Quantify Mercury Sulfide in Sediments for Bioavailability Assessment

While studies have shown that heavy metal sulfides in sediments are not bioavailable, most of them are acid-extractable (AE) and inseparable from non-sulfides in sediments. Exceptions were found recently that mercury and copper sulfides precipitated in sediments are non-AE because they are exclusively bi-sulfides. Therefore, quantifying mercury sulfide in sediments is possible for bioavailability assessment. To illustrate the application of this new approach for mercury bioavailability assessment, we quantified the distribution of mercury sulfide in sediments of the Apalachicola Bay, North Florida, USA. We extracted sediment cores and determined the total mercury, non-sulfide mercury, and sulfide mercury as well as the total sulfides, bulk density, and organic matter. The results showed that the mercury in the top 45 cm sediments were, on average, 13.3 ± 5.4 ng/cm3, of which 97.1 ± 2.5% were sulfide. The potentially bioavailable (non-sulfide) mercury was on average only 0.28 ± 0.22 ng/cm3 (2.9% of the total mercury). The total mercury and sulfide mercury were significantly correlated with the organic matter, which were dictated by the discharge pattern of the river input. This study demonstrates that aquatic sediments accumulate terrestrial mercury, and sulfidic sediments sequester most of it as sulfide. This new approach for mercury bioavailability assessment is simple yet chemically rigorous.