An Investigation of Cadmium, Copper and Nickel in the Colloidal Size Range in Seawater

Abstract

This thesis contains the results of an investigation of Cd, Cu and Ni in the colloidal size-range in seawater. The main objectives are: 1) to evaluate an Osmonics cross-flow filtration system for submicron sampling of Cd, Cu and Ni; and 2) to directly assess the concentration of Cd, Cu and Ni in truly dissolved (<1,000 NMW), colloidal (1,000 NMW-1 μm) and particulate (>1 μm) size-fractions in the Gulf of Maine. A 1,000 nominal molecular weight (NMW) spiral wound cross-flow filtration (CFF) membrane has been evaluated for sampling Cd, Cu and Ni in <1,000 NMW and >1,000 NMW fractions of prefiltered seawater. Mass balance results averaged 93 ± 6% (Cd), 81 ± 19% (Cu), and 99 ± 18% (Ni) in a multi-institution CFF intercomparison experiment and 56 ± 17% (Cd), 119 ± 38% (Cu), and 91 ± 6% (Ni) for samples collected in the Gulf of Maine. Differences in mass balance between these studies are suggested to be related to removal of submicron particles and associated trace metals during prefiltration. Results from the Gulf of Maine show a decrease in trace metal recovery for Cu and Ni with increasing suspended particle concentration (Cp) and a minor dependence of recovery on CFF volume processed. Model calculations based on particle impaction with a filter membrane indicate that a 100% contact opportunity exists between submicron particles and a 1,000 NMW Osmonics CFF membrane or conventional (Nuclepore) filters. Size-fractionated samples for trace metal analysis were collected in the Gulf of Maine covering a range of Cp = 0.1-1 mg 1-1. Results show that colloidal Cd, Cu and Ni concentrations represent <1-7% of the total in these shelf waters and increase with increasing particle concentration. By comparison, results for the strongly particle-reactive radionuclide 234Th indicate that <1-10% of the total is associated with the colloidal size fraction. Calculated distribution coefficients (Kd's) for Cd, Cu and Ni suggest that colloidal metal concentrations measured in this study do not constitute a sufficient portion of the total to account for previous observations of an apparent inverse dependence of Kd on Cp. Results indicate that for most ocean waters (Cp = 0.01-10 mg 1-1), the fraction of total dissolved metal associated with colloids is <1-20% and increases with increasing particle concentration.