Tracing the Fate of Rubber-Derived Chemicals (RDCs) in Subtropical Aquatic Environments in Hong Kong and Guangdong Coastal Areas: Leaching, Transformation, Partition, Bioaccumulation, and Ecological Impacts

In the past two decades, global car production has increased from 56.3 million in 2001 to 1.55 billion in 2020 (OICA, 2022). The wide use of rubber products, such as tires, has led to a substantial release of tire wear particles (TWPs) into the environment due to the friction between vehicle tires and road surfaces. After TWPs are generated on roads, they can enter urban surface water through road runoff. In 2015, China released 400 million tons of TWPs, with 10.8% entering the aquatic environment, making it the second-largest source of microplastics. Rubber additives (RAs) include vulcanizing agents, corrosion inhibitors, antioxidants, light stabilizers, and plasticizers, which are usually incorporated into rubber during the manufacturing process to improve its performance and durability. Such chemicals can leach into aquatic environments and further derive other transformation products (TPs). Rubber-derived chemicals (RDCs), including RAs and their TPs, have shown variable degrees of toxicity towards aquatic organisms. However, these RDCs and associated ecological risks have not been studied as much in aquatic environments. Hong Kong, a bustling international metropolis on China's southern coast, is characterized by its dense urbanization, high population density, and one of the world's highest vehicle densities. This makes Hong Kong an ideal place to address the knowledge gaps.

In light of the above, lab-based simulation experiments were conducted in this work to reveal the environmental transformation behaviors of typical RAs in experimental river water. Comprehensive field investigations were then conducted to investigate RAs and their TPs (RDCs) in water, sediment, and biotic samples from Hong Kong and the adjacent coastal area, including sewage, stormwater, river water, coastal seawater and sediments, and marine mammals. The aims of this study are to obtain fingerprints of RDCs in multiple aquatic environments of a typical coastal metropolis and its adjacent areas, and to investigate the leaching, transformation, transportation, water-sediment partitioning behaviors, bioaccumulation characteristics, and ecological risks of RDCs in these environments. To understand the leaching and transformation mechanisms of typical RAs from TWPs into the aquatic environment, lab-based simulation experiments were conducted that exposed TWPs and RAs separately in experimental river water under an environmental condition. After being placed in river water for 24 h, about 7%–95% of 12 investigated RAs in TWPs were leached, with the 2,4,6-tris[bis(methoxymethyl)amino]-1,3,5-triazine (HMMM) and N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) predominant in leachate. Seven investigated antioxidants were more readily degraded than seven most vulcanization agents and one corrosion inhibitor. Forty-eight TPs from eight RAs were tentatively identified in exposure liquid of TWPs and RAs, along with different transformation pathways via suspect screening using high-resolution mass spectrometry. Semiquantitative results indicated that TPs derived from 6PPD were predominant in leachates, while aryl hydrolysis and quinone pathways were the main transformation pathways. Field investigations on urban surface water samples in Hong Kong further identified sixteen TPs from six RAs via suspect screening with N-(1,3-dimethylbutyl)-N'-phenyl-p-quinonediimine (6QDI) identified as a persistent-bioaccumulative-toxic substance.

However, the occurrence and transportation characteristics of RDCs in aquatic environments, from their source to receiving waters, remain poorly understood. Accordingly, water samples from the urban water system of Hong Kong, including stormwater, sewage influent and effluent, river water, and coastal seawater, were collected and analyzed. The vulcanizing agents and corrosion inhibitors were found to be predominant in the water samples, accounting for 26.0-66.1% and 29.3-71.7% of total concentrations of 45 RDCs (Σ₄₅RDC), respectively, while antioxidants and their TPs presented in smaller quantities, accounting for 0.214-26.7% and 0-15.2% of Σ₄₅RDC, respectively. Ten RAs from five classes were additionally identified by suspect screening. An estimated mass load of Σ₄₅RDC amounting to 1690 kg/month was discharged into the coastal marine environment of Hong Kong, with sewage effluent being the primary source. Population density and vehicle-related factors (e.g., traffic load) were the major drivers shaping the spatial distribution of RDCs in urban surface water. Based on the ecological risk assessment outcomes, 16 of 45 RDCs exhibited medium to high risks in investigated waterbodies, and priority lists of RDCs for various water bodies were established to support their risk management for safeguarding water quality.

Additionally, the occurrence and partitioning behaviors of RDCs between water-suspended particulate matter (SPM)-sediments in coastal marine environments have not been extensively studied. Paired water, SPM and sediment samples were collected from three marine protected areas (MPAs) along Guangdong coast. The results showed that RDC levels in these MPAs were relatively low compared to those reported in natural aquatic environments. The observed spatial and temporal variability in Σ₂₉RDC in the dissolved water phase, influenced by sampling area and season, suggests that most RDCs during the wet season likely originate from land-based sources. In contrast, Σ₂₉RDC in dissolved water phase in the dry season were similar across the MPAs, indicating a uniform baseline level of coastal marine pollution and reduced influence from land-based RDCs via freshwater discharge. Variations in Σ₂₉RDC in SPM and sediment were observed only among the MPAs, and their levels remained stable during both wet and dry seasons. Additionally, water-SPM (K_poc) and water-sediment (K_soc) partitioning coefficients of RDCs based on their concentrations in paired water-SPM-sediment samples, which were higher than those predicted. Although low RDC contamination levels in these MPAs, a preliminary ecological risk assessment indicated medium to high risks from N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPDQ), which should be considered for future continuous monitoring and control in coastal marine environments.

Considering the bioaccumulation potential of RDCs, marine mammals that occupy high trophic levels serve as biological sinks for these pollutants in aquatic environments. This chapter investigated the pollution status of RDCs in five tissues of finless porpoises (Neophocaena phocaenoides), a resident marine mammal in the waters adjacent to Hong Kong. The results showed that although RA levels were higher than TP levels in aquatic environments these porpoises inhabited, the levels of RAs in porpoise tissues were significantly lower than those of TPs (p < 0.05). Analysis of relative body burdens revealed that blubber and muscle tissues serve as the primary reservoirs for RDCs. Furthermore, bioaccumulation factors (BAFs) for 30 RDCs ranged from -1.05 to 7.26. Notably, TPs exhibited significantly higher BAFs compared to the other three categories of RAs—vulcanizing agents, corrosion inhibitors, and antioxidants (p < 0.05). This suggests that TPs are more persistent, less likely to be metabolized, and thus more prone to accumulation in porpoise tissues.

In summary, this PhD study investigated the pollution status of RDCs across multiple aquatic environments in Hong Kong and MPAs along Guangdong coast, as well as in a resident marine mammal species. This study further examined transformation behaviors of RDCs in the riverine environment, their partitioning behaviors in the coastal marine environment, and their bioaccumulation characteristics in marine mammals. Although TPs derived from antioxidants represent a small proportion of total RDCs in aquatic environments, they exhibit relatively high ecological risks in the investigated environments and high bioaccumulation potential in marine mammals, warranting future concern. The results are useful to environmental authorities for making informed decisions on source control and regulation of RDCs to make aquatic environments cleaner and safer for wildlife.