Aging Significantly Affects Mobility and Contaminant-Mobilizing Ability

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Environmental Processes

Aging Significantly Affects Mobility and Contaminantmobilizing Ability of Nanoplastics in Saturated Loamy Sand Jin Liu, Tong Zhang, Lili Tian, Xinlei Liu, Zhichong Qi, Yini Ma, Rong Ji, and Wei Chen Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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Environmental Science & Technology

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Aging Significantly Affects Mobility and

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Contaminant-mobilizing Ability of Nanoplastics in Saturated

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Loamy Sand

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Jin Liu,1,2 Tong Zhang,1 Lili Tian,2 Xinlei Liu,1 Zhichong Qi,1,3 Yini Ma,2 Rong Ji,2* Wei Chen1*

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1 College

of Environmental Science and Engineering, Ministry of Education Key Laboratory of

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Pollution Processes and Environmental Criteria, Tianjin Key Laboratory of Environmental

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

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2 State

Key Laboratory of Pollution Control and Resource Reuse, School of the Environment,

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Nanjing University, Nanjing 210023, China 3 College

of Chemistry and Chemical Engineering, Henan Joint International Research

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Laboratory of Environmental Pollution Control Materials, Henan University, Kaifeng 475004,

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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, 86-22-6622-9516;

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(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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Plastic debris, in particular, microplastics and nanoplastics, is becoming an emerging class

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of pollutants of global concern. Aging can significantly affect the physicochemical properties of

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plastics, and therefore, may influence the fate, transport and effects of these materials. Here, we

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show that aging by UV or O3 exposure drastically enhanced the mobility and

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contaminant-mobilizing ability of spherical polystyrene nanoplastics (PSNPs, 487.3  18.3 nm in

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diameter) in saturated loamy sand. Extended Derjaguin–Landau–Verwey–Overbeek calculations

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and pH-dependent transport experiments demonstrated that the greater mobility of the aged

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PSNPs was mainly the result of surface oxidation of the nanoplastics, which increased not only

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the surface charge negativity, but more importantly, hydrophilicity of the materials. The

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increased mobility of the aged PSNPs significantly contributed to their elevated

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contaminant-mobilizing abilities. Moreover, aging of PSNPs enhanced the binding of both

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nonpolar and polar contaminants, further increasing the contaminant-mobilizing ability of PSNPs.

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Interestingly, aging enhanced binding of nonpolar versus polar compounds via distinctly

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different mechanisms: increased binding of nonpolar contaminants (tested using pyrene) was

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mainly the result of the modification of the polymeric structure of PSNPs that exacerbated slow

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desorption kinetics; for polar compounds (4-nonylphenol), aging induced changes in surface

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properties also resulted in irreversible adsorption of contaminants through polar interactions,

42

such as hydrogen bonding. The findings further underline the significant effects of aging on

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environmental fate and implications of nanoplastics.

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INTRODUCTION

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The emergence of plastic debris is rapidly becoming a pressing environmental pollution

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issue globally. These discarded materials are prevalent in both aquatic and terrestrial ecosystems,

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and have been detected from a wide variety of environmental media, including freshwater

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lake/river,1 soil,2 coastal areas,3 open seas,4 and even from remote areas, such as polar regions5-7

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and deep-sea trenches8. While microplastics, referring to plastic debris smaller than 5 mm,9,10

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have been the focus of environmental research, environmental processes and biological effects of

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nanoplastics (commonly defined as particles with sizes less than 1 μm,11,12 while the threshold of

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100 nm has also been proposed13) are raising concern due to their potentially greater mobility14,

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higher exposure15,16 and toxicity to organisms.17-20 For example, nanoplastics appeared to cause

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significant developmental neurotoxicity to zebrafish larvae, whereas no obvious effects were

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observed for microplastics.21 Additionally, the production and use of consumer products (e.g.,

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paints, coatings, and biomedical products) also contribute to the environmental release of

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nanoplastics.13

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One potential environmental implication of nanoplastics is that these materials may

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significantly enhance the migration of environmental contaminants. Previous research indicated

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high adsorption affinities of nanoplastics toward a number of environmental contaminants,

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including polycyclic aromatic hydrocarbons, polychlorinated biphenyls, perfluoroalkyl

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substances and antibiotics.22-29 Several studies have also shown that nanoplastics possess high

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colloidal stability and mobility.30-32 For example, even in seawater-saturated sand, breakthrough

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of polystyrene nanoplastics still reached over 40%.14 The combined strong adsorption affinities

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and high mobility render nanoplastics potentially potent contaminant carriers that exacerbate the

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environmental risks of nanoplastics as well as organic contaminants.31,33-38 In our previous study, 4


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we found that the presence of low-concentration polystyrene nanoplastics (80.4 ± 7.9 nm in size)

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significantly enhanced the transport of nonpolar and weakly polar organic contaminants in

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saturated soil, due to the high adsorption affinities and entrapment of organic molecules in the

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microporous regimes of the nanoplastics.39

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Nanoplastics can undergo aging processes in the environment, as previously demonstrated

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for microplastics,40 which modifies their physicochemical properties and consequently affects

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both adsorption properties and colloidal stability and mobility of these materials. Previous

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studies conducted using microplastics showed that aging led to significant surface oxidation and

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the formation of localized microcracks,41,42 which appeared to be the main factors contributing to

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the altered adsorption affinities of aged microplastics.43-47 For example, it was reported that

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aging resulted in enhanced adsorption of ciprofloxacin and oxytetracycline to polystyrene, as the

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results of several altered adsorptive interactions, such as electrostatic interaction, H-bonding, and

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increased surface area.48,49 Even though no published studies on how aging may affect the

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mobility of nanoplastics are available in the literature, it can be expected that aging likely can

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affect the colloidal stability and transport properties of nanoplastics, mainly by modifying the

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surface chemistry (e.g., surface charge and surface functional groups), a critical property

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controlling particle transport. To date, the effects of aging-induced changes in adsorption

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properties and mobility of nanoplastics on their abilities to mobilize environmental contaminants

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are poorly understood.

