Baseline analysis of metal(loid)s on microplastics collected from the Australian shoreline using citizen science

Abstract

Microplastics represent a growing concern as pollutants in aquatic ecosystems. Currently, there is limited data available regarding the presence and characteristics of microplastics in Australian environments, and their interactions with other chemical contaminants have not been adequately explored. Consequently, the primary objective of this study was to establish baseline information on the physical and chemical attributes of microplastics found on Australian shorelines.

This foundational data is intended to support more detailed risk assessments in the future. Microplastic samples collected from the field were categorized based on their color, shape, and polymer type. The predominant colors of the plastic particles were clear, blue, white, and green. In terms of shape, the majority of particles were fragments (57.80%) and pellets (30.68%). Polymer characterization through laboratory analysis revealed that the shoreline microplastics were primarily composed of polyethylene (53.17%), followed by polypropylene (35.17%), polystyrene (6.61%), and polyethylene terephthalate (1.85%). Analysis of metal(loid) concentrations on the microplastic samples indicated that the levels of manganese (Mn), chromium (Cr), copper (Cu), arsenic (As), zinc (Zn), and lead (Pb) were significantly higher on microplastics collected from shorelines associated with industrial locations compared to those from other land use types. This finding suggests that aged microplastics have the capacity to adsorb toxic metals, and that the levels of these metals on the plastics may vary depending on the location.

Introduction

Modern-day plastics entered widespread use almost a century ago. However, it wasn’t until the early 1970s, with the appearance of small, weathered, and fouled plastic particles in neuston trawls (nets towed at the surface of the water), that the potential chemical and biological impacts of microplastics (MPs, defined as plastic particles smaller than 5 millimeters) began to be understood. In aquatic environments, plastics naturally distribute themselves across different compartments based on their specific density and the prevailing environmental conditions.

Most plastics, having a density lower than that of freshwater or seawater, tend to float on the surface. Nevertheless, processes such as wind mixing, the growth of organisms on the plastic surface (biofouling), and other factors can cause buoyant particles and heavier plastics to sink. Plastic materials are also known to accumulate on shorelines, where weathering processes, driven by variations in dryness, ultraviolet (UV) radiation, and temperature, are accelerated. Monitoring marine plastic debris is frequently conducted on beaches and shorelines due to their potential to act as pathways for anthropogenic waste to enter water bodies and their relative ease of access. Consequently, the presence and distribution of both large plastics and microplastics on shorelines have garnered significant global attention. Previous studies have reported mean concentrations of shoreline MPs ranging from 0.1 to 5500 MPs per kilogram of dry weight, with the substantial differences likely arising from variations in the sources of plastic pollution and inconsistencies in sampling methodologies.

Increasingly, research is focusing on the chemical contaminants associated with small plastic particles to determine the role of MPs as vectors for anthropogenic pollutants in the environment. Generally, the chemicals of interest include polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethane (DDT), and various other hydrophobic organic contaminants (HOCs). These substances are targeted due to their known toxicological effects and their tendency to adhere to hydrophobic surfaces. Fewer studies have examined the interaction between metals and MPs, as plastics have historically been considered inert towards metals, leading to a less established understanding of the mechanisms governing their interaction.

The adsorption of metals onto plastic surfaces may be facilitated by the presence of charged substances such as biofilms, organic matter, and other contaminants, or through interactions with organic matter that alter the surface area and hydrophobicity of the plastic materials. Microplastics are unique among contaminants because they have the potential to exert both physical and chemical impacts on living organisms. While the physical impacts of MP ingestion, such as gut blockage, have been studied, the chemical impacts are less understood.

However, a recent study demonstrated the transfer of benzo(a)pyrene, a PAH, from virgin polyethylene (PE) MPs (smaller than 20 micrometers) within a freshwater food web. This indicates that PAHs can desorb from MPs in the intestines of fish and reach the intestinal lining and liver, thereby increasing the overall body burden of these toxic chemicals. Given that metals can be adsorbed onto the surface of plastic particles or associated with attached biological material, they are likely to be in a bioaccessible form, meaning they can be taken up by living organisms. Therefore, it is crucial to develop baseline data on the physical and chemical characteristics of MPs through environmental surveys to provide a foundation for future studies on exposure and risk.

