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. 2025 May 22;5(6):3111–3117. doi: 10.1021/acsestwater.5c00017

Optimized Extraction Methods for Pristine and Aged Microplastics from Complex Water Samples

Razegheh Akhbarizadeh †, Yan Jin Xu †, Freya Boerner †, Paul A Helm ‡,§, Miriam L Diamond †,§,*
PMCID: PMC12172039  PMID: 40535157

Abstract

Efficient and replicable extraction of microplastics (MPs) and other anthropogenic particles from complex environmental matrices remains challenging. We tested and optimized the extraction of water samples with and without organic matter (OM) spiked with 9 MP polymers with 16 different morphologies and/or colors, that were pristine (63–1000 μm) and aged (300–1000 μm). Statistical analyses showed that OM presence most significantly influenced MP (300–1000 μm) recoveries, followed by the strength of digestion reagents, temperature, and exposure time. Optimal recovery of MPs in a matrix with OM of <2 g/L can be obtained with a single-step digestion of Fenton’s reagent. A sequential combination of two or more digestion solutions (e.g., Fenton’s reagent +10% potassium hydroxide) is recommended when OM >10 g/L. Recoveries of aged MPs susceptible to degradation were up to 6 times lower than those of their pristine version after applying the same digestion method. Thus, while the digestion method may be nondestructive for pristine MPs, weathered MPs could be partially or completely digested. We recommend that the characteristics of the spiked MPs closely match those of the targeted particles in real samples during quality control tests, which allows for the generation of robust and reliable monitoring data sets.

Keywords: quality control/quality assurance, recovery testing, sequential digestion, stormwater, surface water, harmonized methods, polymer degradation

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1. Introduction

Ensuring quantitative extraction of microplastics (MPs) and other anthropogenic particles from complex environmental samples containing natural organic matter (OM) and inorganic materials remains a challenge. Various methods have been developed for extracting MPs from water samples, but establishing harmonized methods continues to be an area of development. , In particular, the identification of MPs from complex water samples after extraction, especially MPs < 500 μm, is prone to misidentification without suitable digestion methods. Chemical digestion is the most common method for extracting MPs from water samples, with density separation added to enhance extraction rates. −

Various digestion methods to extract MPs from environmental samples (e.g., biota, sediments, soils, wastewaters) have used acids, − oxidation solutions, alkaline solutions, , enzymes, , and a combination of the above. , However, as highlighted by Monteiro and Pinto da Costa, effective extraction methods for MPs from complex water samples such as stormwater, untreated wastewater, and streamwater after rain with high organic content has received less attention. These high-OM waters require optimization of digestion methods so that the OM is destroyed while preserving MPs.

Most studies of digestion methods have used pristine MPs. However, most MPs in real samples are aged to some degree, with associated physiochemical changes, such as increased brittleness, loss of color, surface changes, and changed chemical composition. − These alterations may result in the fragmentation of aged MPs during digestion and/or misidentification due to altered spectral signatures, which could yield over- or underestimation of specific MP types in samples, respectively.

This research aimed to fill the gap of determining digestion methods to optimize MP recovery in water samples based on the systematic validation of different digestion methods on nine polymers across 16 shapes/colors that were pristine or aged MPs. Specifically, we tested sequential methods to effectively extract and maximize the recovery of spiked pristine and aged MPs of sizes 63–1000 μm found in complex water samples with OM, such as stormwater and urban-impacted receiving waters. This study contributes to developing harmonized sample treatment methods for MPs extractions in “real world” samples.

2. Materials and Methods

Figure S1 summarizes the study design, including different digestion methods used and statistical testing strategy.

2.1. Chemicals and Spike Materials

Chemicals used are provided in Supporting Information SI 1.1. MPs for spiking recovery tests were prepared in sizes of 63–1000 μm, and microfibers with diameters >17 μm. Diverse MPs for testing included purchased primary and generated secondary MPs (Table S1). The number of pristine and aged MPs in each spike for treatments and controls is provided in SI 1.2.