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The objective of this study was to examine the effects of aging on the mobility and

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contaminant-mobilizing ability of nanoplastics in saturated porous media. Polystyrene

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nanoplastics (PSNPs) were selected as the representative model nanoplastics, as polystyrene is

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one of the most widely used plastics, with annual production rate of multiple million tons.50 An 5



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as-synthesized PSNPs material was chemically aged by treatment with ozone or UV

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irradiation,51,52 to obtain three aged PSNPs with varied surface and structural properties. The

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transport properties of the as-synthesized and aged PSNPs in a loamy sand were compared under

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varied solution chemical conditions (i.e., cation species and concentrations, as well as pH). The

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interaction energy profiles between the nanoplastics and soil were evaluated to understand the

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dominant factors controlling the deposition of aged versus as-synthesized PSNPs. Transport

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properties of two model organic compounds, pyrene and 4-nonylphenol, mediated by the

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as-synthesized and three aged PSNPs, were examined. (The two model compounds were selected

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to represent organic contaminants of different polarity and for their environmental relevance, as

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the nonpolar, highly hydrophobic pyrene is a persistent organic pollutant, and the polar, less

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hydrophobic 4-nonylphenol a common endocrine disruptor often used as additives during the

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manufacturing of plastics.53) Batch adsorption and desorption experiments of the contaminants

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were carried out to further reveal how aging differentially affected the contaminant-mobilizing

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abilities of PSNPs for nonpolar, hydrophobic organic contaminants versus polar organic

104

contaminants, by modulating nanoplastics–contaminant interactions.

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MATERIALS AND METHODS

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Materials. A polystyrene polymer was synthesized from styrene using the emulsion

108

polymerization approach.54,55 The obtained product (referred to as “PS” hereafter) was washed

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sequentially using deionized (DI) water and ethanol to remove the impurities. To obtain aged

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PSNPs, a stock suspension of the as-synthesized polymer was first prepared by magnetically

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stirring 60 mg of the PS powder in 300 mL of DI water for 12 h at 20 °C, followed by 6


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ultrasonication at 100 W (Vibra-Cell VCX800, Sonics & Material, Newtown, CT) for 30 min.

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The obtained suspension was filtered with 1-μm glass fiber membrane filters (Millipore Co.,

114

Billerica, MA) to remove large aggregates, and then aged using two different approaches. In the

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first aging approach, the PS suspension in 5 mM NaNO3 was added to a-50 mL quartz tube and

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UV-irradiated (500 W mercury lamps) at 20 °C for 6 h or 12 h; the two products are referred to

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as UV-PS1 and UV-PS2, respectively. Another aging approach involved bubbling O3 gas (using

118

a 3S-A15 ozone generator, Tonglin Technologies Co. Ltd., Beijing, China) into 500 mL of the

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PS suspension at a rate of 0.1 g/min for 3 h; this product is referred to as O3-PS. The aged PSNPs

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suspensions were washed repeatedly with DI water using an ultra-filter system with a molecular

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weight cut off of 10000 daltons (Millipore 8400, Merck, Boston, MA).

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14C-labeled

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

123

Chemicals (St. Louis, MO). Non-labeled pyrene (with purity >99%) was purchased from

124

Sigma−Aldrich (St. Louis, MO). 14C-labeled 4-nonylphenol (2.78 GBq/mmol) and non-labeled

125

4-nonylphenol were synthesized using 14C-labeled and non-labeled phenol, respectively, as the

126

precursor.56

127

Lufa soil (a loamy sand; standard soil type no. 2.1) was purchased from Lufa Speyer

128

(Speyer, Germany). The detailed properties of the soil are given by the supplier

129

(www.lufa-speyer.de). Briefly, the soil contained 86.0% sand, 11.5% silt and 2.5% clay. The

130

fractional organic carbon (fOC) value of the soil was 0.71%. The particle size distribution of the

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soil (see Supporting Information (SI) Figure S1) was measured using a laser diffraction particle

132

size analyzer (Mastersizer 2000, Malvern, U.K.), and the determined uniformity (a parameter

133

describing the symmetry of particle size distribution) was 0.46.

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Characterization of As-synthesized and Aged PSNPs. The physical dimensions and 7



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morphologies of the as-synthesized and aged PSNPs were characterized by scanning electron

136

microscopy (SEM) (S-3400 N II, Hitachi, Japan) and transmission electron microscopy (TEM)

137

(JEM-2100, JEOL, Japan). The specific surface area and the micropore volume were determined

138

by N2 adsorption/desorption at 77 K using an accelerated surface area and porosimetry system

139

(ASAP 2460, Micromeritics, Norcross, GA). Gel permeation chromatography (GPC) was carried

140

out on a SECurity GPC system (PL-GPC 120, Agilent technologies, USA) to calculate the

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number average molecular weight (Mn) and the weight average molecular weight (Mw). Two

142

coupled polystyrene gel columns (PL gel-MIXED B (10 μm, 300 mm×7.5 mm) and PL

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gel-MIXED C (5 μm, 300 mm×7.5 mm)) were used to separate the molecules using

144

tetrahydrofuran (THF) as eluent at a flow rate of 1.0 mL/min at 40 °C. The columns were

145

calibrated using molecular weight standards of polystyrene ranging from 580 Da to 2581 kDa.

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Glass transition temperature (Tg) was determined using differential scanning calorimetry (DSC)

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(204F1, Netzsch, Germany), with a heating rate of 10 °C/min.

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Surface elemental compositions of the PSNPs were determined using X-ray photoelectron

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spectroscopy (XPS) (PHI 5000 VersaProbe, Ulvac-Phi, Japan). Fourier transform infrared (FTIR)

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transmission spectra of the PSNPs were obtained by using a Thermo Nicolet NEXUS 870

151

spectrometer (Thermo Nicolet Corporation, Madison, WI). The relative hydrophobicity of

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PSNPs was assessed using a hydrocarbon partitioning test with laboratory-grade n-dodecane.57-59

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The contact angles of the PSNPs in three probing liquids (water, glycerol, and diiodomethane)

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were measured using an OCA-20 contact angle system (Dataphysics Instruments GmbH,

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Germany) at ambient temperature.

