Characteristics and Relevance of Reduced Inorganic Sulphur Forms and Heavy Metal Elements in Surface Sediments of the Dongzhai Harbour Mangrove Wetland

The mangrove wetland of the Dongzhai Harbour, located in northeastern Hainan, is of high ecological value. However, environmental pollution brought by the current rapid economic development, especially the pollution of heavy metals and reduced inorganic sulphur, puts the ecological environment of this region at risk. As the main carrier of these two kinds of pollution, sediments are deeply involved in their biogeochemical cycles. Therefore, in this study, surface sediments of mangrove wetlands in the Dongzhai Harbour were collected, and their concentration characteristics and sources of reductive inorganic sulphur (RIS) forms and heavy metals were investigated. The average RIS concentrations of the sediments in the study area were about 22.31 μmol/g, among which acid volatile sulphide (AVS) had the highest weight (ave. 10.01 μmol/g), followed by chromium (II)-reducible sulphur (CRS) with the average concentrations of 8.46 μmol/g, and elemental sulphur (ES) (3.84 μmol/g), which had the lowest concentration. The spatial distributions of different RIS forms were differentiated, reflecting the diversity of environments in the study area. Correlation analyses showed that the RIS forms, such as AVS and CRS, were mainly enriched in clay-sized sediments with p < 0.05. Compared to other areas of the world, the present study area was less contaminated by RIS. Ratio calculation revealed that the reducing environment in the study area limited the conversion of AVS to CRS in the sediments. Among the heavy metals, Cr (70.03 mg/kg) was the most abundant, followed by Zn, Pb, and Ni with an average of >20 mg/kg, while other elements such as As, Cd, Cu, Hg, and Co were found to be low (<18 mg/kg). Correlation analyses revealed that clay-sized sediments significantly constrained Ni, Zn, and Co and could effectively bind to AVS (p < 0.05). Based on cluster analysis, it was found that agricultural activities, mariculture, and transport mainly influenced heavy metals and RIS of sediments in the study area. The study helps understand the pollutant cycle in shelf wetland sediments and provides practical guidance for the sustainable development of ecosystems. In the future, it is recommended that reasonable control of pollution emissions be implemented in relevant areas.