Fe3O4-biochar amendment

Water Research 169 (2020) 115284

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Transport behaviors of plastic particles in saturated quartz sand without and with biochar/Fe3O4-biochar amendment Meiping Tong a, *, Lei He a, Haifeng Rong a, Meng Li a, Hyunjung Kim b a

The Key Laboratory of Water and Sediment Sciences, Ministry of Education, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, PR China b Department of Mineral Resources and Energy Engineering, Chonbuk National University, Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do, 561-756, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 August 2019 Received in revised form 22 October 2019 Accepted 4 November 2019 Available online 7 November 2019

As an environmentally friendly material, biochar has been widely used to remediate soil/water contaminants such as heavy metals and organic pollutants. The addition of biochar or modified biochar to porous media might affect the retention of plastic particles and thus influence their fate in natural environment. In this study, both biochar and magnetic biochar (Fe3O4-biochar) were synthesized via a facile precipitation method at room temperature. To determine the significance of biochar and Fe3O4biochar amendment on the transport and deposition behaviors of plastic particles, the breakthrough curves and retained profiles of three different sized plastic particles (0.02 mm nano-plastic particles, and 0.2 mm and 2 mm micro-plastic particles) in quartz sand were compared with those obtained in quartz sand either with biochar or Fe3O4-biochar amendment in both 5 mM and 25 mM NaCl solutions. The results show that for all three different sized plastic particles under both examined solution conditions, the addition of biochar and Fe3O4-biochar in quartz sand decreases the transport and increases the retention of plastic particles in porous media. Fe3O4-biochar more effectively inhibits the transport of plastic particles than biochar. We found that the addition of biochar/Fe3O4-biochar could change the suspension property and increase the adsorption capacity of porous media (due to the increase of porous media surface roughness and negatively decrease the zeta potentials of porous media), contributing to the enhanced deposition of plastic particles. Moreover, we found that negligible amount of biochar and Fe3O4-biochar (<1%) were released from the columns following the plastic particle transport when the columns were eluted with very low ionic strength solution at high flow rate (to simulate a sudden rainstorm). Similarly, small amount of plastic particles were detached from the porous media under this extreme condition (16.5% for quartz sand, 14.6% for quartz sand with biochar amendment, and 7.5% for quartz sand with Fe3O4-biochar amendment). We found that over 74% of the Fe3O4-biochar can be recovered from the porous media after the retention of plastic particles by using a magnet and 87% plastic particles could be desorbed from Fe3O4-biochar by dispersing the Fe3O4-biochar into 10 mM NaOH solution. In addition, we found that the amendment of unsaturated porous media with biochar/ Fe3O4-biochar also decreased the transport of plastic particles. When biochar/Fe3O4-biochar were added into porous media as one layer of permeable barrier near to column inlet, the decreased transport of plastic particles could be also obtained. The results of this study indicate that magnetic biochar can be potentially applied to immobilize plastic particles in terrestrial ecosystems such as in soil or groundwater. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Plastic particles Magnetic biochar Retention Adsorption Porous media

1. Introduction

* Corresponding author. E-mail address: [email protected] (M. Tong). https://doi.org/10.1016/j.watres.2019.115284 0043-1354/© 2019 Elsevier Ltd. All rights reserved.

The use of agricultural mulch and greenhouse, the irrigation of water with plastics, as well as the application of sewage sludge containing microplastic particles on the agricultural land lead to the ubiquitous presence of plastic particles in terrestrial ecosystems

