Transport of micron-sized polyethylene particles in confined aquifer: Effects of size, ageing, and confining pressure

doi:10.21203/rs.3.rs-2224623/v1

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Transport of micron-sized polyethylene particles in confined aquifer: Effects of size, ageing, and confining pressure

https://doi.org/10.21203/rs.3.rs-2224623/v1

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Contamination of groundwater by microplastics (MPs) is increasingly reported and draw growing attention due to their potential risks. To understand how MPs migrate into the groundwater, many previous works have investigated the transport of MPs using man-made microspheres of few microns or smaller as models. However, MPs observed in the environment are more diverse in size, shape, and types, which may have different migration behaviors. In this work, transport of irregular shaped polyethylene (PE) particles in sand packed column was studied. Small MPs (22-37 μm) generally have a high mobility than large MPs (44-74 μm) but can also be affected by aging. Ageing decreased the hydrophobicity of the MPs and increased their surface negative charge, which could have facilitated the transport of MPs. However, the physical barrier of space in the porous medium might have a greater influence on the transport of the MPs. The retention of the MPs was enhanced with the increase in pressure due to compression that decreased pore size. Results from this study showed that MPs of environmental features can also be transported in the groundwater but the processes could be governed by different factors from that of man-made plastic microspheres.

Microplastics

Groundwater

Ageing

Confining pressure

Transport

Mankind is now in the plastic age with the world’s plastic production increasing from 1.7 million tons in 1950 to 367 million tons in 2020 (PlasticEurope, 2021; Tan et al., 2021). The improper disposal of waste plastics has caused serious environmental pollution (Thompson et al., 2009). The plastic debris can break down into microplastics (MPs, < 5 mm in size) through abiotic and biotic processes, leading to MPs pollution being a more serious problem (Barnes et al., 2009; Qi et al., 2020).

Studies showed that MPs are ubiquitous in all environmental compartments (Ali et al., 2021; Liao et al., 2021; Wan et al., 2022). Most of the previous works on microplastics focused on aquatic environment while recent research indicated that microplastic pollution in terrestrial environment are even more serious (Wong et al., 2020). It’s reported that annual release of plastic wastes into the terrestrial system was 4 to 23 times higher than that into the ocean (Horton et al., 2017). Recent evidences demonstrated that MPs in terrestrial environment can be transported into groundwater (Panno et al., 2019; Peng et al., 2021; Wan et al., 2022), which aroused potential concern on drinking water safety issues.

Several works have investigated the vertical transport of microplastics in porous media. Transports of MPs were found to be influenced by many external factors, such as flow rate, water chemistry, porous media properties, etc. (He et al., 2020; Hou et al., 2020; Tan et al., 2021). In addition, internal factors like the ageing of plastic can also have an effect (Ren et al., 2021a). However, most of the works use spherical MPs as a model, while MPs in the environment have various shapes that may affect their transport (Sun et al., 2015). In addition, the influence of head pressure on the transport of MPs in groundwater is unclear.

Therefore, the purpose of this work is to study the transport of irregular-shaped MPs in confined aquifers. The effects of MPs size, ageing, and confining pressure were examined using a simulation device. Results from this work provide a better understanding on the transport of MPs in groundwater environment and the controlling factors.

2.1 Materials

The natural river sand collected from Yueyang City (Hunan Province, China) was used for the experiment. The sample was air-dried and sieved through a 2 mm sieve to remove large particles and impurities, and then baking at 400℃ for 3 hours to remove organic materials that may interfere the quantification of MPs. Prepared sand sample was stored in a desiccator before being used in the experiment.

Polyethylene (PE) MPs were purchased from Huachuang Plastic Company (Dongguan, China). According to the supplier, the MPs were prepared by pulverizing large plastics in a mill with a liquid nitrogen and passed through a 6.5 µm sieve in frozen state. However, we measured the size distribution of the purchased MPs was 22–74 µm at room temperature. Due to a wide size range, the MPs was further passed through a 45 µm sieve in the laboratory, and MPs of two different size ranges (22–37 µm and 44–74 µm) were obtained.

A half of the MPs were aged in an UV aging chamber (30 W, 365 nm irradiation) for 40 days to simulate photo weathering. MPs were placed in a petri dish with ultrapure water 15 cm from the UV lamps. Water loss was replenished every 2 days.

