Volatile sulfides and their toxicity in freshwater sediments

Three forms of volatile sulfides (free hydrogen sulfide, acid-volatile sulfides [AVS] and a new form, heat-volatile sulfides [HVS]), were measured and their relationships discussed. Purging of some contaminated sediments with nitrogen at 22°C failed to remove all of the free H2S, even after 6 h. With freshly H2S-spiked uncontaminated sediment, purging of H2S was complete after 2 h; however, if the spiked sediment was allowed to stand for 53 d, H2S continued to be purged, even after 43 h. The H2S likely originates from equilibrium reactions involving reduced sulfur species in the sediment. Uncontaminated sediment spiked with H2S was found to be highly toxic using a sediment-contact bioassay employing Photobacterium phosphoreum. Addition of Fe3+, which sequesters the S2−, reduced the toxicity. Two other bioassays (ATP-TOX and Toxi-Chromotest), both involving a DMSO/methanol extract of the spiked sediment, showed greatest toxicity with spiked sediment that had been standing for 22 d. The results indicate that reduced sulfur species in sediment may be a cause of toxicity observed in contaminated anoxic sediments.