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especially in soil (Nizzetto et al., 2016; Rillig, 2012; Scheurer and Bigalke, 2018; Zhang and Liu, 2018). For example, Nizzetto et al. (2016) estimated that 63,000e43,0000 and 44,000e300,000 tonnes of microplastics are likely added annually to farmlands in Europe and North America, respectively. Scheurer and Bigalke (2018) reported that 90% of Swiss floodplain soils contain plastic particles with concentrations of up to 55.5 mg kg
1. Zhang and Liu (2018) found that the concentrations of plastic particles in four cropped areas and an established riparian forest buffer zone at Dian Lake (in southwestern China) ranged from 7100 to 42,960 particles kg
1 (mean 18,760 particles kg
1). Since they contain different surface functional groups (Alimi et al., 2018; Wang et al., 2018), plastic particles can adsorb pollutants such as heavy metals and organic pollutants on their surfaces (Brennecke et al., 2016; Hirai et al., 2011; Hu et al., 2017). Thus, the release of plastic particles into the natural environment such as soil system could induce the environmental risk. The presence of plastic particles has been found to change soil properties (Qi et al., 2018), have an influence on the soil biota (Huerta Lwanga et al., 2016), and have negative effects on crop growth (Maaß et al., 2017; Qi et al., 2018; Rillig et al., 2017). For example, Huerta Lwanga et al. (2016) found that plastic particles significantly reduce the growth rate of the earthworm Lumbricus terrestris. Plastic particles have been found to be able to enter into the terrestrial food chain (Huerta Lwanga et al., 2017) and thus could pose health risks of humans (Bouwmeester et al., 2015; Sharma and Chatterjee, 2017). Therefore, it is essential to inhibit the transport behaviors of plastic particles in porous media after they are released into natural environment. As an environmentally friendly material, biochar or modified biochar has been widely used to remediate soil contaminants such as heavy metals (Rajendran et al., 2019; Shen et al., 2019; Yang and Jiang, 2014) and organic pollutants (Liang et al., 2019; Rong et al., 2019; Zhang et al., 2010). For example, Zhang et al. (2010) found that the addition of Pinus radiate-derived biochars to soil could enhance the sorption of phenanthrene in soil. Rajendran et al. (2019) found that the concentrations of cadmium in rice were reduced when adding sulfur and sulfur-iron modified biochar to paddy soils. Shen et al. (2019) reported that the introduction of MgO-coated biochar to soil significantly facilitated lead removal. Owing to the numerous oxygen functional groups on biochar, it is high likely that biochar could interact with plastic particles. The addition of biochar into porous media might reduce the transport behaviors of plastic particles and thus decrease their environmental risks. It should be noted that it is commonly difficult to recover biochar after they are added into porous media. Due to their magnetic property, Fe3O4 nanoparticles could be efficiently recycled after use via the employment of external magnetic force (Shan et al., 2015; Zhang et al., 2014). Thus, the modification of biochar by magnetic Fe3O4 could improve the potential recovery of the biochar being added into porous media. Moreover, Fe3O4 nanoparticles commonly contain positive surface charge under pH < 6.5 (Jin et al., 2015). Thus, they can adsorb negatively charged particles via attractive electrostatic interactions at pH < 6.5. It is expected that the modification of biochar by magnetic Fe3O4 could also increase the adsorption of plastic particles. The addition of magnetic Fe3O4modified biochar to porous media might further increase the retention of plastic particles in the porous media. In addition, by using magnetic force, Fe3O4-modified biochar might be recoverable from porous media after its application to retain plastic particles. Herein, biochar and the Fe3O4 modified biochar (Fe3O4-biochar) were synthesized and characterized. Their performances towards the retention of plastic particles in porous media were investigated. In present study, carboxylate-modified polystyrene latex microspheres with diameter of 0.02 mm (as model nano-plastic particles), 0.2 mm and 2 mm (as model microplastic particles) were employed.

Both the breakthrough curves and retained profiles of all three different sized plastic particles under environmentally relevant conditions in 5 mM and 25 mM NaCl solutions in quartz sand were compared with those in quartz sand with 0.5% biochar amendment and in quartz sand with 0.5% Fe3O4-biochar amendment. The possible mechanisms enhancing the deposition of plastic particles by biochar and Fe3O4-biochar are proposed and discussed. The release of biochar, Fe3O4-biochar, and plastic particles from the columns with increasing flow rate and decreasing ionic strength (to stimulate a rainstorm) were investigated. The recovery efficiency of Fe3O4-biochar from the porous media after the plastic particle transport was determined. The desorption of plastic particles after they were adsorbed onto Fe3O4-biochar was testified. In addition, the transport behaviors of plastic particles in unsaturated porous media with biochar/Fe3O4-biochar amendment as well as in saturated porous media with biochar/Fe3O4-biochar as permeable barrier near to the column inlet were also investigated. 2. Materials and methods 2.1. Synthesis of biochar and Fe3O4-biochar The biochar was prepared by heating cellulose (purchased from J&K Scientific) under oxygen-limited conditions in a muffle furnace at 400  C for 6 h. The detailed synthesis protocol was described in detail in a previous study (Yang et al., 2018). The Fe3O4-biochar was synthesized following a previous study (Shan et al., 2015). Briefly, 0.932 g FeCl3$6H2O and 0.985 g FeSO4$7H2O were dissolved in 200 mL Milli-Q water in a 500 mL flask, and then 2 g synthesized biochar was dispersed into the solution by ultrasonication for 5 min. N2 was continually bubbled through the 200 mL solution to expel oxygen under vigorous mechanical agitation. NaOH (2 M L
1) was added dropwise to the flask at room temperature until the pH reached 10. The black precipitate was then formed and separated with an external magnet, which was washed repeatedly with deionized water until the supernatant was neutral (pH 7). Then, the composites (referred as Fe3O4-biochar) were acquired after vacuum freeze drying for 24 h. The mass ratio of biochar to Fe3O4 was set to be 5:1 for the transport experiments. The detailed information regarding why we chose this mass ratio is provided in Fig. S1. Composites with different mass ratios of biochar to Fe3O4 (1:1, 2:1, and 10:1) were also prepared according to the similar procedure. 2.2. Biochar and Fe3O4-biochar characterization The biochar and Fe3O4-biochar were characterized by powder Xray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy spectrometer (FT-IR), scanning electron microscope (SEM), Brunauere- Emmette-Teller (BET) adsorption isotherms, as well as hysteresis loop analysis. The zeta potentials and the sizes of biochar and Fe3O4-biochar under experimental conditions were characterized using dynamic light scattering (DLS). The concentration of biochar and Fe3O4biochar were determined by an UV spectrophotometer. The detailed information is provided in the Supplementary Materials. 2.3. Preparation of the plastic particle suspension Polystyrene is one of the most widely distributed and abundant plastic types in the environment (Alimi et al., 2018; Carpente et al., 1972; Ivleva et al., 2017). Thus, similar as previous studies (Alimi et al., 2018; Cai et al., 2019; Jeong et al., 2017; Li et al., 2019), carboxylate-modified polystyrene latex microspheres (Molecular Probes, Invitrogen Canada Inc.) with diameter of 0.02 mm (as model nano-plastic particles), 0.2 mm and 2 mm (as model microplastic

