Nanoplastics and microplastics

As a plastic material we used polystyrene (PS) particles since this type of plastic is used in many applications for example in packaging, building and construction, electrical and electronics industries, and paints. The demand on polystyrene in Europe, including expanded polystyrene, reaches the level of 3.4·10⁶ tonnes per year (Plastics-the Facts, 2016), thus, making this plastic materials extensively present in the environment. In addition, PS plastics are difficult to recycle and are highly resistant to biodegradation (Chaukura et al., 2016; Kaplan et al., 1979).

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PS particles which we use in this study are available as dispersions and serve as model particles to assess the behaviour of PS nanoplastics in the environment. Latex polystyrene nanospheres were provided from Molecular Probes^® (Life Technologies Corporation, USA) with a mean diameter equal to 20 ± 0.3 nm, density equal to 1.055 g/cm³ (20°C) and specific surface area of 2.8·10⁶ cm²/g (provided by the manufacturer).

Using SEM image analysis (Fig. II.5B) estimated size of the pristine nanoplastics was found between 25 and 63 nm.

3 4 5 6 7 8 9 10 11

0 10 20 30 40 50

60 ^-potential

D_Z

pH

Zeta potential (mV)

A

30 40 50 60 70 80 90 100 Z-average diameter (nm)

Fig. II.5. (A) Zeta potential and z-average hydrodynamic diameter of PS nanoplastic particles at different pH in ultrapure water. Zeta potential is decreasing with increase of pH. Z-average hydrodynamic diameter is found constant with a mean value equal to 53.1

± 4.3 nm. Experimental conditions: [PS] = 50 mg/L, initial pH 3. (B) SEM image of pristine nanoplastics in ultrapure water: [PS] = 5 mg/L, the diameter of PS particles is found between 25 and 63 nm.

These PS nanoplastics are positively charged in a large pH domain (Fig. II.5A) with a surface charge density equal to 3.0 μC/cm², due to the presence of amidine groups. In aquatic systems, positive charges of nanoplastics favours the interaction with negatively charged natural colloids. A 400 mg/L stock suspension of nanoplastics at pH 3.0 was prepared by diluting original suspension with ultrapure water (R > 18 MΩ cm, Millipore, Switzerland) for which pH was previously adjusted to 3.

On the other hand, we used microplastic particles which are also made of polystyrene. In that case we used negatively charged microplastics. Their negative charge is due to the presence of sulphate functional groups on the surface. IDC Latex particles (Life Technologies Corporation, USA) have a diameter 0.99 ± 0.03 µm (TEM measurement, provided by manufacturer), initial concentration 78 g/L, density 1.055 g/cm³ (20°C), specific surface area 5.7·10⁴ cm/g and were free from surfactants. A stock suspension of

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1 g/L microplastic was prepared and then diluted to the appropriate concentration with Milli Q water (R > 18 MΩ·cm). The suspension pH was adjusted by adding small amount of diluted HCl and NaOH (Merck, Germany). Negative values of zeta potential through full pH range (Fig. II.6) confirms presence of negatively charged functional groups.

Microplastic hydrodynamic diameter was found in good agreement with data provided by manufacturer and was found equal to 1098 ± 224 nm (Fig. II.4 Inset).

2 3 4 5 6 7 8 9 10

-80 -70 -60 -50 -40 -30 -20 -10 0 10

400 800 1200 1600 2000

0 5 10 15 20 25

Size (nm)

Intensity (%)



-po te n ti al (mV )

pH

Fig. II.6. Zeta potential of sulphate polystyrene microplastic particles as a function of pH.

Zeta potential is found negative in all pHs. Size distribution (inset) of particles using DLS method. Z-average hydrodynamic diameter is found equal to 1000 nm.

II.2.1.3 NOM

To evaluate the effect of NOM on the stability of ENMs released to natural water two types of materials have been considered, humic substances and polysaccharides.

As a surrogate of humic substance, the Suwannee River Fulvic acids (Standard II, 2S101F) were used and supplied by International Humic Substance Society (USA). FAs are mixtures of different types of organic acids and are more reactive compared to HAs due to the high concentration of hydroxyl (-OH) and carboxyl (-COOH) groups. Three structural models of FA molecules are presented in Fig. II.7 (Leenheer et al., 1995). FAs have low molecular weights and are soluble in a large pH range (Pettit, 2004). A 1 g/L suspension was first prepared from powder and then diluted to 100 mg/L by ultrapure

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water. This suspension was our stock suspension from which we made our further dilutions.

Fig. II.7. Structural models of Suwannee River fulvic acid molecules (Leenheer et al., 1995).

To characterize FAs a 50 mg/L suspension with addition of 0.001 M NaCl as background electrolyte was prepared at pH 3.0 ± 0.1. The titration was performed with diluted 0.01 M NaOH solution. FA surface charge was found negative through all pH range (Fig. II.8). All solutions were maintained in a dark place with temperature between 0 and 4 °C.

