Plastic is an important basic material developed by humans. Owing to its high-quality characteristics, such as its portability, affordability, great plasticity, and endurance, it is extensively employed in daily life, agriculture, industry, and the military (Tan et al. 2020), bringing great convenience to production and people's lives. According to statistics, global plastic manufacturing will reach 367 million tons in 2020 (Chen et al. 2023), and by 2050, global plastic output is anticipate to total 34 billion tons (Petersen & Hubbart 2021). Despite the huge output, only 6–14% of plastics are recycled (Alimi et al. 2018). Most plastics enter the environment without proper treatment and gradually break and disintegrate into plastic fragments with smaller particles under environmental stress, such as weathering, mechanical crushing, UV irradiation, chemical reactions, and microbial degradation (De Gisi et al. 2022, Lionetto et al. 2023). When plastic fragments or particles are smaller than 5 mm in diameter, they are generlly called MPs ( Thompson et al. 2004). When the plastic debris or particle size is less than 5 mm, it is called MPs.
MPs are a new global environmental pollutant, widely distributed in oceans, rivers, lakes, sediments, soils, and other environmental media (Fiore et al. 2022, Hu et al. 2021, Li et al. 2018, Li et al. 2021, Shen et al. 2018), and are even found in polar areas (Bergmann et al. 2019, Ding et al. 2023). MPs have also entered the food chain (Wang et al. 2021a). Humans ingest MPs through their diet, respiration, or skin contact, and they could harm human health (Carbery et al. 2018, Rubio et al. 2020), which also presents an immense risk to ecological environmental security and biodiversity (Rahman et al. 2021). Compared with large plastics, MPs have a small particle size, high SBET, high hydrophobicity, and are difficult to degrade (Leng et al. 2023, Liu et al. 2018), and they readily serve as carriers to adsorb and enrich various organic compounds, heavy metals, and microorganisms from external phases (Ahechti et al. 2022, Loncarski et al. 2021, Torres et al. 2021, Upadhyay et al. 2022). These pollutants also enter the food chain with MPs, and are transmitted and enriched between food chains, posing a serious risk to human wellness (Luo et al. 2023, Zhuang & Wang 2023). MPs help pollutants migrate into the environment through water transport or atmospheric flow, and can also interact with pollutants to produce combined effects, causing joint toxicity to organisms and ecosystems (Lv et al. 2022, Mejias et al. 2023). Consequently, the high concentrations of MPs in the environment and their environmental risks have attracted worldwide attention in recent years. MPs inevitably experience UV irradiation, thermal radiation, physical wear, chemical oxidation, and biological action in nature (Lang et al. 2020, Liu et al. 2020, Wang et al. 2021c), leading to different degrees of aging and resulting in changes in the surface morphology, functional group composition, crystallinity, and hydrophobicity of MPs (Liu et al. 2021b, Wright et al. 2013). The adsorbability and mechanism of MPs will also change, and their toxicity to organisms may also change accordingly. Cracks and gullies appear on MPs after aging (Liu et al. 2019), increasing both the pore structure and SBET (Fan et al. 2021b), and oxygen-functional groups (Brennecke et al. 2016, Kalcikova et al. 2020). Thus, there are more oxidation-binding sites on MPs and their adsorbability is improved (Wang et al. 2023), both of which indirectly raise ecological and environmental concerns. The aging of MPs affects their ability to absorb contaminants (Lin et al. 2021), which, in turn, affects the mechanism by which MPs adsorb pollutants.
Due to the rapid socioeconomic development and ongoing growth of metal electroplating, fertilizer manufacturing, mining, papermaking, and other industries, heavy metal ion-containing wastewater is being released into the environment in increasing amounts. (Dong et al. 2019, Wu et al. 2020, Xie et al. 2022), and heavy metal pollution in water frequently occurs; Cu(II) is a common heavy metal pollutant in water (Cherono et al. 2021, Wang et al. 2021b). Heavy metals are persistent contaminants that can bioaccumulate, persist in the environment, and are toxic (Sheng et al. 2022, Velusamy et al. 2021). At lower concentrations, heavy metals as Cu, Cd, Cr, As, and Pb can also have harmful consequences. Heavy metals are extremely difficult to biodegrade; thus, they can be enriched in the human body after diffusion, transfer, and dispersion under the amplification of organisms in the food chain, resulting in significant toxic effects (O 'Connor et al. 2020). Among the toxic pollutants adsorbed on MPs, heavy metals are representative of toxic inorganic pollutants (Foshtomi et al. 2019, Marsic-Lucic et al. 2018), and their physical and chemical characteristics, such as their structure, SBET, and age, affect their adsorption ability (Luo et al. 2022). Recently, the reciprocity between heavy metals and MPs has attracted extensive attention from the academic community. The potential hazards to the aquatic environment could be worsened by heavy metal ion adsorption on MPs (Khalid et al. 2021) and produce a compound toxic effect on organisms. The foundation for disclosing the environmental behavior and dangers of MPs and coexisting heavy metal ions in a combined contaminated aquatic environment is delineating the interaction strength and mechanism between MPs and coexisting heavy metal ions.
Owing to its good mechanical properties, heat resistance, and chemical stability, polyamide (PA) is widely used in the fields of automobiles, electronics, machinery, rail transit, sports equipment, and so on (Crespo et al. 2019, Reinaldo et al. 2020). Polylactic acid (PLA) is a transparent and environmentally friendly biomaterial with high strength, high modulus, and good formability with great application value in various fields (Geng et al. 2020, Sun et al. 2022). PA and PLA MPs have been detected in many aquatic environments (Jang et al. 2020, Naji et al. 2021, Obbard et al. 2014, Yan et al. 2019). At present, investigation on MPs in the environment has mainly focused on original MPs, whereas the carrier effect of aged MPs remains to be studied. Therefore, two types of polymer particles with different functional groups, PA and PLA, were used as the target MPs in this study. Cu(II) was selected as the coexisting MPs pollutant with the highest detection rate in the aquatic environments. Scanning electron microscopy (SEM), Brunauer-EmmettTeller (BET), Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD) measurements were used to describe the structure of MPs both before and after simulated UV aging. The isotherms and kinetics of the adsorption characteristics of Cu(II) in an aquatic environment by the two types of MPs were compared. From the viewpoint of the physicochemical properties of MPs, the affecting factors of pH, salinity, and MPs dosage on Cu(II) adsorption were investigated and the relevant mechanisms were elucidated. To provide basic data with scientific value for a deeper understanding of the environmental performance of the interactions between MPs and traditional heavy metal contaminants in nature, it is helpful to expand our understanding of the ecological and environmental effects of MPs.