Surface interactions of model microplastic particles in seawater

Introduction

Microplastics are fragments of various types of polymers of irregular geometries with sizes ranging from 1 to 500 μm. ¹ Microplastics are widely recognized as a global environmental contaminant. ² With millions of tons of plastic entering the marine environment annually, it is no longer shocking that plastic wastes break down into microparticles, detected in the marine environment threatening marine organisms. Microplastics have been found in surface water and sediments globally, including in freshwater, soil, coastal ecosystems, open seas, and even the polar regions. ³

Efforts to curb microplastics have been undertaken just in the past few years. Despite scientific evidence of microplastic contamination discovered dating back to the early 1970s, ⁴ scientists have only recently conducted systematic studies establishing the existence of primary and secondary microplastics in the marine environment. According to recent studies, microplastics may represent a higher danger than macroplastics. ⁵

Microplastic aggregation is a key physicochemical mechanism that regulates microplastic diffusion and transport in water bodies. ⁶ The aggregation behaviour of microplastics in water is driven by the electrical and chemical surface properties of the microplastics when suspended in water. ⁷ The physical properties of the water systems such as pH, conductivity and salinity values, and the amount of dissolved organic matter (DOM) are also vital in determining the interactions of microplastic in water. ⁸ The surface interactions between microplastics in water determine the fate of water ecosystems and environmental sustainability. This work envisions to provide information on surface properties and interactions of microplastic in water for the scientific development of the microplastic removal devices and technologies based on surface capturing such as coagulation, flocculation, filtrations, and membranes.

In this work, various types of common microplastics were employed as a model system to investigate their surface interactions in seawater. An anionic surfactant was also used to alter the strength of microplastic interactions, and these surface responses were correlated with the aggregation behaviour of microplastics in seawater.

Experimental

Preparation of micropolymer suspensions and seawater characterization

The suspensions of seawater containing polymer microparticles (PP, PVC, and PVA), and NR in distilled water at a fixed concentration (0.02 wt.%) were prepared via sonication using an ultrasonic bath at room temperature for an hour. The sonication process proceeds until cloudy solutions obtained. The concentrations of AOT were varied from 0 to 30 wt.%. Similar mixing parameter was applied for AOT-laden solutions. The physical properties of the seawater that include pH, electrical conductivity, salinity, and density were measured and tabulated in Table 1. The pH and conductivity values of the seawater and distilled water were measured using Mettler Toledo pH-conductivity meter operating at 1 mV. The water salinity was determined using the Shinwa Rules handheld salinity meter, and the water density was determined using the hydrometer. The data for distilled water is supplied as a comparison with the values for seawater.

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Table 1.

Physical properties of seawater and distilled water.

	pH	Conductivity (mS/cm)	Salinity (ppt)	Density (g/cm3)
Seawater	8.12 ± 0.9	56.6 ± 3.2	30.3 ± 0.2	1.02 ± 0.03
Distilled water	7.03 ± 0.04	0.002 ± 0.0003	0	0.99 ± 0.005

Dynamic light scattering

The hydrodynamic diameter D _h of microplastic suspension was measured using the Malvern Zetasizer Nano S operating at a wavelength of 532 nm. The suspension was diluted with the seawater to eliminate dust and micron-sized impurities before testing, where the samples were placed in square 3.5 mm glass cuvettes with four-polished windows. The cuvette was then inserted into the instrument with the incident laser set up at 90^o. The count rate recorded was observed at a scattering intensity of about 100 kcps. The D _h is calculated using the Stokes-Einstein relation of particle size to its motion (Equation (1)) with k _B is the Boltzmann constant (1.38 x 10⁻²³ m²kgs⁻¹K⁻¹), D _t is the translational diffusion coefficient and T is temperature,

D h = k B T 3 π η D t

(1)

Results and discussion

The optical turbidity observation of different types of micro-sized polymeric suspension in water is illustrated in Figure 1. The natural rubber (NR) latex suspension was used as a benchmark for the optical turbidity observation. The suspension of NR latex in distilled water was turbid due to NR latex being white sap by nature. This is also owing to the good dispersion of NR Latex in distilled water arising from exceptional particle-solvent interactions. ⁹ While PVA, PP and PVC suspensions were initially clear, predominantly due to density mismatch between the microplastics and seawater. Dispersed micro polymers samples in seawater is cloudy, similar to NR latex solution. From the visual evidence, all micro polymers portray poor affinity for seawater due to lacking hydrophilic groups except for PVA and NR latex. ¹⁰ Nevertheless, the ions present in the seawater (e.g., SO42−, Cl−, Na+, Ca2+, K+) experienced a so-called salting-out effect, thus decreasing the solubility of PVA in seawater. ¹¹ The salting out phenomenon occurs because of the formation of ion-water hydration complexes results from a cation binding with the polymer ether oxygens. ¹² This can be confirmed via the UV-Vis spectrum of various micro polymers in seawater as depicted in Figure 2. PP, PVC and PVA in seawater showed lower absorbance peaks possibly contributed by the poor distribution of micro polymers throughout the seawater. This also suggests that microplastics are not surface active initially due to their hydrophobic nature. ¹³ In this work, only PP and PVC were selected to investigate their surface interactions on a colloidal scale, where AOT anionic surfactant was added to alter the electrokinetic behaviour of the micro polymers in seawater.OPEN IN VIEWER

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Figure 2.

