Sorption and leaching behaviors between aged MPs and BPA in water: The role of BPA binding modes within plastic matrix

Highlights

• Structural properties of LDPE and PC were changed in UV/H₂O, UV/H₂O₂ and UV/Cl₂ aging systems.

• Effects of BPA binding mode within MPs matrix on aged behaviors of MPs were firstly explored.

• Aged LDPE can serve as an environmental sink for BPA comparing with virgin one.

• Virgin and Aged PC was a persistent releasing source of BPA monomers, especially in UV/Cl₂ aging systems.

Abstract

Due to the hydrophobicity and large specific surface area microplastics (MPs) have become the vector for the migration of environmental organic pollutants. Environmental aging process affects the physiochemical structure of MPs and their corresponding environmental behaviors, in which the effect of bisphenol A (BPA) binding mode within plastic matrix on aging behaviors of MPs is not reported. In this work, the structural properties and BPA sorption behaviors of low density polyethylene (LDPE) MPs with BPA additives and polycarbonate (PC) MPs with BPA monomers exposed to three types of artificial accelerated aging processes including UV/H₂O, UV/H₂O₂, and UV/Cl₂ systems were comparatively investigated. Virgin LDPE and PC exhibited obvious leakage of BPA additives or monomers. Aged LDPE had stronger sorption ability towards BPA in water environment with no observed leakage of BPA additives. While, aged PC had extremely high leakage of BPA monomers, which is similar to virgin PCs and was proved to be a persistent source of BPA release. The BPA sorption on aged LDPE or leaching from aged PC was influenced by aging processes, water pH, salinity, co-existing estradiol (E2), and water sources. This study reveals the potential ecological and environmental risks of MPs containing toxic additives/monomers during aging processes from a new perspective.

Introduction

With the increasing use of one-off plastic product, the plastic debris pollution has become more and more serious owing to the mismanagement of plastic waste. Microplastics (MPs, plastics with particle size less than 5 mm), a worldwide emerging contaminants, threat the growth and reproduction of aquatic organisms and numerous marine-industries (e.g. fishing, shipping, energy production, aquaculture) (Barnes, 2002; Cole et al., 2011). In addition to being physical contaminants, MPs are manmade organic polymers consisting of plastic resins and a range of toxic chemicals/additives (Gassel and Rochman, 2019). Moreover, it is possible that the plastic product contains either polymerized monomers or residual monomers because the monomer compositions can't be completely polymerized during plastic production. Under the action of ultraviolet radiation, chemical oxidation, mechanical wear and biological forces, these toxic chemical monomers/additives might be released from aged MPs noumenon (Ateia et al., 2020; Auta et al., 2017; Bandow et al., 2017; Mao et al., 2020).

The aging process causes structural changes in the plastic, such as oxygen content, molecular weight, surface morphology, then affects interfacial sorption behaviors of MPs (Gardette et al., 2013; Li et al., 2020; Lv et al., 2017). Previous studies have shown that the aging of MPs leads to an arrangement of molecular chains, breaks the relatively vulnerable bonds in the molecular chain and has a lasting effect on the properties of MPs (Redjala et al., 2019; Senden et al., 2012). Gardette's work has shown that a C–H bond breaks and forms free radicals during aging process of MPs, which reacts with oxygen to form oxygen free radicals leading to the formation of alcohols, aldehydes, acids and unsaturated groups (Gardette et al., 2013). Fan's work found that the adsorption capacity of aged polylactic acid (PLA) towards tetracycline (TC) and ciprofloxacin (CIP) increased due to the changes of its surface structure, the increased intensity of oxygen-containing functional groups and hydrophilicity (Fan et al., 2021). Antunes's work indicated that aged MPs contained high concentration of polychlorinated biphenyls (PCBs) owing to the increase of the specific surface area of MPs and had a long reaction time with pollutants (Antunes et al., 2013). By contrast, Endo's results demonstrated that aging of PE and PP MPs had no effect on interaction with PCBs, weathering and crystallinity may have no clear relationship with PCBs sorption (Endo et al., 2005). At present, there is no consensus of the relationship between the structural properties of aged plastics and their sorption behaviors. There is still a lot of important work to be explored on the aging of MPs.

