The use of dilute HCI for the selective extraction of mercury from suboxic and anoxic sediments has been evaluated. The effect of acid concentration (1-6 M HCI) on the extraction of Hg and on the recovery of aqueous Hg preextraction spikes from sediments containing various amounts of AVS has been studied. The results obtained by using 1 M HCI as extractant (～90 of added Hg was recovered in the solid phase after extraction) indicated that acid-soluble Hg may react with H{sub}2S released from AVS phases (e.g. FeS, MnS) and precipitate in the form of HgS. The modeling of Hg speciation in HCI solutions in the presence of sulfide predicted the HgS(s) precipitation in 1 M HCI, observed experimentally. The precipitation of HgS(s) was found to be considerably lower when greater (2-6 M) HCI concentrations were used for Hg extraction. The modeling calculations confirmed that the extent of HgS(s) formation in the solution was controlled by the applied HCI concentration. The difference between HCI-soluble Hg fraction in an estuarine sediment core extracted by 1 M (1-5 of Hg{sub}(tot)) and 6 M (50-60 of Hg{sub}(tot)) HCI was consistent with the model experiments. These results suggest that the use of 1 M HCI extraction procedures must be questioned for methodological artifacts and that 6 M HCI could advantageously replace 1 M HCI for extracting Hg in the presence of AVS.
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