Environ. Sci. Technol. 2000, 34, 3420-3427
A Critical Evaluation of Tangential-Flow Ultrafiltration for Trace Metal Studies in Freshwater Systems. 1. Organic Carbon
and DOC) (4, 5). Because colloidal phases are important in natural waters (6), a description of the distribution including only dissolved and particulate fractions is inadequate to describe or predict the impact of metals in aquatic systems. One approach for quantifying the colloidal speciation of carbon and trace metals is ultrafiltration (UF), which separates particles at the nanometer scale, differentiating macromolecules from small “dissolved” species.
STEPHEN R. HOFFMANN,* MARTIN M. SHAFER, CHRISTOPHER L. BABIARZ, AND DAVID E. ARMSTRONG Water Chemistry Program, University of WisconsinsMadison, 660 North Park Street, Madison, Wisconsin 53706-1484
Although ultrafiltration principally fractionates according to molecular size and is therefore not a true chemical speciation technique, many important physicochemical properties of colloids including chemical functionality may be partially dependent on size (7). Aggregation and coagulation, sedimentation, transport processes, binding site availability, and metal-vector sorption and desorption kinetics are all influenced by colloidal size (8, 9). Additionally, ultrafiltration is an effective method for investigating the influence of colloidal particles on trace metal partitioning constants (8, 10, 11).
The utility of tangential flow ultrafiltration (TFUF) for size fractionation of natural organic matter (NOM) in freshwater streams was investigated, focusing on characterization or elimination of potential artifacts. Spiral-wound polyethersulfone (PES) and regenerated cellulose (RCL) membranes with nominal molecular weight limits of 10 kilodalton (kDa) and 100 kDa were compared as part of a large project assessing the utility of large volume (>5 L) ultrafiltration for determining trace metal speciation in freshwaters. With careful cleaning, reliable fractionations of carbon and trace metals in freshwater can be obtained, and a detailed protocol necessary to avoid potentially significant biases is presented. Both PES and RCL membranes can be cleaned efficiently to provide low carbon blanks (10 kDa on either membrane at either ionic strength. However, the PES membrane showed significant sorption: almost 20% was recovered in the MQ or base rinses or was unrecoverable. The mechanism of this sorption is unknown, but the greater sorption on PES than RCL is consistent with natural DOC observations. The 18 kDa tests were similar for RCL and PES: for both, 85-90% was >10 kDa, which is consistent with the NMWL rating. The remainder either passed the membrane or sorbed, with no identifiable differences between membrane types or ionic strength level. Extrapolation of these results to natural DOC must be done with care. The test proteins are globular and therefore may not be good surrogates for the behavior of natural compounds of similar molecular weight, but dissimilar structure or shape. There may also be specific functional group interactions between the proteins and the membrane that could be different for natural colloids. However, the primary goal of these tests was validation of membrane performance under our specific operating protocol.
