Polystyrene Nanoplastics-Enhanced Contaminant Transport: Role of

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Polystyrene Nanoplastics-enhanced Contaminant Transport: Role of Irreversible Adsorption in Glassy Polymeric Domain Jin Liu, Yini Ma, Dongqiang Zhu, Tianjiao Xia, Yu Qi, Yao Yao, Xiaoran Guo, Rong Ji, and Wei Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05211 • Publication Date (Web): 08 Feb 2018 Downloaded from http://pubs.acs.org on February 8, 2018

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Environmental Science & Technology

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Polystyrene Nanoplastics-enhanced Contaminant Transport:

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Role of Irreversible Adsorption in Glassy Polymeric

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Domain

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Jin Liu,1,2 Yini Ma,2 Dongqiang Zhu,3 Tianjiao Xia,1 Yu Qi,1 Yao Yao,2 Xiaoran Guo,2

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Rong Ji,2* Wei Chen1*

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College of Environmental Science and Engineering, Ministry of Education Key

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Laboratory of Pollution Processes and Environmental Criteria, Tianjin Key Laboratory of

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Environmental Remediation and Pollution Control, Nankai University, Tianjin 300350,

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China 2

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State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, China

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School of Urban and Environmental Sciences, Peking University, Beijing 100871, China

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Manuscript prepared for Environmental Science & Technology

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*To whom correspondence may be addressed: (Phone/fax) 86-25-8968-0581,

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86-22-6622-9516; (e-mail) [email protected], [email protected].

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ACS Paragon Plus Environment


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TOC Art

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ABSTRACT

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Nanoplastics (NPs) are becoming an emerging pollutant of global concern. A

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potential risk is that NPs may serve as carriers to increase the spreading of co-existing

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contaminants. In this study, we examined the effects of polystyrene nanoplastics (PSNPs,

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100 nm), used as a model NP, on the transport of five organic contaminants of different

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polarity in saturated soil. The presence of low concentrations of PSNPs significantly

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enhanced the transport of nonpolar (pyrene) and weakly polar

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(2,2',4,4'-tetrabromodiphenyl ether) compounds, but had essentially no effects on the

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transport of three polar compounds (bisphenol A, bisphenol F and 4-nonylphenol). The

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strikingly different effects of NPs on the transport of nonpolar/weakly polar versus polar

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contaminants could not be explained with different adsorption affinities, but was

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consistent with the polarity-dependent extents of desorption hysteresis. Notably,

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desorption hysteresis was only observed for nonpolar/weakly polar contaminants, likely

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because nonpolar compounds tended to adsorb in the inner matrices of glassy polymeric

39

structure of polystyrene (resulting in physical entrapment of adsorbates), whereas polar

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compounds favored surface adsorption. This hypothesis was verified with supplemental

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adsorption and desorption experiments of pyrene and 4-nonylphenol using a dense,

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glassy polystyrene polymer and a flexible, rubbery polyethylene polymer. Overall, the

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findings of this study underscore the potentially significant environmental implication of

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NPs as contaminant carriers.

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INTRODUCTION

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The occurrence of microplastics (MPs) and nanoplastics (NPs) in the environment is

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becoming an increasing concern, as large quantities of these materials have been detected

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in environmental media ranging from surface waters and sediments to beach sands and

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deep-sea waters all over the world.1-4 MPs are operationally defined as plastic particles

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smaller than 5 mm.5, 6 In the environment MPs can further break down to form NPs (with

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sizes less than 1 µm or 100 nm,7, 8 depending on different classifications) through

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prolonged mechanical abrasion, UV radiation, and microbial activity.8, 9 Moreover, NPs

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may also be introduced to the natural environment from use of consumer products.10 It has

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been shown that MPs and NPs can affect the metabolism, growth, mortality, and

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reproduction of aquatic organisms,11, 12 in similar ways as many engineered

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nanomaterials.13-15 Additionally, accumulation and persistence of MPs and NPs may

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eventually cause these materials to reach the levels that can affect the functioning and

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biodiversity of soil.16, 17

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Owing to their high surface-to-volume ratio and high surface hydrophobicity, MPs

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and NPs have strong adsorption affinities for a range of environmental contaminants, in

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particular, highly hydrophobic organic chemicals such as polychlorinated biphenyls,

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polycyclic aromatic hydrocarbons, polybrominated diphenyl ethers, and perfluorinated

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surfactants.18-23 Thus, there is a growing concern on an “indirect” effect of MPs and NPs,

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that is, these materials may serve as carriers to enhance the bioaccumulation of

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contaminants in living organisms,11, 24-27 and may also result in the so-called “Trojan

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Horse” effects.28-30 Similarly, MPs and NPs may serve as carriers for environmental

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contaminants in soil, facilitating the spreading of contaminants.31, 32 Between MPs and 4


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NPs, the latter will likely have stronger effects on contaminant transport. Specifically,

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NPs are much smaller in size, which gives them not only greater adsorption affinities for

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contaminants (e.g., due to the larger surface areas),18, 27 but also high colloidal stability

