Interaction of metal oxide nanoparticles with microplastics: Impact of weathering under riverine conditions

Highlights

• Microplastics (MPs) shows interaction with CeO₂ nanoparticle (CeNP).

• Weathered MPs showed higher sorption than pristine MPs.

• Salinity, pH, and organic matter play a significant role in stability and sorption.

• Aggregation and surface charge governed the interaction mechanism.

Abstract

The ubiquitous distribution of microplastics (MPs) leads to inevitable interactions with the toxic pollutants present in the environment including metal-oxide nanoparticles. This study investigates the interaction of CeO₂ nanoparticles (CeNPs) with MPs generated from a disposable plastic container. Further, rough MPs (R-MPs), generated through mechanical abrasion of MPs with sand, were used to probe the impact of roughness. To understand the sorption kinetics and underlying interaction processes, batch experiments were carried out. The results distinctly indicate that CeNPs sorption occurred on MPs surfaces and was consistent with the pseudo-second order kinetics model. For pristine MPs, the sorption capacity was as high as 12.9 mg/g while for R-MPs kinetic equilibrium was achieved faster and an enhanced sorption capacity (13.4 mg/g) was identified. A rise in sorption with an increase in salinity was noted while pH and humic acid exhibited a negative correlation. The observed interactions were attributed to the aggregation profile and surface charge of CeNPs and MPs. Surprisingly, CeNPs also got loaded onto MPs in non-agitated and undisturbed conditions. The sorption process was influenced by the type of aqueous matrix and the sorption capacity at equilibrium followed the trend: distilled water> synthetic freshwater> river water. FTIR spectra, zeta potential, SEM imaging, and elemental mapping revealed electrostatic interaction as the dominant mechanism. This work contributes towards the knowledge gap on the environmental risk of MPs.

Introduction

The increased production and consumption of plastics with high durability, versatility, structural strength, resistance, and proficiency in manufacturing will soon leave the earth engulfed by plastic waste. The exponential rise in demand for plastics in 2015, has led to an approximately 620% increase in plastic pellet resin production as compared to 1975 (Jambeck et al., 2015). Very recently tiny plastic particles have attracted a lot of attention in the scientific community. This includes microplastics (MPs), i.e. plastic particles of size smaller than 5 mm (Barnes et al., 2009; Thompson et al., 2004). MPs are categorized as primary and secondary based on their origin. Primary MPs are intentionally produced for end-user consumption (Derraik, 2002; Fendall and Sewell, 2009) whereas secondary MPs are generated by the exertion of various environmental forces such as physical, chemical, and biological (Andrady, 2011). The small plastic fragments are found to be omnipresent in all components of the earth, i.e. air, water, glaciers, soil, and aquatic lifeforms (Karbalaei et al., 2018; Kumar et al., 2018; Luoto et al., 2019; Tiwari et al., 2019). On account of reduced size and colorful appearance, plastic particles are mistakenly ingested and then reside within the guts of aquatic organisms (Cole et al., 2013; Redondo-Hasselerharm et al., 2018; Rillig and Bonkowski, 2018). Furthermore, MPs are known to interfere with reproduction, gene expression, and cause mortality (Alimba and Faggio, 2019; Bhagat et al., 2020).

MPs are of great concern as they have the propensity to bind contaminants from its surrounding based on contaminant properties, partition coefficients, and they are well known to co-transport pollutants including pathogens (Browne et al., 2007; Hüffer et al., 2018; Koelmans et al., 2016; L Teuten et al., 2007; Wang et al., 2018). Phenanthrene, a hydrophobic polycyclic aromatic hydrocarbon (PAH), was reported to accumulate on plastics in a greater amount than in sediments (Teuten et al., 2007). Wang et al. reported partitioning as the major interaction for contaminants such as perfluorooctanesulfonamide and perfluorooctanesulfonate with MPs, not hydrophobicity (Wang et al., 2015). Unlike glassy polymers, rubbery polymers such as polyethylene can accommodate a higher concentration of pollutants (Rodrigues et al., 2019). Studies have reported significantly higher adsorption of pollutants on MP than in the environmental matrix including heavy metals (Brennecke et al., 2016; Hüffer et al., 2018; Munier and Bendell, 2018). Few metals were found to be concentrated on MPs owing to their proximity to the contaminated site (Carbery et al., 2020) while another study has highlighted the local accumulation of heavy metals on MPs to be higher than its ambient environment (Ashton et al., 2010). Another report has emphasized a higher adsorption affinity of weathered MPs for metals due to the development of anionic sites on the surface (Holmes et al., 2014). Though there are studies on MPs-aided metal ion sorption and transport, not much attention has been given to the interaction between MPs and novel emerging nanoparticles (NPs) in the environment.

