Effects of acid volatile sulfides on the use of hydrochloric acid for determining solid-phase associations of mercury in sediments

The use of dilute HCl for the selective extraction of mercury from suboxic and anoxic sediments has been evaluated. The effect of acid concentration ( 1-6 M HCl) on the extraction of Hg and on the recovery of aqueous Hg preext raction spikes from sediments containing various amounts of AVS has been st udied. The results obtained by using 1 M HCl as extractant (similar to 90% of added Hg was recovered in the solid phase after extraction) indicated th at acid-soluble Hg may react with H2S released from AVS phases (e.g. FeS, M nS) and precipitate in the form of HSS. The modeling of Hg speciation in HC l solutions in the presence of sulfide predicted the HgS(s) precipitation i n 1 M HCl, observed experimentally. The precipitation of HgS(s) was found t o be considerably lower when greater (2-6 M) HCl concentrations were used f or Hg extraction. The modeling calculations confirmed that the extent of Hg S(s) formation in the solution was controlled by the applied HCl concentrat ion. The difference between HCl-soluble Hg fraction in an estuarine sedimen t core extracted by 1 M (1-5% of Hg-tot) and 6 M (50-60% of Hg-tot) HCl was consistent with the model experiments. These results suggest that the use of 1 M HCl extraction procedures must be questioned for methodological arti facts and that 6 M HCl could advantageously replace 1 M HCl for extracting tig in the presence of AVS.