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Column Transport Experiments. Column transport experiments were conducted following the procedures developed in our previous study.39 Briefly, approximately 8 g Lufa soil 8


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were dry-packed into Omnifit borosilicate glass columns (10 cm × 1.0 cm, Bio-Chem Valve Inc.,

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Boonton, NJ) with 10-μm stainless-steel screens (Valco Instruments Inc., Houston, TX) on both

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ends.14,60,61 The columns were operated in an upward direction using syringe pumps (KD

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Scientific, Holliston, MA). Once packed, the column was flushed at a flow rate of 3 mL/h with at

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least 100 mL DI water followed by 300 mL background electrolyte solution. The porosity and

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dead volume were determined by inverse-fitting the breakthrough curves (BTCs) of a

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conservative tracer (KBr).

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Column transport experiments involved either transport experiments of the PSNPs, or

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co-transport experiments of PSNPs and a contaminant, pyrene or 4-nonylphenol (see SI Tables

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S1 and S2 for the column and influent properties of the experiments). Immediately prior to

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initiating the column experiments, the influents were prepared by first ultrasonicating a PSNPs

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

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(the mass concentrations of the PSNPs in the stock suspensions were determined based on the

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measured total organic carbon content (Shimadzu Scientific Instruments, Columbia, MD) and the

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elemental compositions (CHN-O-Rapid, Heraeus, Germany)) and then diluting with a

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background electrolyte in amber glass vials to give a concentration of working PSNPs

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suspension of 15 mg/L. For the influents containing an organic contaminant, a stock solution of

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an organic contaminant in methanol was added immediately after a working PSNPs suspension

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was prepared, to give a contaminant concentration of approximately 10 μg/L. The volume

177

percentage of methanol was kept below 0.1% (v/v) to minimize the cosolvent effects. The vials

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were sealed with Teflon-lined screw caps and equilibrated by tumbling end-over-end at 3 rpm for

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7 d. The hydrodynamic diameter (Dh) and ζ potential values of the PSNPs in different influents

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were determined using a ZetaSizer Nano ZS system (Malvern Instruments, Worcestershire, 9



181

U.K.). Within the time frame of the particle transport experiments the suspensions were stable, as

182

the average hydrodynamic diameters of the PSNPs changed little with time (see representative

183

data in SI Figure S2).

184

To initiate a column transport experiment, the influent was pumped into the column from a

185

100-mL glass syringe (SGE Analytical Science, Victoria, Australia). After 30 pore volumes (PV),

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the influent was switched to the respective background solution to flush the column, until PSNPs

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and contaminant concentration in the effluent was below the detection limits. Effluent samples

188

were collected every 2–3 PV to measure the concentrations of PSNPs and contaminants, when

189

applicable. The contaminants were quantified by determining radioactivity using a liquid

190

scintillation counter (LSC) (LS6500, Beckman Coulter, Fullerton, CA). The concentrations of

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the PSNPs in effluents were determined using a flow cytometer (LSRFortessa, Becton,

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Dickinson and Company, Franklin Lakes, NJ), based on pre-established calibration curves of the

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as-synthesized and aged PSNPs that correlate the mass concentrations of PSNPs to number

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concentrations (SI Figure S3).

195

To qualitatively understand the effects of aging-induced changes in the physicochemical

196

properties of PSNPs on PSNPs–soil interactions, the Derjaguin–Landau–Verwey–Overbeek

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(DLVO) and extended DLVO (XDLVO) interaction energy between nanoplastics particles and

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porous medium were calculated and analyzed (see SI for detailed equations).62,63

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Batch Adsorption and Desorption Experiments. The adsorption and desorption isotherms

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of pyrene and 4-nonylphenol to and from the as-synthesized and aged PSNPs were obtained

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using a batch adsorption/desorption approach developed in our previous studies.39,64 First,

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aliquots of a 50 mg/L PSNPs suspension in 0.5 mM NaCl were added to a series of 20-mL amber

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glass vials. Next, different amounts of 14C-labled pyrene or 4-nonylphenol stock solution were 10


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added. The vials were sealed using Teflon-lined screw caps and tumbled end-over-end at 3 rpm

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for 7 d at 25 °C to reach equilibrium. Afterward, the suspensions were filtered using 0.22-μm

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glass fiber filters to remove the PSNPs, and contaminant concentrations in the aqueous phase

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were determined (the loss of contaminants to the filter was predetermined and accounted for). To

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initiate a desorption experiment, approximately one half of the suspension was withdrawn from

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the vial at the end of the adsorption experiment, and an equal volume of adsorbate-free

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background electrolyte was added. The diluted suspensions were equilibrated for 7 d, and then

211

the aqueous-phase concentrations were measured. Three data points of each adsorption isotherm

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were selected to do the desorption experiments. All the adsorption and desorption experiments

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were run in duplicate.

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For the convenience of quantification and comparison of adsorption irreversibility, a thermodynamical index of irreversibility (TII) was calculated as:65

ln C γ  ln C D TII  ln C S  ln C D

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(1)

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where CS is the aqueous-phase concentration corresponding to an experimental adsorption point

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at which desorption is initiated; CD is the experimentally observed aqueous-phase concentration

219

during desorption (in equilibrium with an adsorbed-phase concentration of qD); and Cγ is the

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hypothetical aqueous-phase concentration in equilibrium with qD assuming desorption is

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completely reversible. Theoretically, the value of TII varies between 0 and 1, corresponding to

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the two boundary states of completely reversible adsorption and completely irreversible

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adsorption, respectively.