The Australian coastal environment is subject to various pressures, including coal mining, shipping activities, oil and gas exploration, agriculture, development, and consumerism. These activities have the potential to introduce both plastics and other contaminants into the environment. Previous studies on plastic pellets collected from beaches worldwide have shown a correlation between the concentrations of HOCs on the plastic surfaces and those found in the surrounding sediments, as well as regional land use patterns. To the best of our knowledge, this relationship has not yet been investigated internationally for MPs and metals.

Therefore, this study aims to generate baseline information on the occurrence and characteristics of MPs on Australian shorelines, to assess the potential for MPs to adsorb metals, and to determine whether the metals found on the MPs reflect the land use at the specific collection sites. The overall goal is to better understand the risks associated with small plastic particles and their role as vectors for chemical contaminants. Furthermore, we examine the effectiveness of using citizen science initiatives for sampling MPs on a national scale.

A total of 3559 MPs, ranging in size from greater than 1 millimeter to 5 millimeters, were collected from 37 locations along the Australian coastline between October 2016 and December 2018. Volunteers from the Tangaroa Blue Foundation hand-picked suspected plastic particles from shoreline sediments and placed them in paper envelopes. These envelopes were labeled with GPS coordinates, the date of collection, and a brief description of the site, and then mailed to The University of Newcastle (UoN) for chemical analysis. Microplastic samples from the Hunter Region were collected by the UoN research team using the same methodology. All samples were stored at room temperature in the paper envelopes prior to analysis.

The GPS coordinates provided for each microplastic sample were used to assign the collection locations to appropriate land use categories based on the Australian Government’s geospatial database, National Map ([https://www.google.com/search?q=https://nationalmap.gov.au/](https://www.google.com/search?q=https://nationalmap.gov.au/)).

To achieve finer spatial resolution and investigate any noticeable features suggesting an alternate land use in the ‘Catchment Scale Land Use 2017′ layer, Google Earth ([https://earth.google.com/web](https://earth.google.com/web)) was further utilized. Based on this analysis, samples were categorized into industrial, urban, residential, or rural land use types. Due to significant variability observed in the initial data, industrial locations were subsequently divided into two subcategories: industrial (Hunter Region, NSW, Australia) and industrial (Swan River, WA, Australia).

In total, microplastic samples from 23 locations, having sufficient mass for chemical extractions, were included in the analysis of metals adsorbed onto shoreline microplastics. These microplastics were visually examined to confirm their synthetic origin, and those identified as plastic were categorized by their shape and color. Before analysis, all samples were rinsed with Milli-Q water, weighed, and photographed. A representative subsample from each collection was then selected for Attenuated Total Reflectance Fourier Transform Infrared spectroscopy (ATR-FTIR) analysis using a Spectrum Two PerkinElmer FTIR Spectrometer.

Spectra were recorded in reflectance mode across the wavenumber range of 4000–400 cm⁻¹ with a resolution of 4 cm⁻¹, averaging 8 scans per sample. The polymer type of each microplastic was determined by comparing its spectrum against an in-house spectral library created using spectra of virgin plastic materials obtained from commercial suppliers.

To determine the concentration of metals adsorbed onto the surface of the plastic samples, microplastic samples of sufficient mass (n = 23) were weighed (in grams) and divided into triplicates to serve as analytical replicates. Each of these subsamples was digested in a 20% Aqua Regia solution (a mixture of nitric acid (HNO₃) and hydrochloric acid (HCl) in a 1:3 ratio) and agitated for 24 hours at 100 revolutions per minute. The resulting sample extracts were then analyzed using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) with an Agilent 7500ce instrument equipped with an octopole reaction system for multi-elemental analysis.