UV-Vis absorption spectrum of micro polymers in water systems.

Figure 3 shows the electrophoretic mobility trend of PP and PVC at varied AOT surfactant dosages. For both PP and PVC systems, with and without AOT, the electrophoretic mobility values were negative. This indicates that both PP and PVC were negatively charged in seawater. The Coulombic force acting on the microparticles is directly proportional to the carried charge, hence the reduction in the electrophoretic mobility value results in surface charge reduction and microparticles stability. Interestingly, at 5 wt.% of AOT, the electrophoretic mobility values of both PP and PVC were drifted to positive values, indicating the micropolymer surfaces bearing positive charge. The switchover from the negative to positively charged surfaces of PP and PVC is causing by the presence of excess resultant counterions within the particle surfaces, arising from free ions in the seawater and the formation of complex multivalence ions. ¹⁴ Increasing AOT concentration to 10 wt.% reduced the mobility to a sub-zero regime which also implies the surface charge of PP and PVC was close to neutral or point of zero charge (pzc). Increasing AOT concentration from 5 wt.% to up to 30 wt.% showed a monotonous decline in mobility value, where charge reversal phenomenon at 5 wt.% ≤ [AOT] ≤ 10 wt.%. Further addition of AOT to the system showed an advancement of mobility value towards negative values. After the charge reversal and neutralization phenomena observed, the restabilization of the micro polymers was reported when the AOT concentration was increased at up to 30 wt.%. The negative surface charge of PP and PVC micropolymer in seawater was mainly due to adsorption of specific ion and surface-active organic matter with carboxylic (-COOH) and phenolic (-OH) groups. ¹⁵ OPEN IN VIEWER

The specific conductivity trend of the PP/SW and PVC/SW systems with increasing loading of AOT is illustrated in Figure 4. The value decreased with increasing AOT concentration. The conductivity value of the suspensions exhibits the number of free ions (cations and anions) presence in bulk solution, ¹⁶ and the reduction of the conductivity value indicates that the free ions contributed by the seawater and AOT were adsorbed on the microparticle surfaces, altering the electrophoretic mobility and surface charge of the particles. The AOT-charging in aqueous polymeric colloidal system was also observed in work performed by Cao and co-workers. ¹⁷ It is also probable that the reduction of conductivity value of the suspensions when AOT was added is mainly due to the sulfonate (SO³⁻) anion of AOT formed a structure with counterions such as Na⁺, K⁺, or Mg²⁺ in seawater via ionic recombination, resulting in a lower concentration of free ions in seawater. ¹⁸ OPEN IN VIEWER

Figure 5 shows the average hydrodynamic diameter D_h and polydispersity index PdI of PP and PVC in seawater at varied AOT concentrations. At 10 wt.% of AOT, the D_h for both PP and PVC increased, similarly to PdI value. Further increment of AOT concentration resulted in the decline in D_h value of micro polymers by an order of magnitude. When AOT was added at more than 20 wt.%, both PP and PVC samples were more monodispersed compared to AOT at lower concentrations. This suggests that at higher concentration of AOT, the surfactant can stabilize the microplastic particles as also observed in mineral-based colloid. ¹⁹ At 10 wt.%, it was also revealed in Figure 3 that both PP and PVC particles were at the point of zero charges (pzc) vicinity. The low value of electrophoretic mobility consequences in lower magnitude of surface charge, ²⁰ enabling the microparticles to attract and form aggregates. The preferential ion adsorption on microparticle surfaces resulted in altered surface charge, ²¹ and eventually electrostatic interactions between the microplastic, which can be observed at high AOT concentrations regime.OPEN IN VIEWER

Conclusion

In this study, the strength of interactions between micro polymers in seawater was investigated via observation, UV-Vis spectroscopy, electrophoretic mobility and hydrodynamic size measurements. The strength of interactions was altered by the addition of AOT surfactant in both PP and PVC/seawater suspensions. The addition of AOT surfactant to both systems resulted in a non-monotonous trend in electrophoretic mobility and hydrodynamic sizes of the microparticles and a decrease in suspension conductivity. The exploration of surface interactions between microplastic in water is useful to understanding their impacts and consequences on water ecosystems and the environment.