Transfer 9.0 g of PA MPs into a 150 mm diameter glass petri dish, ensuring uniform distribution. Thereafter, the dish was placed in a dark chamber and the PA MPs were subjected to UV aging using two UV-C lamps (254 nm/20 W) for a duration of one month. During aging, the PA MPs were periodically turned to ensure uniform aging.

Seawater samples were collected from the area around the East Romantic Line in Xiamen, Fujian Province (118°8.4′ E, 24°37.2′ N), and its salinity was determined to be 35‰. The seawater collected was passed through a 0.45 µm pore size membrane, and the filtered seawater was then stored for future use. Follow the method described in Section 2.2.1, filtered seawater was added to completely submerge the PA MPs in the petri dishes. UV-C irradiation was subsequently initiated, with regular replenishment of seawater conducted during the experiment. After one month of aging, the PA MPs mixed with seawater were filtered through a 0.22 µm membrane filter and then placed in an oven at 60℃ for 24 hours.

A chlorine disinfectant with an active chlorine concentration of 30 mg/L was prepared using NaClO. A mixture was made by combining 2 g of PA MPs with 100 mL of the disinfectant in a 150 mL conical flask, which was shaken at 160 rpm for 24 hours at 25℃ in a dark environment. After the reaction, the mixture was washed six times with ultrapure water and then dried in an oven at 60℃ for 24 hours[19].

Compared with other aging methods, ultraviolet aging causes a significant yellowing in appearance(Fig. S1). The surface morphology of PA MPs was examined using scanning electron microscopy (SEM) to assess the impact of various aging conditions. As shown in Fig. 1a, the surface of untreated PA MPs exhibits a generally smooth, uniform, and flat morphology. However, an increased presence of cracks, elevated formations, micropores, and a wrinkled texture was noted on the surfaces of PA MPs subjected to various aging methods (Fig. 1b-d). As shown in Fig. 1, the surfaces of UV-PA and UV-Sea-PA exhibited higher roughness compared to the Cl-PA surface. Specifically, the folds on the UV-PA surface are deep and numerous, while the UV-Sea-PA surface is distributed with dense and fine micropores; in contrast, the Cl-PA surface has relatively few cracks and folds. UV-C radiation has a high energy level and can induce rapid aging of PA MPs in a short period[7]. When oxygen is present, a photo-oxidation reaction takes place, and the interaction of UV-C light with oxygen intensifies the aging of PA MPs, leading to an increased occurrence of surface cracks and yellowing[8]. In this study, due to the method of covering the petri dish to reduce evaporation, the UV-Sea-PA has relatively low contact with air and is also affected by factors such as the corrosive effect of seawater salinity, so its surface shows fewer deep folds and a relatively high number of micropores[28]. The aging treatments increased surface roughness and porosity of PA MPs, making them more susceptible to fragmentation. This significantly enlarged their specific surface area, thereby promoting Cr(VI) adsorption[29, 30].

Figure 2 compares the FTIR spectra of pristine and aged PA MPs. FTIR spectral analysis showed that although both original and aged PA MPs had distinct characteristic absorption bands at 1642 cm^− 1(Amide I), 1551 cm^− 1(Amide II), and 1254 cm^− 1(Amide III) indicating the presence of amide groups[22, 30, 31]. However, aging still caused damage to the chemical bonds of PA MPs. The N-H/O-H stretching vibration band at 3423 cm^− 1 shifted to 3308 cm^− 1 with a decrease in intensity, indicating N-H bond cleavage in PA MPs during aging[22]. The reduced transmittance at 2930 cm⁻¹ (C-H) and 2865 cm⁻¹ (C-H) indicates oxidative cleavage of C-H bonds during aging, which may lead to the formation of carbonyl groups or other oxidation products[32]. These spectral changes demonstrate significant chemical composition modifications in aged PA MPs, warranting detailed investigation combined with 2D-COS analysis.