Standard Reference Materials (NIST 1640a – trace elements in natural water and NIST 1643e – trace elements in water) from the National Institute of Standards and Technology (NIST) were used to assess the recovery of all elements during the analytical process. The recovery rates for all metals analyzed were confirmed to be within the highest percentage range of recovery.

The obtained metals data were assessed for normality and homogeneity of variance using the Shapiro-Wilk and Levene’s tests in IBM SPSS v25. As the assumptions of normality and equal variances were violated for most of the metals, the data were transformed using a log(x + 1) transformation, and the assessments were re-examined. Since the transformed data still violated these assumptions, a decision was made to proceed with a non-parametric statistical test.

Significant differences in the concentration of individual metals on microplastics among the different land use categories were analyzed using the Kruskal-Wallis test, followed by Dunn’s post hoc test to identify which specific land use categories differed significantly, with location serving as independent replicates. Non-parametric multidimensional scaling (nMDS) with permutational multivariate analysis (PERMANOVA) was applied to a log(x + 1) transformed Euclidean Distance resemblance matrix to visually represent the differences in the overall metal assemblages on microplastics among the various land use categories.

This analysis was performed using Primer v7 software. The trace statistic for the null hypothesis of no difference was calculated using 999 permutations. A vector plot based on Pearson correlations was overlaid on the nMDS plot to indicate which specific metals were most responsible for contributing to the observed differences among locations based on their assigned land use category.

Differences in the distribution of microplastic colors, shapes, and polymer types among the different land use categories were tested using a series of two-way Chi-Square analyses in IBM SPSS v25. Locations were grouped by their land use category to determine whether differences in metal concentrations among land uses could be attributed to variations in the adsorption affinities of environmental particles based on their color, shape, or polymer type, or whether they were primarily a result of anthropogenic metal sources specific to those land use types.

The visual examination of the collected particles revealed that over half of all suspected plastic particles were fragments (57.80%), followed by pellets (30.68%), foams (7.87%), films and fibers (less than 2%), and other miscellaneous shapes (less than 1%). The primary colors of the particles were clear, blue, white, and green. ATR-FTIR analysis confirmed the polymer composition of a subset of the selected particles (378), revealing that they consisted of polyethylene (PE) (53.17%), polypropylene (PP) (35.17%), polystyrene (PS) (6.61%), and polyethylene terephthalate (PET) (1.85%). Ten particles (less than 3%) could not be conclusively identified.

Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) analysis detected nine different metals and metalloids on the shoreline microplastic samples (as shown in Table 1). Overall, the concentration of metals decreased in the following order: Barium (Ba) \> Manganese (Mn) \> Zinc (Zn) \> Copper (Cu) \> Lead (Pb) \> Cadmium (Cd) \> Chromium (Cr) \> Arsenic (As) \> Selenium (Se). The maximum concentrations (in mg kg⁻¹) of Manganese (174.84), Lead (18.37), Chromium (2.77), and Selenium (0.03) were detected on microplastics collected from Cockle Creek.

In contrast, the highest concentrations of Zinc (95.20), Copper (142.66), Cadmium (11.46), and Arsenic (1.53) were found on microplastics collected from Dora Creek. Both Cockle Creek and Dora Creek are located within the Hunter industrial region of New South Wales. The highest concentration of Barium (59.08) was detected on microplastics found at Bather’s Beach in Western Australia. Notably, metal(loid) concentrations were consistently high on microplastics collected from industrial locations.

Statistical analysis revealed that the levels of Manganese, Chromium, Copper, Arsenic, Zinc, and Lead sorbed to microplastics were significantly different among the various land use categories (as illustrated in Figure 3). Post hoc analysis further identified which specific land use categories exhibited significant differences in their respective metal concentrations. The results suggested that microplastics from industrial locations within the Hunter region contained higher concentrations of most of the analyzed metals compared to microplastics from other land use categories, with the exception of urban locations.