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particles) were employed in present study. The influent concentrations of the NPs or MPs were maintained at 4 mg L
1 (±15%) by diluting the stock suspensions. A fluorescence spectrophotometer was used to analyze the concentrations of the NPs and MPs. The detailed information is provided in the Supplementary Materials. The zeta potentials and the sizes of plastic particles, biochar, Fe3O4biochar, and quartz sand in both 5 mM and 25 mM NaCl solutions at pH 6 were determined by a Zetasizer Nano ZS90 (Malvern Instruments, UK). Each measurement was repeated 9e12 times at room temperature (25  C). The corresponding results are provided in Table S1. 2.4. Column transport experiments Porous media (quartz sand without and with the addition of biochar/Fe3O4-biochar) were wet-packed into cylindrical Plexiglas columns (10 cm long and 2 cm inner diameter). The detailed column packing protocol is provided in Supplementary Materials. After packing, all sand columns were first equilibrated using ten pore volumes (PVs) of salt solutions at the desired ionic strength and pH. Following pre-equilibration, 3 PVs of the plastic particle suspension were injected into the column. This was followed by elution with 5 PVs of salt solution at the same ionic strength. The suspension and salt solutions were injected in an up-flow orientation at 8 m day-1 (0.73 mL min-1), using a syringe pump (Harvard PHD 2000; Harvard Apparatus Inc., Holliston, USA), to represent fluid velocities in coarse aquifer sediments, forced-gradient conditions, or engineered filtration Systems (Cai et al., 2013; Harter et al., 2000). For selected experiments after the entire plastic particle transport process, 10 PVs of 0.1 mM NaCl solution was injected into the column with increasing flow rate up to 4 mL min
1 to imitate the flow rate of a heavy rain event. Column effluent samples were collected and analyzed to yield the breakthrough curves, and the quartz sand was dissected into 10 segments and employed to yield the retained profiles. The protocol for obtaining the mass balance is provided in the Supplementary Materials. The detailed overall recovery (mass balance) for the plastic particles in each experiment is provided in Table S2. 3. Results and discussion 3.1. Characterization of biochar and Fe3O4-biochar The phase and crystal structures of biochar and Fe3O4-biochar were characterized by XRD analyses, and the results are shown in Fig. 1a. The distinct peak at 2q ¼ 23.0 observed in the XRD pattern of biochar (fabricated at 400  C) is consistent with previous observations (its pyrolytic temperature was higher than 350  C) (Hao et al., 2013; Keiluweit et al., 2010). After being modified by Fe3O4, the peaks at 2q ¼ 30.1, 35.5 , 43.1, 57.1, and 62.6 , which correspond to the planes of (112), (103), (004), (321), and (224) of magnetite (JCPDS 75e1609) (Liang et al., 2018), are present in the XRD pattern of Fe3O4-biochar. The FT-IR results (Fig. S2) show that both biochar and Fe3O4-biochar contain hydrophilic functional groups, such as C¼O, C¼C, C-H, and C-O, which is consistent with previous studies (Chen et al., 2019; Wan and Li, 2018). Moreover, a strong adsorption peak at 561 cm
1 (Fe-O) is observed in the FT-IR spectra of Fe3O4-biochar, suggesting that iron oxides are present on the surfaces of the biochar. As shown in the XPS results (Fig. S3a), the main characteristic peaks of Fe3O4-biochar are Fe2p (724 eV), O1s (531 eV), and C1s (285 eV). As shown in the high-resolution XPS spectrum of C (Fig. S3b), the sharp peak at 284.5 eV is corresponded to the C-C bonds (Li and Li, 2019), while the peaks at 285.6 eV and 288.9 eV could be assigned to C-O bonds and O-C¼O bonds, respectively

