Relation of acid-volatile sulfides (AVS) with metals in sediments from eutrophicated estuaries: Is it limited by metal-to-AVS ratios?

Abstract

Purpose

The role of acid-volatile sulfide (AVS) as a factor affecting (or reflecting the environmental conditions influencing) the behavior of metals has been evidenced in anoxic sediments. However, sediment quality studies tend not to consider any potential AVS role when sum of simultaneously extracted metal (ΣSEM) concentrations exceed AVS concentrations, restricting the application of the AVS model to predict toxicity, rather than coupling this model (when applicable) with other possible interpretations of metal–AVS relations.

Materials and methods

The relations between total organic carbon (TOC), AVS, and simultaneously extracted Fe, Cd, Cu, Ni, Pb, and Zn were investigated in sediment cores from two eutrophicated estuaries in Guanabara Bay (SE Brazil) in order to contribute to our understanding about metal–AVS relations in conditions of variable ΣSEM and AVS levels, due to gradients of eutrophication and metal contamination.

Results and discussion

Correlation analyses indicated a more important role of TOC, AVS, and Fe in the mechanisms affecting trace metal distribution in the less eutrophicated estuary. This suggests that AVS was a more important binding phase, or at least a better proxy for biogeochemical conditions affecting metal distribution, when it exceeds the sum of simultaneously extracted Cd, Cu, Ni, Pb, and Zn concentrations (ΣSEM). However, this potential role of AVS in anoxic estuarine sediments should not be discarded before evaluating individual metals–AVS relations even when ΣSEM levels exceed AVS levels, since the biogeochemical controls on the behavior of individual metals may be also related to metal–AVS associations in this situation (as indicated for Cu and Ni in the more eutrophicated estuary). The same is suggested for all AVS-based approaches, such as the (ΣSEM − AVS)/f _OC (organic carbon normalization of excess ΣSEM), since (ΣSEM − AVS)/f _OC values were mostly positive and significantly higher in the more eutrophicated estuary.

Conclusions

Although the importance of metal relations to AVS in evaluating individual metal behavior in anoxic estuarine sediments may not be restricted to situations in which AVS levels exceed ΣSEM levels (as observed for Ni and Cu in the present study), metal–AVS relations were apparently favored in this situation. Approaches in this way are recommended for future research, coupling the possibilities of metal behavior interpretations (and often predictions) allowed by AVS models.