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Adsorption and desorption kinetics of pyrene or 4-nonylphenol to and from PSNPs were assessed using single batch experiments, carried out using 250-mL bottles with repeated 11



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sampling (approximately 0.5 mL aliquot each time).66 The adsorption/desorption kinetics results

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were fitted using first-order, two-compartment models:67,68

228

S t /S0  f1_ad (1  e

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S t /S0  f1_de e

 k1_de t

 k1_ad t

)  f 2_ad (1  e

 f 2_de e

 k2_ad t

)

(2)

 k2_de t

(3)

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where St is the contaminant mass adsorbed on PSNPs at time t (h) and S0 mass on PSNPs either

231

at adsorption equilibrium (for eq. 2) or in the beginning of a desorption experiment (eq. 3);

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f1_ad/f1_de and f2_ad/f2_de (f1 + f2 =1) are the fractions of contaminants residing in the rapidly and

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slowly adsorbing/desorbing compartments of PSNPs; and k1_ad/k1_de (h-1) and k2_ad/k2_de (h-1) are

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the rate constants describing rapid and slow adsorption/desorption.

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The sorption isotherms of pyrene and 4-nonylphenol to Lufa soil were obtained using a batch approach developed in our previous study.69

237 238 239

RESULTS AND DISCUSSION Physicochemical Characteristics of As-synthesized and Aged PSNPs. Aging processes

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(i.e., UV or O3 exposure) drastically modified the physicochemical characteristics of PSNPs

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(Table 1). The as-synthesized PSNPs were spherical in shape, with smooth surfaces (SI Figures

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S4 and S5) and an average particle size of 487.3 ± 18.3 nm. The morphological features (i.e.,

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shape, size and surface smoothness) of UV-PS1 did not differ noticeably from those of the

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untreated PS, likely due to the relative less intensive treatment involved. In comparison, UV-PS2

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appeared to have much rougher surfaces, as evidenced by the SEM and TEM images (Figures S4

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and S5), and nano-debris was observed on the surface of this UV-treated PS, probably

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attributable to the embrittlement and fragmentation of materials.40,41,70 The particle sizes of 12


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O3-PS and UV-PS2 were noticeably smaller compared with those of the as-synthesized and

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UV-PS1 (Table 1). The glass transition temperature, Tg, of the two UV-treated PSNPs (103.1°C

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for UV-PS1 and 102.1°C for UV-PS2) appeared to be similar to that of PS (102.8 °C), whereas

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the Tg of O3-PS could not be determined accurately (as the DSC curves did not show a distinct

252

transition temperature), indicating that the crosslinking structure of the polymer was likely

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damaged and the material became relatively amorphous. This significant structural change was

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consistent with the abovementioned reduction in particle size. Furthermore, the number average

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molecular weight (Mn) values of UV-PS2 and O3-PS were smaller than that of PS, in line with

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the chain scission process.41,71 Interestingly, the micropore volumes of the aged PS materials

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were approximately 2 to 5 times that of the as-synthesized PS, with the highest pore volume

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observed for UV-PS2.

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The as-synthesized PS had a high C/O ratio of 22.8, and therefore, very low content of

260

surface O-functional groups, likely in the form of epoxy/hydroxyl (Table 1). In comparison, the

261

UV- and O3-treated PSNPs had abundant surface O-functional groups, varying in

262

epoxy/hydroxyl, carbonyl and carboxyl groups (Table 1 and SI Figure S6). The relative degrees

263

of surface oxidation among the three aged PSNPs followed the order of UV-PS2 > O3-PS >>

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UV-PS1. Notably, the UV-treated and O3-treated PSNPs also differed in the concentration and

265

distribution of surface O-functionality; in particular, O3-PS had higher content of surface

266

carboxyl group. The FTIR spectra data (SI Figure S7) were generally consistent with the

267

abovementioned differences in the degree of surface oxidation among the four different materials.

268

Specifically, the spectrum of the as-synthesized PS showed five main peaks within the

269

2800–3010 cm−1 range, due to C–H stretching vibrations in aromatic rings and in the main chain,

270

whereas the peaks at 1602.6, 1492.7, 1452.2, 757.2 and 698.6 cm−1 are attributed to the 13



271

deformation and skeletal vibrations of C–H.72 The spectra of the aged PSNPs exhibited

272

additional bands. For example, a band at ~1739 cm−1 (C=O stretching) was observed for the three

273

aged PSNPs, and one at 1373 cm−1 (O–H bending) was observed for UV-PS2. Furthermore, the

274

O–C=O and C=O stretching bands at 1716.4 cm−1 and the peaks of C–O–C at 1183 cm−1 were

275

observed for O3-PS. Overall, the observed differences in surface chemistry in this study are

276

consistent with the findings of artificial aging of polystyrene microplastics43-45,48 and the surface

277

chemistry properties of the polystyrene microplastics samples collected in the environment.73

278

Owing to the introduction of surface hydrophilic functional groups, the hydrophilicity of the

279

aged nanoplastics was substantially higher than that of the as-synthesized PS, indicated by the

280

n-dodecane–water partition coefficients and the water contact angles (Table 1).

281

Aging Significantly Increased the Mobility of PSNPs in Saturated Porous Media. The

282

aged PSNPs exhibited much greater mobility, as indicated by the different extents of

283

breakthrough of the as-synthesized and the aged PSNPs from saturated loamy sand at different

284

cation concentrations and species (Figure 1). Under all the conditions tested, the mobility of the

285

PSNPs followed the order of PS < UV-PS1 < UV-PS2 < O3-PS (Figure 1). For example, when

286

the background electrolyte was 10 mM NaCl (Figure 1b), the maximum breakthrough (i.e., C/C0)

287

of PS only reached 31.1%, whereas those of the aged ones were 53.3%, 75.6% and 94.1% for

288

UV-PS1, UV-PS2 and O3-PS, respectively. The mobility of the two UV-treated PSNPs (UV-PS1

289

and UV-PS2) was subdued at the highest NaCl concentration tested (i.e., 50 mM, Figure 1c) and

290

at 0.5 mM MgCl2 (Figure 1d), whereas the mobility of O3-PS remained high, reaching

291

approximately 80% in both cases (Figure 1c and 1d).