Differences in the overall metal assemblages on microplastics among the different land use categories were visually represented using non-parametric multidimensional scaling (nMDS) overlaid with a vector plot (as shown in Figure 4). The metal concentrations on microplastics associated with industrial locations in the Hunter region appeared to be distinct from those found at locations belonging to other land use categories. Permutational analysis of variance (PERMANOVA) based on a Euclidean Distance dissimilarity matrix confirmed that land use had a significant effect on the overall assemblages of metals sorbed to the microplastics (as shown in Table 2).

The distribution of microplastic particle shapes [χ²(20, N = 248) = 86.964, p \< 0.001] and polymer types [χ²(16, N = 248) = 34.248, p = 0.05] showed significant differences among the land use categories. However, no statistically significant association was detected between the distribution of microplastic colors and the land use category [χ²(48, N = 248) = 61.705, p = 0.088].

Our findings indicate that the characteristics of microplastics on Australian shorelines are consistent with those previously reported in Australian surface waters, supporting the movement of plastics between terrestrial and aquatic environments. A comprehensive survey of surface waters around Australia revealed a high abundance of hard pieces of polyethylene (PE) (67.5%) and polypropylene (PP) (31.0%), while our shoreline samples showed that PE and PP accounted for 53.17% and 35.17% of all identified plastic polymer types. Both studies identified PE, PP, and PS as the three most common polymer types, which is expected due to their relative density in seawater and their widespread global production.

The variation in the proportion of fragments and pellets between surface water and shoreline samples may be attributed to aging and weathering factors, such as the distance from the plastic source and the amount of time spent in the environment. While hard plastic pieces (fragments) and pellets represented 75.44% and less than 1% of microplastics in Australian surface water samples, respectively, fragments and pellets accounted for 57.80% and 30.68% of microplastics on shorelines. The higher proportion of pellets in shoreline samples is likely due to the direct loss of plastic pre-production pellets during transportation and processing by plastic manufacturing industries.

Nevertheless, over half of all shoreline microplastics were fragments, suggesting that the primary source of microplastics (1 mm–5 mm) to Australian coastal environments is the degradation of larger plastic waste through UV radiation and physical abrasion. This finding is supported by a substantial body of research that reports a positive correlation between the density of marine debris and regional population, indicating deposition as a significant driving factor. It is estimated that approximately 80% of all marine litter originates from land-based sources, with global estimates ranging between 4.8 and 12.7 million tonnes each year.

Some studies suggest that metal contamination of microplastics is primarily due to inherent loads present in the plastics from manufacturing or reclamation processes. Metals can come into contact with plastic materials during the addition of additives and through contact with machinery during manufacturing and recycling. Analysis using ICP-OES has demonstrated elevated levels of aluminum (Al), chromium (Cr), iron (Fe), tin (Sn), and zinc (Zn) on the surface of virgin high-density PE and PP pellets, and SEM-EDX has revealed the presence of titanium oxide (TiO₂), barium (Ba), and zinc (Zn) compounds within the internal structure of marine microplastic samples of various polymer types.

Plastic pigments contain metallic elements, including copper (Cu), cadmium (Cd), manganese (Mn), lead (Pb), tin (Sn), and zinc (Zn), particularly in deep colors such as black, dark green, blue, and red. For example, copper, the main metal associated with blue and green phthalocyanine pigments, has been detected at concentrations up to 373.71 ± 6.98 μg g⁻¹ in virgin plastics, while an aged PE particle containing green pigment was found to have 219.70 μg g⁻¹ of lead and 23.64 μg g⁻¹ of cadmium. In our study, clear, blue, and white were the dominant colors among Australian shoreline microplastics, suggesting that the presence of the phthalocyanine blue pigment could potentially contribute to the observed copper levels.

