A Critical Evaluation of Tangential-Flow Ultrafiltration for Trace Metal Studies in Freshwater Systems. 2. Total Mercury and Methylmercury

Abstract

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 river-water 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 (<100 μS cm-1). Increases in permeate concentration were observed as the UF separation progressed, and preconditioning the membrane with a subsample of the feed solution did not substantially improve the performance. Overall, RCL outperformed PES membranes in low ionic strength waters and therefore is preferred for investigations of mercury partitioning under ambient freshwater conditions.