292 293

The aged PSNPs, in particular, UV-PS2 and O3-PS, had much higher surface oxygen contents, due to the introduction of surface O-functional groups, than the as-synthesized one 14


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(Table 1). Consequently, the aged PSNPs were more negatively charged under all the conditions

295

tested (SI Table S3), a trait that would facilitate the transport of PSNPs in porous media. The

296

breakthrough data of the four PSNPs under different pH (SI Figure S8) further show the effects

297

of increased surface negativity on the higher mobility of the aged PSNPs. Specifically, the

298

mobility of PS and UV-PS1 increased considerably with the increase of pH from 5 to 9, likely

299

because the surface O-functional groups of these two materials were mainly in the form of

300

hydroxyl group, which underwent significant protonation–deprotonation within the pH range

301

tested. Accordingly, these two materials became more negatively charged at higher pH (SI

302

Figure S9). In comparison, the mobility of UV-PS2 and O3-PS were less affected by the changes

303

of pH (Figure S8), likely because their dominant surface functional groups were carboxyl groups,

304

which had much lower pKa values.74 The majority of the surface O-functional groups on UV-PS2

305

and O3-PS were deprotonated at relatively low pH, and thus increasing pH exhibited less

306

significant effect on their surface negativity (Figure S9). However, the differences in surface

307

charge negativity among the four PSNPs alone did not fully explain the variation in the mobility

308

of these materials. For instance, the DLVO particle–collector interaction profiles show that at 0.5

309

and 10 mM NaCl, the differences in both the height of the primary energy barrier (Φmax) and the

310

depth of the secondary minimum well (Φsec) were very small among the four different PSNPs (SI

311

Figure S10), inconsistent with the observed large differences in mobility (Figure 1). This

312

indicated that other important mechanism(s) were responsible for the enhanced mobility of the

313

aged materials.

314

The changes of particle aggregation, as the results of aging-induced surface modification of

315

the PSNPs, was not a viable explanation for the greater mobility of the aged PSNPs. For example,

316

the as-synthesized PSNPs already dispersed well in 0.5 and 10 mM NaCl, as indicated by the Dh 15



317

values (Table S3), and there were no significant differences in particle size among the four

318

PSNPs. Even though noticeable aggregation was observed at 50 mM NaCl and 0.5 mM MgCl2

319

for PS, as well as for UV-PS1 and UV-PS2 but to slightly lower degrees (Table S3), the sizes of

320

the aggregated particles were not sufficiently large to cause significant straining, based on the

321

calculated values dp/dc (the ratio of the particle to median grain diameter, Table S3).75,76

322

Moreover, for all four PSNPs the particle sizes in the effluent and influent were similar (SI

323

Figure S11, and SI Figure S12 vs. Figure S5), and the SEM images of dissected soil columns (SI

324

Figure S13) did not indicate size fractionation.

325

Aging-induced Increase in Hydrophilicity Was the Primary Cause for the Enhanced

326

Mobility of PSNPs. An interesting observation was that the relative mobility of the four PSNPs

327

correlated well with their degree of hydrophobicity, as indicated by the n-dodecane–water

328

partition coefficients and the water contact angles (Table 1). To understand if the enhanced

329

mobility of the aged PSNPs was related to their greater hydrophilicity, we calculated the

330

particle–collector interaction profiles using the XDLVO model.63,77-79 The calculated total

331

PSNPs–soil interaction energy based on XDLVO theory, which took into account the

332

electrostatic and van der Waals interactions, as well as the hydrophobic effect (SI Figures

333

S14-S18),80-82 were consistent with the observed relative mobility among the four PSNPs (Figure

334

1). In particular, the simulation clearly showed the important contribution of the hydrophobic

335

effect in the transport of PSNPs, as the Lewis acid–base interaction (VAB) profiles—which

336

indicates the extent of hydrophobic attraction or hydrophilic repulsion80—were significantly

337

different among the four PSNPs, whereas the van der Waals interaction (VVDW) and electrostatic

338

double layer interaction (VEDL) profiles were rather similar (Figures S15-S18). Overall, when

339

taking into account the effects of increased hydrophilicity of the aged PSNPs, the height of the 16


Page 16 of 41

Page 17 of 41


340

primary energy barrier (Φmax) became markedly different among different PSNPs, in the order of

341

O3-PS > UV-PS2 > UV-PS1 > PS (SI Table S4 and Figure S14). For example, in 50 mM NaCl

342

the primary energy barrier was essentially diminished for PS and UV-PS1, substantially subdued

343

for UV-PS2 (to 75.1 KBT), whereas it remained over 460 KBT for O3-PS and the total interaction

344

remained repulsive (Figure S14), consistent with the high mobility of O3-PS and the significantly

345

lower mobility of the other three (Figure 1c). Thus, it can be concluded that the aged PSNPs

346

exhibited higher mobility in saturated soil largely because of their greater surface hydrophilicity,

347

whereas the contribution from increased surface charge negativity was relatively small. Note that

348

aging appeared to also increase the resilience of PSNPs mobility to the changes of solution

349

chemistry (e.g., lower pH that is unfavorable for transport of negatively charged particles)

350

(Figure S8), further underscoring the significant effects of aging on mobility of nanoplastics.

351

Aging Significantly Enhanced Contaminant-mobilizing Ability of PSNPs by Increasing

352

both Mobility and Contaminant Binding. All three aged PSNPs facilitated the transport of the

353

two model contaminants, pyrene and 4-nonylphenol, to much higher extents than did the

354

as-synthesized PSNPs (Figure 2). In the absence of nanoplastics, both pyrene and 4-nonylphenol

355

exhibited minimal breakthrough in saturated loamy sand (Figure 2), due to the strong sorption of

356

these two compounds to Lufa soil (SI Figure S19). The as-synthesized PSNPs, PS, increased the

357

breakthrough of the nonionic, nonpolar pyrene to approximately 11% (Figure 2a), whereas had

358

essentially no effect on the transport of 4-nonylphenol, the polar contaminant (Figure 2b). These

359

polarity-dependent effects were consistent with our previous findings using a commercially

360

available polystyrene nanoplastics material of 80.4  7.9 nm.39 In comparison, at the same

361

nanoplastics concentration, the aged PSNPs were able to increase the breakthrough of pyrene to

362

24.2–65.6% and the breakthrough of 4-nonylphenol to 8.1–44.4% (Figure 2). The much greater 17



363

contaminant-mobilizing effects of the three aged PSNPs than the as-synthesized one on the polar

364

compound were particularly striking, because PSNPs were previously found to be unable to

365

enhance the transport of all the polar organic compounds tested, including bisphenol A,

366

bisphenol F and 4-nonlyphenol.39 This highlights the significant effects of aging on the

367

contaminant-mobilizing ability of PSNPs.