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and mobility.33

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Previous studies on nanoparticles-facilitated contaminant transport indicate that the

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extent of facilitated transport of contaminants relies largely on the nature of

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nanoparticles–contaminant interaction.30, 34, 35 In particular, at low nanoparticle (i.e.,

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“carrier”) concentrations significant facilitated transport of organic contaminants requires

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not only strong adsorption of contaminants to the carriers, but also significant desorption

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hysteresis (a collective term referring to both slow desorption kinetics and

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thermodynamically irreversible adsorption36-38) of contaminants from the carriers.30, 39-41

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Desorption hysteresis can either be due to the physical entrapment of contaminants in the

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complex matrices of the carriers, or strong specific adsorptive interactions that lead to

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irreversible binding of contaminants to the carriers.35 To date, little is known about how

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the capability of NPs as contaminant carriers varies as a function of contaminant and

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plastics properties. Even though lessons can be learned from previous work on other

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nanoparticles (e.g., engineered carbon nanomaterials),29, 30, 42, 43 it is not always possible

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to extrapolate the specific effects of NPs on contaminant transport from findings using

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other nanoparticles, considering their differences in size, shape, chemical compositions,

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pore structures, and aggregation properties in aqueous solutions, to mention a few.

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The objective of this study was to examine the capabilities of NPs to enhance the

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transport of common organic contaminants in saturated porous media, and to reveal the

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predominant mechanisms responsible for NPs-facilitated transport. Polystyrene 5



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nanoplastics (PSNPs) were selected as representative model NPs, because polystyrene

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accounts for approximately 90% of the total plastic demand and is widely found in the

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environment.27, 44 Additionally, polystyrene particles have been used as a probe MPs and

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NPs in many research to investigate the detrimental effects of MPs and NPs on

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organisms.11, 27, 45, 46 Five model organic compounds, including pyrene,

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2,2',4,4'-tetrabromodiphenyl ether (BDE47), bisphenol A (BPA), bisphenol F (BPF) and

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4-nonylphenol (4-NP) were selected as the test contaminants to represent organic

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contaminants of varied polarity and hydrophobicity. Additionally, pyrene and BDE47 are

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persistent organic contaminants, and BPA, BPF and 4-NP are common endocrine

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disruptor compounds.47-49 The effects of PSNPs on the transport of the five contaminants

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were examined at different PSNPs concentrations, using column transport experiments.

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Batch adsorption and desorption experiments of the contaminants were carried out to

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understand the differential effects of PSNPs on the transport of nonpolar/weakly polar

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compounds versus that of polar compounds. Environmental implications are discussed.

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MATERIALS AND METHODS Materials. Fluorescent PSNPs, supplied as an aqueous suspension of polymeric

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particles (1% solids by weight), were purchased from Thermo Fisher Scientific Inc.

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(Fremont, CA). The average particle size, confirmed with scanning electron microscopy

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(SEM) (S-3400 N II, Hitachi, Japan) by measuring more than 200 individual particles

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(see Supporting Information (SI) Figure S1), was 80.4 ± 7.9 nm. The Fourier transform

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infrared (FTIR) transmission spectra of the PSNPs (SI Figure S2), obtained using a

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Thermo Nicolet NEXUS 870 spectrometer (Thermo Nicolet Corporation, Madison, WI), 6


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confirmed that material was free of surface functional groups. Two micro-sized (50−100

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µm) polymers, including a polystyrene and a low-density polyethylene, were obtained

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from J&K Chemical (Beijing, China) and Sigma−Aldrich (St. Louis, MO), respectively.

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14

C-labeled pyrene (2.18 GBq/mmol) was purchased from American Radiolabeled

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Chemicals (St. Louis, MO). 14C-labeled BDE47 (2.44 GBq/mmol), BPA (0.74 GBq/mmol),

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BPF (2.82 GBq/mmol) and 4-NP (2.78 GBq/mmol) were synthesized using 14C-labeled

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phenol as a precursor. The physicochemical characteristics of the compounds are given in

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SI Table S1. Non-labeled pyrene, BDE47, BPA, BPF and 4-NP (all with purity >99%) were

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purchased from Sigma−Aldrich (St. Louis, MO).

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Lula soil was collected from a ranch near Lula, OK, USA. The soil contained 45%

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sand, 36% silt and 19% clay. The fractional organic carbon (fOC) value of the soil was

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0.37%. The particle size distribution of the soil (SI Figure S3) was measured using a laser

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diffraction particle size analyzer (Mastersizer 3000, Malvern, U.K.). The uniformity of the

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soil was 0.65, and the coefficient of uniformity was 8.50.

