SOLID PHASE HG SPECIATION IN SEDIMENTS FROM PATROON CREEK, A TRIBUTARY OF THE HUDSON RIVER

Studies of Hg in watersheds have demonstrated Hg’s strong affinity for the sediment compartment. However, Hg in sediments can be mobilized by several processes, including diagenetic redox changes, bacterial methylation and physical transport of Hg-bearing particles. Since the potential for mobilization depends to a significant extent on the solid phase speciation of Hg, considerable effort has been directed to the problem of finding a reliable method of characterizing Hg speciation. Of these methods, sequential chemical extraction is the only one that is widely practical for the determination of Hg speciation in large numbers of samples. However, there is presently no consensus on the most suitable extraction protocol, nor on precisely what species of Hg is determined at each step.

Here we present a new protocol that uses separate steps for extracting metallic Hg (4M nitric acid) and sulfidic Hg (saturated sodium sulfate), each of which has been extensively tested and found to be selective for extraction of its nominal phase. The protocol was tailored to sediment samples from 3-m cores collected from a reservoir along Patroon Creek. These sediments were contaminated with elemental Hg, have total Hg concentrations as high as 6.24 μg/g dw, and contain clay and organic matter, with lesser amounts of sand.

Preliminary extractions of several samples yielded excellent recoveries of total Hg with < 1% in the “residual” fraction, which is considered to be silicate lattice-bound Hg and other forms that failed to extract in the proper step. 60-72% of total Hg was found in the nominal “metallic Hg” phase, which includes elemental Hg and amalgams. This is consistent with the source contamination (elemental Hg) and with the fact that the sediments contain elevated levels of amalgamable metals (Cu, Zn, Ag, Pb). The next largest proportion (17-21%) was extracted into the “sulfidic fraction”, which could include authigenic cinnabar or metacinnabar (HgS) as well as Hg-(poly)sulfide complexes. This, too, is consistent with the high levels of organic matter and elevated sulfate levels found in the water of the Creek and the reservoir. These results suggest that elemental Hg is relatively unreactive in fluvial sediments such as those found in the Patroon Creek.