Tests with OM involved adding the spike materials to a consistent complex matrix (CM) consisting of OM and inorganic material (e.g., sand). CM was prepared from a well-homogenized mixture of peat (Fluval-ART#A-696) and composted sheep manure (Voilà) with a ratio of 2:1 (<500 μm). Sheep manure contains both inorganic materials and OM. Details are provided in SI 1.3.

2.2. Preparation of Aged MPs

Polyethylene terephthalate (PET), polypropylene (PP), polystyrene (PS), polyester (PES), polyurethane (PU), polyamide (PA), and tire rubber particles 300–1000 μm, were aged in a modified ultraviolet (UV) chamber equipped with UVA light (UVA, 365 nm, 2 × 15 W/m2, Model XX-15A, Spectroline Inc., Westbury, NY). More details are given in SI 1.4 and Table S2.

2.3. Experimental Design

2.3.1. Testing Effects of Sequential Digestion Methods on MP Spikes

We conducted sequential combinations of oxidative-alkaline methods using potassium hydroxide (KOH, 10%), hydrogen peroxide (H2O2, 30%), and Fenton’s reagent (FR) on pristine MPs (63–300 and 300–1000 μm) and aged MPs (300–1000 μm) with exposure times of 48 h for KOH (10%) and H2O2 (30%) and 2 h for FR (Figure S1a and Table S3). Each of the seven treatment combinations and controls was run in triplicate (n = 57 for treatments with n = 9 controls). Details of the digestion treatments are presented in SI 2. An additional density separation step using a sodium iodide (NaI) solution (ρ = 1.6 g/cm3) was added as needed (see SI 2.5).

2.3.2. Impacts of Various Factors on MP Recoveries

Using a multilevel fractional factorial design (eq 1, SI 3.1), we tested factors including temperature, exposure time, concentration of digestion solutions, and OM concentrations (Figure S1b and Table S4) to determine which factors most influenced recoveries of pristine MPs. We focused on the polymers and morphologies that were most susceptible to losses, such as PA fibers, PES fibers, PS foams, glitters (flat PET flakes), PU foams, and tire rubbers, as identified in the sequential testing (referred to as “susceptible MPs”). Briefly, 40 spiked samples were tested in duplicate for recoveries with treatments randomly assigned to minimize bias. Additionally, we conducted six controls consisting of spike MPs and filtered RO water. A stepwise regression model was used to identify the main effects (p > 95%; SI 3.1-eq 2), , and Pareto charts were used to illustrate the most significant factors. Regression equations and more details can be found in SI 3.1.

The multifactorial response optimization function in Minitab 19.2020.1 software was used to determine the treatment(s) that maximized recoveries of MPs susceptible to digestion conditions. − Data input to this function were recoveries of spiked MPs and the concentrations of digestion agents, temperature, exposure time, and level of OM. Target recoveries of 100%, with a minimum of 70% for the susceptible polymers, were considered adequate. However, the target recovery was set at >50% for PES fiber and PU foam because of their greater susceptibility to disintegrate during digestion. Details of the algorithms and formulas are presented in SI 3.2. Desirability scores (D, eq 16, SI 3.2) greater than 0.7 were used to select the optimal digestion methods.

2.3.3. Method Validation of Optimized Methods

Method validation was undertaken using pristine (63–300 and 300–1000 μm) and aged (300–1000 μm) MPs to be more representative of the mix of MPs likely to be found in the environment (Figure S1c). MPs were spiked into samples containing ∼16 g OM/L and tested using optimal digestion methods identified using the multifactorial response optimizer model for the same OM level. These methods were FR + KOH (10%) at 50 °C for 2 + 48 h, H2O2 (30%) + KOH (10%) at 50 °C for 24 + 48 h, and KOH (10%) + H2O2 (15%) at 50 °C for 48 + 48 h (each digestion method was tested in triplicate for pristine and aged MPs to give n = 27). Density separation was also applied using NaI (ρ = 1.6 g/cm3) for pristine MPs (63–300 μm). Triplicates of control samples consisting of spike MPs and filtered RO water were included in each round (n = 9).