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Column Transport Experiments. Lula soil was dry-packed into Omnifit

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borosilicate glass columns (10 cm × 0.66 cm, Bio-Chem Valve Inc., Boonton, NJ) with

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10-µm stainless-steel screens (Valco Instruments Inc., Houston, TX) on both ends. Each

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column contained approximately 3.5 g of soil (dry-weight) with an average length of

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approximately 7.0 cm. The columns were operated in an upward direction using syringe

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pumps (KD Scientific, Holliston, MA). Once packed, the column was flushed at a flow rate

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of 3 mL/h with at least 100 mL deionized water followed by 180 mL background

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electrolyte solution (0.5 mM NaCl). The porosity and dead volume were determined by

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inverse-fitting the breakthrough curves (BTCs) of KBr (used as a conservative tracer). 7



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The experimental protocols of the column experiments are summarized in Table 1 and

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SI Table S2. To prepare the influents, the as-purchased PSNPs suspension was first

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ultrasonicated at 100 W (Vibra-Cell VCX800, Sonics & Material, Newtown, CT) for 5 min

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and then diluted with a background electrolyte of 0.5 mM NaCl in amber glass vials to give

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the working PSNPs concentrations of 5.0−20.3 mg/L. Immediately after adding the PSNPs

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suspension, a stock solution of an organic contaminant in methanol was added to each vial

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to give a contaminant concentration of approximately 10 µg/L. The volume percentage of

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methanol was kept below 0.1% (v/v) to minimize cosolvent effects. The vials were sealed

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with Teflon-lined screw caps and equilibrated by tumbling end-over-end at 3 rpm. The

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concentrations of dissolved and PSNPs-adsorbed contaminants in the influents were

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determined using a negligible depletion solid-phase micro-extraction approach (see SI for

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detailed procedures). The transmission electron microscopy (TEM) images (JEM-2100,

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JEOL, Tokyo, Japan) of the working suspensions showed that PSNPs were well dispersed

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(SI Figure S4) and were stable during the course of column experiments, as indicated by

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the essentially overlapping dynamic light scattering (DLS) (ZetaSizer Nano ZS, Malvern

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Instruments, Worcestershire, U.K.) profiles over 14 d (SI Figure S5). The average

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hydrodynamic diameters of PSNPs in different influents were characterized with DLS,

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and the ζ potential values was measured using a ZetaSizer Nano ZS system (Malvern

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Instruments, Worcestershire, U.K.); these data are summarized in Table S2.

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In a typical column experiment, the influent was pumped into the column from a

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100-mL glass syringe (SGE Analytical Science, Victoria, Australia). After 60 pore

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volumes (PV), a background electrolyte of 0.5 mM NaCl was used to flush the columns,

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until contaminant concentration in the effluent was below the detection limit. Effluent 8


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samples were collected every 2–3 PV. Each collected sample was split into two aliquots to

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measure the concentrations of PSNPs and contaminants. The concentrations of PSNPs

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were determined using a fluorescence spectrometer (FluoroMax-4, Horiba Scientific,

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Edison, NJ) (SI Figure S6), based on a pre-established calibration curve of PSNPs (SI

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Figure S7). (Adsorption of the five contaminants had no effects on the fluorescence

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intensity of PSNPs; SI Figure S8). The contaminants were quantified by determining

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radioactivity using a liquid scintillation counter (LS6500, Beckman Coulter, Fullerton, CA)

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(the detailed procedures are given in SI). In the experiments of pyrene and BDE47,

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selected effluent samples were taken to verify that the contaminants in the effluents were

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mainly associated with the PSNPs, using a previously developed method.41 In the

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experiments of BPA and BPF, the contaminants in the influent and effluent of randomly

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selected samples were analyzed using reversed-phase high-performance liquid

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chromatography (Agilent HPLC Series 1100, Agilent Technologies, Germany) with a

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radio flow detector to confirm that no degradation of BPA and BPF occurred in the soil

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column (representative chromatograms are shown in SI Figure S9).

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Batch Adsorption and Desorption Experiments. Adsorption experiments to

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PSNPs were carried out using a batch adsorption approach.50 Aliquots of 5, 10, or 20 mg/L

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PSNPs suspension in 0.5 mM NaCl were added to a series of 20-mL amber glass vials.

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Then, a certain amount of a contaminant stock solution was added to each of the vials. The

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vials were sealed with Teflon-lined screw caps and tumbled end-over-end at 3 rpm for 2 d

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(for BPA and BPF) or 14 d (for pyrene, BDE47, and 4-NP) to reach adsorption

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equilibrium.27, 51, 52 Afterward, the above-mentioned negligible depletion solid-phase

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microextraction approach was used to determine the concentrations of dissolved and 9



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PSNPs-adsorbed contaminants. Each adsorption isotherm data point was run in duplicate.

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In desorption experiments the suspensions in selected vials in the adsorption isotherm

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experiments were each split into two aliquots of equal volumes (approximately 10 mL) in

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two 20-mL amber glass vials. Then, adsorbate-free background electrolyte was added to

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each vial. The diluted suspensions were equilibrated by tumbling the vials end-over-end at

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3 rpm, to initiate desorption of the adsorbate from PSNPs. The aqueous-phase

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concentrations of the contaminants were measured using the negligible depletion

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solid-phase microextraction approach mentioned above. The total mass of the adsorbate in

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each vial was also measured. The concentrations in the adsorbed phase were obtained

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based on mass balance. Two data points of each adsorption isotherm were selected to do

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the desorption experiments. Each desorption experiment was run in duplicate.

