screw caps and tumbledend-over-end at 3 rpm for 2 d (for BPA and BPF) or 14 d (forpyrene, BDE47, and 4-NP) to reach adsorption equili-brium.27,51,52 Afterward, the above-mentioned negligibledepletion solid-phase microextraction approach was used todetermine the concentrations of dissolved and PSNPs-adsorbedcontaminants. Each adsorption isotherm data point was run induplicate.In desorption experiments the suspensions in selected vials in
the adsorption isotherm experiments were each split into twoaliquots of equal volumes (approximately 10 mL) in two 20 mLamber glass vials. Then, adsorbate-free background electrolytewas added to each vial. The diluted suspensions wereequilibrated by tumbling the vials end-over-end at 3 rpm, toinitiate desorption of the adsorbate from PSNPs. The aqueous-phase concentrations of the contaminants were measured usingthe negligible depletion solid-phase microextraction approachmentioned above. The total mass of the adsorbate in each vialwas also measured. The concentrations in the adsorbed phasewere obtained based on mass balance. Two data points of eachadsorption isotherm were selected to do the desorptionexperiments. Each desorption experiment was run in duplicate.For the convenience of quantification and comparison of
desorption hysteresis, the hysteresis index (HI) was calcu-lated:53
=−
|q q
qHI T C
ed
es
es , e
(1)
where qed is adsorbed-phase concentration observed in the
desorption experiment that is in equilibrium with an aqueous-phase concentration Ce, and qe
s is the adsorbed-phaseconcentration calculated from Ce assuming that desorption isreversible. If desorption is completely reversible, then HI is
equal to 0; the higher the HI value, the greater degree ofdesorption hysteresis. The subscripts T and Ce specify constantconditions of temperature and equilibrium concentration ofsolute.The sorption isotherms of the contaminants to Lula soil, as
well as adsorption and desorption experiments of pyrene and 4-NP to and from microsized polystyrene and polyethylene, wereobtained using a batch approach developed in our previousstudy (see SI for detailed procedures).54
■ RESULTS AND DISCUSSIONNanoplastics Significantly Enhance Transport of
Nonpolar and Weakly Polar Organic Contaminants.The presence of small amount of PSNPs (5.0 to 20.0 mg/L)significantly enhanced the transport of the model nonpolarorganic contaminant, pyrene, and the weakly polar organiccontaminant, BDE47 (Figure 1). In the absence of PSNPs in
the influent, negligible breakthrough of pyrene or BDE47 wasobserved after 80 PV. However, the presence of 5.0−20.0 mg/LPSNPs in the influent resulted in significantly increasedtransport of both pyrene and BDE47. For pyrene, themaximum breakthrough (as indicated by C/C0) increased to33.9 ± 0.2% in the presence of 5.1 mg/L PSNPs, and to 61.8 ±0.2% when the concentration of PSNPs was 19.4 mg/L.Similarly, the maximum breakthrough of BDE47 reached 29.7−49.8% in the presence of 5.0−20.0 mg/L PSNPs.Nanoplastics Had Negligible Effects on Transport of
Polar Organic Contaminants. In contrast to the significanttransport enhancement effects of PSNPs on pyrene andBDE47, the presence of PSNPs (5.0−20.3 mg/L) in theinfluent had essentially no effects on the transport of the threepolar compounds, i.e., BPA, BPF, and 4-NP (Figure 2). In theabsence of PSNPs, the three compounds exhibited different
degrees of mobility: BPA was relatively mobile, with maximumbreakthrough of 86.7%; the less mobile BPF reached amaximum breakthrough of 57.0%; 4-NP exhibited the lowestmobility, with maximum breakthrough reaching only 4.6%.Interestingly, for all three compounds the BTCs in the presenceof PSNPs overlapped with the one without PSNPs, indicatingthat PSNPs played minimal roles in the transport of these threecompounds. It is particularly intriguing and counterintuitivethat PSNPs had essentially no effects on the transport of 4-NP,in that, 4-NP exhibited similar low mobility to pyrene andBDE47 and PSNPs significantly facilitated the transport ofpyrene and BDE47. Thus, the remarkable difference in thetransport-enhancement effects between 4-NP and pyrene/BDE47 indicates that different transport-enhancement mecha-nisms were in play for polar versus nonpolar/weakly polarorganic contaminants.
