Determination of sulfide ligands and association with natural organic matter

Group B metals, such as Hg, Cu, Ag, Pb and Cd bind strongly to reduced inorganic and organic S(II−) ligands. These S(II−) ligands, stable in oxic waters for significant periods of time, occur at the <1–100 s nM concentrations. It is hypothesized that S(II−) ligands are stabilized as Cu–S molecules associated with organic matter by multi-ligand binding or in nano-pore encapsulations in organic matter. S(II−) ligands are estimated by two methods: purge/trap analysis as Cr-reducible sulfide (CRS), and strong ligand (SL_T) from a competitive ligand titration with Ag(I). The CRS/SL_T ratio is nearly one for selected samples. CRS correlates reasonably well (r² ∼ 0.5) with organic C with a slope of 14.6 nM per mg C. The conditional binding constant of Ag–SL is 11.3 for effluent associated with waste-water and decreases for river waters from about 12–8.8 as the strong sites are occupied with Ag(I).