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For the convenience of quantification and comparison of desorption hysteresis, the hysteresis index (HI) was calculated:53 HI =

qed − qes qes

(1)

T ,Ce

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where qed is adsorbed-phase concentration observed in the desorption experiment that is

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in equilibrium with an aqueous-phase concentration Ce, and qes is the adsorbed-phase

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concentration calculated from Ce assuming that desorption is reversible. If desorption is

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completely reversible, HI is equal to 0; the higher the HI value, the greater degree of

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desorption hysteresis. The subscripts T and Ce specify constant conditions of temperature

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and equilibrium concentration of solute.

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The sorption isotherms of the contaminants to Lula soil, as well as adsorption and desorption experiments of pyrene and 4-NP to and from micro-sized polystyrene and 10


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polyethylene, were obtained using a batch approach developed in our previous study (see

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SI for detailed procedures).54

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RESULTS AND DISCUSSION Nanoplastics Significantly Enhance Transport of Nonpolar and Weakly Polar

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Organic Contaminants. The presence of small amount of PSNPs (5.0 to 20.0 mg/L)

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significantly enhanced the transport of the model nonpolar organic contaminant, pyrene,

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and the weakly polar organic contaminant, BDE47 (Figure 1). In the absence of PSNPs in

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the influent, negligible breakthrough of pyrene or BDE47 was observed after 80 PV.

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However, the presence of 5.0−20.0 mg/L PSNPs in the influent resulted in significantly

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increased transport of both pyrene and BDE47. For pyrene, the maximum breakthrough

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(as indicated by C/C0) increased to 33.9 ± 0.2% in the presence of 5.1 mg/L PSNPs, and

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to 61.8 ± 0.2% when the concentration of PSNPs was 19.4 mg/L. Similarly, the

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maximum breakthrough of BDE47 reached 29.7–49.8% in the presence of 5.0−20.0 mg/L

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PSNPs.

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Nanoplastics Had Negligible Effects on Transport of Polar Organic

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Contaminants. In contrast to the significant transport enhancement effects of PSNPs on

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pyrene and BDE47, the presence of PSNPs (5.0–20.3 mg/L) in the influent had

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essentially no effects on the transport of the three polar compounds, i.e., BPA, BPF, and

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4-NP (Figure 2). In the absence of PSNPs, the three compounds exhibited different

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degrees of mobility: BPA was relatively mobile, with maximum breakthrough of 86.7%;

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the less mobile BPF reached a maximum breakthrough of 57.0%; 4-NP exhibited the

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lowest mobility, with maximum breakthrough reaching only 4.6%. Interestingly, for all 11



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three compounds the BTCs in the presence of PSNPs overlapped with the one without

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PSNPs, indicating that PSNPs played minimal roles in the transport of these three

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compounds. It is particularly intriguing and counterintuitive that PSNPs had essentially

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no effects on the transport of 4-NP, in that, 4-NP exhibited similar low mobility to pyrene

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and BDE47 and PSNPs significantly facilitated the transport of pyrene and BDE47. Thus,

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the remarkable difference in the transport-enhancement effects between 4-NP and

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pyrene/BDE47 indicates that different transport-enhancement mechanisms were in play

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for polar vs. nonpolar/weakly polar organic contaminants.

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Differential Effects on Transport of Nonpolar/Weakly Polar vs. Polar

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Contaminants Cannot Be Explained with Different Adsorption Affinities. While

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strong binding of contaminants to colloidal particles is a prerequisite for

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colloid-enhanced transport of contaminants, the remarkable differences in the effects of

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PSNPs on the transport of nonpolar/weakly polar vs. polar contaminants cannot be

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explained with the differences in adsorption affinities of PSNPs for these compounds.

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Among the five compounds tested, both of the nonpolar/weakly polar compounds

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(pyrene and BDE47), as well as one of the polar compounds (4-NP) exhibited very low

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mobility in the absence of PSNPs (Figures 1 and 2). Thus, for these three compounds,

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any significant transport enhancement in the presence of PSNPs should be largely

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attributable to the co-transport of the contaminant with PSNPs (this was verified

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experimentally with selected effluent samples of pyrene and BDE47, as the mass of these

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contaminants in the dissolved phase was largely below the detection limits, i.e., most of

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the contaminants detected in the effluent should have been those originally adsorbed to

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PSNPs in the influent). Since 92–98% of pyrene in the influent was bound to PSNPs 12


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(Table 1) and the breakthrough of PSNPs reached ~69.0% (Figure 1a), the high

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breakthrough of pyrene (33.9–61.8%) (Table 1) is justified. Similarly, the 29.7–49.8%

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breakthrough of BDE47 was consistent with its strong adsorption to PSNPs (Table 1).

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Intriguingly, 4-NP also adsorbed to PSNPs strongly, in that 44–70% of 4-NP in the

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influent was adsorbed to PSNPs (Table 1), and in the column experiments of 4-NP the

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breakthrough of the carriers (i.e., PSNPs) also reached ~64.7%. Thus, it would be

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reasonable to expect that PSNPs should also markedly enhance the transport of 4-NP,

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which was not the case (Figure 2c).