Size distribution of the MPs was measured by S3500 Particle Size Analyzer (Microtrac, USA) (Fig. S1). The infrared spectra of the MPs were analyzed using a Nicolet iN10 attenuated total reflection Fourier-transform infrared spectrometer (ATR-FTIR) (Thermo, USA) (Fig. S2). The composition of the natural river sand was measured using a SmartLab SE X-Ray Diffractometer (Rigaku, USA) (Fig. S3). The morphology of the MPs was examined using a JSM-IT 300HR Scanning Electron Microscope (SEM) (JEOL, Japan) (Fig. S4). The contact angle of unaged and aged PE MPs was measured by an OCA Contact Angle Tester (Dataphysics, Germany) (Fig. S5). The zeta potentials of PE MPs and river sand were determined by Zetasizer Nano ZEN2600 (Malvern Instruments, UK). Properties of the tested materials are presented in Table 1.

Table 1

Properties of the tested materials.
Materials	Ageing	Size (µm)	Contact angle (°)	Zeta Potential (mV)
PE	Unaged	22–37	141.37	-10.00
	Unaged	44–74	141.37	-11.31
	Aged	22–37	138.28	-15.53
	Aged	44–74	130.77	-17.23
River sand	-	704–2000	-	-23.68

2.2 Experimental device

A flexible wall permeameter (FWP) was employed for the experiment (Kandalai et al., 2018). Confining pressure was adjusted to simulate different head pressures of confined aquifer. The latex membrane with an inner diameter of 5 cm and filled with 10 cm sand was supported by a hollow drilled polyvinyl chloride (PVC) pipe, to form a three-dimensional pressure. Once the initialization phase was completed, the background solution started to inflow. The integral device is depicted in Fig. 1.

2.3 Column experiment

We selected three confining pressures (0.1, 0.3, 0.5 MPa), two sizes (22–37 µm, 44–74 µm), and non-ageing and ageing processing as variables to generate 12 treatment groups. Details on the three sets of matching variables are described in Table 1. The sand sample was packed into the column with a porosity of 0.46 under atmospheric pressure. A Mariotte bottle served as an inlet device to deliver a constant flux for the permeameter (Kireš, 2006). A magnetic agitator was placed under the Mariotte bottle to mix the MPs with water. At the beginning of the experiment, 5 pore volumes (PVs) of deionized water were loaded into the column for pre-equilibration, followed by 3 PVs of MPs suspension. To ensure the counting is more accurate, a concentration of 20 mg/L, about 1.5 billion PE MPs/L was chosen. Then, the column was eluted with 3 PVs of deionized water. The effluent samples were collected in glass bottles from the outlet every 0.5 PVs. At the end, the sand column was sliced at an increment of 1 cm and MPs remained in each layer were analyzed. Subsequently, nonreactive tracer tests were carried out with 1 mM KBr to determine the hydrodynamic properties of the sand columns with no retardation and details are provided in the supporting information (S5). All experiments were conducted in duplicate.

2.4 Analysis

The sand samples were excavated from the columns and then were transferred into a glass bottle and weighed. MPs were extracted following a previous method (Zhang et al., 2017). Nile red (NR) stain was used for the quantification of MPs referring to Shim et al. (2016). Concentrations were calculated by counting under a fluorescence microscope. Changes of pore volume of the sand column was determined using a Model WG Single Lever Consolidmeter by measuring the amount of compression deformation and the compression process. Details are provided in Supplementary Materials.

2.5 Modeling

The transport of MPs was modeled using Hydrus 1D software. The advection-dispersion equation was used to describe the transport of MPs in the sand column. The dispersion coefficient (cm² min^− 1) was obtained by fitting the tracer breakthrough curves. The first-order retention coefficient (K, min^− 1) and the maximum solid phase particle concentration (S_max) were used as the fitting parameters.

3.1 Transport and retention of MPs

Breakthrough curves (BTCs) of the MPs at different tested conditions are presented in Fig. 2. MPs were detected in the effluent after loading of 0.5 PV MPs suspension and reached plateau after loading of 1 PV suspension. After 3 PV suspension, MPs in the effluent began to decrease and reached equilibrium after 2 PV of deionized water. Our results demonstrated that irregular shaped MPs could be transported in conferred aquifer. Our results agree with previous observations, which showed that spherical MPs can be transported in natural soils and sand (Dong et al., 2019; Tong et al., 2020; Wu et al., 2020).

Retained profiles of MPs in the column at the end of the experiment were presented in Fig. 3. In all treatments, a large portion of the MPs were retained in the column after elution with 3 PVs of deionized water, which could be attributed to the presence of small pores in the column that blocked MPs physically. The RPs scatter considerably within a small dynamic range. This can be attributed to non-uniform size of sand (Fig. S4a), which provides the different deposition sites in each section of the column. The incomplete symmetrical shape of the tracer BTC (Fig. S6) indicates that the non-homogeneous sand column may also cause the scattering.