Differential Effects on Transport of Nonpolar/WeaklyPolar versus Polar Contaminants Cannot Be Explainedwith Different Adsorption Affinities. While strong bindingof contaminants to colloidal particles is a prerequisite forcolloid-enhanced transport of contaminants, the remarkabledifferences in the effects of PSNPs on the transport ofnonpolar/weakly polar versus polar contaminants cannot beexplained with the differences in adsorption affinities of PSNPs
Figure 1. Effects of PSNPs on transport of pyrene (Columns 1−4)and BDE47 (Columns 5−8) in saturated soil. The left panel shows theBTCs of PSNPs, and the right panel shows the BTCs of thecontaminants.
Figure 2. Effects of PSNPs on transport of BPA (Columns 9−12),BPF (Columns 13−16), and 4-NP (Columns 17−20) in saturated soil.The left panel shows the BTCs of PSNPs, and the right panel showsthe BTCs of the contaminants.
for these compounds. Among the five compounds tested, bothof the nonpolar/weakly polar compounds (pyrene andBDE47), as well as one of the polar compounds (4-NP)exhibited very low mobility in the absence of PSNPs (Figures 1and 2). Thus, for these three compounds, any significanttransport enhancement in the presence of PSNPs should belargely attributable to the cotransport of the contaminant withPSNPs (this was verified experimentally with selected effluentsamples of pyrene and BDE47, as the mass of thesecontaminants in the dissolved phase was largely below thedetection limits, i.e., most of the contaminants detected in theeffluent should have been those originally adsorbed to PSNPsin the influent). Since 92−98% of pyrene in the influent wasbound to PSNPs (Table 1) and the breakthrough of PSNPsreached ∼69.0% (Figure 1a), the high breakthrough of pyrene(33.9−61.8%) (Table 1) is justified. Similarly, the 29.7−49.8%breakthrough of BDE47 was consistent with its strongadsorption to PSNPs (Table 1). Intriguingly, 4-NP alsoadsorbed to PSNPs strongly, in that 44−70% of 4-NP in theinfluent was adsorbed to PSNPs (Table 1), and in the columnexperiments of 4-NP the breakthrough of the carriers (i.e.,PSNPs) also reached ∼64.7%. Thus, it would be reasonable toexpect that PSNPs should also markedly enhance the transportof 4-NP, which was not the case (Figure 2c).Differential Effects on Transport of Nonpolar/Weakly
Polar versus Polar Contaminants Are Attributable toDifferent Extents of Desorption Hysteresis. In ourprevious studies we demonstrated that at low nanoparticleconcentrations the significance of transport-enhancementeffects largely depends on how irreversibly a contaminant isadsorbed to the nanoparticles.35 Thus, one explanation for thedifferent effects of PSNPs on the transport of pyrene/BDE47versus 4-NP is that pyrene and BDE47 exhibited strongerirreversible adsorption to PSNPs than did 4-NP. This can beunderstood with the following analysis on the effects of PSNPson contaminant transport, considering two extreme cases: (1)desorption of contaminants from PSNPs is instantaneous andcompletely reversible; and (2) desorption is completelyirreversible. If assuming desorption is instantaneous andcompletely reversible, then the maximum contaminant break-through can be estimated using the following equation:35
=+ × ×
+ × + × ×_
_ _C C
V C V KV m K C V K
/ 0PSNPs d PSNPs
soil d soil PSNPs d PSNPs(2)
where CPSNPs (kg/L) is the concentration of PSNPs in theeffluent; V (mL) is the volume of the suspension flowedthrough the column; msoil (g) is the mass of soil in the column;and Kd_PSNPs (L/kg) and Kd_soil (L/kg) are the distributioncoefficients of 4-NP, pyrene, or BDE47 to PSNPs and soil,respectively (Kd_PSNPs and Kd_soil can be obtained from thesorption isotherms in Figure 3). The simulation results (Figure4) indicate that for all three contaminants if desorption fromPSNPs is completely reversible, then PSNPs (even at 20 mg/L,the highest tested concentration) would have little effect on thetransport of these contaminants. This is because the mass ofPSNPs was too low compared with that of soil organic matter,which competed with PSNPs for contaminants (even thoughadsorption affinities of pyrene, BDE47, and 4-NP to PSNPswere approximately 3−4 orders of magnitude higher than thoseto the soil). If assuming desorption of contaminants fromPSNPs is completely irreversible, then the BTCs ofcontaminants can be estimated based on the BTCs of PSNPs
Figure 3. Adsorption isotherms of pyrene (a), BDE47 (b), BPA (c),BPF (d), and 4-NP (e) to Lula soil and PSNPs. The error barsrepresent the mean deviations of duplicates.