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Differential Effects on Transport of Nonpolar/Weakly Polar vs. Polar

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Contaminants Are Attributable to Different Extents of Desorption Hysteresis. In our

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previous studies we demonstrated that at low nanoparticle concentrations the significance

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of transport-enhancement effects largely depends on how irreversibly a contaminant is

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adsorbed to the nanoparticles.35 Thus, one explanation for the different effects of PSNPs

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on the transport of pyrene/BDE47 versus 4-NP is that pyrene and BDE47 exhibited

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stronger irreversible adsorption to PSNPs than did 4-NP. This can be understood with the

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following analysis on the effects of PSNPs on contaminant transport, considering two

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extreme cases: 1) desorption of contaminants from PSNPs is instantaneous and

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completely reversible; and 2) desorption is completely irreversible. If assuming

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desorption is instantaneous and completely reversible, then the maximum contaminant

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breakthrough can be estimated using the following equation:35

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C / C0 =

V + CPSNPs ⋅ V ⋅ K d_PSNPs V + msoil ⋅ K d_soil + CPSNPs ⋅ V ⋅ K d_PSNPs

(2)

where CPSNPs (kg/L) is the concentration of PSNPs in the effluent; V (mL) is the volume 13



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of the suspension flowed through the column; msoil (g) is the mass of soil in the column;

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and Kd_PSNPs (L/kg) and Kd_soil (L/kg) are the distribution coefficients of 4-NP, pyrene, or

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BDE47 to PSNPs and soil, respectively (Kd_PSNPs and Kd_soil can be obtained from the

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sorption isotherms in Figure 3). The simulation results (Figure 4) indicate that for all

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three contaminants if desorption from PSNPs is completely reversible, then PSNPs (even

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at 20 mg/L, the highest tested concentration) would have little effect on the transport of

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these contaminants. This is because the mass of PSNPs was too low compared with that

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of soil organic matter, which competed with PSNPs for contaminants (even though

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adsorption affinities of pyrene, BDE47, and 4-NP to PSNPs were approximately 3–4

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orders of magnitude higher than those to the soil). If assuming desorption of

283

contaminants from PSNPs is completely irreversible, then the BTCs of contaminants can

284

be estimated based on the BTCs of PSNPs and the mass fractions of contaminants

285

adsorbed to PSNPs in the influents (Table 1). For instance, the BTCs of pyrene and

286

BDE47 would overlap with the BTCs of the PSNPs, since essentially all the pyrene and

287

BDE47 in the influents were bound to PSNPs. Accordingly, by comparing the BTCs of a

288

contaminant with the estimated ones assuming the two extreme cases, one can

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qualitatively understand how irreversibly a contaminant was bound to PSNPs during the

290

transport in the column.

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Figure 4 shows that the BTC of pyrene nearly overlaps with (sits only slightly below)

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the estimated one assuming completely irreversible adsorption. In comparison, the BTC

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of BED47 falls considerably below the estimated one assuming completely irreversible

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adsorption, but is substantially above that assuming desorption is completely reversible.

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Interestingly, the BTC of 4-NP is only slightly upshifted compared with that assuming 14


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instantaneous and reversible desorption. Evidently, the differential effects of PSNPs on

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the transport of nonpolar/weakly polar compounds versus polar contaminants were

298

attributable to the different extents of desorption hysteresis of these compounds from

299

PSNPs.

300

We confirmed the dependency of irreversible adsorption to PSNPs on contaminant

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polarity using batch desorption experiments. Figure 5 compares the desorption of the five

302

contaminants from PSNPs (10 mg/L). Remarkable desorption hysteresis of pyrene and

303

BDE47 were observed, whereas the desorption of all three polar contaminants was

304

essentially reversible. Strikingly, among pyrene, BDE47 and 4-NP the hysteresis index

305

values follow the order of pyrene (0.52–0.63) > BDE47 (0.31–0.43) >> 4-NP (0.04–0.16),

306

corroborating the different extents of desorption hysteresis estimated based on Figure 4.

307

Thus, the fact that transport enhancement by PSNPs was only observed for

308

nonpolar/weakly polar contaminants is attributable to the strong desorption hysteresis of

309

such compounds to PSNPs. Note that even though only pyrene and BDE47 were selected

310

for the nonpolar and weakly polar group, the difference between these two compounds

311

appears to indicate that the degree of desorption hysteresis depended on the polarity of

312

the compounds (i.e., pyrene is less polar than BDE47, see SI Table S1).