2.4. Quantitation and MP Characterization

MPs were quantified by counting particles under a ZEISS stereoscope (magnification up to 80×). MP recoveries after the digestion treatments were calculated as the percentage of the number of MPs spiked into each treatment/sample. The physical-chemical changes of MPs were assessed before and after the aging process and treatments, first by visual observations using the ZEISS stereoscope, followed by SEM (JSM 6610LV, JEOL, Japan), and FTIR spectroscopy using a Hyperion 3000 microFTIR (Bruker Optik GmbH, Germany). Full details are provided in SI 4. We observed and classified the disintegration stage of MPs in terms of visual, spectral (FTIR), and surface morphological (SEM) changes according to the literature, , with minor revisions (Table S5).

2.5. Quality Assurance and Quality Control (QA/QC)

We took a series of precautions during sample preparation and sieving to ensure the accuracy of MPs quantification and minimize potential issues such as MPs loss, cross-sample contamination, and fragmentation, particularly for aged MPs. We followed best practices for preventing lab contamination as described previously. Sample preparation was conducted in a laminar flow hood (Air Science). All experiments were run in triplicate to test the methods and in duplicate to test the factors. In each batch of tests, controls of the spike samples with filtered RO water were run in triplicate (n = 23). Temperatures were kept below 50 ± 5 °C as suggested in previous studies , and the volume of all digestion solutions was kept consistent. At least one-third of pristine samples and two-thirds of aged plastic samples (300–1000 μm) were checked by two operators to account for interoperator variability. The independent counts and categorization results from two operators for the same samples (pristine, n = 36; aged, n = 18) were assessed for relative differences in abundances across morphological categories.

2.6. Data Analysis

All experimental design, regression analysis, and statistical analyses were performed in Minitab 19.2020.1, R version 4.3.1, and RStudio (2023.06.0 + 421) software. Data normality was checked using a Shapiro-Wilk test. Due to the non-normality of data, we conducted nonparametric statistical tests. Significant differences among the groups (i.e., treatments, influenced factors, recoveries) and pairwise comparisons were performed using nonparametric tests such as the Mann–Whitney test. We used cluster analysis to explore similarities between applied digestion methods and various MPs recoveries, employing correlation distance and average linkage for clustering. The choice of average linkage was based on its ability to create compact and cohesive clusters. Stepwise regression analysis was used to identify the main factors controlling the recoveries of susceptible MPs, based on p values with a confidence level greater than 95%.

3. Results and Discussion

3.1. QA/QC

Median percentage variations between sample replicates and two operators were zero for most targeted plastic types with several exceptions (Figure S2). The median percentage differences between the two operators for spiked water samples were 12% for pristine PS foams, 6% for tire rubbers (Figure S2a), and increased to 5–18% for pristine glitters, irregular PE beads, PU foams, and tire rubbers with the addition of CM (Figure S2b). The differences between the two operators were significantly higher for aged vs pristine MPs and with vs without CM (Mann–Whitney test, p < 0.05). The median percentage differences were 8–15% for aged MPs when only MPs were in the samples (Figure S2c), and 8–40% for aged MPs with CM (Figure S2d). Recoveries were reported as the average of two operators except when percentage variations were >100%, for which results from the less experienced operator were omitted.

For pristine MPs, control samples showed no loss or fragmentation during the process (recoveries >90% for all MPs, Figure a), however, aged PS foams fragmented to give elevated recoveries >100% even in control samples (Figure b, control and PS foam intersection), likely due to increasing brittleness during UV exposure and abrasion during sample transfers, rinses and sieving. Hence, we recommend avoiding sonication or any strong mechanical force during the preparation of environmental samples containing weathered MPs. The remaining aged particles in the control samples showed recoveries of >80%.

1.

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Heatmap of 300–1000 μm MP recoveries after treatments for (a) pristine MPs, and (b) aged MPs. Rows and columns are clustered using correlation distance and average linkage. The color key range represents average percentage recoveries of triplicate tests and density plot shows the relative frequency of data points across the range of data (unitless) with lighter colors indicating higher recoveries. H2O2: Hydrogen Peroxide (30%), KOH: Potassium hydroxide (10%), FR: Fenton’s reagent.