FIGURE 2. Sorption results. (a) and (b) show the average and range of (a) overall recovery, where 10 kDa (colloidal) fraction for carbon than the mass balance approach, generally by 5-8% (Figure 3). This difference results from the increasing permeate DOC levels as the separation continues, an effect which has been observed previously and can be described by a permeation model (18, 26, 29). This trend in permeate concentration necessitates careful description of the method used to calculate fractionation (Table 2). The literature permeation model describes the permeate concentration as a function of the concentration factor (CF) and a permeation coefficient (Pc), which is the ratio of solute (10 kDa was within about 5% for replicates, and other fractions also showed good agreement (Table 3). Precision was not quite as good as observed for seawater ultrafiltrations (26), where replication is within 2%. On a concentration basis, the difference between the replicates corresponds to 1.9 mg L-1 COC for the Suwanee R., 0.9 mg L-1 COC for the Tahquamenon R., and 10 kDa is shown. The drawn line is the 1:1 line, representing equivalent results from the two methods. possibly lead to charge-related interactions. For both membranes, we assessed the importance of sorption of natural carbon in our ultrafiltration protocols using two approaches: (1) measurement of both base-extractable sorbed, and residual (unrecoverable) fractions of the overall mass 3424
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 16, 2000
balance and (2) experiments passing preultrafiltered natural water through membranes of significantly larger NMWL, so losses could be attributed to sorption. Sorption losses to RCL and PES can be investigated by comparing, for the entire data set of natural sample filtrations (n ) 67), the percent of feed carbon recovered in base-labile and unrecoverable fractions of the total mass balance. PES 10 kDa base-extractable sorbed fractions are almost twice as high as for 10 kDa RCL (Figure 2(b)), suggesting more sorption on PES. Additionally, sorption is more important for 10 kDa membranes than for 100 kDa membranes of either type. Unrecoverable sorption, indicated by a recovery of less than 100% (Figure 2(a)), was observed only for PES (average unrecoverable fraction of 4-5% for both 10 and 100 kDa). Four natural matrix sorption experiments were performed (Figure 2(c)). In these tests, a natural sample 10 kDa permeate was processed through a 100 kDa ultrafilter. The two filtrations were done as quickly as possible, to minimize retention on the 100 kDa membrane which could result from reformation of >100 kDa colloids from the 10 kDa permeate. All of the carbon in the 10 kDa feed solution should pass the 100 kDa filter; therefore measuring the fraction retained allows direct assessment of sorption of natural carbon. This evaluation was performed on both PES and RCL and at different levels of DOC and specific conductance: (1) a mixed river water of high conductivity and moderate DOC, run on PES, (2) Moose River (WI) water, low conductivity and high DOC, on PES, (3) the same Moose River water run on RCL, and (4) Presque Isle River (MI) water, moderate conductivity
FIGURE 4. Mass balance recovery of all field samples. In all graphs, circles are 100 kDa filtrations, triangles are 10 kDa filtrations. Filled symbols are PES membrane; open symbols are RCL membrane. All plots are recovery (%) vs (a) UF chronological order, (b) specific conductance, (c) DOC, and (d) percent colloidal carbon. Shaded area represents one standard deviation from the mean. ART ) Acid Removal Treatment.
TABLE 3. Replication of DOC Ultrafiltration at Three Field Sites site and date Fish Creek, WI 10-1-97 Tahquamenon R., MI 4-8-98 Suwanee R., GA 7-9-97
>10 [DOC] kDa (mg L-1) (%) 0.8 14.2 47.0
24.9 20.8 46.5 51.7 44.7 40.6
100 kDa) was low (10 kDa fraction for RCL relative to PES. Therefore, the fractions of DOC 10 kDa and specific conductance (logarithmic) for natural samples. Filled circles are RCL membrane, filled triangles are PES membrane. Open squares and fit line labeled ECR are from the increased conductivity experiment on the East Creek.
FIGURE 5. Comparison of PES to RCL membranes for ultrafiltrations repeated on both membranes. Bar label ) percent < 10kDa (of 0.4 µm filterable). DOC values (in mg L-1) and specific conductance (in µS cm-1) are listed for each river. of the two membranes was identical within analytical error. The similar charge would suggest that the charge-determining moieties had been modified (presumably by DOC sorption), giving the two membranes similar surface character. The observed fractionation differences, therefore, are likely not charge related. Two other observations may be important in selecting between membranes: color and colloidal metal retention. Qualitatively, the color of the PES retentates was darker than the RCL retentates, suggesting the opposite of the DOC measurements: that PES retains more carbon. This observation is purely qualitative, and no spectrophotometry was performed. For most metals measured, higher levels of colloidal metals were observed with PES than with RCL (25), which matches the color observations but not the DOC measurements. We can reconcile the difference between the color and metal results and DOC results by postulating nonsize dependent fractionation effects for both membranes. Because of the stronger hydrophobic character of PES than RCL (PES has an aromatic monomer; RCL does not), sorption of highly hydrophobic DOC is likely greater on PES than RCL. This fraction is likely to be highly chromophoric (hence the color observations) and also likely to form stronger associations with many trace metals (e.g., through aromatic acid or phenolic binding sites). Because of the slightly more negative charge of RCL (at least at the beginning of the separation), rejection of negatively charged,