To further investigate the dynamic evolution of functional groups during the aging process of PA MPs, a 2D-COS analysis was performed using FTIR spectral data. The synchronous and asynchronous 2D-COS patterns of aged PA MPs are presented in Fig. 3.The reaction sequence was analyzed based on the Noda[33] rule, with a focus on the dynamic changes of six characteristic peaks at 1252 cm^− 1(C-N), 1551 cm^− 1(-NH-CO-), 1642 cm^− 1(C = O), 2865 cm^− 1(C-H), 2930 cm^− 1(C-H), and 3423 cm^− 1 (N-H) in PA MPs (Table S3). The functional group reaction sequences of PA MPs vary depending on the aging method. The UV-PA reaction sequence is: -NH-CO- >C-H > C-H > N-H > C-N > C = O. For UV-Sea-PA, it follows: -NH-CO- >C = O > C-N > C-H > C-H > N-H. In contrast, the reaction order of Cl-PA is: N-H >-NH-CO- >C-N > C = O > C-H > C-H. The sequences of reactions observed in the three aging techniques show some resemblances, as they all reveal the initial breaking of the -NH-CO- bond. This is because in PA MPs, the -NH-CO- bond has the weakest binding force and is prone to breakage under the oxidation of ultraviolet radiation or hypochlorite[34].

(Note: red signifies a positive correlation (+), while blue denotes a negative correlation (-). Darker shades indicate a greater intensity and stronger relationships.)

The XPS method was employed to perform a comprehensive characterization of PA MPs both prior to and after aging, in order to explore the chemical changes caused by different aging processes. As illustrated in Fig. 4, the C1s spectrum is decomposed into three distinct components: 284.93 eV (C-C), 285.31 eV (C-N), and 287.98 eV (C = O). Similarly, the O1s spectrum is characterized by three components: 531.23 eV (O-H), 532.09 eV (O = C-N), and 533.36 eV (O = C-O)[22, 35]. Different aging methods have significantly impacted the functional group content of PA MPs. C1s spectral analysis results indicate that aging leads to a considerable reduction in the amount of C-N bonds in PA MPs. Additionally, the content of the O = C-N bond in the O1s spectrum also showed a significant decrease. These phenomena collectively suggest that the -NH-CO- bonds in PA MPs undergo cleavage during the aging process. Meanwhile, in the O1s spectrum, it was observed that both UV-PA and Cl-PA exhibited characteristic peaks of O = C-O after the aging treatment. This suggests that aging promotes the formation of oxygen-containing functional groups in PA MPs. Notably, while carboxyl group (O = C-O) formation was absent in UV-Sea-PA, a marked enhancement in O-H peak intensity was observed. Mao et al.[36] suggest that under UV-air conditions, MPs absorb sufficient ultraviolet light to induce bond cleavage, favoring the formation of carbonyl groups (C = O). The research indicates that in aquatic environments, the higher concentration of hydrogen atoms reduces the aging for ultraviolet light, promoting a greater tendency for the formation of hydroxyl groups (-OH) compared to terrestrial settings. Conversely, the oxidation process of PA MPs was accelerated by Cl-PA, attributed to the powerful oxidizing characteristics of elemental chlorine, resulting in a rise in oxygen-containing functional groups[19].

The solution pH significantly influences ionic contaminant adsorption, as shown in Fig. 5a, which compares Cr(VI) adsorption before and after PA aging. The ability of PA MPs to absorb Cr(VI) decreased with rising pH levels, reaching its highest point at pH 3.0 for both prior and aged PA MPs. This result is attributed to the surface charge properties of PA and the pH sensitivity of Cr(VI). Cr(VI) exists as HCrO₄⁻ at pH < 6.7 and CrO₄²⁻ at pH > 6.7[37]. Huang et al.[22] discovered that the pH at the point of zero charge (pH_pzc) for PA is 3.68. If the solution pH drops below pHpzc, the amino groups on the PA MP surface become protonated, forming positively charged ions (-NH₂⁺)[22]. As a result, the positively charged PA has the ability to draw in the oxygenated anion of Cr(VI) through electrostatic forces and may also interact with it through hydrogen bonding[38].

