Identification of naturally weathering microplastics and their interactions with ion dyes in aquatic environments

Highlights

• Irregular lamellar and distorted texture surface with small grains was confirmed.

• Polypropylene microplastics exhibited better affinity toward crystal Violet.

• Sodium chloride and humic acid inhibited dye capture.

• Higher desorption efficiency in simulated seawater than in fresh water.

Abstract

Here, naturally weathering plastics were collected and identified to understand the effect of the weathering conditions on their properties. The adsorption and desorption of crystal Violet (CV), a candidate of cationic organic pollutant, on microplastics (MPs) under various environmental conditions were simultaneously explored. The first type was the aged polyethylene (PE-MPs), which exhibited a fibrous structure with a smoother and uniform surface attached some flaps, while the second type was recognized as the aged polypropylene (PP-MPs), which showed a lamellar structure with less smooth and distorted texture surface attached with fragments and small grains. The experiment data fitted well with the pseudo-second-order kinetic and Freundlich isotherm models. The possible interactions between CV and MPs included electrostatic attraction, hydrogen bonding and hydrophobic interactions. This study demonstrates that the high possibility of transport ionic pollutants from fresh water to ocean by the naturally weathering microplastics, highlighting their potential risk in various environments.

Introduction

Plastics are widely used in many fields due to their high durability, high structural strength and low cost. It was estimated that the global plastic production exceeded 320 million tons per year and was expected to double in the next 20 years (Wright & Kelly, 2017). However, most of the plastics are discarded into the environment due to mismanagement and disposal. Thus the increase in plastic production and consumption makes the earth soon to be engulfed by plastic waste, and the plastics released into the environment will further fragment or decompose into macro-, meso-, micro- or nano-plastics due to the physical, chemical and biological reasons (Alimi et al., 2018; Ding et al., 2020; Wright & Kelly, 2017).

Microplastics (MPs) are usually defined as plastic particles or fragments with diameter no greater than 5 mm (Thompson et al., 2004). As one of the most common pollutants and waste solids, MPs pose a greater threat to the environment. On one hand, MPs can persist in the environment for a long period of time. During this period, they are susceptible to light and heat exposure, thermal degradation, bio- and oxidation reactions, called weathering process. These processes have the potential to alter the specific physical and chemical properties of plastics, including morphology, specific surface area, hydrophobicity, durability, mechanical strength, and elemental composition (Liu et al., 2019a; Liu et al., 2019b; Liu et al., 2019c), thus allow them to act as contaminants carriers transformed and transported in the water environment through floating, sedimentation, adsorption, or bio-uptake (Xue et al., 2021). On the other hand, various types of additives such as phthalate esters, brominated flame retardants, and benzophenones for the intended purpose (Fred-Ahmadu et al., 2020), are physically mixed with plastics, and likely to be released into the surroundings during use and weathering (Zhang et al., 2018a). The weathering of MPs not only brings about the release of toxic monomers, metals and inorganic additives, causing greater environmental hazards, but it has been demonstrated that aging polypropylene promotes greater uptake of MPs by marine organisms (Liu et al., 2019a; Liu et al., 2019b; Liu et al., 2019c; Vroom et al., 2017). Moreover, the presence of polybrominated diphenyl ethers, a plastic additive, has been reported in the intestinal tissue of lugworms inhibiting the biological activity (Browne et al., 2013). Furthermore, several reports have claimed that MPs are a major source of some additives, e.g., plasticizers, flame retardants, and antioxidants, in the aquatic environment (Gouin et al., 2011; Hahladakis et al., 2018; Hammer et al., 2012).

At the same time, dye effluents are a major environmental concern because these colorants are not only aesthetically unpleasant in the aquatic environment, but also many organic compounds and aerobic substances derived from dyes are dangerous for aquatic organisms, and some of them are carcinogenic, leading to acute or chronic diseases in living organisms (Chen et al., 2010). In addition, the presence of dyes also absorbs or reflects sunlight, preventing the entry of sunlight and further hindering photosynthesis in water (Mittal et al., 2007). Moreover, dyes are increasingly used in the plastics, coatings and paints industries to meet the various requirements of product appearance (Liu et al., 2021), thus dye contaminants are prevalent and highly concentrated in the environment, and there is a high potential for MPs to encounter with dyes in aqueous environment. You et al. investigated the adsorption mechanism of methylene blue on PE (You et al., 2021). Lin et al. studied the adsorption of malachite green by initial and aged nylon microplastics (Lin et al., 2020). More importantly, studying the interaction of MPs with dyes can help to explore its carrier effects, thus evaluate the environmental behavior and ecological risks under coexistence. However, studies on the adsorption of dyes by naturally aged microplastics are still limited so far.

We assume that the effect of the natural field aging process on the surface composition, structure and adsorption properties of microplastics is more complex than the simulated single weathering process in laboratory, here, two types of commonly collected weathering plastics from the actual laboratory field scenarios were chosen, the main objectives of this study are to (1) identify the types of weathering plastics by FT-IR and EDS spectra; (2) evaluate the effect of the weathering process on their surface morphology, composition, and structural changes and adsorption properties; and (3) investigate the possible interactions and the release behavior in various aquatic environments, to better understand the potential harm to the environment and further indicate the fate of aging MPs.