The Risk Assessment Of Chromium Pollution And Magnetic Characteristics Of Iron-bearing Minerals In The Mangrove Sediments

Mangroves are forested wetlands situated at the dynamic interface between land and ocean.Mangrove sediments are sources or sinks of various human-made heavy elements.High concentrations of sediment-bound heavy metals in the mangrove wetlands have seriously affected the ecological service value of mangrove ecosystems.Here,the sediment specimens were collected in the mangrove wetlands,Southeast,China and determined the pollution level of total chromium and the fractionations of sediment-bound chromium,the possible mechanism of hexavalent chromium adsorption on the studied sediments and the probabilistic human health risk of sediment-bound hexavalent chromium;Alternatively,the change of sediment-bound iron-bearing minerals and its correlation with the geochemical processes of heavy metals were also explored.The results suggested that:Sediment-bound chromium concentrations in the mangrove wetlands(60.81 ±1.67 mg kg-1)were much higher than the background values of southeast coastal sediments,China(40.70 mg kg-1),and the residual fraction was 77.80?96.20%of the total chromium concentration.There is almost no anthropogenic input of high chromium concentration existed in mangrove sediments.The results of human risk assessment indicated that the target hazard quotients of sediment-bound hexavalent chromium via oral ingestion,dermal contact and fish consumption were less than unity,respectively.The carcinogenic risk values of sediment-bound hexavalent chromium via oral ingestion,dermal contact and fish consumption were less than 1.0×10-4 as well.Prolonged exposure to sedminent-bound hexavalent chromium could pose little human health risk for the coastal fishermen folk in the mangrove wetlands.Adsorption processes of hexavalent chromium to the mangrove sediment were characterized by rapid adsorption,slow adsorption and equilibrium phases.There was no "g" factor of trivalent chromium on the surface of the studied specimen after high concentration of hexavalent chromium exposure.Plus,the X-ray photoelectron spectrometer failed to detect the absorption peaks corresponding to hexavalent chromium and trivalent chromium at 588.7,579.4,586.9 and 577.3 eV as well.The sediment environment is not favored to hexavalent chromium-absorption and its reduction,hexavalent chromium has a strong mobility.A much lower concentrations of magnetic minerals were recorded in the mangrove core sediments.Sediment iron-bearing minerals were mainly associated with pseudosingle domain soft magnetic components.Sediment percentage frequency magnetic susceptibility(?fd%)was 3.51±0.32 and the waek correlation between magnetic susceptibility(?LF)and?fd%(R2=0.065,p>0.05)indicated that the superparamagnetic particles made little contribution to ?LF.The sediment-bound magnetic mineral contained partial information of sediment parent materials.Multivariate statistical results indicated that Ti,V,Co,Cr,and Ni were mainly related to the sediment lithogenic minerals(Factor ?),Cu,Zn,Pb were linked to the level of terrigenous minerals(Factor ?).Sediment-bound heavy metals,e.g.,Cu,Zn,and Pb were associated with magnetic mineral of pseudo-single-domain sizes.Alternatively,a strong positive correlation(r=0.36-0.71,p<0.05)was seen between metal concentrations(e.g.,Cu,Pb and Zn)and magnetic concentration-related parameters(e.g.,?LF,SIRM and ?ARM).Sediment magnetic concentration-related parameters are an alternative analytical approach for the determination of exogenous heavy metals concentration in the mangrove welands.