However, other studies investigating metal loads in virgin plastic particles indicate that the inherent metal load is negligible for both colored and clear microplastics. Furthermore, the results of our two-way Chi-Square analysis showed no significant association between the distribution of microplastic particle colors among the different land use categories. This indicates that particle color was not a primary factor driving the significant differences observed in metal concentrations on microplastics from different land use areas.

The results of this initial survey indicate that microplastics (MPs) are capable of accumulating metals beyond their inherent load, suggesting that MPs may act as a vector for metals within the Australian coastal environment. The levels of toxic metals found on MPs collected from the Australian shoreline were comparable to or higher than those reported in other published studies (Table 3), with the exception of one study that utilized field-portable X-ray fluorescence (FP-XRF) to determine metal levels associated with environmental MPs. The elevated metal levels observed in our study could be attributed to the interaction between MPs and legacy metals in areas that have historically been subject to intense industrial activity. Alternatively, it could be a result of oceanic currents depositing aged plastics on the Australian coastline, plastics that have an unknown history of metal exposure.

Land use appears to be a significant factor influencing the accumulation of metals by MPs. The elevated levels of chromium (Cr), manganese (Mn), copper (Cu), arsenic (As), lead (Pb), and zinc (Zn) on MPs collected from industrial locations in the Hunter Region, compared to various other land use types, suggest that the metal loads on MPs most strongly reflect their recent environmental history. Given that the maximum concentration for eight of the nine metals analyzed was reported for Cockle Creek and Dora Creek, locations situated to the north and south-west of Lake Macquarie, respectively, it is reasonable to infer that MPs are somewhat representative of anthropogenic metal contamination in these areas.

At the Dora Creek location, the levels of cadmium (Cd) and copper (Cu) on MPs exceeded the upper guideline value (GV-High) and the recommended toxicant default value guideline (DVG) for metals in marine sediments, respectively. Furthermore, the levels of cadmium, copper, lead, and zinc on MPs also exceeded the guideline values for marine water quality (slightly-moderately disturbed systems) at all locations, and chromium exceeded these guidelines at most locations. Lake Macquarie was historically a key site for heavy industry, including the Pasminco lead–zinc smelter at Cockle Creek, a fertilizer plant, a steel foundry, collieries, and sewage treatment works.

Additionally, two coal-fired power stations still operate in the southern part of the lake’s catchment. Heavy metals are known to be transported in fly ash, a byproduct of coal-fired power generation, such as that occurring near Dora Creek. Despite reports indicating a decline in sediment metal loads within Lake Macquarie, the metal levels found on MPs suggest their ongoing input into the system.

Moreover, the Port of Newcastle, located at the mouth of the Hunter River, was previously home to BHP Steelworks for nearly a century. Consequently, the sediments of the Throsby Creek basin and surrounding land, which were infilled with contaminated waste, are likely contributing metals to the Hunter River estuary. With the port still operating as a major international coal shipping terminal, the leaching of metals from the antifouling paints on ship hulls is likely another significant source of metal contamination in the aquatic environment.

The results of the Chi-Square analysis suggest that differences in the distribution of particle shapes and polymer types among the land use categories may also influence the enrichment of metals on the MP surface, thereby contributing to the observed differences in metal levels across these categories. Fragments were the predominant MP shape across all land use categories (comprising less than 50.00%), with the exception of the industrial locations along the Swan River, where pellets were the most abundant shape in shoreline samples (46.43% pellets; 39.29% fragments).

However, this deviation did not appear to significantly influence metal sorption to MPs at the industrial (Swan River) locations, as the metal levels were not statistically dissimilar from most other land use categories. The lower proportion of pellets (4.08%) and the increased proportions of films (10.20%) and fibers (8.16%) in samples from industrial (Hunter) locations compared to other land use categories may help explain the increased sorption of metals due to differences in the overall surface area available for metals to bind with the plastic surface.

Size and shape are directly related to the surface area-to-volume ratio, which determines the rate of contaminant adsorption and the time required to reach equilibrium. For instance, a controlled laboratory experiment demonstrated that the absorbance of metals to polypropylene (PP) particles of 2 mm, 3 mm, 4 mm, and 5 mm decreased with increasing particle size, due to a reduced specific surface area and fewer available sorption sites.