313

Polarity-dependent Desorption Hysteresis Is Linked to Glassy Polymeric

314

Structure of Polystyrene Nanoplastics. Our previous studies showed that desorption

315

hysteresis of organic contaminants from nanoparticles is attributable to two processes, i.e.,

316

entrapment of contaminants in porous nanoparticle aggregates, and irreversible

317

adsorption due specific polar interactions between contaminant and nanoparticles (e.g.,

318

hydrogen bonding).35, 41 Thus, nanoparticles in the form of nano-aggregates (e.g., C60 15



319

aggregates, or graphene oxide under solution chemistry conditions favoring aggregation,

320

e.g., at high ionic strength) can enhance the transport of nonpolar compounds without

321

incurring any specific polar interactions. In contrast, well dispersed nanoparticles (e.g.,

322

graphene oxide at low ionic strength) can only enhance the transport of highly polar

323

compounds (e.g., 1-naphthol, which exhibits irreversible adsorption to graphene oxide

324

through H-bonding).35 The fact that PSNPs were free of surface functional groups (FTIR

325

spectrum, SI Figure S2), as well as the observation that desorption hysteresis was only

326

observed for nonpolar and weakly polar compounds, indicate that the cause of the strong

327

desorption hysteresis observed for pyrene and BDE47 had to be the physical entrapment

328

of these contaminants in PSNPs.

329

Interestingly, the PSNPs dispersed well in the influents of column experiments (SI

330

Figure S4), as the hydrodynamic diameters of PSNPs were around 100 nm (only slightly

331

higher than the true sizes of the particles (SI Figure S1)), with very low polydispersity

332

index (SI Table S2). Furthermore, neither prolonged sitting time nor increased particle

333

and contaminant concentrations resulted in noticeably increased tendency of particle

334

aggregation (SI Figure S5; SI Table S3 and Figure S10). Based on the above analysis, we

335

offer the following hypothesis to explain the vastly different degrees of desorption

336

hysteresis between nonpolar and polar compounds from PSNPs. That is, adsorption of

337

nonpolar versus polar compounds occurred in different domains of PSNPS, leading to

338

polarity-dependent desorption hysteresis. Polystyrene is known to have dense, glassy

339

structure, due to the cross-linking of chains.55, 56 For nonpolar, highly hydrophobic

340

organic compounds the inner spaces of such glassy structure are favorable adsorption

341

sites due to micropore-filling,57 and desorption from these inner spaces into the bulk 16


Page 16 of 34

Page 17 of 34


342

aqueous solution is energetically unfavorable, and can be highly hysteretic.51, 58, 59 Note

343

that physical entrapment in the glassy domains of soil organic matter has been considered

344

a major mechanism controlling desorption hysteresis of hydrophobic organic compounds

345

from soil.37, 59 In comparison, polar and less hydrophobic organic contaminants have a

346

much lower tendency in entering the glassy polymeric domain and likely favor surface

347

adsorption. Accordingly, desorption of polar contaminants from polystyrene would be

348

much more reversible.

349

To verify this hypothesis, we conducted additional adsorption and desorption

350

experiments of pyrene and 4-NP using two micro-sized polystyrene and polyethylene

351

polymers. The two polymers were similar in size and shape, but varied in polymeric

352

structure. In particular, the polyethylene material had relatively rubbery and flexible

353

structure, whereas the polystyrene material had dense, glassy polymeric structure. As

354

expected, desorption hysteresis of pyrene was only observed on the glassy polystyrene

355

but not on the rubbery polyethylene (Figure 6), consistent with the hypothesis that the

356

significant desorption hysteresis of nonpolar compounds was linked to the rigid glassy

357

polymeric inner structure of PSNPs. In contrast, desorption of 4-NP from both

358

polystyrene and polyethylene was essentially reversible, in line with the hypothesis that

359

polar compounds favor surface adsorption. Moreover, the desorption kinetics data show

360

that desorption of pyrene from the rubbery polyethylene was very fast, in that apparent

361

desorption equilibrium was reached within 2 h, whereas desorption from the glassy

362

polystyrene was much slower (Figure 7a). This striking difference further corroborates

363

the physical entrapment of pyrene within glassy polymeric structures, as compared with

364

partitioning driven desorption from rubbery polymers. Notably, rapid desorption kinetics 17



365

of 4-NP was observed not only on the rubbery polyethylene, but also on the glassy

366

polystyrene, as apparent desorption equilibrium was reach within 2 h (Figure 7b),

367

consistent with the proposed surface adsorption mechanism for 4-NP.

368

Environmental Implications. The wide spreading of MPs and NPs in the

369

environment has drawn concerns about their potential detrimental environmental effects.

370

The findings of this study underscore a potentially significant environmental implication

371

of NPs, that is, NPs may significantly enhance the spreading of organic contaminants in

372

the environment. Given the potentially high concentrations of NPs in the environment,42

373

these materials may become one of the most important contaminant carriers. Even though

374

only one specific type of nanoplastics was tested in this study, the interesting observation

375

that the polymeric structures of nanoplastics fundamentally determine the significance of

376

NP-enhanced contaminant transport (and consequently, resulting in highly

377

compound-specific effects) warrants further studies using nanoplastics covering a wide

378

range of variables of polymeric properties.

379 380

Acknowledgments. This project was supported by the National Natural Science Foundation

381

of China (Grants 21425729 and 21237002), the National Key Research and Development

382

Program of China (2016YFC1402203), and the Ministry of Science and Technology of

383

China (Grant 2014CB932001).

