Competitive displacement reactions of cadmium, copper, and zinc added to apolluted, 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-wa ter metal measurements indicated that added Cd and Zn reacted quantitativel y with the acid-volatile sulfide (AVS) fraction (primarily FeS) of the sedi ments to form CdS and ZnS, whereas Cu reacted to form Cu2S, not CuS as is o ften assumed. The titration of a synthetic FeS phase, with Cu(II) in a stri ctly 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 o f 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 phas es. Likewise, Cd added in excess of the molar AVS (FeS) concentration displ aced Zn from less stable solid phases. These observations, although commonl y predicted by thermodynamics, have seldom been demonstrated in real sedime nts and provide evidence that AVS (FeS) is the most reactive phase in anoxi c sediments. The metal additions caused large disturbances to the sediment redox potential. This was attributed to disruption of the HS-/SO42- redox e quilibrium position by precipitation of HS- as metal sulfide phases. Althou gh these redox changes did not seem to affect the partitioning of Cd, Cu, o r Zn to the reactive sulfide phase (AVS), significant changes to the pore-w ater metal speciation may be expected. The effects of these observations on metal bioavailability for sediment toxicology studies that utilize metal-s piked sediments during method development are discussed.