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(Wan and Li, 2018). In the high resolution XPS spectra of Fe (Fig. S3c), the binding energy of Fe 2p 1/2 appears at 723.4 eV, while that of Fe 2p 3/2 located at 713.5 eV, both of which are consistent with previously reported values of Fe3O4 (Ma et al., 2019). The peaks at 531.6 eV, 533.2 eV, and 530.3 eV observed in the XPS spectra of O (Fig. S3d) can be attributed to C¼O, O-C¼O, and Fe-O, respectively (Ma et al., 2019; Wan and Li, 2018). The results further confirm that Fe3O4 has been successfully anchored on the biochar. The EDS spectrum (Fig. 1b) shows the Fe distribute areas aligned with those of O, which further confirms the presence of Fe3O4 on the biochar surface. Moreover, the contentes of S and Cl are very low (data not shown), which agrees well with the XPS results and indicates the well crystallized Fe3O4-biochar. The BET surface area of biochar (Fig. S4) is calculated to be 2.0 m2 g-1, which is similar as those synthesized at about 400  C reported in previous studies (Park et al., 2013; Zhang et al., 2017). After modification with Fe3O4, the BET surface area of the Fe3O4biochar increases to 24.2 m2 g
1, which suggests that Fe3O4-biochar might have more adsorption sites and provide stronger adsorption capability than biochar. The SEM image (Fig. 1d) shows that the biochar have a rough surface. As shown in Fig. 1e, Fe3O4 particles are deposited onto the biochar surface. The mass percentage of Fe3O4 on the Fe3O4-biochar is determined to be 0.1733. Both Fe3O4 and Fe3O4-biochar are found to be superparamagnetic (Fig. S5). The magnetic saturation (MS) values of Fe3O4 and Fe3O4-biochar are determined to be 56.2 emu g
1 and 10.7 emu g
1, respectively. The Fe3O4-biochar can be conveniently and easily separated from water by a magnet within 2 min (Fig. S6). In 5 and 25 mM NaCl solutions at pH 6, the zeta potentials of biochar are determined to be negative (Table S1), while the overall zeta potentials of Fe3O4-biochar are positive due to the deposition of positively charged Fe3O4 onto the biochar surface. 3.2. Effects of biochar and Fe3O4-biochar amendment on plastic particle transport To understand whether the addition of biochar and Fe3O4-biochar into quartz sand would affect the transport and retention of plastic particles, transport experiments of three different sized plastic particles (0.02 mm NPs, 0.2 mm MPs, and 2 mm MPs) in quartz sand without and with biochar/Fe3O4-biochar amendment were conducted at low (5 mM) and high ionic strength (25 mM) in NaCl solutions at pH 6. Without biochar/Fe3O4-biochar amendment, the breakthrough curves of all three different sized plastic particles in clean quartz sand are high at both examined ionic strength conditions (Fig. 2). Specifically, over 75% of plastic particles (for all three different sizes) pass through the quartz sand columns in 5 mM NaCl solution. The increase of solution ionic strength decreases the zeta potentials of all three different sized plastic particles, as well as those of quartz sand (Table S1), which decreases the overall DLVO interaction between the plastic particles and the quartz sand (Fig. S7). Thus, the breakthrough curves of all three different sized plastic particles at high ionic strength (25 mM NaCl) are relatively lower than those at low ionic strength (5 mM NaCl). The lower breakthrough curves of colloids with increasing ionic strength have been widely reported (Bradford et al., 2007; Dong et al., 2018; Peng et al., 2017). Although the breakthrough curves of all three different sized plastic particles in 25 mM NaCl are lower relative to those in 5 mM NaCl, the percentages of plastic particles that passed through the quartz sand columns are still high in 25 mM NaCl solution. Around 58e70% of plastic particles break through the quartz sand columns in 25 mM NaCl solution (Fig. 2 and Table S2). Under both examined ionic strength conditions, the breakthrough curves of all three different sized plastic particles in quartz

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Fig. 1. XRD patterns of biochar and Fe3O4-biochar samples (a). SEM-EDX elemental mapping of the Fe3O4-biochar sample (b). SEM images of sand (c), biochar (d), and Fe3O4-biochar samples (e).

sand with biochar/Fe3O4-biochar amendment at an application rate of 0.5% are lower than those in clean quartz sand (Figs. 2 and 3). For instance, in 5 mM NaCl solutions, the breakthrough percentage for 0.02 mm NPs, 0.2 mm MPs, and 2 mm MPs in clean quartz sand is 78.5%, 77.7% and 75.2%, respectively. The corresponding percentage with the addition of biochar in quartz sand yet decreases to 68.5%, 37.4% and 36.9%, respectively. The introduction of Fe3O4-biochar to quartz sand further decrease the breakthrough percentage of 0.02 mm NPs, 0.2 mm MPs, and 2 mm MPs to 0.5%, 1.3% and 2.6%, respectively. These observations demonstrate that the addition of small amount of biochar/Fe3O4-biochar into quartz sand decreases the transport of all three different sized plastic particle in quartz sand under both ionic strength conditions. Abit et al. (2014) also reported that pine chip biochars decreased the transport of 1 mm diameter carboxylated polystyrene latex microspheres. Note that compared to those with biochar amendment, the breakthrough curves of all three different sized plastic particles in quartz sand with Fe3O4-biochar amendment are obviously lower under both examined ionic strength conditions. The observation indicates that although the addition of biochar to quartz sand also decrease the transport of plastic particles, Fe3O4-biochar amendment is much more effective in inhibiting the transport of plastic particles. Comparison of the retained plastic particle profiles in quartz sand versus those in biochar/Fe3O4-biochar amendment quartz sand shows that for all three different sized plastic particles at both ionic strengths, the amounts of plastic particles retained in quartz sand with the addition of biochar/Fe3O4-biochar are greater than those in bare quartz sand (Table S2) and thus the corresponding retained profiles are higher in quartz sand with biochar/Fe3O4biochar amendment relative to those in bare quartz sand (Figs. 2 and 3, right). The observations clearly show that the application of biochar/Fe3O4-biochar increases the retention of all three

