Polyvinyl chloride (PVC) was one of the most common plastic materials in water pipes, toys and packaging, due to its great mechanical properties, chemical resistance and recyclability, and variety of customizable rigidity/color by adding different additives such as Pb salts (Europe 2020; Pfaendner 2006). Pb has now been banned or restricted in PVC production (council and EU 2003, 2009), however it is still dispersed in current plastics through recycling of historical material without proper regulations (Turner and Filella 2021). Released plastic wastes undergo weathering and degrades into small particles such as microplastics (MPs), which are ubiquitous in the environment (Zhang et al. 2019). MPs are commonly defined as a plastic particle size less than 5 mm which is not only a type of contaminant, but also a vector for other contaminants (Alimi et al. 2018; Li et al. 2018). Compared to the understanding of PVC risk to environment, little is known about the release of Pb from PVC and its potential risk. In fact, Pb is a harmful substance which can seriously affect the nervous systems, kidneys and blood pressure, especially children’s cognitive performance (Needleman 2004). We hypothesize that the recycled plastic is an important source of Pb exposure, especially under the weathering process of the Pb-contained MPs in the environment.

Among the various weathering processes such as hydrolysis, mechanical abrasion, biodegradation and the natural oxidation, photo-irradiation is one of the most important factors (Alimi et al. 2018). PVC is sensitive to the UV light (Gewert et al. 2015). Upon exposure to the sunlight, the dechlorination of PVC occurs, accompanied with the generation of the unsaturated C=C double bonds, and the cleavage of PVC backbones into small fragments (Gewert et al. 2015). Moreover, declorination is affected by dissolved organic matter (DOM) including low molecular weight organic acids (LMWOAs) in natural water and soil (Zhang et al. 2009). Gallic acid (GA) is frequently detected and reactive natural organic matter with low molecular weight and often used in many studies as model LMWOAs to investigate the influence of LMWOAs on different pollutant transformation processes (Wang et al. 2020).

There is an increasing interest in the potential risk of MPs in the environment, but little information is available on the release of additives in MPs (Li et al. 2018). Herein, we mainly explore the release process of the endogenous chemical additives, particularly, Pb from recycled PVC MPs under the UV-irradiation. The study revealed the importance of the environmental ageing processes, especially the photochemical process on the release and fate of endogenous Pb from recycled PVC MPs, which provide important information on the risk assessments of MPs.

Materials and Methods

Micro-sized PVC MPs were obtained from plastic board milled with a grinder (FW100, Tianjin taste, China) (Fig. S1). PVC particle was sieved through 150 μm sieves, and the size distribution was between 20 and 100 μm. The resultant PVC MPs were washed with 70% alcohol to remove any surface adsorbed soluble plastic monomers and allogenic materials (Chen et al. 2020), dried at 60 ˚C for 5 h and stored at 4 °C until usage. A photochemical reactor (CEL-LAB500E4, CEAULIGHT, China) with a 500 W lamp was used as the reactor. Acetic acid (CH3COOH, ≥ 99%), sodium acetate (CH3COONa, ≥ 99%), gallic acid (C7H6O5, ≥ 99%), were purchased from China National Pharmaceutical Group Co., Ltd. (Sinopharm). The 5,5-dimethyl-1-pyrroline N-oxide (DMPO, ≥ 97%) was purchased from Dojindo Molecular Technologies, Inc. (Japan). The superoxide dismutase (SOD, ≥ 2500 μ/mg protein) was bought from Shanghai Yuanye Bio-Technology Co., Ltd. All solutions were prepared with Ultrapure water (18.2 MΩ cm-1 at 25 °C) obtained from Milli-Q system (Millipore).

Scanning electron microscope (SEM, JSM-6510, JEOL, Japan) was used to analyze the particle size and morphology of MPs, as well as Pb distribution among MPs by the energy dispersive X-ray spectroscopy for scanning electron microscopes (SEM/EDX). The particle size was calculated using the image J software. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR, Thermo Fisher, Nicolet iS50, U.S.A.) was used to identify the surface functional groups. Total Pb content in the prepared plastic was analyzed by inductively coupled plasma mass spectrometry (ICP-MS, 7700x, Agilent, U.S.A.). Briefly, 0.05 g MPs in triplicate was digested in a mixture of 6 mL nitrate (68%), 2 mL hydrofluoric acid (40%) and 2 mL perchloric acid (70%) in a microwave digestion system (ETHOS UP, Milestone, Italy), and the average analytical method recovery of Pb for the reference material (Soil, GBW07404) reached 97.6%. The total concentrations of different metals in MPs were listed in Table S1.

10 mg MPs was suspended in 20 mL sodium acetate-acetic acid buffer solution at pH 5.5, the suspension was added into the 50 mL transparent quartz tubes, stirred under a 300 W UV365 light irradiation or in dark (Fig. S2) (Chen et al. 2018), and sampled at 0.5, 1, 2, 5, 8, 16, 24, 32 and 40 h. At each time point, triplicates were harvested, where 5 mL suspension was sampled, filtered through a 0.22 μm membrane (PES, Millipore), and analyzed for Pb.