368

As shown earlier, the aged PSNPs had considerably greater mobility than the as-synthesized

369

PSNPs. This seemed to be a critical factor contributing to the much greater

370

contaminant-mobilizing abilities of the aged PSNPs.39,64,83,84 However, enhanced nanoplastics

371

mobility alone cannot fully explain the differential extents of enhanced contaminant transport by

372

different PSNPs. In particular, O3-PS had the highest mobility among the four PSNPs (Figure 1),

373

but it was not as effective in enhancing the transport of either pyrene or 4-nonylphenol as was

374

UV-PS2 (Figure 2), which consistently exhibited lower mobility (Figure 1). Previous research

375

has shown that in addition to the mobility of nanoparticles, the extent of nanoparticles-enhanced

376

contaminant transport strongly depends on how strongly nanoplastics can bind contaminants.64,85

377

Since aging resulted in significant alteration of both surface chemistry and physical structure of

378

PSNPs, it very likely could have influenced the binding strength of contaminants to PSNPs. Thus,

379

the inconsistency observed between the relative mobility of the PSNPs and their

380

contaminant-mobilizing abilities was probably due to the differential effects of aging on their

381

abilities to bind contaminants.

382

To more quantitatively understand the relative contaminant-binding abilities as affected by

383

different aging processes, we operationally defined an apparent contaminant-binding ability

384

index (Ibinding_app) of PSNPs, by normalizing the PSNPs-facilitated contaminant breakthrough

385

with the breakthrough of the respective PSNPs (see detailed derivation in SI): 18


Page 18 of 41

Page 19 of 41


I binding_app 

C /C0_cont  (1  ads%)  (C /C0_cont_free )

386

ads%  (C /C0_PSNPs )

387

where C/C0_cont and C/C0_PSNPs are the breakthrough of contaminant and PSNPs, respectively;

388

C/C0_cont_free is the breakthrough of contaminant without PSNPs; and ads% is the mass fraction of

389

PSNPs-bound contaminant in the influent. The values of C/C0_cont_free of pyrene and

390

4-nonylphenol were assumed to be 1.8% and 3.1%, respectively, based on the breakthrough of

391

these two compounds in the absence of PSNPs (Table S2, Columns 25 and 30). The estimated

392

Ibinding_app values (Figure 3) clearly show that aging significantly altered the ability of PSNPs to

393

bind contaminants, as the contaminant-binding ability followed the order of UV-PS2 > O3-PS >

394

UV-PS1 > PS, for both pyrene and 4-nonylphenol. This is consistent with the trends observed in

395

nanoplastics-facilitated transport of contaminants, particularly the nonpolar pyrene (Figure 2).

396

(4)

Aging Enhanced Binding of Nonpolar and Polar Contaminants to PSNPs via Distinctly

397

Different Mechanisms. Interestingly, while aging affected the binding of both nonpolar and

398

polar contaminants to PSNPs, the underlying mechanisms appeared to be compound-specific.

399

Aging appeared to have only small effects on the adsorption of pyrene, the nonpolar model

400

compound, as the adsorption isotherms to the four PSNPs nearly overlap, especially at relatively

401

high pyrene concentrations (Figure 4 and SI Table S5). The rather small differences in the

402

adsorption of nonpolar compounds were consistent with the findings in the literature using

403

microplastics,44 and were likely due to the combined effects of decreased surface hydrophobicity

404

of the PSNPs (Table 1),43 which would inhibit pyrene adsorption, and the increased surface area

405

and pore volume (Table 1), which would enhance adsorption through hydrophobic-effect-driven

406

micropore filling.86

19



407

Strikingly, the adsorption affinity of the polar compound, 4-nonylphenol, to the four

408

different PSNPs differed markedly, and followed the order of UV-PS2 > O3-PS > UV-PS1/PS

409

(Figure 4 and Table S5). The much more significant enhancement in the adsorption affinity for

410

4-nonylphenol than for pyrene was consistent with the significantly higher concentrations of

411

surface O-functional groups of the aged PSNPs, which allowed stronger adsorption of polar

412

compounds to the surfaces of PSNPs via enhanced hydrogen bonding.39,83 One possible

413

explanation for the higher adsorption affinity of UV-PS2 than O3-PS is that the latter was

414

considerably more hydrophilic, which was unfavorable for the adsorption of 4-nonylphenol via

415

the hydrophobic effect (note that the log KOW value of 4-nonylphenol is 4.28,87 indicating that

416

the hydrophobic effect was an important factor driving its adsorption).

417

Note that even though aging markedly increased the adsorption affinity of PSNPs for

418

4-nonylphenol, increased adsorption affinity could not fully account for the significantly greater

419

contaminant-mobilizing abilities of the aged PSNPs. In fact, if assuming that desorption of

420

contaminant from PSNPs was instantaneous and completely reversible, the increased adsorption

421

affinity of 4-nonylphenol to the aged PSNPs would have little effect on the breakthrough of the

422

contaminant (SI Figure S20), as the competitive sorption by porous media could easily

423

overshadow the increased adsorption of 4-nonylphenol to the aged PSNPs.39,83 Previous studies

424

have shown that desorption hysteresis, stemmed from thermodynamically irreversible adsorption

425

or slow desorption kinetics or both, is often the most important mechanism responsible for

426

nanoparticles-enhanced contaminant transport.83,84 Thus, it is reasoned that the increased

427

contaminant-binding abilities of the aged PSNPs were primarily attributable to the increased

428

desorption hysteresis of contaminants on the aged PSNPs, as the results of increased extent of

429

thermodynamic adsorption irreversibility and/or inhibited desorption kinetics. 20


Page 20 of 41

Page 21 of 41

430


Intriguingly, aging did not significantly increase the extent of irreversible adsorption of