Figure 4. Comparison between experimentally observed BTCs(Columns 4, 8, and 20) and the estimated ones assuming one ofthe two idealized scenarios: (1) desorption of contaminant fromPSNPs is instantaneous and completely reversible (eq 2); and (2)desorption from PSNPs is completely irreversible.
and the mass fractions of contaminants adsorbed to PSNPs inthe influents (Table 1). For instance, the BTCs of pyrene andBDE47 would overlap with the BTCs of the PSNPs, sinceessentially all the pyrene and BDE47 in the influents werebound to PSNPs. Accordingly, by comparing the BTCs of acontaminant with the estimated ones assuming the two extremecases, one can qualitatively understand how irreversibly acontaminant was bound to PSNPs during the transport in thecolumn.Figure 4 shows that the BTC of pyrene nearly overlaps with
(sits only slightly below) the estimated one assumingcompletely irreversible adsorption. In comparison, the BTCof BED47 falls considerably below the estimated one assumingcompletely irreversible adsorption, but is substantially abovethat assuming desorption is completely reversible. Interestingly,the BTC of 4-NP is only slightly upshifted compared with thatassuming instantaneous and reversible desorption. Evidently,the differential effects of PSNPs on the transport of nonpolar/weakly polar compounds versus polar contaminants wereattributable to the different extents of desorption hysteresis ofthese compounds from PSNPs.We confirmed the dependency of irreversible adsorption to
PSNPs on contaminant polarity using batch desorptionexperiments. Figure 5 compares the desorption of the fivecontaminants from PSNPs (10 mg/L). Remarkable desorptionhysteresis of pyrene and BDE47 were observed, whereas thedesorption of all three polar contaminants was essentiallyreversible. Strikingly, among pyrene, BDE47 and 4-NP thehysteresis index values follow the order of pyrene (0.52−0.63)> BDE47 (0.31−0.43) ≫ 4-NP (0.04−0.16), corroborating thedifferent extents of desorption hysteresis estimated based onFigure 4. Thus, the fact that transport enhancement by PSNPswas only observed for nonpolar/weakly polar contaminants isattributable to the strong desorption hysteresis of suchcompounds to PSNPs. Note that even though only pyreneand BDE47 were selected for the nonpolar and weakly polargroup, the difference between these two compounds appears toindicate that the degree of desorption hysteresis depended onthe polarity of the compounds (i.e., pyrene is less polar thanBDE47, see SI Table S1).Polarity-Dependent Desorption Hysteresis Is Linked
to Glassy Polymeric Structure of Polystyrene Nano-plastics. Our previous studies showed that desorptionhysteresis of organic contaminants from nanoparticles isattributable to two processes, i.e., entrapment of contaminantsin porous nanoparticle aggregates, and irreversible adsorptiondue to specific polar interactions between contaminant andnanoparticles (e.g., hydrogen bonding).35,41 Thus, nano-particles in the form of nanoaggregates (e.g., C60 aggregates,or graphene oxide under solution chemistry conditions favoringaggregation, e.g., at high ionic strength) can enhance thetransport of nonpolar compounds without incurring anyspecific polar interactions. In contrast, well dispersed nano-particles (e.g., graphene oxide at low ionic strength) can onlyenhance the transport of highly polar compounds (e.g., 1-naphthol, which exhibits irreversible adsorption to grapheneoxide through H-bonding).35 The fact that PSNPs were free ofsurface functional groups (FTIR spectrum, SI Figure S2), aswell as the observation that desorption hysteresis was onlyobserved for nonpolar and weakly polar compounds, indicatethat the cause of the strong desorption hysteresis observed forpyrene and BDE47 had to be the physical entrapment of thesecontaminants in PSNPs.
Interestingly, the PSNPs dispersed well in the influents ofcolumn experiments (SI Figure S4), as the hydrodynamicdiameters of PSNPs were around 100 nm (only slightly higherthan the true sizes of the particles (SI Figure S1)), with verylow polydispersity index (SI Table S2). Furthermore, neitherprolonged sitting time nor increased particle and contaminantconcentrations resulted in noticeably increased tendency ofparticle aggregation (SI Figure S5; SI Table S3 and Figure S10).On the basis of the above analysis, we offer the followinghypothesis to explain the vastly different degrees of desorptionhysteresis between nonpolar and polar compounds fromPSNPs. That is, adsorption of nonpolar versus polarcompounds occurred in different domains of PSNPS, leadingto polarity-dependent desorption hysteresis. Polystyrene isknown to have dense, glassy structure, due to the cross-linkingof chains.55,56 For nonpolar, highly hydrophobic organiccompounds the inner spaces of such glassy structure arefavorable adsorption sites due to micropore-filling,57 anddesorption from these inner spaces into the bulk aqueoussolution is energetically unfavorable, and can be highlyhysteretic.51,58,59 Note that physical entrapment in the glassydomains of soil organic matter has been considered a majormechanism controlling desorption hysteresis of hydrophobicorganic compounds from soil.37,59 In comparison, polar and less
Figure 5. Desorption data of pyrene (a), BDE47 (b), BPA (c), BPF(d), and 4-NP (e) from PSNPs (10 mg/L), using one-step desorptionexperiments. For each contaminant two desorption data points wereobtained, one at a relatively high contaminant concentration (hollowsquares) and one at a low concentration (hollow triangles). The filledcircles are adsorption data. Hysteresis index (HI) values werecalculated using eq 1.