3.2. Effects of Combinations of Digestion Methods on MP Recoveries

3.2.1. Recoveries of Pristine MPs

We found the highest recoveries of >70% MPs 300–1000 μm and >60% MPs 63–300 μm for FR + FR, FR + KOH (10%), and KOH (10%) + FR treatments, as shown in Figures a and S3, respectively. In contrast, two sets of H2O2 (30%) and a combination of H2O2 (30%) and KOH (10%) gave lower recoveries. The order of digestion solutions did not significantly affect the recoveries of pristine MPs in both size classes (Mann–Whitney test, p > 0.05). Other studies have also demonstrated that a single-step FR method minimizes damage to the most susceptible MPs, including PA, PU, and tire rubbers. ,−

For all the treatments, 300–1000 μm PET, PP, and PE fragments had maximum recoveries of 100%. However, recoveries ranged from 0 to 100% for PES and PA fibers, PU and PS foams, glitters, and tire rubber, depending on the digestion solution and their particle size (Figures a and S3). For example, recoveries of smaller (63–300 μm) PS foams and PET fragments were significantly lower than larger pristine (p < 0.05) after FR + KOH (10%) at 50 °C for 2 + 48 h, with percentages of 34 vs 100% for PS foams and 63 vs 99% for PET fragments, for small and large MPs, respectively. We attribute the relative sensitivity of the small MPs (lower recoveries) to their higher surface-to-volume ratio compared to large fragments, fibers, and films. ,,

Our results showed low recoveries for PA fibers, PU foams, and tire rubbers for H2O2 (30%), whereas PS foams, PES fibers, and glitters were sensitive for KOH (10%), as reported by Munno et al. for PS foams (125–1000 μm). However, they reported high recoveries of PA fibers (∼88%) after treatment with KOH (20%) + H2O2 (35%), which was inconsistent with our results and those of others. , This difference could be due to a shorter contact time (2 h) compared to the 24–48 h used here and by others. Olsen et al. also reported significantly lower recoveries of PET/PES fibers compared to PET fragments after exposure to alkaline methods (10 M NaOH at 60 °C for 24 h). Schrank et al. observed that the molar mass of PA films (10 mm × 10 mm × 100 μm) decreased after H2O2 (30%) at 37 °C for 24 h, which could be due to surficial polymer degradation.

3.2.2. Recoveries of Aged MPs

Recoveries were lower vs pristine MPs (Figure ), particularly for the more susceptible polymers, presumably caused by greater deterioration caused by aging. As with pristine MPs, the highest recoveries of aged MPs were achieved with FR + KOH (10%), KOH (10%) + FR, and FR + FR (>72%), except for 46–60% for PES in KOH-related treatments and PU (Figure b). Here, the order of treatment became important, where recoveries dropped from 120 to 45, 60 to 2, and 50 to 0% for aged PS foams, PU foams, and PA fibers, respectively, after H2O2 (30%) + KOH (10%) vs after KOH (10%) + H2O2 (30%) treatments (Mann–Whitney test, p < 0.05).

Recoveries of aged PU foams, PET glitters, and PES fibers were up to 6 times lower than that of their pristine version after applying the same digestion method (2–45 vs 12–79%, without OM). Recoveries of tire rubbers and PS foams exceeded 100% after H2O2 (30%) and KOH (10%) treatments due to fragmentation (Figure b). Conversely, the recoveries of pristine and aged PET and PP fragments were similar due to their high stability against the treatments, e.g., oxidation. Savino et al. also found no significant changes in the recovery of aged PE, PP, and PET fragments (500–5000 μm) compared to pristine ones following digestion with FR at 30–50 °C.

