The global production of plastics has increased substantially to nearly 400 million tons per year since their widespread use in 1950, and the amount is projected to more than double by 2050. However, approximately 60% of all the plastics ever produced are disposed of due to their endurance, low-recycling rates, and improper management; they end up in landfills or natural environments. The progressive weathering and fragmentation of these discarded plastic wastes generate considerable small plastic debris, resulting in the environment's omnipresence of microplastics (<5 mm). Once ingested by organisms, microplastics might cause both physical and chemical damages. For the latter, harmful chemicals are not only from pollutants adsorbed on their surface, but also from those additives (i.e., plasticizers) incorporated into plastic products. These additives are physically bound to plastic products, thus potentially migrating out and releasing into the environment. Phthalate esters (PAEs) are one of the predominant additives extensively integrated into consumer plastic products as plasticizers. Over 11 tons of PAEs are produced worldwide per minute to manufacture plastic products. As they have been proven to jeopardize the endocrine system, several PAEs have been classified as priority pollutants by the U.S. Environmental Protection Agency (USEPA). Over the past few decades, PAEs have been widely detected in the natural environment worldwide. Although the release of PAEs from microplastics with effects of light, bacterial and hydrostatic pressure has been reported, data about the leaching behaviors of PAEs under other possible environmental conditions are still insufficient.

The Pearl River Delta (PRD) region is one of the most developed, urbanized, and industrialized areas in China. As the world's workshop and manufacturing base for plastic and electronic products, textiles, etc., PAEs discharged from the PRD can be continuously transported to the northern South China Sea (NSCS). Investigating the transport of PAEs from the PRD can provide essential information for source prevention and implementation of relevant remediation measures. Additionally, contamination characteristics of PAEs in the NSCS and their potential risks to marine organisms have yet to be well studied. Consequently, this study aimed to investigate the leaching ability and mechanisms of PAEs from microplastics to reveal the potential source of PAEs in the aquatic environment. Besides, field investigations of the spatiotemporal variations of PAEs in the water samples from the PRD were conducted to assess the possible input of PAEs to the NSCS. Finally, the occurrence and distribution of PAEs in the NSCS and their potential risks to marine organisms were examined. Agilent 7890A gas chromatograph coupled with 5975C mass spectrometer (GC–MS, Agilent Technologies, CA, USA) was adopted to identify and quantify the levels of PAEs in relevant samples after pretreatment with solid phase extraction method.

Twelve commonly used plastic products were prepared as microplastics to investigate the release behaviors of PAEs. Six out of 15 PAEs were quantified after 14 days of incubation in seawater. The leaching potentials were plastic type-specific, where the pencil case (polyvinyl chloride, PVC) represented the highest migrations with a total Σ₁₅PAEs concentration of 6,660 ± 513 ng/g, followed by the cleaning brush-1 (polyamide, PA, ~1,830 ng/g) and rubber glove (1,390 ± 57.5 ng/g). Conversely, the straw (polypropylene, PP), cleaning brush-2 (polyethylene terephthalate, PET) and shampoo bottle (PET) released the lowest amounts of PAEs, with 50.3 ± 8.21, 93.9 ± 91.8, and 104.35 ng/g, respectively. The release patterns of PAE congeners were related to polymer types, where di-n-butyl phthalate (DBP) dominated the leaching from PA, PP, and PET microplastics (47-84%), diethyl phthalate (DEP) leached the most from PVC and rubber microplastics (45-
92%), while diisobutyl phthalate (DiBP) and DBP dominated the leaching from PE microplastics (68-94%). Water chemical properties such as pH and salinity could affect PAEs migration, and the kinetic leaching process was well-fitted with the pseudo-first-order model. This study revealed that microplastics are the potential source of PAEs in aquatic environments, with an estimated annual mass load of up to 16,100 kg.

