# Particle Size and Pre-Treatment Effects on Polystyrene Microplastic Settlement in Water: Implications for Environmental Behavior and Ecotoxicological Tests

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## Abstract

**:**

_{D}) as the ratio between calculated volume and theoretical volume of suspended particles was used to compare techniques. PS dispersions (20 mg/L) treated for 90 min in an ultrasonic bath (120 W, 35 kHz) were evenly dispersed with a particle concentration of 140,000 particles/mL and a high reproducibility (rel. SD = 2.1%, n = 6). Automated horizontal shaking for 754 h (250 rpm) reached similar particle numbers (122,000/mL) but with a lower reproducibility (rel. SD = 9.1%, n = 6). Manual shaking by hand dispersed the lowest number of particles (55,000/mL) and was therefore found to be unsuitable to counteract homo-agglomeration. E

_{D}was calculated as 127%, 104% and 69% for ultrasonic treatment, horizontal shaking and manual shaking, respectively, showing an overestimation of volume assuming spherical shaped particles.

## 1. Introduction

_{D}was determined to evaluate the methods of dispersing.

## 2. Materials and Methods

_{D}was used (for formulae see SI, Section C). E

_{D}is the ratio between calculated suspended solid volume V

_{css}and theoretical suspended solid volume V

_{tss}and indicates the state of the dispersion (fully dispersed vs. agglomerated/attached to surfaces etc.). V

_{tss}was calculated using the suspended solid mass and a PS density of 1.05 g/cm

^{3}. V

_{css}was calculated using the particle counter data and the lower channel boundary (see SI, Section D) and volume formula for spherical shape.

## 3. Results

#### 3.1. Horizontal Shaker vs. Ultrasonic Treatment

_{D}after 754 h was 104%, additional ultrasonic treatment decreased E

_{D}to 98%. Long-term horizontal shaking only resulted in a partial deagglomeration. This has to be seen critically since usual contact times of adsorption experiments are in the magnitude of 48–72 h [28,29] with even less efficient dispersion.

#### 3.2. Manual Shaking vs. Ultrasonic Treatment

_{D}of 69% after 250 agitations. After ultrasonic treatment, batches G and H reached E

_{D}of 106% and 104%, respectively. Almost complete dispersion was reached after 25 min of ultrasonic treatment. Measurements at ultrasonic durations of 5, 10, 15, and 30 min show that manual shaking before sonication did not improve dispersibility. The final particle concentrations of both batches are almost identical after sonication, showing the reproducibility of ultrasonic treatment. This strongly emphasizes that manual shaking only is not a sufficient way of dispersing particles that tend to agglomerate.

#### 3.3. Reproducibility of Ultrasonic Treatment

_{D}after 90–120 min was 125%, which implies an overestimation of the suspended solid volume.

#### 3.4. Changes in Particle Size Distribution

#### 3.5. Changes in Calculated Volume Distribution during Ultrasonic Treatment

^{6}µm

^{3}/mL (calculated from values 120–180 min) with rel. SD of 3.2%, whereas the total theoretical suspended volume of the PS is 19.6 × 10

^{6}µm

^{3}/mL. E

_{D}calculated with the mean volume of 120–180 min was therefore 127%, which indicates that calculation of the volume overestimated the volume in dispersion.

#### 3.6. Settling of Particles after Ultrasonic Treatment

#### 3.7. Environmental Implications

## 4. Discussion

_{D}values of above 100% indicated that the presumption of spherical shape for cryogenically produced particles is a simplification, as cryomilled particles are more shard-like fragments [5]. This overestimated the calculated volume of measured particles when using the Feret diameter given by the particle counter, which was only partly counteracted by using the lower boundaries of the measurement channels.