3.2.3. Changes to the Surface Morphology and Spectral Signatures of Pristine and Aged MPs

Since recoveries (visual counting) do not indicate the level of degradation of MPs after digestion, we examined changes in surface morphology and spectral signatures to understand reasons for over- and under-estimating recoveries after digestion. Digestion caused the surface morphology to deteriorate and changes in the chemistry of both pristine and aged tire rubbers, PS foams, glitters, and PU foams, as shown by SEM imaging and shifted FTIR peak locations and, in some cases, additional peaks (Figures S4–S8). For example, tire rubbers subject to H2O2 (30%) had recoveries >50% for both aged and pristine MPs, but their texture became soft and gummy (Figure S4). Their spectra differed from reference spectra by the position of the silicon dioxide (a common reinforcement in tire rubbers) peaks within the 1130–975 cm–1 (Si–O–Si) range which led to a low hit quality index (<0.3), consistent with the leaching of silicon dioxide content (Figure S5a). FTIR spectra of aged tire rubber were misclassified as hemp seed or black ink after H2O2 treatment, which could be due to the oxidation of the unsaturated C–C bonds and Si–C bonds (Figure S5b). SEM images of aged tire rubber revealed that cracks, which developed during the aging process and expanded after H2O2 treatment (Figure S4), resulted in particle deformation or fragmentation. Our results suggest that the aging process further complicates the identification of tire rubber particles in environmental samples due to changes upon oxidation under environmental conditions and during digestion.

PS foams after KOH (10%) digestions, for which the lowest recoveries were 79 and 45% for pristine and aged MPs, respectively, also had altered FTIR spectra (Figure S6a,c), which led to the misclassification as PE with a low hit quality of ∼0.4, after KOH (10%) + KOH (10%) treatment (Figure S6d). These changes after digestion of aged PS could cause over- or underestimation of recoveries depending on the size of generated particles and the size detection limits.

While pristine glitter’s spectrum (PET) after KOH (10%) + KOH (10%) was the same as the control sample, the particles had physically altered edges (Figure S7; recovery = 69 ± 5%). Aged glitters separated into single-layer PET films (Figure S4) with a low recovery of 44 ± 3%, and the FTIR spectrum showed changes in intensity along with formation of new peaks and/or disappearance of existing ones (Figure S7b). The separation of PET films from glitters could also cause over- or underestimation following mild vs harsh KOH digestion, respectively.

For pristine PU, H2O2-based treatments caused the surface to become rough (Figure S4) and the FTIR spectra to change (Figure S8a). Those spectral changes were more pronounced in aged PU, particularly with significant shifts in CO stretching bond (1700–1800 cm–1), C–O stretching bond (1000–1300 cm–1), and C–H stretching bond (2800–3100 cm–1), which led to a low hit quality index (<0.3). These treatments also caused aged PU to break into small fragments (Figure S4), which could result in an overestimation of particle number or an underestimation if they were misidentified or lost during sampling handling, such as sieving. The same deterioration/fragmentation occurred for PA fibers. Savino et al. also reported a fraying of aged PA fibers (1–5 mm) after FR treatments at temperatures higher than 50 °C.

As noted above, pristine and aged PET, PE, and PP fragments (300–1000 μm) had high recoveries along with minimal morphological or spectral changes (Figure S9). Lessa Belone et al. also found no significant changes in PET (10 mm) surface features upon digestion with H2O2 (30%) at 40 °C for 24 h. As well, Schrank et al. reported no chemical changes of PP films (10 mm × 10 mm × 100 μm) after FR treatment at 75 °C for 1.5–2 h. However, we found that small pristine (63–300 μm) and aged (300–1000 μm) PET fragments showed significant surface (physical) changes and slight chemical changes after KOH (10%) treatments (Figures S9 and S10). Their recoveries were also significantly lower than large pristine PET fragments. Highly alkaline solutions can cause PES and PET degradation by saponification of ester (−CO–O−) linkages at the particle surface area, especially for those with higher surface-to-volume ratios. ,

3.3. Factors Influencing MP Recoveries

We tested the influence of various factors using a stepwise regression model (eq 2, SI 3.1) to allow us to systematically analyze and understand the relationships between different factors and their impacts on recoveries of susceptible MPs. Stepwise first and second order regression models were determined for each MP type (eqs 3–14; SI 3.1) across the tested factors. We found that OM concentration influenced recoveries more than all factors tested, regardless of the digestion method. This is shown in Pareto charts (Figure S11) that were derived from the slopes of the first-order regression relationships (eqs 4, 6, 8, 10, 12 in SI 3.1). In particular, the presence of OM in tests increased the recoveries of susceptible MPs since OM competes with plastic polymers for the reactive species in digestion solutions.

After OM, concentrations of H2O2 and KOH, followed by temperature, were most influential on the recoveries of susceptible MPs (Figure S11). Exposure time was only important for preserving PU foams (Figure S11d). FR did not impact the susceptible polymers, resulting in nonsignificant contributions in the regression models, and consequently did not surpass the reference line. These results indicate that FR is the safest digestion reagent for preserving susceptible MPs.

