Sediment and interstitial water heavy metals in mangrove restoration wetland and preliminary exploration of microplastics in interstitial water.

• The distribution of heavy metals and microplastics was significantly different under different planting densities. • The same heavy metal element presents different contamination levels in sediment and interstitial water, with Cd is high contamination levels in sediment and potentially less harmful to biota in interstitial water. • Mangrove wetland restoration could promote the abundance of MPs in interstitial water. • PE, PP, and PET were the most common and dominant microplastics in the study area. Compared with heavy metals in sediments, those in interstitial water can act as better indicators of bioavailability. Based on different planting densities, the distribution and risk of heavy metals (Zn, Cu, Ni, Cd) in the sediments and interstitial waters of " Kandelia obovata - Rhizophora stylosa - Bruguiera gymnorhiza " in a mangrove wetland restoration were studied, and the identification and analysis of microplastics in interstitial water were preliminarily explored. The results showed that the spatial distribution of heavy metals in sediments and interstitial waters in mangrove restoration wetlands varied greatly and was primarily affected by planting density and sediment properties. The geo-accumulation index, potential ecological risk index of a single trace metal, and comprehensive potential ecological risk index indicated that the level of Cd pollution in sediments was high. The results of the interstitial water criteria toxicity unit (IWCTU) and Nemerow index (NI) showed that the pollution level of heavy metals in interstitial water was generally high. Among them, the IWCTU values of Cu and Zn were>1, whereas the average value of the NI reached 2.67, which exerted adverse effects on the survival and growth of environmental organisms. In addition, the types and abundance distribution characteristics of microplastics in the interstitial waters of mangrove restoration wetlands were explored. The abundance of microplastics in the interstitial waters of the entire study area was 306.7–553.3 L-1. Moreover, different planting densities of mangroves could substantially affect the abundance distribution of microplastics, and the maximum value appeared when the planting density was 0.5 × 0.5 m. Polyethylene (49 %), polypropylene (21 %) and polyethylene terephthalate (11 %) were the most common and main microplastics in interstitial water. Scanning electron microscopy/energy-dispersive X-ray spectroscopy analysis showed that interstitial water microplastics had characteristics similar to those of sediment microplastics, such as fracture, tear, sag, and material adhesion, and the surface of the interstitial water microplastics could absorb a large amount of metal elements. [ABSTRACT FROM AUTHOR]