HEAVY METAL COMPLEXATION WITH NATURALLY OCCURRING ORGANIC LIGANDS IN WETLAND ECOSYSTEMS.

摘要

The mode, rate, and extent of heavy metal uptake by freshwater wetlands were investigated as well as heavy metal complexation by a variety of soluble organics. In one study, the levels of Cd, Cu, Mn, Ni, Pb, and Zn entering Basin Swamp from the sewage treatment plant in Jasper, Florida, were low (0.1-30 (mu)g/L). Consequently, no conclusions could be made regarding heavy metal cycling in Basin Swamp. Another study involved adding Cd, Cu, Mn, and Zn to septage from Waldo, Florida, and pumping the wastewater through a 10 x 40-m corridor in a cypress swamp. Soluble concentrations of all four metals decreased within the corridor to levels close to those considered acceptable for discharge to surface water. Rates of immobilization were calculated to be a minimum of 7.2, 36, and 72, and 72 g metal/ha(.)day('-1) for Cd, Cu, Mn, and Zn, respectively. In addition, microcosms (model ecosystems that simulated a swamp) were constructed for a laboratory investigation of factors that control metal immobilization rates. Of the factors--dissolved organic carbon, iron, calcium, sulfide, and pH--the latter two most greatly affected metal immobilization rates.;Heavy metal interactions with naturally occurring organics and model compounds were examined by complexometric titrations using five separate techniques. Three of the techniques used--anodic stripping voltammetry (ASV), ion-selective electrode, and fluorescence quenching--were previously published methods, and the other two methods--continuous flow ultrafiltration and competing ligand/differential spectroscopy--were developed for this study. Each procedure was used to determine the available metal binding ability of the organic solutions and the conditional stability constants of the metal-organic complexes. For each solution, agreement among the procedures was good except for the ASV method, which produced results lower than the others. Additionally, the ASV method yielded a conditional stability constant for copper with ethylenedinitrilotetraacetic acid that was five orders of magnitude lower than other published values. The low ASV results for copper with swamp-water organics were found to be caused by the reducible nature of organically complexed copper.