different sized plastic particles in porous media. Moreover, for all three different sized plastic particles in both 5 and 25 mM NaCl solutions, the Fe3O4-biochar results in greater plastic particle deposition than biochar. The results indicate that both biochar and Fe3O4-biochar amendment could decrease the transport and increase the deposition of all three different sized plastic particles in quartz sand. Moreover, Fe3O4-biochar amendment is more effective to retain plastic particles than biochar. 3.3. Mechanisms for the enhanced deposition of plastic particles 3.3.1. Alteration of the effluent properties Previous studies have found that the application of biochar to soil could change the soil effluent properties (Abit et al., 2012; Bolster and Abit, 2012; Laird et al., 2010). To determine whether the addition of biochar and Fe3O4-biochar would also affect the properties of column effluents, both influents and effluents from columns packed with only quartz sand, quartz sand with biochar amendment, and quartz sand with Fe3O4-biochar amendment prior to the injection of plastic particle suspension (after preequilibration for 10 PVs of 5 mM NaCl solutions) were collected. The concentrations of metal ions (Al, Ca, Fe, Mg, and K), pH, and conductivity in both influents and effluents were analyzed. The results (Table S3) show that for column packed only with quartz sand, the pH value, conductivity, and the concentrations of Al, Fe, Mg, and K ions in both influent and effluent are similar, which indicates that the properties of the salt suspension do not change during its passage through the quartz sand columns. Although the pH value and the concentrations of Al, Fe, Mg, and K ions are comparable in both influent and effluent of the columns packed with quartz sands with either biochar or Fe3O4-biochar amendment, the concentrations of Ca ions and the conductivity are higher

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Fig. 2. Breakthrough curves (left) and retained profiles (right) of three different sized plastic particles without (circle) and with biochar (square) and Fe3O4-biochar (triangle) amendment in quartz sand in 5 mM NaCl solutions at pH 6. Here, “w/” refers to “with”. The error bars represent standard deviations from replicate experiments (n  2).

in the effluents of these columns. The observation indicates that the addition of either biochar or Fe3O4-biochar to quartz sand slightly changes the effluent properties. The increased conductivity and concentration of Ca ions in the effluents of biochar or Fe3O4-biochar amendment quartz sand suggests that compared with those in bare quartz sand, the ionic strength of plastic particle suspension during its passage through the amendment columns would be slightly increased, which would thus affect the zeta potentials of plastic particles. To verify this, zeta potentials of all three different sized plastic particles in both influent and effluent suspensions in 5 mM NaCl solutions were determined. The results (Table S4) show that for clean quartz sand columns without biochar amendment, the zeta potentials of all three different sized plastic particles in the effluents are equivalent to those in the influents, which is consistent with the negligible change of suspension properties. However, the zeta potentials of all three different sized plastic particles in the effluents collected from the columns with biochar and Fe3O4-biochar are slightly less negative relative to those in the influents. The observation indicates that biochar or Fe3O4-biochar amendment negatively decreases the zeta potentials of plastic particles. Thus, less repulsion between the plastic particle and the quartz sand with biochar amendment and even attraction between the plastic particle and the quartz sand with Fe3O4-biochar amendment are expected, which theoretically supports the observation of the increased retention of plastic particles in the columns with biochar and Fe3O4-biochar amendment relative to those without this modification. However, it should be noted that both the effluent properties and the zeta potentials of plastic particles in the effluents collected from the biochar amendment columns are similar to those collected from the Fe3O4biochar amendment columns, yet the addition of Fe3O4-biochar

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Fig. 3. Breakthrough curves (left) and retained profiles (right) of three different sized plastic particles without (circle) and with biochar (square) and Fe3O4-biochar (triangle) amendment in quartz sand in 25 mM NaCl solutions at pH 6. Here, “w/” refers to “with”. The error bars represent standard deviations from replicate experiments (n  2).

induces more effective retention of plastic particles than the addition of biochar. This observation suggests that other mechanisms also contribute to the increased deposition of plastic particles when Fe3O4-biochar is added to quartz sand. 3.3.2. Alteration of the porous media porosity Previous studies have shown that the porosity of porous media affects the transport of colloids (decreasing porosity increases colloid deposition) (Moussaoui et al., 1992; Peulen and Wilkinson, 2011). The porosity of columns packed only with quartz sand, quartz sand with biochar amendment, and quartz sand with Fe3O4biochar amendment was determined by the gravimetric method (Chen et al., 2011). The porosity of quartz sand columns without amendment is 0.42 ± 0.019, while the porosity of columns both with biochar amendment and with Fe3O4-biochar amendment are found to be approximately 0.41 ± 0.012. The addition of biochar and Fe3O4-biochar to quartz sand slightly decrease the porosity by 0.01. This slightly decreased porosity (only by 0.01) might have negligible contribution to the increased deposition of plastic particles in the columns. Note that the porosities of the columns with biochar and Fe3O4-biochar amendment are equal, yet the increased plastic particle deposition is more obvious in the Fe3O4-biochar amendment columns. The observation suggests that other mechanisms might contribute to the more obvious enhancement of the plastic particle deposition due to the application of Fe3O4-biochar to quartz sand. 3.3.3. Alteration in the porous media adsorption capacity Biochar and modified biochar have been reported to have superior adsorption capabilities for soil contaminates such as organic

