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)su lfides, (b) Fe, Zn, and Ph 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 d issociating at pH > 7.0 during depositions, which allows for identification in certain pH zones. The concentrations of the specific metal sulfide comp lexes were linked to the extent of watershed development and proximity to s ource areas. At sampling sites impacted by treated sewage effluent, the con centrations of Cu and Zn sulfide complexes accounted for over 30% of the to tal metals passing through a 0.45-mu m filter. Ultrafiltration revealed tha t between 30% and 60% of these Cu and Zn sulfide complexes were >3000 MW an d probably associated with organic matter. A kinetic loss experiment showed that the Cu and Zn sulfide complexes had half-lives >15 days, demonstratin g the importance of these complexes as metal carrier in small- and medium-s ized river systems.