The adsorption of UV-PA at pH 4–5 was improved in comparison to Original-PA, attributable to UV-induced formation of oxygen-containing functional groups. Under acidic conditions, protonation of these groups and the formation of hydrogen bonds promote enhanced adsorption[39]. The adsorption capacity of UV-Sea-PA was lower compared to Original-PA across various pH levels. This phenomenon may be related to the reduction of oxygen-containing functional groups. Meanwhile, the competitive adsorption of anions and cations in seawater on the surface further hinders Cr(VI) from reaching the active sites[11, 20]. At pH levels of 3–4, Cl-PA exhibited a greater adsorption capacity compared to UV-Sea-PA and UV-PA. This enhanced phenomenon can be linked to its higher content of oxygen-containing functional groups and amide bonds. As shown in Fig. S2, under conditions of pH 3–5, the pH of the solution increased after PA MPs adsorbed Cr(VI). Research data indicates that the protonation of amide bonds and oxygen-containing functional groups in acidic environments enhances proton consumption, thereby improving adsorption efficiency.

Figure 5b shows that temperature significantly affects the adsorption of Cr(VI) by PA MPs. Numerous studies have shown that the adsorption of heavy metals by MPs is an endothermic process. The study by Tang et al.[40] investigated the interaction of nylon MPs with Cu(II), Ni(II), and Zn(II) in water. The results showed that the adsorption of these metal ions was both endothermic and spontaneous. Similarly, Wang et al.[41] found that the uptake of Cu(II) by both polystyrene (PS) and polyethylene terephthalate (PET) increased significantly with rising temperature. The ability of Original-PA, UV-PA, and UV-Sea-PA to adsorb Cr(VI) demonstrated a positive relationship with temperature, indicating that higher temperatures resulted in greater uptake. The maximum Cr(VI) adsorption capacities at 308 K were 0.21 mg/g (Original-PA), 0.27 mg/g (UV-PA), and 0.14 mg/g (UV-Sea-PA). As temperature increases, heavy metal ions are more likely to move toward the active sites on the surface of PA MPs, enhancing the capacity of these materials to adsorb Cr(VI)[42]. The findings are consistent with those of Li et al.[20], who observed enhanced Cr(VI) adsorption by both Original-PA and UV-aged PA at higher temperatures. However, the adsorption behavior of UV-Sea-PA was the opposite of the other three. This effect can be linked to the improved affinity of the UV-Sea-PA surface for metal ions (such as Na⁺ and Mg²⁺) at elevated temperatures, resulting in a reduction of its effective adsorption sites. The experimental findings suggest that the impact of temperature on Cr(VI) adsorption by PA MPs is significantly influenced by the different aging methods.

Heavy metal interactions in aquatic systems are inherently influenced by multiple environmental factors, particularly salinity and DOM. DOM is prevalent in aquatic environments, and the HA was selected for this study. The influence of salinity and HA on the adsorption of Cr(VI) by PA MPs is clearly illustrated in Fig. 5c. The adsorption amounts of PA MPs in various water environments exhibit notable differences, with the following sequence of magnitude: Freshwater > DOM water > Semi-saline water > Seawater. It has been demonstrated that the molecular composition of HA exhibits numerous functional groups (such as carboxyl) capable of coordinating with Cr(Ⅵ) via complexation[43]. Moreover, these functional groups can exploit the adsorption sites found on the surface of PA MPs through competitive adsorption. It has been shown that HA enhances the negative charge on the surface of PA MPs, increasing the electrostatic repulsion between PA MPs and the negatively charged Cr(VI)[30]. Seawater contains abundant ions ((such as Na⁺ and Cl⁻) that significantly influence Cr(VI) adsorption. You et al.[44] reported that Na⁺ and Cl⁻ ions increased solution viscosity and density, thereby hindering Cr(VI) adsorption by PA through weakened electrostatic interactions. Studies indicate Na⁺ can displace acidic group hydrogens, suppressing hydrogen bonding, while Cl⁻ may form coordination complexes with Cr(VI) during adsorption[45, 46]. In summary, the Cr(VI) adsorption capacity of PA MPs is significantly reduced by both DOM and salinity in aquatic systems.

