Geochemistry of Cd, Cr, and Zn in highly contaminated sediments and its influences on assimilation by marine bivalves

We tested the controls of metal geochemistry in sediments collected from an extremely contaminated Chinese bay on metal assimilation by marine mussels and clams. Metal speciation in the contaminated sediments, quantified by the Tessier operational extraction method, was significantly dependent on metal concentrations in the sediments. The fractions of Cd in the easily exchangeable and carbonate phases increased, while the reducible and residue phases decreased with increasing Cd concentration. The majority (72-91%) of Cr was associated with the residue component, with the remainder of Cr in the organic matter and reducible phases. Zn in carbonate phase increased, whereas in the organic matter and residue phases it decreased with increasing Zn concentration. The bioavailability of Cd, Cr, and Zn to marine green mussels (Perna viridis) and clams (Ruditapes philippinarum) was quantified using radiotracer spiked technique with concurrent measurements of speciation of spiked metals. There was a significant correlation between the Cd assimilation efficiency (AE) by both mussels and clams and Cd partitioning in the easily exchangeable and reducible phases. In contrast to previous studies, a negative correlation was found between the Cd AE and its total concentration in sediment, likely caused by the saturation of Cd binding sites in the gut or by its antagonistic interaction with a very high Zn concentration in these collected sediments. In contrast, there was no significant correlation between the AEs of Cr or Zn and any of their geochemical phases or their concentrations. The metal AEs were further quantified by experimentally manipulating different concentrations and ratios of acid volatile sulfide (AVS) and simultaneously extractable metals (SEM). There was no statistically significant relationship between the AEs of the three metals and the concentrations of AVS and SEM or [SEM-AVS]. Geochemical controls on metal assimilation from contaminated sediment are therefore only relatively apparent for Cd. The influences of metal speciation on metal bioavailability can be confounded by the degree to which sediments are contaminated with metals.