Environ. Sci. Technol. 2000, 34, 3428-3434
A Critical Evaluation of Tangential-Flow Ultrafiltration for Trace Metal Studies in Freshwater Systems. 2. Total Mercury and Methylmercury C H R I S T O P H E R L . B A B I A R Z , * ,† STEPHEN R. HOFFMANN,† MARTIN M. SHAFER,† J A M E S P . H U R L E Y , ‡,§ ANDERS W. ANDREN,† AND DAVID E. ARMSTRONG† Water Chemistry Program, University of WisconsinsMadison, 660 North Park Street, Madison, Wisconsin 53706-1484, Bureau of Integrated Science Services, Wisconsin Department of Natural Resources, 1350 Femrite Drive, Monona, Wisconsin 53716, and Water Resources Center, University of Wisconsins Madison, 1975 Willow Drive, Madison, Wisconsin 53706
Laboratory and field investigations of 10 kDa polyethersulfone (PES) and regenerated cellulose (RCL) membranes were conducted to evaluate the utility of ultrafiltration (UF) for low-level mercury determinations in freshwaters. Laboratory investigations focused on blank levels, sorption loss, and charge rejection; while field investigations addressed mass balance closure, replicate precision, and permeate trends in concentration. A thorough mass balance approach was used throughout the investigation, and experiments were conducted across typical gradients in freshwater pH, specific conductance, and dissolved organic carbon (DOC). The minimum source-water HgT concentration that could be confidently processed from a 5 L volume was 0.5 ng L-1 for PES and 1.4 ng L-1 for RCL as determined by three times the standard deviation of our method blank. The minimum source-water MeHg concentration was 0.5 ng L-1 for PES and 0.2 ng L-1 for RCL based on riverwater mass balance results. Mass balance closures were generally better for RCL than PES, especially for MeHg where 95% of the residuals (n ) 21) were within (30% of closure. Duplicate UF separations on RCL membranes, and between RCL and PES membranes, agreed within 16% in the colloidal and dissolved fractions for both total mercury (HgT) and MeHg. Mercury sorption loss and charge rejection were greater for PES membranes than RCL membranes, especially for low ionic strength waters (