The XPS analysis results of the Cr 2p orbitals presented in Fig. S4 demonstrate that different aging methods of PA MPs significantly influence the morphological changes of Cr(VI). The Cr2p3/2 orbitals were divided, showing two peaks. The first, at 576 eV, is Cr(III). The second, at 579 eV, is Cr(VI)[51]. The generation of Cr(III) confirms that Cr(VI) undergoes partial reduction during the adsorption process. The Cr(III) content serves as an indicator of the reduction potential. The results showed that the PA reduction capacity before and after aging showed significant differences, as follows: UV-PA (66.59%) > UV-Sea-PA (63.35%) > Original-PA (61.49%) > Cl-PA (61.41%). The reduction efficiency of Cr(VI) was significantly elevated for both UV-PA and UV-Sea-PA, indicating that the UV aging process was essential in improving the Cr(VI) reduction. Therefore, after a 2-month secondary aging treatment on the UV-PA sample, it was detected that the Cr(III) content significantly increased to 81.1% (Fig. S6). The formation of Cr(III) is linked to the reducing properties of nitrogen-containing functional groups on the surface of PA MPs[52]. Huang et al.[22] suggested that the lone pair electrons on the oxygen atom of the carbonyl group and the nitrogen atom of the amino group within the amide bond of PA MPs serve as electron donors. Zhu et al.[53] reported that the presence of not only superoxide anion radicals (O₂^•−) but also other reactive species such as persistent amino oxygen radicals (PAORs) was observed after PA was treated with photoaging. The O₂^•− produced by PA MPs during aging assist in reducing Cr(VI). This is supported by Zhang et al.[21], who confirmed that O₂^•− enhances the transformation of Cr(VI) to Cr(III) on the surface of PS under simulated light conditions. The low Cr(III) content in Cl-PA is due to the inhibitory effect of Cl⁻ on O₂^•− generation, reducing the efficiency of Cr(III) reduction by Cr(VI)[54]. In summary, both before and after aging, PA MPs can reduce Cr(VI).

PA MPs not only mainly immobilize Cr(III) on their surface but also promote the reduction of Cr(VI) in aquatic solutions. Figure 9 illustrates that the Cr(VI) adsorption and reduction capacity by PA different significantly under different conditions of UV light, DOM, and DO. The results suggest that the environmental factors examined can promote the reductive conversion of Cr(VI) by PA MPs. Exposure to UV light significantly increased Cr(VI) adsorption on all PA MPs. Additionally, Original-PA and Cl-PA promoted the conversion of Cr(VI) to Cr(III) in the surrounding solution. As demonstrated by Zhu et al.[55], the aging process of PA MPs in solution under light exposure leads to the production of a significant amount of ROS, such as O₂^•−, ¹O₂and •OH, in the MP suspension. And O₂^•− can promote the reduction of Cr(VI). The presence of HA promoted the reduction of Cr(VI) by PA MPs. Rose et al.[56] demonstrated that the quinone structure in HA is the primary contributor to its redox activity. Lv et al.[57] reported that HA acts as an electron shuttling agent, facilitating electron transfer between Cr(VI) and the adsorbent, thus accelerating Cr(VI) reduction. In the absence of oxygen, PA MPs demonstrated reduction potential. DO, functioning as a highly active oxidizing agent, plays a crucial role in regulating the redox system. DO is an efficient electron trapping agent, competitively capturing surface electrons on PA MPs and thereby suppressing electron transfer to Cr(VI), finally inhibiting its reduction rate[58]. In conclusion, the reduction of Cr(VI) is influenced by the complex interaction of environmental factors such as UV radiation, DOM, and DO.