To investigate the riverine input of PAEs from PRD to NSCS, eight major riverine outlets, which mainly discharge water from upstream tributaries of the PRD to the NSCS, were selected. The occurrence, spatial, seasonal, and tidal variations of PAEs in suspended particulate matter (SPM)-water systems in the surface and bottom layers of the eight riverine outlets were investigated. The total concentrations of Σ₁₅PAEs, including both the dissolved and particulate phases, ranged from 562-1,460 ng/L and 679 ng/L-2,830 ng/L in the surface and bottom layers, respectively. DBP and di(2-ethylhexyl) phthalate (DEHP) dominated in the dissolved and SPM phases, respectively, accounting for > 50% and > 80% of Σ₁₅PAEs. Riverine input of wastewater from the PRD was possibly the primary source of the contamination. Higher levels of PAEs occurred at the eastern outlets than at the western ones. The dissolved and particulate PAEs varied seasonally, with significantly higher concentrations observed in the dry season than in the wet season. However, no significant differences in PAE levels in both phases were observed among low, medium, and high tides. The partitioning results demonstrated that SPM is important in the transport of pollutants in estuaries, where more hydrophobic DEHP was predominantly transported by the SPM phase, while those more hydrophilic ones were regularly transported by the dissolved phase. As estimated, the total annual flux of Σ₁₅PAEs through the eight outlets to the NSCS reached 1,390 tons.

Apart from the riverine input, urban stormwater is another important pathway for transporting anthropogenic pollutants to receiving water bodies. However, the transport of PAEs by urban stormwater has been largely overlooked. This study, for the first time, investigated 15 PAEs in stormwater from six major stormwater drains in the highly urbanized Hong Kong, a major metropolitan city in China. The results showed that PAEs were ubiquitous in the stormwater of Hong Kong, with total concentrations spanning from 195 to 80,500 ng/L. Bis(2-n-butoxyethyl) phthalate (DBEP), diisopentyl phthalate (DiPP), dicyclohexyl phthalate (DCHP) and di-n-pentyl phthalate (DnPP) were detected in stormwater for the first time. Spatial variations in PAEs were observed among different stormwater drains, possibly due to the different land use patterns and intensities of human activities in their respective catchments. The highest and lowest levels of Σ₁₅PAEs were found in Kwai Chung (3,860 ± 1,960 ng/L) and the Ng Tung River (672 ± 557 ng/L), respectively. Additionally, significantly higher concentrations of Σ₁₅PAEs in stormwater were found in the wet season (2,520 ± 2,050 ng/L) than in the dry season (947 ± 904 ng/L). Principal component analysis classified domestic and industrial origins as two important sources of PAEs in the stormwater of Hong Kong. Stormwater was crucial in transporting PAEs, with an estimated annual flux of 0.705-29.4 kg in Hong Kong. Based on the results, possible stormwater management measures were proposed to protect the receiving aquatic ecosystems from exposure to PAEs and other chemical contaminants.

Oceans are considered an immense sink for various chemical compounds. To better understand the contamination status of PAEs in the NSCS and their potential risks to marine organisms, the occurrence and distribution of PAEs in seawater and sediment from NSCS were investigated for the first time. The concentrations of Σ₁₅PAEs ranged from 68.8 to 1,500 ng/L, 46.0 to 7,800 ng/L, and 49.2 to 440 ng/g dry weight in surface seawater, bottom seawater, and sediment, respectively. Among the 15 PAEs, DBP and DEHP were the predominant PAE congeners, with mean contributions of 44.7% and 24.0% in surface water, and 42.7% and 25.8% in the bottom water, respectively. Moreover, DiBP constituted the majority of Σ₁₅PAEs in the sediment (61.3%). Generally, relatively higher concentrations of Σ₁₅PAEs were detected in the seawater and sediments at the eastern NSCS than the western sides. River input and atmospheric deposition could be the main sources of PAEs in the NSCS. Preliminary risk assessment implied that DBP, DiBP, and DEHP posed low to high potential risks for marine organisms at different trophic levels. These results would be valuable for implementing effective control measures and remediation strategies for PAEs contamination in the region.

In summary, the estimated mass load of PAEs from microplastics in this study suggested that microplastics are the potential source of PAEs in aquatic environments. Considering the toxicities of PAEs to aquatic organisms and humans, effective management and proper disposal of plastic waste should be established and implemented. Additionally, this study also provides an in-depth understanding of the riverine transport of PAEs to the NSCS from SPM-water systems of eight riverine outlets and from the stormwater in the major stormwater drains of Hong Kong, and ultimately evaluates the potential risks of PAEs to marine organisms in the NSCS. Considering the important transport pathways of PAEs from terrestrial sources to oceans, more attention and effective management measures should be taken for riverine input, including stormwater drains.