3 4 5 6 7 8 9 10 11

-50 -40 -30 -20

-10 zeta potential

z-average diameter

pH

-potential (mV)

200 300 400 500 600 700 z-average diameter (nm) A

0 100 200 300 400 500

0.00 0.25 0.50 0.75 1.00 1.25

Concentration (106 particles/ml)

Hydrodynamic diameter (nm)

B

Fig. II.8. (A) FA zeta potential and z-average hydrodynamic diameter variation as a function of pH. Surface charges of FAs are negative at all pHs. Z-average hydrodynamic diameter varies from 200 to 400 nm. (B) FA size distribution is determined using NTA method. Average hydrodynamic diameter was found equal to 194 ± 89 nm. We observed the presence of polydisperse particles.

As a surrogate of natural polysaccharide we used alginate (A2158, Sigma Aldrich, Switzerland). The molecular weight of low viscosity alginate is equal to 50 kDa.(LeRoux et al., 1999). Alginate is a natural anionic polymer usually extracted from brown algae

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such as Laminaria hyperborea, Laminaria digitata, Laminaria japonica, Ascophyllum nodosum, and Macrocystis pyrifera.

Alginate is a mixture of linear copolymers blocks of β-d-mannuronate (M) and α-l-guluronate (G) residues (Lee and Mooney, 2012). The carboxyl groups of mannuronic acid has a pKa =3.38 and of guloronic acid pKa =3.65 and can be protonated by inorganic acid.

When the pH of solution is below the pKa alginate tends to form a hydrogel (Rehm, 2009).

The blocks of copolymers are composed of the individual or mixed residues (Fig. II.9A).

We prepared a 100 mg/L stock solution in ultrapure water which was used for further dilution. To characterise alginate titration of 50 mg/L alginate solution was done. The variation of z-average diameters and zeta potential versus pH was recorded and presented in Fig. II.9B. To adjust the pH, diluted sodium hydroxide and hydrochloride acid 0.01 M (NaOH and HCl, Titrisol^®113, Merck, Switzerland) were used. For suspension homogenisation, gentle agitation was applied during all the experiments with a magnet vortex and a rotational speed equal to 200 rpm.

3 4 5 6 7 8 9 10

-60 -50 -40 -30 -20 -10 0

zeta potential z-average diameter

pH

Zeta potential (mV)

0 100 200 300 400 500 Z-average diameter (nm)

B

Fig. II.9. (A) Alginate structure composed from consecutive blocks of G, M and mixed MGM residues (Lee and Mooney, 2012). (B) Variation of alginate zeta potential and z-average hydrodynamic diameters as a function of pH. Zeta potential is negative in all pHs. Z-average diameters vary from 150 to 250 nm. Experimental conditions: [Alginate] = 50 mg/L.

II.2.1.4 ICs

As an analogue of inorganic colloids from river Rhône, iron (III) oxide (α-Fe2O3, 99%) (Nanoamor, Inc., USA) as a powder was used. The main characteristics of Fe2O3 are given in Table II.2. First, a 1 g/L suspension was prepared at pH 10. Such pH allows a better resuspension and higher stability of dispersed particles. Then, to obtain homogenised

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suspension and well dispersed particles, sonication with an ultrasonic probe (CV18, Sonics Vibra cell, Blanc Labo S.A., Switzerland) during 15 min was performed. To prepare a 100 mg/L stock suspension, dilution with ultrapure water, for which pH was also previously adjusted to 10, was done. To characterise Fe2O3 particles we used SEM imaging (Fig. II.10A) and we measured z-average hydrodynamic diameter and zeta-potential as a function of pH (Fig. II.10B). Fe2O3 particles were found to have needle shapes. Zeta potential changed from negative to positive passing through a PZC at pH 5.8 ± 0.1. At environmentally relevant pH (pH 8.0 ± 0.2) Fe2O3 had a negative surface charge. Below the pH 5.0 and above the pH 8.0 nanoparticles are stable with z-average diameters less than 100 nm.

Table II.2. Characterisation analysis of Fe2O3 provided by manufacturer

Parameters Value

Appearance Red powder

pH value 6.7

Crystal α

Original particle size, nm 30-50nm

Surface area, m²/g 28

Purity, % 99.2

3 4 5 6 7 8 9 10 11

-45 -30 -15 0 15 30

zeta potential diameter

pH

Zeta potential (mV)

B

0 750 1500 2250 3000 3750 4500 Z-average diameter (nm)

Fig. II.10. (A) SEM image of pristine Fe2O3 ICs in ultrapure water. (B) Fe2O3 IC zeta potential and z-average hydrodynamic diameter variation as a function of pH.

Experimental conditions: [Fe2O3] = 10 mg/L. At environmental pH = 8.0 ± 0.2 Fe2O3 ICs are negatively charged.

33 II.2.2 Experimental technique

II.2.2.1 Dynamic light scattering (DLS) method and laser Doppler electrophoresis