Effects Of Mangrove Wetland Restoration On The Heavy Metals And Microplastics Of Soils In Jinjiang Estuary

The subtropical estuarine wetlands have been seriously degraded due to human activities and biological invasions recent years.Restoring mangrove plants in degraded estuarine wetlands is of practical significance.In addition,the heavy metals and microplastics pollution in the estuarine wetland were also focused increasing attention due to their potential ecological risks.This study was aimed to explore the effects of different planting patterns on distribution characteristics of total heavy metals,acidvolatile sulfide(AVS),simultaneously extracted metals(SEM)and microplastics during mangrove restoration.Then revealing the bioavailability and potential ecological risk of heavy metals in restored mangrove wetland at Jinjiang Estuary,and preliminary clarifying the sources of microplastics in wetland soils and its interaction with heavy metals.We hope that this study will provide a scientific basis for the restoration and protection of similar estuarine wetlands in the future.The results shown as follows:The total concentrations of Cr,Ni,Cu,Zn,Pb,Cd,As and Hg all increased during restoration,and the restored regions were higher than control,which indicated that mangrove wetland restoration benefited for accumulation of heavy metals.And different mixed-planting types and planting densities could significantly affect the distribution of heavy metals.The geo-accumulation index and the potential ecological risk index results indicated that mangrove restoration did not reduce the heavy metals pollution,Cd and Hg were high ecological risk.The AVS contents increased first and then stabilized with depth during restoration,but the vertical distributions of SEM were more evenly than AVS.SEM and AVS in restored regions were much higher than control,indicating that mangrove restoration was contributed to the generation of AVS and accumulation of SEM.Different mixedplanting types and planting densities could significantly affect the distributions of AVS and SEM.It was found that the bioavailability of heavy metals in the surface and subsurface soils(0-20 cm)increased after restoration,and due to the fixation of AVSs,no potential adverse effects in deeper layers.The abundances of microplastic in restored wetland soils was 490 to 1170 n/500 g dry soils(n presented the number of microplastics),small(< 1mm,81.93%)fiber(68.58%)microplastics were the major,and mangrove restoration could promote their gathering and accumulation.One-way ANOVA results shown only planting densities could affect microplastic distribution.Polyethylene terephthalate(PET),polyethylene(PE)and polypropylene(PP)were the main polymer types of microplastics after Raman identification.SEM images illustrated that fractures,pits,flakes and adhering other matters were common characteristics of microplastics degradation,demonstrating that the examined microplastics experienced different levels of mechanical erosion and chemical weathering in environment.EDS spectra analysis shown that there were heavy metals like Cr,Zn,Pb and Cd absorbed on surface of microplastics.The correlation analysis indicated that the absorbed heavy metals might not necessarily derive from wetland soils,and microplastics might be carriers transferred heavy metals to soils.