Mercury distribution and transfer in sediment-mangrove system in urban mangroves of fast-developing coastal region, Southern China

urban mangroves affected by surrounding urbanization and anthropogenic activities, the fate of mercury (Hg) in sediment-mangrove system remains unclear. In this study, four urban mangroves with different urban functional zonings in fast-developing Shenzhen of China were selected: Shajing mangrove (SJM) and Xixiang mangrove (XXM) featured with industry district, Futian mangrove (FTM) and Baguang mangrove (BGM) featured with central business district and ecological preserve, respectively. The distributions of Hg in sediment, iron plaque (IP), and Kandelia obovata were determined. The mean concentrations of total Hg in mangrove sediments were SJM > XXM, FTM > BGM. Higher ecological risk was also detected in SJM based on geo-accumulation index, potential ecological risk factor, and contamination factor. The speciation analysis of Hg in non-rhizosphere sediments showed that residual, oxidizable, and volatile Hg acted as dominant fractions in urban mangroves, with volatile Hg to be important for Hg bioaccumulation in K. obovata. IP acted as a natural barrier to limit transfer of Hg from sediment to K. obovate due to lower Hg concentrations in IP compared to that in sediment. In terms of the total and bioavailable Hg, translocation factors (TF) of Hg in mangroves featured with different urban functions were: TFtotal: BGM (0.23) approximate to FTM (0.23) > SJM (0.15) approximate to XXM (0.14); TFavail: BGM (0.81) approximate to FTM (0.74) > SJM (0.40) approximate to XXM (0.37). The reduced TFs of Hg were important for protecting K. obovata from heavy Hg pollution in urban mangroves featured with industry district.