Peter K. Ndiba

Zn sorption mechanisms onto sheathed Leptothrix discophora and the impact of the nanoparticulate biogenic Mn oxide coating.

Zinc sorption on sheathed Leptothrix discophora bacterium, the isolated extracellular polymeric substances (EPS) sheath, and Mn oxide-coated bacteria was investigated with macroscopic and spectroscopic techniques. Complexation with L. discophora was dominated by the outer membrane phosphoryl groups of the phospholipid bilayer while sorption to isolated EPS was dominated by carboxyl groups. Precipitation of nanoparticulate Mn oxide coatings on the cell surface increased site capacity by over twenty times with significant increase in metal sorption. XAS analysis of Zn sorption in the coated system showed Mn oxide phase contributions of 18 to 43% through mononuclear inner-sphere complexes. The coordination environments in coprecipitation samples were identical to those of sorption samples, indicating that, even in coprecipitation, Zn is not incorporated into the Mn oxide structure. Rather, through enzymatic oxidation by L. discophora, Mn(II) is oxidized and precipitated onto the biofilm providing a large surface for metal sequestration. The nanoparticulate Mn oxide coating exhibited significant microporosity (75%) suggesting contributions from intraparticle diffusion. Transient studies conducted over 7 months revealed a 170% increase in Zn loading. However, the intraparticle diffusivity of 10(-19) cm(2) s(-1) is two orders of magnitude smaller than that for abiotic Mn oxide which we attribute to morphological changes such as reduced pore sizes in the nanoparticulate oxide. Our results demonstrate that the cell-bound Mn oxide particles can sorb significant amounts of Zn over long periods of time representing an important surface for sequestration of metal contaminants.

Risk Assessment of Metal Leaching into Groundwater from Phosphate and Thermal Treated Sediments

Beneficial reuse as an alternative to disposal is increasingly being considered in management of contaminated sediments dredged from harbors. The risk of metal leaching into groundwater in reuse of phosphate and thermal treated sediments was assessed with sequential extraction, synthetic precipitation leaching procedure (SPLP), and leaching as a function of pH and liquid to solid ratio (L/S). Sequential extraction revealed that phosphate addition at 5% by dry weight, followed by calcination at 700°C reduced metal association with exchangeable/carbonate and organic phases and increased that with sparingly soluble residuals. Over the pH range 4 to 9, metal leachability varied by two to four orders of magnitude while varying L/S over 5 to 100 showed little difference. The SPLP revealed that risk to groundwater criteria based on applying a dilution attenuation factor (DAF) of 13 (New Jersey) to groundwater quality levels were achieved. Risk criteria based on a DAF of 1 (Florida and Wisconsin) were exceeded fo...

METAL SPECIATION IN PHOSPHATE AND THERMAL STABILIZATION OF CONTAMINATED DREDGED SEDIMENTS

Sediments dredged from harbors and waterways are often contaminated with metal and organic toxins, therefore, their disposal is not only expensive but unsustainable. Treatment and reuse in construction as an alternative to disposal would reduce costs and conserve resources. While thermal treatment followed by carbon adsorption theoretically addresses organics, the fate of metals is of concern. Phosphate addition followed by thermal treatment at 700 degree C was investigated for immobilization of metal contaminants with X-ray absorption spectroscopy (XAS). In sediments from Belgium where Zn concentrations were elevated, Zn was precipitated as ZnCO3 and adsorbed to hydrous manganese oxide and hydrous iron oxide. Phosphate and thermal treatment resulted in sparingly soluble minerals, hopeite (Zn3(PO4)2.4H2O), and spinels, gahnite (ZnAl2O4) and franklinite (ZnFe2O4). Leaching assessments with the U.S. EPA toxicity characteristic leaching procedure (TCLP) confirmed Zn and other heavy metals were immobilized. Results of synthetic precipitation leaching procedure (SPLP) showed compliance with the New Jersey State impact to groundwater criteria.