Coupling mechanisms of S-Fe-P in surface sediments under the stresses of high salinity and heavy metals in coastal rivers

Purpose The aim of the study was to (1) investigate the distributions of sulfur (S), iron (Fe), and phosphorus (P) in coastal surface sediments under the stresses of high salinity and heavy metals; (2) identify potential sources and the environmental geochemical process of S-Fe mineral accumulation; and (3) elucidate the coupling mechanisms of S-Fe-P in coastal sediments under environmental stresses. Materials and methods The distributions of reduced inorganic sulfur (RIS), inorganic P, and reactive Fe (RFe) in surface sediments in two coastal rivers with heavy metals and high salinity (similar to 50 psu, brine drainage) and in offshore areas in the Bohai Sea were investigated. Results and discussion Pyrite sulfur (CRS) was the predominant fraction of RIS in two rivers (Jiaolai River (JL), 56%; Jiehe River (JH), 72%), whereas acid volatile sulfur (AVS) dominated RIS in their offshore sediments (L, 66%; J, 45%). RFe(II) dominated RFe in JH (70%) and JL (66%), while RFe(III) dominated the fractions in their offshore areas (53%). Coastal surface sediments under the stresses of high salinity and heavy metals were dominated by HCl-P (Ca-bound P) and NaOH-P (Fe- and Al-bound P), respectively. Conclusions Accumulated Fe sulfide was limited by elemental sulfur (ES) availability under high salinity, while it was limited by total organic carbon (TOC) availability when the river was heavily polluted. High salinity produced sulfide-dominated environments, which could reduce the NaOH-P and facilitate HCl-P immobilization. However, the presence of heavy metals resulted in Fe-dominated environments, which could promote metal sulfides formation and increase the NaOH-P fixation