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pollutants and heavy metals (Mohan et al., 2014; Qian et al., 2016; Shen et al., 2019). The addition of biochar/Fe3O4-biochar to quartz sand might increase the adsorption capability for plastic particles, and thus lead to the enhanced deposition of plastic particles. To test this, adsorption experiments were conducted by choosing 0.02 mm NPs and 0.2 mm MPs as representative plastic particles. It can be clearly seen from Fig. 4 that, for both 0.02 mm NPs and 0.2 mm MPs in both 5 and 25 mM NaCl solutions after 2 h adsorption, the amounts of plastic particles adsorbed onto the different materials have the following order: Fe3O4-biochar > biochar > quartz sand. For instance, the adsorption fraction of 0.02 mm NPs by Fe3O4-biochar, biochar, and quartz sand in 5 mM NaCl solution is 31.7%, 12.8%, and 5.1%, respectively. The orders of the adsorption capacity for plastic particles are consistent with the deposition trends of the plastic particles in porous media (quartz sand with Fe3O4-biochar amendment > quartz sand with biochar amendment > quartz sand without amendment). Clearly, the transport and deposition behaviors of plastic particles in porous media have correlation with the adsorption capacity of porous media. The addition of biochar/Fe3O4-biochar into quartz sand might alter the zeta potentials of porous media, leading to the increased plastic particle adsorption capacity of treated sand relative to bare

Fig. 4. Adsorption capacity of 0.02 mm NPs (a) and 0.2 mm MPs (b) by sand, biochar, and Fe3O4-biochar in 5 mM and 25 mM NaCl solutions after 2 h of adsorption. “B” and “FeB00 refer to “biochar” and “Fe3O4-biochar”, respectively.

quartz sand (increased deposition of plastic particles in amended porous media). Table S1 shows that under all examined conditions, zeta potentials of both biochar and Fe3O4-biochar treated quartz sand are less negative relative to those of clean quartz sand, which suggests that the electrostatic repulsion between the plastic particles and the quartz sands with biochar and Fe3O4-biochar amendment is lower relative to those between the plastic particles and the bare quartz sand without amendment (Fig. S7), which theoretically supports the larger adsorption capacity for plastic particles by quartz sands with biochar and Fe3O4-biochar amendment relative to bare quartz sand. As a result, the decreased transport and enhanced deposition of all three different sized plastic particles is observed in the quartz sand with biochar and Fe3O4-biochar amendment. Moreover, the zeta potentials of sand with Fe3O4-biochar amendment are less negative than those of sand with biochar amendment (due to the less negative zeta potentials of Fe3O4-biochar than biochar) (Table S1). The electrostatic repulsion (or energy barrier) between plastic particles and quartz sand surfaces with Fe3O4-biochar amendment is therefore lower than that between plastic particles and quartz sand with biochar amendment (Fig. S7). Accordingly, greater deposition (lager adsorption) of plastic particles in quartz sand with the addition of Fe3O4-biochar than in quartz sand with the addition of biochar is observed. Previous studies have also found that the transport and deposition of bio-colloids (Sasidharan et al., 2016), plastic particles (Li et al., 2019), and graphene oxides (Wang et al., 2017) would be greatly affected by the alteration in the zeta potentials of porous media. Surface roughness of porous media might be altered by the addition of biochar/Fe3O4-biochar into quartz sand, leading to the change of adsorption (deposition) capacity of plastic particles. Previous studies have shown that surface roughness (both nanoand micro-scale) plays critical roles on the retention of colloids in porous media (Bradford and Torkzaban, 2013, 2015; Jin et al., 2017; Rasmuson et al., 2019; Shen et al., 2014; Torkzaban and Bradford, 2016). SEM images of quartz sand, biochar, and Fe3O4-biochar (Fig. 1c, d, and 1e) show that, for these three surfaces, the roughness appears in the following order: Fe3O4-biochar > biochar > quartz sand. Note that the greater surface area of Fe3O4-biochar (24.2 m2 g1 ) than that of biochar (2.0 m2 g-1) (Fig. S5) also indicates the larger surface roughness of Fe3O4-biochar relative to biochar. The increased surface roughness might contribute to the enhanced deposition of plastic particles in quartz sand with biochar/Fe3O4biochar relative to that in the bare quartz sand. To investigate whether the roughness would influence interaction force, we calculated the DLVO interaction with the consideration of the surface roughness (Fig. S8). Details on the interaction energy calculation are provided in the Supplementary Materials. The results show that the consideration of roughness significantly decreases the values of the energy barrier between plastic and all of the porous media (bare quartz sand and quartz sands with biochar and Fe3O4biochar amendment). For example, without considering the surface roughness, the energy barriers between the 0.02 mm NPs, 0.2 mm MPs, and 2 mm MPs and the bare quartz sand in 5 mM NaCl solution are about 27 kT, 290 kT and 3220 kT, respectively (Fig. S7); taking the roughness into consideration, the values of energy barrier decrease to about 10 kT, 30 kT, and 220 kT, respectively (Fig. S8). More importantly, when considering the surface roughness, the values of energy barrier between plastic particle and sand with the addition of biochar/Fe3O4-biochar under all examined solution conditions are lower than those between the plastics and the quartz sand without amendment, which theoretically supports the enhanced deposition (adsorption) of plastic particles in quartz sand with biochar/Fe3O4-biochar relative to that in bare quartz sand. Moreover, when considering the roughness, the energy barrier