Temperature was more important than exposure time for PA fibers (Figure S11a), PS foams (Figure S11c), and tire rubber (Figure S10f), while exposure time was a critical factor for PU foams (Figure S11d). Previous studies have suggested temperatures below 50 °C to remain within the heat deflection limits of most susceptible MPs. ,, However, we found that even at 50 °C, H2O2 significantly affected recoveries of PA fibers and tire rubber, while KOH significantly decreased the recoveries of PS foams at low OM concentrations. Since heating is critical to initiate the reaction and reduce the reaction time, , we recommend using a maximum temperature of 50 °C for at most 48 h to prevent damage to the susceptible MPs during a highly exothermic reaction or a longer exposure time.

We found that OM plays a protective role in mitigating the negative effects of other variables on the recoveries of MPs. This was indicated by the negative slope for all factors except OM or any interactions involving OM in eqs 3–14 (SI 3.1).

3.4. Optimal Digestion Methods

It is essential to optimize protocols of complex environmental samples based on OM content, while balancing temperature, exposure time, and digestion reagent concentrations to effectively remove OM while preserving MP integrity. Although we found that the presence of OM was protective of susceptible MPs, it is still necessary to effectively remove the OM for accurate quantitative analysis of all MPs. More repeat and aggressive digestion steps may be necessary to reduce OM in samples to enable MP identification; these steps can degrade susceptible MPs. To balance these competing needs, we employed a multifactorial response optimizer based on our factorial design to indicate which digestion treatments yielded the highest recoveries of all six susceptible MPs with desirability (D score) of >0.7 as criteria. Among those methods meeting this criterion, we selected the top three methods with the highest D scores for each level of OM (Figure S12 and Table S6). At low OM concentrations of 1.6 g/L, a single-step digestion solution (e.g., FR) for a shorter time (2 h) and a lower temperature (<50 °C) can be used. For higher OM levels of <16 g/L, we recommend a combination of two digestion solutions, e.g., FR + KOH (10%) at 50 °C for 2 + 48 h. Finally, for OM ∼ 82 g/L, more than two steps of digestion are needed, e.g., FR + KOH (10%) + FR at 50 °C for 2 + 48 + 2 h.

Table S6 summarizes predicted recoveries of pristine MPs (300–1000 μm) for optimal digestion methods obtained using a response optimization model. At OM 82 g/L, repeated digestion is necessary to ensure complete breakdown of OM, but this process can significantly reduce the recovery of PU foams and PES fibers to 50–70% from 70 to 90% at OM 16 and 1.6 g/L. To minimize underestimation, we recommend testing the digestion method on these MPs beforehand for recovery rates.

3.5. Validation of the Optimal Digestion Methods

The recoveries of pristine (63–1000 μm) and aged (300–1000 μm) MPs, following the predicted optimal digestion methods for OM = 16 g/L (highlighted as a red box in Figure S12), were experimentally tested in the presence of OM to validate the recommended digestion protocols, e.g., FR + KOH (10%), H2O2 (30%) + KOH (10%), and KOH (10%) + H2O2 (15%). The overall recoveries of susceptible MPs after FR + KOH (10%) were higher than the other two digestion methods for all types of MPs tested (Figure and Table S7). These findings underscore the robustness and applicability of these optimal methods under realistic environmental conditions, particularly when the OM level is up to 16 g/L. Density separation significantly enhanced the recoveries of small-sized PU foams, PET fragments, Irregular PE microbeads, and tire rubbers by removing residual inorganic material that hid some of the MPs (p < 0.05, Kruskal–Wallis test; Figure S13).

2.

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Recoveries of the most susceptible (to degradation) MPs in the presence of organic matter (OM; 16 g/L) in three groups of aged MPs (300–1000 μm, n = 9), large pristine MPs (300–1000 μm, n = 9), and small pristine MPs (63–300 μm, n = 9). Dashed line shows recovery = 70%. H2O2: hydrogen peroxide, KOH: potassium hydroxide, FR: Fenton’s reagent. All digestions were conducted at 50 °C with times varying from 2 to 48 h (see Table S7 for full details).