Due to the precipitation of Pb ion in alkaline condition, acidic pHs of 4.0 and 5.5 were chosen to examine the effects of pH on Pb release from MPs as described above. Meanwhile, GA was chosen as a model LMWOAs to examine its effect on the Pb release kinetics from the MPs. The buffered reaction suspension at pH 5.5 with/without 1 mM GA were prepared, and the liquid-to-solid ratio and time interval were same as described above.

The electron paramagnetic resonance (EPR) analysis was used to determine the free radicals during the UV-irradiation at room temperature by an EPR (E500, Bruker, Germany) instrument operated at the following conditions: X-Band, microwave power of 6.325 mW, sweep width of 50 G, modulation width of 1 G and modulation frequency of 100 kHz. The time intervals of the experiment were set as 0, 5, 10, 15 and 20 min. The DMPO was selected as spin trap agent and a 180 W MP-Hg lamp was selected as the light source. In order to identify the contribution of different free radical species in the plastic ageing process, quenching experiments using superoxide dismutase (SOD) for superoxide (O2.−) and methanol for both O2.− and ·OH were conducted. Meanwhile, the effects of dissolved oxygen on the radical process were investigated under anaerobic and open-air conditions with otherwise same experiment settings. The Pb concentration changes during the reaction was used as an indicator to evaluate the influence of each free radical species.

In this study, the dissolution kinetics of the Pb from MPs under different pHs were described by the two-site dissolution model. The site 1 dominated during the initial phase of the Pb dissolution, the site 2 dominated during later stage.

$$\frac{{{\text{d}}C}}{{{\text{dt}}}} = k_{1} \left( {C_{s1} - C} \right) + k_{2} \left( {C_{s2} - C} \right)$$
(1)
$$C\left[ {{\text{Pb}}^{2 + } } \right]_{{\text{t}}} = C\left[ {{\text{Pb}}^{2 + } } \right]_{{{\text{max}}1}} \times \left( {1 - {\text{e}}^{{ - k_{1} {\text{t}}}} } \right) + C\left[ {{\text{Pb}}^{2 + } } \right]_{{{\text{max}}2}} \times \left( {1 - {\text{e}}^{{ - k_{2} {\text{t}}}} } \right)$$
(2)

where C was the concentration (mg/L) of the dissolved Pb2+; Cs1 and Cs2 were the apparent concentration (mg/L) of Pb associated with the site 1 and site 2 in MPs, respectively; k1 and k2 were the release rate constants (h−1); C[Pb2+]max1 and C[Pb2+]max2 were the total concentrations of released Pb2+ for site 1 and site 2 when the reaction reached a plateau (Wang et al. 2016).

Results and Discussion

The morphology of MPs was characterized by SEM. As shown in Fig. 1, the particle size of MPs was about 76 ± 25 μm for UV-irradiated MPs which is the same with the pristine MPs. Meanwhile, the ageing process smoothed the wrinkles but added more cracks on the surface of the MPs (Fig. 1a, b). Similar morphological changes were also reported in previous studies (Zhu et al. 2020). The elemental mapping by SEM/EDX showed that Pb was uniformly distributed in the pristine MPs (Fig. 1d). The total Pb concentration in the MPs determined by digestion method was 11.3 mg/g. The surface functional groups of the MPs before and after UV-irradiation were characterized by ATR-FTIR (Fig. 1c). Similar peak patterns corresponding to C–Cl, C–H and C=O stretching vibrations were observed in both pristine and UV-aged MPs. However, their relative intensity changed significantly: the intensity of C=O increased by 30.2% and intensity of C–Cl decreased by 12.5% after UV-irradiation, indicating dechlorination of PVCs (Shi et al. 2008). No peak corresponding to C=C stretching mode was observed in both conditions, which might be ascribed to that the amount of formed C=C was lower than the detection limit of ATR-FTIR (Chen et al. 2019).

Fig. 1
figure 1

The morphology of the MPs before and after ageing process, a pristine PVC MPs; b UV-aged PVC MPs, c the ATR-FTIR spectra of pristine and UV-aged PVC MPs, d SEM/EDX mapping for Pb on the surface of MPs

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The kinetics experiments of Pb dissolution under different conditions showed that compared to dark condition, UV-irradiation increased the Pb concentration in solution significantly (Fig. 2a, b). All Pb release kinetic curves can be well fitted by the two-site dissolution model (Table S2), suggesting that Pb was controlled by two processes, including a fast H+ dominated process and a slow ·OH dominated process. The Pb release rate constant in the H+ dominated process was about 3.49 h−1 at pH 5.5 but increase to 5.09 h−1 at pH 4.0, suggesting a pH-dependent H+ dominated process. Similar pattern was also observed under dark condition, with k1 increased from 3.07 h−1 at pH 5.5 to 3.69 h−1 at pH 4.0 at dark condition. The slow ·OH dominated process also had a pH-dependent Pb release, with the rate constant k2 ranging between 0.046 and 0.090 h−1. However, the ·OH dominated process was a vital factor for the ageing of MPs since it might involve coupled diffusion–reaction process on the inner surface inside the MPs through the surface cracks (Zhu et al. 2020). The process increased the solution of Pb in MPs at prolonged reaction times.