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between the plastic particles and the Fe3O4-biochar-modified sand are lower than that between the plastics and the biochar-modified sand. As a result, greater deposition of plastic particles is obtained in the Fe3O4-biochar modified sand than in the biochar-treated sand. Although both biochar and Fe3O4 (positive charge) could adsorb plastic particles, deposition of Fe3O4 onto the biochar surfaces yet might have better adsorption performance than physical mixture of biochar and Fe3O4. Comparison of the breakthrough curves of plastic particles in quartz sand with Fe3O4-biochar amendment (0.5%) versus those without amendment, with the addition of 0.41% (w/w) biochar, with the addition of 0.09% (w/w) Fe3O4, and with the addition of physical mixture of 0.41% (w/w) biochar and 0.09% (w/w) Fe3O4 indicates that among the five amendments, the Fe3O4biochar amendment display the best plastic particle retention performance (Fig. 5). Due to its magnetic properties, Fe3O4 tends to form aggregates and their specific surface area thus decreases. However, it would be difficult for Fe3O4 particles deposited onto

Fig. 5. Breakthrough curves (a) and breakthrough efficiency (b) of 0.02 mm NPs with bare sand (blank), sand with 0.5% (w/w) biochar, sand with 0.41% (w/w) biochar, sand with 0.09% Fe3O4, sand with physical mixture of 0.41% (w/w) biochar and 0.09% (w/w) Fe3O4, and sand with 0.5% (w/w) Fe3O4-biochar amendment in 5 mM NaCl solution. “B”, “Fe” and “Fe-B” refer to “biochar”, “Fe3O4” and “Fe3O4-biochar”, respectively.

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biochar surfaces to form aggregates (they were evenly deposited on biochar surfaces) (Fig. 1). Thus, the surface area favorable for plastic particle adsorption is greater for Fe3O4-biochar. As a result, the greatest plastic retention is observed in quartz sand with Fe3O4biochar amendment (especially greater retention than physical mixture of biochar and Fe3O4). Shen et al. (2019) also found that MgO-modified biochar more effectively adsorbs lead cations than a physical mixture of MgO and biochar due to the even distribution of MgO particles on biochar surface. 3.3.4. Immobilization performance at high flow rates and the recovery of Fe3O4-biochar after application A sudden rainstorm would increase the flow rates and decrease the solution ionic strength (Chatra et al., 2019; Hernandez-Crespo et al., 2019), which might lead to the release of the biochar/ Fe3O4-biochar and the detachment of plastic particles. To investigate the release of biochar/Fe3O4-biochar and the detachment of plastic particles from the columns, washing experiments were conducted by choosing 0.02 mm NPs as representative plastic particles and 25 mM NaCl solution as representative deposition experimental solution condition. After the transport experiment of 0.02 mm NPs in 25 mM NaCl solution condition in porous media (quartz sand and quartz sands with either biochar or Fe3O4-biochar amendment), 10 PVs of 0.1 mM NaCl solution at pH 6 was injected into the columns with flow rate of 4 mL min
1 to imitate heavy rain and the results are provided in Fig. S9 and Fig. 6. A negligible amount of biochar and Fe3O4-biochar (<1%) is flushed out of the columns, which indicates that, after they were added to the quartz sand columns, the biochar and Fe3O4-biochar are relatively stable even when flushed with high flow rate (Fig. S9). Small amounts of the retained 0.02 mm NPs are found to detach from the porous media with increasing flow rate and decreasing ionic strength (Fig. 6). Specifically, 16.5%, 14.6%, and 7.5% of plastic particles (based on the total amount of plastics retained in columns) detach from quartz sand, quartz sand with biochar amendment, and quartz sand with Fe3O4-biochar amendment, respectively. Clearly, the addition of Fe3O4-biochar to porous media not only causes more plastic particles to be retained, but also results in less plastic particles being re-mobilized after their retention than in the case of biochar

Fig. 6. Breakthrough curves of 0.02 mm NPs during the transport and flushing experiment.