431

pyrene (Figure 5). In fact, in the case of O3-PS, the observed TII value, which indicates the

432

degree of thermodynamically irreversible adsorption, was even lower than those associated with

433

the untreated PS. This was likely because O3 treatment significantly damaged the cross-linking

434

of the polymer, as indicated by the non-measurable Tg value (Table 1), making the structure of

435

the polymer considerably less glassy, which would decrease the extent of physical entrapment of

436

contaminant molecules.39 This was consistent with the considerably lower contaminant-binding

437

ability of O3-PS than UV-PS2 (Figure 3). One possible explanation for the stronger contaminant

438

binding associated with the aged PSNPs was that the aging-induced increase in the pore volume

439

of PSNPs, especially in the cases of UV-PS2 and O3-PS, likely resulted in slower diffusion of

440

organic molecules from the micropore regime, which is another common mechanism resulting in

441

desorption hysteresis.86,88,89 This hypothesis was verified experimentally, as pyrene exhibited

442

considerably slower adsorption and desorption kinetics to and from UV-PS2 and O3-PS than the

443

as-synthesized PS (Figure 6 and SI Table S6).

444

In contrary to pyrene, the polar compound, 4-nonylphenol, exhibited more significant

445

thermodynamically irreversible adsorption to the aged PSNPs (Figure 5), indicating that

446

enhanced degree of irreversible adsorption was likely an important mechanism contributing to

447

the increased binding of 4-nonylphenol to the aged PSNPs. This trend was particularly evident

448

for UV-PS2 and O3-PS (see the TII values in Figure 5), probably due to the enhanced H-bonding

449

of this polar compound to the more polar, O-functionality-rich surfaces of the two aged

450

materials.83 Thus, it is noteworthy that while enhanced H-bonding was a critical factor

451

responsible for the enhanced binding of 4-nonylphenol to the aged PSNPs, the effect was exerted

452

mainly by increasing the adsorption irreversibility, and less so by enhancing adsorption affinity. 21



453

Additionally, aging-induced decrease in desorption kinetics (Figure 6) also appeared to

454

contribute to the increased binding of 4-nonlyphenol.

455

Overall, even though for both pyrene and 4-nonylphenol the contaminant-binding strength

456

among the four different PSNPs followed the same order (as shown in Figure 3), the specific

457

mechanisms differed between these two types of compounds: increased binding of nonpolar

458

contaminants was mainly the result of aging-induced modification of the polymeric structure of

459

PSNPs that exacerbated slow desorption kinetics; for polar compounds, however, aging induced

460

changes in surface functional groups that rendered more significant polar interactions also played

461

a critical role, as such interactions not only increased adsorption affinity, but more importantly,

462

resulted in significant thermodynamic irreversible adsorption.

463

Environmental Implications. There has been increasing awareness of the potential

464

environmental risks and implications of microplastics and nanoplastics. While large effort has

465

been exerted to understand the potential impact of these materials, the use of pristine materials

466

may limit the values of related research. Recently, the effects of aging have received increasing

467

attention. The findings of this study further showed that aging of nanoplastics can significantly

468

alter their physicochemical properties, including both structural properties (e.g., pore volume,

469

pore structures, and surface roughness) and surface chemistry (e.g., functional groups and

470

surface charge). All of these changes can markedly affect the fate and transport of nanoplastics in

471

the environment, as well as the interactions between nanoplastics and environmental

472

contaminants. Additionally, the highly compound-specific effects call for better understanding of

473

how the interplay of the physicochemical properties of nanoplastics and contaminants, as well as

474

environmental factors (e.g., cations of bridging ability, natural organic matter, etc.), determines

475

the risks of these of materials in the environment. 22


Page 22 of 41

Page 23 of 41


476 477

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

478

China (Grants 21876089, 21425729 and 21876079), the Fundamental Research Funds for the

479

Central Universities, and the 111 Program of Ministry of Education of China (T2017002).

480 481

Supporting Information Available: Calculation of DLVO and XDLVO interaction energy,

482

calculation of apparent contaminant-binding ability index; tables summarizing the experimental

483

setups of column experiments, breakthrough results of nanoplastics-mediated contaminant

484

transport, average hydrodynamic diameter and ζ potential of PSNPs in the influents, calculated

485

results of particle–collector XDLVO interaction energy profiles, adsorption parameters and

486

first-order, two-compartment kinetics models fitting results; figures showing the particle size

487

distribution of Lufa soil, average hydrodynamic diameters of the influents, calibration curves of

488

PSNPs, SEM and TEM images, XPS and FTIR spectra, effects of pH on transport of PSNPs,

489

changes ζ potential of PSNPs with pH, particle size distribution of PSNPs and TEM images of

490

PSNPs in the effluents, SEM image of PSNPs in dissected soil columns, DLVO and XDLVO

491

particle–collector interaction energy profiles, sorption isotherms of pyrene and 4-nonylphenol to

492

Lufa soil, estimated breakthrough of pyrene and 4-nonylphenol assuming the desorption of

493

contaminant from PSNPs is instantaneous and completely reversible. This information is

494

available free of charge via the Internet at http://pubs.acs.org.

495 496

Notes—The authors declare no competing financial interest.

497 498

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34


Page 34 of 41

Page 35 of 41


Table 1. Selected physicochemical properties of as-synthesized and aged polystyrene nanoplastics (PSNPs). Ca (wt%) Carbonyl

Carboxyl

Total Ca (wt%)

Total Oa (wt%)

C/O ratio

n-Dodecane –water partition coefficient

Water contact angle (°)

Tg (°C)

M nb (kg /mol)

M wb (kg /mol)

BET surface area (m2/g)

Micropor e volume (cm3/g)

Average particle sizec (nm)

PSNPs

Aromati c rings

Epoxy/ hydroxy l

PS

99.3

0.701

-d

-d

95.8

4.21

22.8

0.39 ± 0.04

94.4 ± 1.1

102.8

24.8

168.1

7.419

0.000068

487 ± 18

89.7

8.44

1.08

0.75

91.8

8.11

11.3

0.29 ± 0.04

83.7 ± 1.1

103.1

23.8

191.7

7.170

0.000137

473 ± 39

76.0

16.6

3.40

4.01

48.3

49.4

0.978

0.25 ± 0.05

70.5 ± 1.4

102.1

16.8

120.5

8.046

0.000337

441 ± 35

61.1

24.4

1.34

13.2

56.3

34.9

1.61

0.18 ± 0.01

65.3 ± 0.6

-e

18.4

121.2

12.94

0.000147

422 ± 45

UV-PS 1 UV-PS 2 O3-PS

a Analyzed

using XPS.

bM

n

represents the number average molecular weight and Mw represents the weight average molecular

weight. c The

average size

of PSNPs particles was measured using ImageJ software by analyzing at least 150 particles in scanning electron microscopy (SEM) images of each PSNP. d Not

detected. e Not applicable.

