Measuring Metal Sulfide Complexes in Oxic River Waters with Square Wave Voltammetry

A sulfide identification protocol was developed to quantify specific metal sulfides that could exist in river water. Using a series of acid additions, nitrogen purges, and voltammetric analyses, metal sulfides were identified and semiquantified in three specific groups: (a) Co, Fe, Mn, and Ni (bi)sulfides, (b) Fe, Zn, and Pb sulfides, and (c) Cu sulfides. All metal sulfide complexes were measured in low nanomolar concentrations in the oxic waters of four Connecticut rivers, using a thin mercury film rotating disk glassy carbon electrode (TMF-RDGCE). The short residence times associated with a RDE prevents certain strong metal sulfide complexes (Cu, Zn, and Pb) from dissociating at pH > 7.0 during depositions, which allows for identification in certain pH zones. The concentrations of the specific metal sulfide complexes were linked to the extent of watershed development and proximity to source areas. At sampling sites impacted by treated sewage effluent, the concentrations of Cu and Zn sulfide complexes accounted for over 30% of the total metals passing through a 0.45-μm filter. Ultrafiltration revealed that between 30% and 60% of these Cu and Zn sulfide complexes were >3000 MW and probably associated with organic matter. A kinetic loss experiment showed that the Cu and Zn sulfide complexes had half-lives > 15 days, demonstrating the importance of these complexes as metal carrier in small- and medium-sized river systems.