The recoveries of PET fragments, PA fibers, and tire rubbers were >80% for all tested methods (Figure and Table S7). In contrast, the recoveries of pristine PES fibers (both large and small), aged thin PA fibers, larger PU foams, and small PS foams were <75%, with aged PU foams showing the lowest recoveries of 8–43%. Considering the lowest recoveries of aged PU foams after H2O2-based methods (<20%), we recommend avoiding these digestion methods for highly weathered environmental samples. Furthermore, only aged larger PS foams showed recoveries >100% after the harsh digestion method (H2O2 (30%) + KOH (10%) for 24 + 48 h at 50 °C), due to fragmentation. These findings also highlight the significance of ensuring similarity in characteristics between the spiked MPs and the targeted particles in real samples during QA/QC tests. Specifically, using pristine MPs as spikes to test laboratory processing efficiency for highly weathered environmental samples may not account for actual losses, and thus could overestimate counts.

4. Conclusions and Recommendations

Based on comprehensive testing of different reagents, temperatures, and exposure times on spiked pristine (63–1000 μm) and aged (300–1000 μm) MPs of 16 different morphology/types, with and without OM, we recommend the following digestion methods to optimize the recovery of MPs from complex matrices containing OM and sand: a sequential procedure of FR + KOH (10%) at 50 °C for 2 and 48 h, respectively, to remove OM (which was the factor that most influenced recoveries) while minimizing degradation of both pristine and aged MPs. This can be followed with density separation using NaI (ρ = 1.6 g/cm3), to improve recoveries of smaller MPs (e.g., 63–300 μm). Since the characterization of OM in all samples is neither practical nor common, we recommend first starting the digestion with FR to reduce even low concentrations of OM and then adding a second step of KOH (10%) if needed. An extra digestion step should be avoided because it can cause aged MPs to fragment, which could result in an overestimation of particle number or an underestimation if MPs were misidentified or lost during sampling handling, such as sieving. We recommend using ‘matrix spikes’ in addition to blank spikes, with spikes comprised of diverse MPs (different types and pristine and aged), to quantify the effect(s) of the treatments used. Digestion recoveries obtained from blank spikes containing only resistant pristine MPs, such as PE and PP, are unlikely to accurately represent real environmental samples since weathered MPs, especially of polymers susceptible to degradation, could be partially or completely digested. Considering the lowest recoveries of aged PU foams after H2O2-based methods (<20%), we recommend not using these digestion methods for highly weathered environmental samples.

Supplementary Material

ew5c00017_si_001.pdf (3MB, pdf)

Acknowledgments

Funding was provided by a NSERC Alliance Grant, Plastics Science for a Cleaner Future (ALLRP 558429-20), and NSERC Discovery Grant to MLD (RGPIN-2023-05451). We thank Dr. Yanan Liu (University of Toronto) for assistance with the SEM.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsestwater.5c00017.

  • Details of chemicals, MP spiking materials and complex matrix, methods for preparing aged MPs, characterization of MPs before and after UV aging, digestion methods for sequential combinations of oxidative-alkaline solutions, factorial design and response optimizer model, characterization of MPs after extraction methods using scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR), relative differences in MP identification between two counters, heatmap of 63–300 μm pristine MP recoveries after treatments, Pareto charts showing factoring influencing recoveries of susceptible MPs, predicted recoveries and optimal digestion strategies for preserving susceptible MPs, experimentally determined (spiked MPs in matrix of OM and sand) recoveries of pristine (63–1000 μm) and aged (300–1000 μm) MPs following the predicted optimal digestion methods, and effects of density separation on recoveries (PDF)

CRediT: Razegheh Akhbarizadeh conceptualization, data curation, formal analysis, investigation, methodology, project administration, supervision, validation, visualization, writing - original draft, writing - review & editing; Yan Jin Xu investigation; Freya Boerner investigation; Paul A. Helm funding acquisition, project administration, resources, supervision, writing - review & editing; Miriam L. Diamond funding acquisition, project administration, supervision, writing - review & editing.

The authors declare no competing financial interest.

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