hydrophobic organic contaminants have a much lowertendency in entering the glassy polymeric domain and likelyfavor surface adsorption. Accordingly, desorption of polarcontaminants from polystyrene would be much more reversible.To verify this hypothesis, we conducted additional
adsorption and desorption experiments of pyrene and 4-NPusing two microsized polystyrene and polyethylene polymers.The two polymers were similar in size and shape, but varied inpolymeric structure. In particular, the polyethylene material hadrelatively rubbery and flexible structure, whereas the poly-styrene material had dense, glassy polymeric structure. Asexpected, desorption hysteresis of pyrene was only observed onthe glassy polystyrene but not on the rubbery polyethylene(Figure 6), consistent with the hypothesis that the significant
desorption hysteresis of nonpolar compounds was linked to therigid glassy polymeric inner structure of PSNPs. In contrast,desorption of 4-NP from both polystyrene and polyethylenewas essentially reversible, in line with the hypothesis that polarcompounds favor surface adsorption. Moreover, the desorptionkinetics data show that desorption of pyrene from the rubberypolyethylene was very fast, in that apparent desorptionequilibrium was reached within 2 h, whereas desorption fromthe glassy polystyrene was much slower (Figure 7a). Thisstriking difference further corroborates the physical entrapmentof pyrene within glassy polymeric structures, as compared withpartitioning driven desorption from rubbery polymers. Notably,rapid desorption kinetics of 4-NP was observed not only on therubbery polyethylene, but also on the glassy polystyrene, asapparent desorption equilibrium was reach within 2 h (Figure7b), consistent with the proposed surface adsorptionmechanism for 4-NP.Environmental Implications. The wide spreading of MPs
and NPs in the environment has drawn concerns about theirpotential detrimental environmental effects. The findings of thisstudy underscore a potentially significant environmentalimplication of NPs, that is, NPs may significantly enhance the
spreading of organic contaminants in the environment. Giventhe potentially high concentrations of NPs in the environ-ment,42 these materials may become one of the most importantcontaminant carriers. Even though only one specific type ofnanoplastic was tested in this study, the interesting observationthat the polymeric structures of nanoplastics fundamentallydetermine the significance of NP-enhanced contaminanttransport (and consequently, resulting in highly compound-specific effects) warrants further studies using nanoplasticscovering a wide range of variables of polymeric properties.
■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.est.7b05211.
Procedures used to determine of the concentrations ofdissolved and PSNPs-adsorbed organic contaminants,detailed procedures of liquid scintillation counting,procedures of sorption experiments to Lula soil, andadsorption and desorption experiments to and frommicrosized polymers; tables summarizing the physico-chemical characteristics of contaminants, average hydro-dynamic diameter and ζ potential of PSNPs in theinfluents, and particle size distribution of PSNPs asaffected by particle and contaminant concentrations;figures showing the SEM images, FTIR spectra, TEMimages, fluorescence spectra, and calibration curves ofPSNPs, particle size distribution of Lula soil, particle sizedistribution of PSNPs as affected by sitting time andconcentrations of contaminants and particles, effects ofcontaminant adsorption on the measurement offluorescence intensity of PSNPs, as well as representativeradio-chromatograms of BPA and BPF(PDF)
Figure 6. Adsorption and desorption isotherms of pyrene and 4-NP toand from microsized polystyrene and polyethylene. The filled circlesare adsorption data and hollow symbols are desorption data. The errorbars represent the mean deviations of duplicates.
Figure 7. Desorption kinetics of pyrene and 4-NP from microsizedpolystyrene (filled symbols) and polyethylene (hollow symbols). Theerror bars represent the mean deviations of duplicates.
This project was supported by the National Natural ScienceFoundation of China (Grants 21425729 and 21237002), theNational Key Research and Development Program of China(2016YFC1402203), and the Ministry of Science andTechnology of China (Grant 2014CB932001).
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