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amendment or no amendment under conditions of increasing flow rate and decreasing ionic strength. It is worth pointing out that after the addition of Fe3O4-biochar to quartz sand prior to the plastic particle transport experiment, 95.9% ± 1.8% of the Fe3O4-biochar could be recovered by magnet. After the plastic particles transport experiment (0.02 mm NPs in 25 mM NaCl as the representative case), 74.7% ± 1.6% of the Fe3O4biochar (with plastic particles adsorbed onto their surfaces) could still be recovered. Note that the approach that Fe3O4-biochar were recovered after use by employing magnetic separation technique from the porous media slurry is similar as the recovery of nanoscale zero valent iron from soil slurry, which has been well addressed in previous study (Phenrat et al., 2019). Moreover, 87.3% ± 5.4% plastic particles (0.02 mm NPs as representative) could be desorbed from Fe3O4-biochar by dispersing the Fe3O4-biochar (with the adsorption of plastic particles onto their surfaces) into 10 mM NaOH solution with ultrasonication for 10 min. Since the majority of plastic particles could be desorbed from the surfaces of Fe3O4-biochar, it is highly likely that Fe3O4-biochar can be utilized repeatedly for plastic particles immobilization after they were recovered and retreated to desorb plastic particles. The above results clearly show that the addition of Fe3O4-biochar to porous media could increase the immobilization of plastic particles. Moreover, the majority of the Fe3O4-biochar could be recycled after its application to porous media to retain plastic particles. Furthermore, we found that majority of plastic particles can be desorbed from the surfaces of Fe3O4-biochar. The above results indicate that Fe3O4-biochar has the potential application for plastic particle immobilization in soil. 4. Conclusion The results show that with the addition of 0.5% of biochar or Fe3O4-biochar to quartz sand, the transport of three different sized plastic particles in porous media is greatly decreased under two examined solution ionic strength conditions (5 and 25 mM) in NaCl solutions. Compared with biochar, Fe3O4-biochar exhibits more effective performance for the retention of all three different sized plastic particles under examined conditions. The mechanisms driving to the enhanced plastic particle retention induced by the addition of biochar and Fe3O4-biochar were discussed. The change in the suspension property, the increase in the adsorption capacity resulted from the increase in the porous media surface roughness and the negative decrease in the zeta potentials of porous media are found to contribute to the enhanced deposition of plastic particles. Moreover, the potential release of biochar and Fe3O4-biochar from the porous media were tested by increasing flow rate and lowering solution ionic strength (to simulate the heavy rain). We found that increasing the flow rate and decreasing the ionic strength did not lead to the release of biochar and Fe3O4-biochar from the columns. In addition, after being immobilized by Fe3O4-biochar, only 7.5% of the previously deposited plastic particles can be released from column under the extreme flushing condition. Due to its magnetic property, 74% of the Fe3O4-biochar could be recovered from the porous media after the retention of plastic particles using a magnet. Although the results shown in the present study are drawn from the experiments performed in saturated porous media, yet the representative experiments conducted in unsaturated porous media (Fig. S10) also show the similar trend that the addition of biochar/Fe3O4-biochar into unsaturated porous media also decrease the transport of plastic particles. The observations clearly indicate that for both unsaturated and saturated porous media, the addition of biochar/Fe3O4-biochar could inhibit the transport of plastic particles. Furthermore, we found that decreased transport of all three different-sized plastic particles were also observed when the same amount of biochar/Fe3O4-biochar were added into porous

media as one layer of permeable barrier near to column inlet (Fig. S11), indicating that biochar/Fe3O4-biochar especially magnetic biochar could be potentially applied as permeable barrier (Faisal et al., 2018; Johnson et al., 2008) to immobilize plastic particles in porous media. After the adsorption capacity of permeable barrier is fully filled, biochar/Fe3O4-biochar can be extracted and replaced. Meanwhile, preventing groundwater from plastic particle contamination might also be achieved by injecting the biochar/ Fe3O4-biochar into subsurface at high flow rates. The push techniques to inject the nanoparticles into the vicinity of the source zone of contamination via using high flow rates have been well addressed in previous studies (Phenrat et al., 2010a, 2011; Phenrat and Kumloet, 2016). However, it should be noted that the plastic particle transport experiments were conducted in quartz sand under simple solution chemistry conditions in present study. Previous studies (Amirbahman and Olson, 1994; Cai et al., 2019; He et al., 2018; Li et al., 2019; Peng et al., 2017; Phenrat et al., 2010b; Walker and Bob, 2001) have shown that natural organic matter, clay particles, microorganisms, engineered nanoparticles, and so on, which are ubiquitously present in the natural environment, have influence on the transport of plastic particles in porous media. These copresence of colloids definitely would also affect the transport of plastic particles in porous media with Fe3O4-biochar amendment. The recovery of Fe3O4-biochar from soil and the desorption of plastic particles from Fe3O4-biochar would also be affected by the copresence of colloids. Moreover, the recycle process of Fe3O4-biochar after use via magnet-assisted soil washing might lead to the change of certain nutrients in natural environment. To test the feasibility of biochar/Fe3O4-biochar in the inhibition of plastic particle transport in natural environments such as in soil, further studies regarding the effects of copresence of colloids on the plastic particle transport and the recycle of Fe3O4biochar, as well as the nutrients interference should be concerned. Overall, the results of this study clearly showed that magnetic biochar could be potentially applied to immobilize plastic particles in soil or groundwater, yet further studies still required prior to their real applications. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This work was supported by the Beijing Natural Science Foundation under Grant No. JQ18030, National Natural Science Foundation of China under Grant No. 51779001 and Fund for Innovative Research Group of NSFC under Grant No. 51721006. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.watres.2019.115284. References Abit, S.M., Bolster, C.H., Cai, P., Walker, S.L., 2012. Influence of feedstock and pyrolysis temperature of biochar amendments on transport of Escherichia coli in saturated and unsaturated soil. Environ. Sci. Technol. 46, 8097e8105. Abit, S.M., Bolster, C.H., Cantrell, K.B., Flores, J.Q., Walker, S.L., 2014. Transport of Escherichia coli, Salmonella typhimurium, and microspheres in biochar-amended soils with different textures. J. Environ. Qual. 43, 371e378. Alimi, O.S., Budarz, J.F., Hernandez, L.M., Tufenkji, N., 2018. Microplastics and nanoplastics in aquatic environments: aggregation, deposition, and enhanced contaminant transport. Environ. Sci. Technol. 52, 1704e1724.

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