Bisphenol A (BPA) is one of the highest production of industrial chemicals globally, and is the most representative chemicals of the common additives contained in some MPs (Chen et al., 2016; Jalal et al., 2018). In general, different types of additives are added initially in the production to improve specific properties of plastic, for example, phthalates are added to make plastic more flexible (Manoli and Voutsa, 2019; Vandenberg et al., 2007; Yu et al., 2008). In almost all cases, additives are not chemically bound to the plastic polymer. These additives may be released from plastics into the air, water and soil at all stages of the commercial product life cycle, and may undesirably migrate and lead to human exposure through packaging materials for food (Hahladakis et al., 2018). Li et al noticed that dibutyl phthalate (DBP) and di-2-ethylhexyl phthalate (DEHP) were migrated from disposable tableware to drinking water samples (10.13 ng/mL and 5.83 ng/mL, respectively) (Li et al., 2016). Begley et al investigated the leaching of caprolactam and oligomers from nylon 6 and nylon 6/66 polymer into oil (almost oven heating conditions, 176 °C for 30 min), they demonstrated that the total amount of the migrated nylon 6/66 oligomers (15.5 μg g⁻¹) was equivalent to almost 43% of the total oligomers presented in the packaging polymers (Begley et al., 1995). And several studies had confirmed that BPA released from commercial plastics into food, such as can linings, PC bottles and baby bottles (Goodson et al., 2004; Kubwabo et al., 2009). BPA is widely used in the manufacture of canned food and beverage, food packages, infant pacifiers, water bottles, sealants for dental fillings, eyeglasses and hundreds of other commodities in order to enhance the durability, elasticity, transparency, lightweight and outstanding impact resistance during the manufacture of plastic products. BPA is the primary intermediate of polycarbonate plastics and epoxy resin (Yu et al., 2008). During use of those commercial plastic products, BPA can be released under high temperature or acidic/alkaline environments threatening the health of human body (Lin et al., 2017; Wang et al., 2015). Wang's work had shown that ten brands of PC bottled waters were detected to contain BPA with high concentrations from 111.8 to 6452.8 ng/L, which would significantly increase human BPA daily intake and health risks (Wang et al., 2020). Goodson's work indicated that total amounts of migrated BPA from can coating into 4 different food-media products (minced beef in gravy, spring vegetable soup, evaporated milk, carrots in brine) were quite high (80% to 100% of total BPA present in coating) during sealing and sterilization process at 121 °C for 90 min (Goodson et al., 2004). As an endocrine disrupting compound, BPA may have adverse effects on developmental, reproductive, neurological, immune system, and endocrine system, even triggering cancer (Lin et al., 2017; Zhang et al., 2020a). Baby bottles containing BPA have been banned in 2011 by the European Union. Nowadays many field studies have explored the mechanism about the interaction between MPs and BPA. But the information on the interaction between aged MPs and BPA in water is still limited.

Currently, simulated aging condition are limited to thermal activated persulfate oxidation (Liu et al., 2019a), ozone oxidation (Liu et al., 2019b) and ultraviolet radiation (combined with air, seawater, H₂O₂) (Cai et al., 2018; Hüffer et al., 2018). There are few studies on aging of MPs by chlorination which is commonly used to disinfect drinking water and treat sewage. Actually, MPs in wastewater are discharged into natural water body through sewage treatment plants (Li et al., 2018; Ziajahromi et al., 2017). As a powerful means of oxidation, chlorination will change the physiochemical properties and the sorption capacity of MPs. Furthermore, C-Cl bonds which enhance ecological toxicity will be introduced on the surfaces of MPs after chlorination (El-Shahawi et al., 2010). Previous studies mainly focus on environmental distribution and abundance (Bergmann et al., 2017; Ding et al., 2021; Rillig, 2012), sorption/desorption behavior and toxicity of MPs (Hodson et al., 2017; Turner et al., 2020; Zhang et al., 2020b), however, there is insufficient works on the structural changes and aging mechanisms of MPs, especially about chlorination of MPs in water environment.

Therefore, we exposed PC and LDPE to three artificial accelerated aging processes (UV/H₂O, UV/H₂O₂, UV/Cl₂) with the irradiation of UV light. BPA monomer participates in the polycondensation of PC as a reaction unit, while BPA only acts as an additive to change the structural properties of LDPE. Thus, our work mainly deals with (i) the structural changes of LDPE and PC MPs under three types of aging processes; (ii) BPA binding mode within plastic matrix affecting the BPA sorption behaviors on aged MPs; (iii) the sorption and leaching behaviors of BPA by aged MPs in various water chemistry conditions (pH, salinity, co-existing E2 and different water matrices). It would provide a new insight on the environmental behaviors of PC and LDPE MPs and their impact on BPA migration in natural conditions.