Competitive displacement reactions of cadmium, copper, and zinc added to a polluted, sulfidic estuarine sediment

The competitive displacement reactions occurring after the addition of Cd, Cu, and Zn to a polluted, sulfidic estuarine sediment were studied. Port-water metal measurements indicated that added Cd and Zn reacted quantitatively with the acid-volatile sulfide (AVS) fraction (primarily FeS) of the sediments to form CdS and ZnS, whereas Cu reacted to form Cu2S, not CuS as is often assumed. The titration of a synthetic FeS phase, with Cu(II) in a strictly oxygen-free environment was used to provide a mechanistic illustration of the formation of Cu2S involving the reduction of Cu(II) to Cu(I) by Fe(II). Although this reduction reaction is not thermodynamically favorable in acidic solutions, in near-neutral pH environments the dual precipitation of Cu2S (reduced phase) and FeOOH (oxidized phase) is the driving force for the reaction. Only when the sedimentary AVS (FeS) phase had been exhausted did the added Cu displace Zn and Cd ions from their respective binding phases. Likewise, Cd added in excess of the molar AVS (FeS) concentration displaced Zn from less stable solid phases. These observations, although commonly predicted by thermodynamics, have seldom been demonstrated in real sediments and provide evidence that AVS (FeS) is the most reactive phase in anoxic sediments. The metal additions caused large disturbances to the sediment redox potential. This was attributed to disruption of the HS-/SO42- redox equilibrium position by precipitation of HS- as metal sulfide phases. Although these redox changes did not seem to affect the partitioning of Cd, Cu, or Zn to the reactive sulfide phase (AVS), significant changes to the pore-water metal speciation may be expected. The effects of these observations on metal bioavailability for sediment toxicology studies that utilize metal-spiked sediments during method development are discussed.