Metals have also been shown to exhibit a greater affinity for aged plastics compared to virgin pellets, likely due to changes in surface area, surface charge, and the accumulation of organic matter. Therefore, weathered plastic particles may pose a greater risk to aquatic organisms in terms of contaminant transfer, owing to alterations in crystallinity, the formation of cracks on the particle surface, and the shedding of fine plastic particles, which create additional sites for the sorption of chemical contaminants.

To our knowledge, the effects of physical particle characteristics on the adsorption of metals have not yet been studied in real-world environmental settings. It was not feasible to directly assess the differences in metal levels among plastic polymer types for each location in this study due to the limited quantities and homogeneity of the provided samples.

In terms of polymer distribution, all land use categories were dominated by polyethylene (PE), polypropylene (PP), and polystyrene (PS), with differences among land uses primarily attributable to variations in minor components. This suggests that metal loads on MPs are unlikely to be significantly confounded by polymer type. However, the current literature on the effect of plastic polymer type on metal sorption remains inconclusive.

For example, a 14-day simulation experiment reported that polyvinyl chloride (PVC) microplastics consistently sorbed higher concentrations of lead (Pb), copper (Cu), and cadmium (Cd) than did PP, PE, and polyamide (PA) particles. Conversely, a six-month parallel field study found that PP accumulated higher concentrations of metals compared to PVC. The authors of that study reported that metal levels on the microplastics were strongly correlated with metal concentrations in the surrounding seawater, indicating that the sorption of metals by MPs was relative to the local environmental conditions.

Similarly, the elevated levels of metals on MPs in our study were associated with locations known to have metal contamination issues, suggesting that MPs accumulate and transport metals in the marine environment and may serve as indicators of metal contamination. Evidence from a long-term field study indicates that aging processes and site-specific conditions play a more significant role in metal adsorption than the inherent characteristics of the plastic particles.

The in-situ aging of virgin plastic pellets (high-density PE, low-density PE, PP, PET, and PVC) revealed that patterns of metal adsorption were not consistent across different locations or over time, and that the adsorption of metals was similar among the different plastic polymer types. The concentration of most metals (Cr, Mn, Co, Ni, Zn, and Pb) continued to increase over a 12-month period without reaching equilibrium, suggesting that plastics may continue to sorb metals the longer they remain in the aquatic environment. In our study, Australian shoreline MPs consisted mainly of PE and PP polymers, which adsorbed higher concentrations of cadmium, chromium, lead, and zinc compared to PE and PP MPs aged in the UK marine environment for 12 months.

This study represents the first comprehensive survey to provide information on the occurrence and characteristics of MPs on Australian shorelines while simultaneously generating baseline data on the relationship between MPs and metals in the Australian coastal environment. Our results are consistent with findings for small plastic particles in the marine surface waters of Australia, indicating a connectivity between microplastics in the coastal margin and those at sea.

Given their small size, buoyancy, and environmental persistence, MPs may serve as a vector for metals and other sorbed contaminants, with potential implications for the transfer of contaminants into the food chain. Our findings suggest that the adsorption of metals to microplastics is enhanced in areas where metals are present in elevated concentrations, particularly in regions supporting heavy industrial activity, including coal-fired power generation, metal smelting, and shipping. Therefore, efforts to manage plastic waste should be informed by a robust knowledge base regarding the interaction of microplastics with complex chemical mixtures and their fate in the aquatic environment.

This study focused on MPs in the visible size range due to the nature of citizen science data collection; however, smaller MPs may possess more sites for the sorption of metals and other environmental contaminants that can become bioavailable to organisms following ingestion. Terephthalic Future research into the size-dependent sorption capacity and size-dependent toxicity of weathered microplastics is urgently needed to improve our understanding of the risks posed by MPs in the environment.