^{2}polystyrene squares cut from larger sheets and additional FTIR-bands mimicking weathering processes in the environment. We did not investigate changes in the morphology through radical attack during sonification. Such changes were not expected since the duration of ultrasonic treatment was much shorter (180 min compared to 15 h) and pH (unbuffered) was around 5.5 (van der Esch: pH = 13). Therefore, surface oxidation during sonication in this study is unlikely. Nevertheless, it needs to be discussed whether polymers pre-treated in an ultrasonic bath even for short periods can still be regarded as pristine if the surface of particles is potentially altered.

## Supplementary Materials

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Theoretical position of an exemplary cryomilled polystyrene (PS) particle with a density of 1.05 g/cm

^{3}during settling in the course of a measurement with PAMAS particle counter. The particle starts to sink at the water level. Height = 0 cm: water level of glass bottle; height = −15.5 cm: bottom of glass bottle. Full, dashed and dash-dotted lines: theoretical location of particles of different diameters. Horizontal dotted line: suction height of tubing connected to PAMAS device 7 cm from water surface. Highlighted area shows timespan of a measurement (less than 3 min). Stoke’s law with assumption of spherical shape was used for calculations.

**Figure 2.**(

**a**) Left: Particle concentration of six batches (20 mg/L PS particles) during 750 h horizontal shaking (HS). Right: Particle concentration of the same 6 batches during 30 min ultrasonic treatment (UT). (

**b**) Particle concentrations of two batches (20 mg/L PS particles); batch G was manually shaken (MS) for 250 times (left diagram, full squares) and subsequently sonicated (right diagram, empty squares), batch H was directly sonicated (UT; right diagram, empty circles). Error bars show standard deviation of n = 3.

**Figure 3.**Particle concentrations of six batches (A–F = 20 mg/L PS particles) during ultrasonic treatment of 180 min. Error bars show standard deviation of n = 3.

**Figure 4.**(

**a**) Particle concentration of batch A during manual shaking (MS), horizontal shaking (HS) and ultrasonic treatment (UT). (

**b**) Particle concentration of batch A (20 mg/L PS particles) during ultrasonic treatment (UT) for 180 min. Particles in all diagrams were grouped in size fractions <5 µm, 5–10 µm and >10 µm. Error bars show standard deviation of n = 3.

**Figure 5.**(

**a**) Calculated suspended solid volume of six batches (A–F = 20 mg/L PS particles) during ultrasonic treatment (UT). Dotted line at 19,048,000 µm

^{3}/mL indicates an E

_{D}of 100%. Highlighted area was used for calculation of mean volume and E

_{D}. (

**b**) Calculated volume per size channel of batch A during 180 min of ultrasonic treatment. Values were normalized through division by channel width to compare smaller and larger channels. A non-normalized graph can be found in the SI, Figure S2. Error bars show standard deviation of n = 3.

**Figure 6.**Calculated volume of batch A during ultrasonic treatment. Particles were grouped in size fractions <10, 10–50, and >50 µm (please note: different fractions were chosen for volume fractions compared to number fractions). Error bars show standard deviation of n = 3.

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**MDPI and ACS Style**

Eitzen, L.; Ruhl, A.S.; Jekel, M. Particle Size and Pre-Treatment Effects on Polystyrene Microplastic Settlement in Water: Implications for Environmental Behavior and Ecotoxicological Tests. *Water* **2020**, *12*, 3436.
https://doi.org/10.3390/w12123436

**AMA Style**

Eitzen L, Ruhl AS, Jekel M. Particle Size and Pre-Treatment Effects on Polystyrene Microplastic Settlement in Water: Implications for Environmental Behavior and Ecotoxicological Tests. *Water*. 2020; 12(12):3436.
https://doi.org/10.3390/w12123436

**Chicago/Turabian Style**

Eitzen, Lars, Aki Sebastian Ruhl, and Martin Jekel. 2020. "Particle Size and Pre-Treatment Effects on Polystyrene Microplastic Settlement in Water: Implications for Environmental Behavior and Ecotoxicological Tests" *Water* 12, no. 12: 3436.
https://doi.org/10.3390/w12123436