384 385

Supporting Information Available: Procedures used to determine of the concentrations of

386

dissolved and PSNPs-adsorbed organic contaminants, detailed procedures of liquid

387

scintillation counting, procedures of sorption experiments to Lula soil, and adsorption and 18


Page 18 of 34

Page 19 of 34


388

desorption experiments to and from micro-sized polymers; tables summarizing the

389

physicochemical characteristics of contaminants, average hydrodynamic diameter and ζ

390

potential of PSNPs in the influents, and particle size distribution of PSNPs as affected by

391

particle and contaminant concentrations; figures showing the SEM images, FTIR spectra,

392

TEM images, fluorescence spectra, and calibration curves of PSNPs, particle size

393

distribution of Lula soil, particle size distribution of PSNPs as affected by sitting time and

394

concentrations of contaminants and particles, effects of contaminant adsorption on the

395

measurement of fluorescence intensity of PSNPs, as well as representative

396

radio-chromatograms of BPA and BPF. This information is available free of charge via the

397

Internet at http://pubs.acs.org.

398 399

Notes—The authors declare no competing financial interest.

400 401

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402

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569

26


Page 26 of 34

Page 27 of 34


Table 1. Experimental Setups and Breakthrough Results of Column Experiments Column properties No.

1

a

Length

Bulk density

(cm)

(g/cm3)

7.08

1.49

Effluent properties a

Influent properties

Porosity

0.44

Contaminant pyrene

Contaminant

PSNPs

concentration

concentration

(µg/L)

(mg/L)

10.39

0

Mass fraction of pH

contaminant on PSNPs (%)

C/C0_PSNPs

C/C0_cont.

(%)

(%)

6.7

-b

-

1.7 ± 0.1

2

6.81

1.39

0.47

pyrene

8.93

5.1

6.8

92

65.4 ± 0.2

33.9 ± 0.2

3

6.90

1.39

0.48

pyrene

10.16

10.9

6.5

96

69.0 ± 0.3

56.1 ± 0.4

4

6.92

1.40

0.47

pyrene

9.93

19.4

6.9

98

65.2 ± 1.9

61.8 ± 0.2

5

7.21

1.48

0.44

BDE47

10.98

0

6.5

-

-

3.6 ± 0.1

6

6.85

1.39

0.48

BDE47

10.12

5.0

6.7

96

61.6 ± 0.2

29.7 ± 2.5

7

6.98

1.38

0.48

BDE47

8.16

11.4

6.8

98

67.1 ± 0.4

37.9 ± 0.6

8

6.90

1.41

0.47

BDE47

10.60

20.0

6.7

99

67.6 ± 0.6

49.8 ± 0.8

9

7.08

1.51

0.43

BPA

9.86

0

6.8

-

-

86.7 ± 0.5

10

6.85

1.41

0.47

BPA

10.78

5.0

6.7 6.8 6.8 6.9 6.7 6.6 6.7

9.8

65.2 ± 0.1

84.6 ± 0.1

14

62.5 ± 0.2

85.5 ± 0.2

22

64.3 ± 0.1

85.8 ± 0.4

-

-

57.0 ± 0.7

21

64.1 ± 0.8

55.5 ± 0.6

30

64.3 ± 0.1

56.7 ± 0.5

41

63.7 ± 0.7

56.3 ± 0.3

11

7.20

1.47

0.44

BPA

11.15

11.1

12

7.22

1.45

0.45

BPA

11.14

20.3

13

7.15

1.50

0.44

BPF

9.13

0

14

7.02

1.42

0.46

BPF

11.39

5.0

15

7.20

1.44

0.46

BPF

10.39

10.4

16

7.16

1.41

0.47

BPF

9.58

20.2

17

6.82

1.36

0.49

4-NP

9.76

0

6.5

-

-

4.6 ± 0.1

18

6.82

1.35

0.49

4-NP

10.64

5.0

6.7

44

61.9 ± 0.3

6.3 ± 0.4

19

6.90

1.40

0.47

4-NP

9.73

10.0

6.6

61

62.5 ± 0.2

6.9 ± 0.3

20

6.90

1.42

0.46

4-NP

10.32

17.8

6.6

70

64.7 ± 0.5

6.4 ± 0.2

Average value of last three data points of respective BTCs before flushed with background solutions. C/C0_PSNPs and C/C0_cont. represent the ratio of effluent PSNPs and

contaminant conentrations to their initial total concentrations in the influent. b not applicable. 27



Page 28 of 34

1.4

1.4

1.2

1.2

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

C/C0_Pyrene

C/C0_PSNPs

(a) Pyrene

0.0 0

20

40

60

80

0

20

40

PV

60

80

PV 1.4

1.2

1.2

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

20

40

60

80

0

20

40

PV

60

80

PV

0 mg/L PSNPs

5 mg/L PSNPs 20 mg/L PSNPs

10 mg/L 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 the BTCs of PSNPs, and the right panel shows the BTCs of the contaminants.