Unlike other pollutants, nanoparticles such as nanometal oxides have unique physicochemical properties different from their bulk counterparts having a similar chemical composition, which has promoted their widespread applications and commercialization to a great extent. Metal oxide NPs are well known to occur in the environment and can generate reactive oxygen species which are a serious threat to microbiota, plants as well as humans (Garner and Keller, 2014; Yang et al., 2013).

Nanoceria or cerium oxide NPs (CeNPs) have extensive industrial applications as catalysts, glass polishing agents, solar cells as well as in bio-medicals as antioxidants (Bhagat et al., 2019; Hu et al., 2018). Estimations suggest an increased utilization of CeNPs that has led to a higher production amounting to approximately 10,000 t/year in 2010 of which 1800 t were ultimately deposited into air, water, and soil, whereas the rest was dumped into landfills (Keller et al., 2013). Once these NPs are in the environment, they tend to interact with the pre-existing MPs. Several parameters control the partitioning between solid and aqueous phases of contaminants in an aquatic environment. However, no reports have so far addressed the sorption of CeNPs onto MPs, which may affect its potential impact on the ecosystem (Bhagat et al., 2019; Deng et al., 2017).

MPs in the environment are exposed to physical weathering due to mechanical abrasion by sand, wind, or water along with UV light that causes photo-oxidation and leads to chain scission or crosslinking of the polymer bonds (Song et al., 2017; Wang et al., 2017). The decrease in mechanical strength also results in the development of surface features to produce brittle and rough plastics with cracks including alteration in roughness, polarity, surface charge, and porosity (Torkzaban and Bradford, 2016). The nanoscale roughness of a surface governs physicochemical and mechanical properties and is known to alter bio-attachment processes, microbial colonization, and surface area (Pan et al., 2019; Verran and Boyd, 2001). Recent studies demonstrate that collector surface roughness and topography affect colloid retention, and consequently its adhesion on the collector surfaces such as rock (Darbha et al., 2012; Krishna Darbha et al., 2012). Although dramatic changes in the roughness of MPs with varying degrees of micro-cracks have been observed during weathering (Pan et al., 2019), there is still a lack of information about how weathering leads to nanoscale roughness and subsequently affects the sorption of CeNPs on MPs. In addition to surface roughness, other factors that influence sorption include pH, temperature, dissolved organic matter (DOM), and ionic strength in an aqueous environment (Khandelwal et al., 2019). Therefore, it is imperative to comprehend the interaction between NPs and MPs surface in its surrounding environment.

Single-use plastic containers often used to serve beverages can be a significant source of microplastics for the environment. Polyethylene, polypropylene, and expanded polystyrene along with several additives are being used to produce such disposable plastic containers. The huge amount of these adds to the nuisance of plastic waste that is susceptible to form complexes with other pollutants in the environment. This has promoted us to use MPs from single-use plastic containers in this work to uncover its interaction probability with CeNPs.

To the best knowledge of the authors to address the interaction and sorption of metal oxide nanoparticle and MPs for the first time, this study was conducted with the primary objectives (i) to assess the potential of CeNPs sorption to MPs by interpreting sorption kinetic and isotherm studies and (ii) to understand the effect of surface roughness of MPs on the sorption process. Besides, we also investigated the sorption process in complex conditions and relevant aqueous matrices such as in presence of varying degrees of DOM, pH, salinity, absence of agitation, and finally in synthetic freshwater (SFW) and ambient river water (RW).