35



(b) 10 mM NaCl

1.2

1.2

1.0

1.0 C/C0_PSNPs

C/C0_PSNPs

(a) 0.5 mM NaCl

0.8 0.6 0.4

0.8 0.6 0.4 0.2

0.2

0.0

0.0 0

10

20

30

40

0

50

10

20

(c) 50 mM NaCl 1.2

1.0

1.0 C/C0_PSNPs

1.2

0.8 0.6 0.4

0.0 30

40

50

0

10

PV PS

40

50

(d) 0.5 mM MgCl2

0.4

0.0 20

50

0.6

0.2 10

40

0.8

0.2 0

30 PV

PV

C/C0_PSNPs

Page 36 of 41

20

30 PV

UV-PS1

UV-PS2

O3-PS

Figure 1. Effects of aging on mobility of polystyrene nanoplastics (PSNPs) in saturated loamy sand as functions of ionic strength and species: (a) 0.5 mM NaCl (Columns 1–4); (b) 10 mM NaCl (Columns 5–8); (c) 50 mM NaCl (Columns 9–12); and (d) 0.5 mM MgCl2 (Columns 13–16).

36


Page 37 of 41


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

10

20

30

40

50

0

10

20

PV

C/C0_PSNPs

1.2

C/C0_pyrene

1.2

30

40

50

PV

(b) 4-nonylphenol

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_4-nonylphenol

C/C0_PSNPs

(a) pyrene

0.0 0

10

20

30

40

50

0

10

30

40

50

PV

PV PS UV-PS2

20

UV-PS1 O3-PS

control

Figure 2. Effects of aging on contaminant-mobilizing abilities of PSNPs (15 mg/L): (a) transport of pyrene (Columns 25-29); and (b) transport of 4-nonylphenol (Columns 30-34) in saturated loamy sand. The left panel shows the breakthrough curves of nanoplastics, and the right panel breakthrough curves of the respective contaminants. The term “control” represents the contaminant transport experiments carried out in the absence of nanoplastics.

37



(b) 4-nonylphenol

(a) pyrene 1.5

1.5

1.0

1.0

Ibinding_app

Ibinding_app

Page 38 of 41

0.5

0.5

0.0

0.0 0

5

10

15

20

25

0

30

5

10

20

25

30

PV

PV PS

15

UV-PS1

UV-PS2

O3-PS

Figure 3. Comparison of the apparent contaminant-binding ability index (calculated using eq. 4) of as-synthesized vs. aged PSNPs for pyrene and 4-nonlylphenol, as indicated by the mass fraction of a contaminant in the effluent that was co-eluted with PSNPs.

38


Page 39 of 41


(a) pyrene

(b) 4-nonylphenol 4

10

10

103

103

q (mg/kg)

q (mg/kg)

4

102

101 10-4

-3

-2

10

10

102

101 10-4

-1

10

10-2

10-1

C (mg/L)

C (mg/L) PS

10-3

UV-PS2

UV-PS1

O3-PS

Figure 4. Adsorption isotherms of pyrene (a) and 4-nonylphenol (b) to as-synthesized vs. aged PSNPs.

39



(a) PS

(b) UV-PS1

4

4

10

10

TII = 0.89 ± 0.11

103

q (mg/kg)

q (mg/kg)

TII = 0.89 ± 0.07

102

103

102

TII = 0.20 ± 0.07

TII = 0.27 ± 0.09

1

1

10

10

10-4

10-3

10-2

10-1

10-4

10-3

C (mg/L)

10-2

10-1

C (mg/L)

(c) UV-PS2

(d) O3-PS

104

104 TII = 0.82 ± 0.13

TII = 0.70 ± 0.06

103

q (mg/kg)

q (mg/kg)

Page 40 of 41

102

103

102

TII = 0.60 ± 0.11

101 10-4

-3

10

-2

10

TII = 0.43 ± 0.06 -1

10

101 10-4

10-3

C (mg/L)

10-2

10-1

C (mg/L)

pyrene adsorption pyrene desorption

4-nonylphenol adsorption 4-nonylphenol desorption

Figure 5. Comparison of the extents of irreversible adsorption of pyrene and 4-nonylphenol to as-synthesized vs. aged PSNPs. The filled symbols are adsorption data and hollow symbols are desorption data. The thermodynamic index of irreversibility (TII) values were calculated using eq. 1.

40


Page 41 of 41


(b) 4-nonylphenol_adsorption

1.2

1.2

1.0

1.0

0.8

0.8 St/S0

St/S0

(a) pyrene_adsorption

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

20

40

60

80

0

20

t (h)

60

80

t (h)

(c) pyrene_desorption

(d) 4-nonylphenol_desorption

1.00

1.0

0.98

0.9

0.96

St/S0

St/S0

40

0.94

0.8 0.7

0.92 0.90

0.6 0

20

40

60

80

0

20

t (h)

40

60

80

t (h) PS

UV-PS2

O3-PS

Figure 6. Adsorption and desorption kinetics of pyrene (a, c) and 4-nonylphenol (b, d) to and from PS, UV-PS2 and O3-PS. The lines were plotted by curve fitting the data using first-order, two-compartment models (eq. 2 or 3).

41


Aging Significantly Affects Mobility and Contaminant-Mobilizing Ability

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