Surface functionalization determines behavior of nanoplastic solutions in model aquatic environments

Abstract

Plastic debris are classified as a function of their size and recently a new class was proposed, the nanoplastics. Nano-sized plastics have a much greater surface area to volume ratio than larger particles, which increases their reactivity in aquatic environment, making them potentially more toxic. Only little information is available about their behavior whereas it crucially influences their toxicity. Here, we used dynamic light scattering (DLS) to explore the influence of environmental factors (fresh-and saltwater, dissolved organic matter) on the behavior (surface charge and aggregation state) of three different nano-polystyrene beads (50 nm), with (i) no surface functionalization (plain), (ii) a carboxylic or (iii) an amine functionalization. Overall, the positive amine particles were very mildly affected by changes in environmental factors with no effect of the salinity gradient (from 0 to 653 mM) and of a range 1-30 μg.L−1 and 1-10 μg.L−1 of organic matter in artificial seawater and ultrapure water, respectively. These observations are supposedly linked to a coating specificity leading to repulsive mechanisms. In contrast, the stability of the negatively charged carboxylic and plain nanobeads was lost under an increasing ionic strength, resulting in homo-aggregation (up to 10 μm). The increase in organic matter content had negligible effect on these two nanobeads. Analysis performed over several days demonstrated that nanoplastics formed evolving dynamic structures detected mainly with an increase of the homo-aggregation level. Thus, surface properties of given polymers/particles are expected to influence their fate in complex and dynamic aquatic environments. Highlights ► The behavior of different nanopolystyrene beads was investigated by dynamic light scattering. ► Surface functionalization affects the behavior of nanopolystyrene beads. ► Carboxylate and plain nanopolystyrene beads formed microscale aggregates in seawater. ► Organic matter had negligible effect on all nanoplastics tested.► Nanoplastics formed evolving dynamic structure over time.

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

“…Before ultrasonic treatment, the size fractions <5 µm, 5-10 µm and >10 µm made up 66.5%, 18.6% and 14.8% of all particles, respectively. After 30 minutes the size fraction ratios changed to 78.1%, 15.2% and 6.7% for <5 µm, 5-10 µm and >10 µm, respectively. Subsequent ultrasonic treatment increased the number of particles but only led to minor changes in the size composition.…”

“…Natural organic matter (NOM) is also used in some studies to stabilize MP and mimic environmental conditions, but the effect of NOM on dispersion stability has not been fully resolved yet. Reported results indicate a stabilizing as well as no effect using NOM standard samples from the International Humic Substances Society (IHSS) in MP dispersions [12][13][14][15][16].…”

“…Zeta potential measurements are widely used to study the effects of ion strength, pH and humic substances on the stability of MP and nanoplastic (NP) dispersions [12,13,15,16]. This parameter is essential to characterize dispersions, but measurements are elaborate and time-consuming and limited to smaller particle sizes depending on the density of the particle.…”

Toxicological considerations of nano-sized plastics

“…Not only do nanoplastics have a polarization associated with their chemicals construct that may influence the hydrophilicity and hydrophobicity of the particle; but they can also adsorb chemical contaminants to their surface, transporting them within the environment or through a biological system [16–18]. Of the particles analyzed thus far, polyethylene has the greatest chemical sorption rates [19].…”

“…These chemical additions may act as a secondary toxicant or as a functionalized group on the surface of the particle, encouraging or discouraging biological interaction. These differential surface modifications and particle transformations will impact nanoplastic fate and toxicity [9,16,18,20].…”