Fig. 2
figure 2

Pb dissolution kinetic of MPs under different conditions, a Pb release at pH 4.0 under both dark and UV-irradiation; b Pb release at pH 5.5 under both dark and UV-irradiation; c the effect of GA on the Pb release under UV-irradiation at pH 5.5; d the quenching effects of ·OH and O2.− by methanol and SOD on Pb release under UV-irradiation at pH 5.5, e Pb release from MPs in anaerobic and open air condition without UV-irradiation. The data are presented as the mean ± SD (n = 3)

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Unlike the significant pH effects on kinetic rate constants, the sum of C[Pb2+]max1 and C[Pb2+]max2, at the quasi-equilibrium state, kept almost constant at ~ 3.0 mg/L under light and ~ 1.6 mg/L under dark at both pHs 4.0 and 5.5, suggesting that pH could only change the kinetic but not thermodynamic behaviors of Pb release from MPs. The latter is more related to the binding capacity of Pb at surface and inside of MPs. Especially at the initial stage of the lower pH, the more H+ presented in the suspension to dissolve the Pb salt on the surface of MPs (Fig. 2a, b). While the ·OH would dominate the slow phase by attacking the inner surface of MPs so that more Pb was released to the solution.

The release kinetics of Pb was similar in the presence and absence of dissolved oxygen under dark condition (Fig. 2e). It therefore indicated that the dissolved oxygen might be less important under dark conditions during the ageing process of MPs (Xu et al. 2011). Furthermore, the radical quenching experiment results revealed that methanol completely quenched the ·OH production during photochemical ageing process of MPs, where the release kinetics of Pb2+ was almost the same with the dark condition. Meanwhile, since SOD can transform O2.− into ·OH, the combined addition of SOD and methanol could quench O2.− firstly, and then ·OH. Consistently, the kinetic study showed a complete inhibition of light-irradiation induced Pb release after the addition of SOD and methanol as compared to dark condition (Fig. 2c). Together, it was confirmed that the photo-generated ·OH radicals dominated the ageing process and Pb release of MPs under the UV-irradiation.

With 1 mM GA was added to the MPs suspension at pH 5.5, the Pb release was completely inhibited (Fig. 2c), the sum of C[Pb2+]max1 and C[Pb2+]max2, at the quasi-equilibrium state was about 1.7 mg/L, similar to those dark condition. The kinetic constants k1 and k2 in the presence of GA under light-irradiation was also similar to those under dark. Since phenols as an inner filter to absorb UV light and competes with the MPs, it could alleviate the ageing process of plastics induced by UV-irradiation (Song et al. 2021). Therefore, under the UV-irradiation, there were much less Pb released from MPs with the addition of GA solution than that without GA.

The formation of ·OH in MPs suspensions under UV-irradiation was further investigated by EPR analysis using DMPO spin trap. Figure 3 shows the ·OH signal in different time intervals in MPs suspensions during the course of UV-irradiation at pH 5.5. Prior to light-irradiation, there was no signal of the free radical existed; After a 5-min UV-irradiation, the signal of DMPO–OH adduct emerged. The intensity of ·OH increased significantly as the photo-interaction proceeded to 20 min. Meanwhile, the signal of DMPO-alkyl radical adduct was detected by EPR at 10 min, and almost kept constant during the reaction course, which suggested that ·OH produced from light-irradiation initiated the degradation of plastic particle surface polymer chains to generate alkyl radicals. The source of ·OH was plausibly from three reactions: (1) water molecules were split directly by UV-photolysis to generate ·OH (Drzewicz et al. 2010), (2) the acetate molecules in the buffer solution were excited into CH3COO· under UV-irradiation and further reacted with O2 to generate ·OH (Zhang and Huang 2020), (3) light induced the C–H bond cleavage on the surface of MPs initialized by reacting with O2 to generate peroxy radical, which subsequently was transformed into hydrogen peroxide as the source of ·OH (Zhu et al. 2020).

Fig. 3
figure 3

The EPR spectra of the ·OH formation by EPR with spin trap agents during the reaction course between 0 to 20 min in plastic suspension; *characteristic signal of DMPO–OH adduct, ♦ characteristic signal of DMPO-alkyl adduct

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In conclusion, this study has revealed the Pb release kinetics from recycled PVC MPs under the UV-irradiation. Under UV-irradiation, the formed ·OH attacking the surface of MPs was the dominant ageing process of plastic in the environment, which caused both degradation of plastics and the release of endogenous additives such as Pb. Suspension pH has minor effects on total amount of Pb released at the quasi-equilibrium stage, but kinetically, it induced faster equilibrium of soluble Pb salt from MPs surface at lower pH. Meanwhile, GA inhibited the release of Pb under light-irradiation, suggesting that LMWOAs in the surface water could alleviated the photo-induced ageing process of the plastics by serving as a filter to absorb the UV light. This study shows the release of Pb from recycled PVC MPs and indicates the historical and recycled plastics has resulted in Pb contamination of the environment which should be treat carefully.