Isolation of colloidal monomethyl mercury in natural waters using cross-flow ultrafiltration techniques

A series of experiments was conducted to evaluate the appropriateness of cross-flow ultrafiltration (CFUF) techniques for the determination of the phase speciation of monomethyl mercury (MeHg) in natural waters. Spiral-wound cartridge (Amicon S1Y1) and Miniplate (Amicon) were evaluated for their nominal molecular weight cut-offs of I and 10 kDa, respectively. The ultrafiltration behavior of standard macromolecules showed that the permeation of high molecular weight (HMW) organic macromolecules was not significant when a concentration factor (CF) > 15 was used. The retention of low molecular weight (LMW) molecules was significant, especially at a low CF < 5, suggesting that the use of a high CF (similar to 15) will minimize the retention of LMW molecules. Sorptive losses of MeHg in the solution phase to the I kDa membrane were negligible, but MeHg bound to HMW macromolecules was still retained (similar to 20%), even with a preconditioned membrane. The mass balance recovery of MeHg during ultrafiltration averaged 101 +/- 15% (n = 7) and 105 +/- 14% (n = 5) for the 1 and 10 kDa membranes, respectively. Sample storage over 24 h caused significant coagulation (similar to 47%) of the < 10 kDa MeHg into the 10 kDa-0.45 pin colloidal or the particulate MeHg pool. The 1 kDa-0.45 mum colloidal MeHg in Galveston Bay and the Trinity River water samples accounted for 40-48% of the filter-passing MeHg, although the most abundant fraction (52-60%) of MeHg was the truly dissolved fraction (< 1 kDa). The partition coefficient between the colloidal (1 kDa-0.45 mum) and truly dissolved MeHg (average log K-C = 5.2) was higher than the partition coefficient based on particle /filter-passing (average log K-D = 4.6) or particle/truly dissolved MeHg (average log K-P = 4.8), suggesting that MeHg has stronger affinity for natural colloids than macroparticulate materials (> 0.45 mum). (C) 2001 Elsevier Science B.V. All rights reserved.