28


C/C0_BDE47

C/C0_PSNPs

(b) BDE47 1.4

Page 29 of 34


1.4

1.2

1.2

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

C/C0_BPA

C/C0_PSNPs

(a) BPA 1.4

0.0 0

20

40

60

80

0

20

PV

40

60

80

PV

1.2

1.2

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

20

40

60

80

0

20

PV

C/C0_PSNPs

1.4

C/C0_BPF

1.4

40

60

80

PV

(c) 4-NP

1.4

1.2

1.2

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

C/C 0_4-NP

C/C0_PSNPs

(b) BPF 1.4

0.0 0

20

40

60

80

0

20

PV

40

60

80

PV

0 mg/L PSNPs

5 mg/L PSNPs 20 mg/L PSNPs

10 mg/L PSNPs

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 shows the BTCs of the contaminants.

29



(b) BDE47

104

104

103

103

102

102

10

q (mg/kg)

q (mg/kg)

(a) Pyrene

1

100 10-1 10-2

101 100 10-1 10-2

10-3 10-5

10

-4

10

-3

10

-2

10-3 10-6

10-5

C e (mg/L)

10-3

(d) BPF 104

103

103

102

102

q (mg/kg)

q (mg/kg)

(c) BPA

1

100 10-1 10-2

101 100 10-1 10-2

10-3 10-4

10

-3

10

-2

10

-1

10-3 10-4

10-3

C e (mg/L)

104

10-4

Ce (mg/L)

104

10

Page 30 of 34

10-2

10-1

Ce (mg/L)

(e) 4-NP

q (mg/kg)

103 102 101 100 10-1

soil PSNPs

10-2 10-3 10-5

10-4

10-3

10-2

C e (mg/L)

Figure 3. Adsorption isotherms of pyrene (a), BDE47 (b), BPA (c), BPF (d) and 4-NP (e) to Lula soil and PSNPs. The error bars represent the mean deviations of duplicates.

30


Page 31 of 34


(b) BDE47

1.2

1.2

1.0

1.0 C/C 0_BDE47

C/C 0_Pyrene

(a) Pyrene

0.8 0.6 0.4 0.2

0.8 0.6 0.4 0.2

0.0

0.0 0

20

40

60

80

0

20

PV

40

60

80

PV

(c) 4-NP 1.2

C/C0_4-NP

1.0 0.8 0.6 0.4

Experimental Estimated, Scenario 1 Estimated, Scenario 2

0.2 0.0 0

20

40

60

80

PV

Figure 4. Comparison between experimentally observed BTCs (Columns 4, 8, and 20) and the estimated ones assuming one of the two idealized scenarios: 1) desorption of contaminant from PSNPs is instantaneous and completely reversible (eq. 2); and 2) desorption from PSNPs is completely irreversible.

31



(b) BDE47 104

103

103

q (mg/kg)

q (mg/kg)

(a) Pyrene 104

102

102

HI = 0.52 ~ 0.63 101 10-5

10-4

10-3

HI = 0.31 ~ 0.43 101 10-6

10-2

10-5

C e (mg/L)

10-4

10-3

C e (mg/L)

(c) BPA

(d) BPF 103

q (mg/kg)

103

q (mg/kg)

Page 32 of 34

102

102

HI = 0.12 ~ 0.15

HI = 0.02 ~ 0.12 101 10-4

10-3

10-2

101 10-4

10-1

C e (mg/L)

10-3

10-2

10-1

C e (mg/L)

(e) 4-NP

q (mg/kg)

103

102

HI = 0.04 ~ 0.16 101 10-5

10-4

10-3

10-2

C e (mg/L)

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 desorption experiments. For each contaminant two desorption data points were obtained, one at a relatively high contaminant concentration (hollow squares) and one at a low concentration (hollow triangles). The filled circles are adsorption data. Hysteresis index (HI) values were calculated using eq. 1. 32


Page 33 of 34


(a) Pyrene 60

PE 80

40

q (mg/kg)

q (mg/kg)

50

100 PS

30 20

60 40 20

10 0 0.000

0.004

0.008

0 0.000

0.012

0.001

0.002

Ce (mg/L)

C e (mg/L)

(b) 4-NP 60

50

40

q (mg/kg)

q (mg/kg)

50

60 PS

30 20 10 0 0.000

PE

40 30 20 10

0.010

0.020

0 0.000

0.030

0.005

C e (mg/L)

0.010

0.015

0.020

Ce (mg/L)

Figure 6. Adsorption and desorption isotherms of pyrene and 4-NP to and from micro-sized polystyrene and polyethylene. The filled circles are adsorption data and hollow symbols are desorption data. The error bars represent the mean deviations of duplicates.

33



(b) 4-NP

40

30

35

25 qt (mg/kg)

qt (mg/kg)

(a) Pyrene

Page 34 of 34

30 25

20 15

20

10 0

2

4

6

8

10 12 14

0

2

Time (h)

4

6

8

10 12 14

Time (h)

Figure 7. Desorption kinetics of pyrene and 4-NP from micro-sized polystyrene (filled symbols) and polyethylene (hollow symbols). The error bars represent the mean deviations of duplicates.

34


Polystyrene Nanoplastics-Enhanced Contaminant Transport: Role of

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