Mass balance and parameters of the BTCs of MP transport in conferred aquifer are presented in Table 2. The percentage of MPs retained in the column varied from 58–117%, which are much higher than results from previous works with smaller MPs (Dong et al., 2018; Tong et al., 2020).The fitness of the BTCs varied from 0.007 to 0.870, which is relatively low comparing to those works with MP spheres (Dong et al., 2018; Ren et al., 2021a). This may be related to a wider size range and irregular shape (Fig. S4) of the MPs, which caused deviation from the ideal situation.

Table 2

Mass balance and parameters of the BTCs of MP transport in the experiment.
Size (µm)	Pressure (MPa)	Mass Balance			Parameters
Size (µm)	Pressure (MPa)	M_eff^a	M_ret^b	M_tot^c	S_max (mg/kg)	K (min^− 1)	R²
Unaged MPs
22–37	0.1	0.28	0.62	0.90	0.497	0.189	0.633
	0.3	0.27	0.91	1.18	0.107	0.131	0.328
	0.5	0.08	0.98	1.06	0.601	0.152	0.147
44–74	0.1	0.23	0.58	0.81	0.289	0.167	0.472
	0.3	0.13	0.90	1.03	0.216	0.811	0.671
	0.5	0.09	1.17	1.26	0.102	0.172	0.007
Aged MPs
22–37	0.1	0.45	0.58	1.03	0.396	0.154	0.868
	0.3	0.12	0.68	0.80	0.982	0.216	0.858
	0.5	0.12	1.05	1.17	0.601	0.152	0.092
44–74	0.1	0.46	0.63	1.09	0.166	0.168	0.828
	0.3	0.23	0.64	0.87	0.194	0.259	0.870
	0.5	0.15	0.89	1.03	1.022	0.172	0.500

^a Percentage of MPs eluted out of the column in the effluent.

^b Percentage of MPs retained in columns.

^c Total percentage of MPs recovered from each treatment.

3.2 Effect of size on the transport of MPs

Comparing the transport of MPs of two size ranges, more 22–37 µm unaged MPs were eluted out of the column in the effluent than 44–74 µm unaged MPs (Table 1). However, 22–37 µm aged MPs in the effluent were comparable even less than 44–74 µm aged MPs at certain sites. The infiltration of MPs was found depended on the ratio between the diameter of MPs and the diameter of the porous medium d_MP/d_PM (Keller et al., 2020). Our results suggested that transport of MPs in confined aquifer is related to the size of the MPs but can also be affected by aging.

(Dong et al., 2018) studied the transport and retention of MP spheres with diameter ranged from 0.1 to 2.0 µm, complete breakthroughs of all MPs were observed in deionized water while retention of MPs was enhanced in simulated seawater, and increase of MP size from 0.8 to 2.0 mm resulted in reduced MP transport. (Cai et al., 2019) showed that breakthrough of polystyrene (PS) MPs with a diameter of 0.2, 1, and 2 µm in quartz sand decrease with the increase of MP diameter and the presence of TiO₂ nanoparticles enhanced the retention of MPs.

In those previous works, MPs investigated are a few microns to hundreds of nanometers in size. Those MPs can exhibit colloidal properties during transport in porous medium. Aggregation of MPs was observed and has a great influence on their transport in porous medium (Dong et al., 2018; Tong et al., 2020). In this work, sizes of the MPs are much larger and are closer to MPs actually detected in the environment, and more MPs are retained.

Another work studied the transport of irregular shaped polyvinyl chloride (PVC) and low-density polyethylene (LDPE) (125–300 µm) in saturated quartz sand, and found that small sized MPs and fragmentation promoted migration while larger MPs experienced obstruction (Tumwet et al., 2022). It was suggested that secondary microplastics with irregular shapes are more likely to experience fragmentation and infiltrate deeper than primary microplastics when transporting through porous media.

3.3 Effect of ageing on the transport of MPs

Ageing decreased the contact angle of the MPs and the zeta potential became more negative (Table 1), indicating that aged MPs became more hydrophilic and the double layer repulsive force between particles became greater. Thus ageing enhanced the transport of MPs at all conditions (Table 2).

Previously, both chemical and photo ageing were found to be able to increase the transport of MPs in saturated soils (Liu et al., 2019; Ren et al., 2021a). Similar to the result of this work, ageing of MPs caused increase in surface negative charge and hydrophobicity. Model calculation suggested that greater surface hydrophilicity was the mainly responsible for higher mobility of the aged MPs, whereas the contribution from increased surface charge negativity was relatively small (Liu et al., 2019).

