Natural Nitrogen-Bearing and Phosphorus-Bearing Nanoparticles in Surface Sediments of the Pearl River Estuary, China: Implications for Nitrogen and Phosphorus Cycling in Estuarine and Coastal Ecosystems

Eutrophication creates multiple environmental problems, threatening the ecological security and sustainability of estuarine and coastal ecosystems worldwide. Key nutrients of concern are nitrogen (N) and phosphorus (P), which are the main controls in eutrophication. Considering that sediments are inseparable sinks of N and P, concern has grown regarding the forms in which N and P occur in the surface sediments of estuaries and coastal areas. Nonetheless, studies on the natural N-bearing or P-bearing nanoparticles in estuarine and coastal sediments have rarely been reported. Herein, the surface sediments (0–5 cm) of the Pearl River Estuary in China were collected and subjected to analysis. Using high-resolution transmission electron microscopy (HR-TEM) analysis, numerous natural N-bearing and P-bearing nanoparticles were observed. The results revealed that there are some differences in the occurrence forms of N and P in nanoparticles, suggesting that N and P could be adsorbed by nanoparticles of minerals such as hematite, goethite, muscovite, anorthite and quartz in estuarine and coastal environments, and further form N-bearing and P-bearing nanoparticles. These nanoparticles contained small amounts of N (1.52–3.73 wt%) and P (0.22–1.12 wt%), and were mainly single crystal or polycrystalline in form, with sizes ranging from 10 nm × 50 nm to 250 nm × 400 nm. In addition, P was shown to exist in the form of Ca and Fe phosphate nanoparticles in the estuarine sediments. The Ca and Fe phosphate nanoparticles had higher phosphorus content (5.02–9.97 wt%), mainly amorphous, with sizes ranging from 50 nm × 120 nm to 250 nm × 400 nm. Moreover, N-bearing and P-bearing nanoparticles could influence the migration, precipitation and release processes of N and P, and play a certain role in the N-cycling and P-cycling of estuarine and coastal ecosystems. Furthermore, we explored the role of N-bearing and P-bearing nanoparticles in the N-cycling and P-cycling in estuarine and coastal ecosystems. Thus, this study could provide new ideas for water environment management and other related research fields.