In this work, the contact angle of both aged and unaged PE MPs was larger than 90°. Although ageing decreased the contact angle of the MPs, their surface was still hydrophobic. However, the contact angle decreased from > 90° to < 90° for polystyrene (PS) nanoplastics after aging (Liu et al., 2019), which changed from hydrophobic to hydrophilic. The zeta potential of MPs became more negative after ageing in our study and previous works due to the introduction of hydrophilic functional groups as a result of oxidation (Zhang et al., 2022). Both the MPs and the river sand are negatively charged, and the electrostatic repulsion between the aged MPs and between the aged MPs and the river sand increased.

For small MPs, the retention of MPs could be related to the deposition or electrostatic attraction. As demonstrated by (Wu et al., 2020), retention of PS nanoparticles in soils was positively correlated with Fe/Al oxides contents due to electrostatic attraction. In another study, it was found that enhanced deposition of PS latex microspheres (diameter of 0.2, 1, and 2 mm) with carboxylic functional groups in the presence of nano-TiO₂ at pH 5, which resulted in increased retention of the MPs (Cai et al., 2019). However, MPs used in this work are larger and have an irregular shape, the physical barrier of pores in the river sand column may be more important on the retention of the MPs.

3.4 Effect of confining pressure on the transport of MPs

Groundwater is buried underground, and could be under certain pressure in confining aquifer. In this work, the effect of confining pressure on the transport of MPs was assessed up to 0.5 MPa, which corresponds to 50 m water head difference. The porosity of the sand column after pressurization of 100, 300 and 500 MPa was 0.40, 0.37 and 0.33, respectively. For both aged and unaged MPs, increase in confining pressure enhanced the retention of MPs. The retention of unaged MPs increased from 62–91% and from 58–117% for 22–37 µm and 44–74 µm MPs, respectively. The retention of aged MPs increased from 58.3–105% and from 63–89% for 22–37 µm and 44–74 µm MPs, respectively.

The influence of pressure could be related to the deformation of the sand column, which resulted in a decrease in pore size due to compression. Result of the consolidated test is presented in Fig. 4, deformation of the sand columns under pressure reached equilibrium in a few minutes and increased with the increase of pressure. At higher pressure, sand particles are more densely packed, the space between the particles become smaller. Therefore, MPs are more likely be retained. Enhanced retention under higher pressure suggests that MPs could be more difficult to transport in deep groundwater in general. However, characteristics of the aquifer, such as particles size, mineral composition, and hydro-chemical characteristics, could also have an impact on the migration of MPs (Ren et al., 2021b).

Transport of PE MPs in confined aquifer was studied using a flexible wall permeameter with packed sand column in a simulation experiment. Effects of size, ageing, and confining pressure on the MPs were assessed. Results indicated that MPs of two different size ranges (22–37 µm and 44–74 µm) can be transported in the sand column. Small size MPs have a high mobility but their transport was affected by aging. Ageing decreased the hydrophobicity of the MPs and increased surface negative charge, which could facilitate the transport of MPs. However, the physical obstacle of space in the porous medium may have a greater influence on the transport of large MPs compared with those of few microns or less. The retention of the MPs was enhanced with the increase in pressure due to compression that can decrease pore size. Therefore, deep groundwater might be less vulnerable to MPs. Different processes may be involved for MPs of different sized. In future, more attention should be paid to the transport of MPs with environmental features other than man-made plastic spheres.

Ethical Approval: Not applicable

Consent to Participate: Not applicable

Consent to Publish: Not applicable

Authors Contributions: All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Xin Chen, Yong Wan, and Jia Jia. The first draft of the manuscript was written by Xin Chen and Jia Jia. Xue Qiang and Chenxi Wu provided the guidance, corrected, and approved the final manuscript.

Funding: This work was supported by the National Natural Science Foundation of China (41877394 and 51909012) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA23040401).

Competing Interests: The authors have no relevant financial or non-financial interests to disclose.

Availability of data and materials: All data and materials can be provided on request.

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SupplementaryMaterials.docx

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published 31 Jul, 2023

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Transport of micron-sized polyethylene particles in confined aquifer: Effects of size, ageing, and confining pressure

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Materials And Methods

2.1 Materials

2.2 Experimental device

2.3 Column experiment

2.4 Analysis

2.5 Modeling

3. Results And Discussions

3.1 Transport and retention of MPs

3.2 Effect of size on the transport of MPs

3.3 Effect of ageing on the transport of MPs

3.4 Effect of confining pressure on the transport of MPs

4. Conclusion

Statements And Declarations

References

Supplementary Files

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