Anal. Chem. 1995,67,3391-3400
Performance Characteristics of Diffusion Gradients in Thin Films for the in Situ Measurement of Trace Metals in Aqueous Solution Ha0 Zhang and William Davison* Environmental Sciences, IEBS, Lancaster University, Lancaster LA 1 4YQ, UK
The technique of diffusive gradients in thin &ns (DGT) provides an in situ means of quantitatively measuring labile species in aqueous systems. By ensuring that transport of metal ions to an exchange resin is solely by free dii€usion through a membrane, of known thickness, Ag, the concentration in the bulk solution, cb,can be calculated 6rom the measured mass in the resin, M,after time, t, by cb = M&/DAt, where D is the molecular diffusion c d c i e n t and A is the exposure surface area of the membrane. If a sufticientlythick (- 1mm) diffusion layer is selected, the flux of metal to the resin is independent of the hydrodynamics in solution above a threshold level of convection. Deployment for 1 day results in a concentration factor of -300, allowing metals to be measured at extremely low levels (4 pmol L-l). Only labile metal species are measured, the effective time window of typically 2 min being determined by the thickness of the diffusion layer. Because metals are quantified by their kinetics of uptake rather than the attainment of equilibrium, any deployment time can be selected from 1 h to typically 3 months when the resin becomes saturated. The measurement is independentof ionic strength (10 nM-1 M). For Chela-100 as the resin, the measurement is independent of pH in the range of 5-8.3, but a subtheoreticalresponse is obtained at pH 25%). These estimates are broadly in line with the observations (Figure 9). (30) Levich, V. G. Physicochemical Hyirodynamics; RenticeHall, Inc.: E n g l e w d Cliffs, NJ, 1962. (31) Welty, J. R.; Wicks, C. E.; Wilson, R E. Fundamentals ofMomentum,Heat, and Mass Transfer, 3rd ed.; John Wiley and Sons, Inc.: New York, 1984. (32) Snodgrass, W. J. In Sediments and Wuter Interactions; Sly, P. G., Ed.; Springer: New York, 1986.
3398 Analytical Chemistry, Vol. 67, No. 19, October 1, 1995
l.* 1
0.4
0.2
[
0
$.
1
0
1
0
200
600 stining Rate (Qm) 400
800
lo00
Figure 9. Effect of flow convection (presented as stirring rate) on DGT measurements assessed by the ratio of Cd concentrations measured by DGT (Get)to the concentrations obtained by direct ASV measurement in seawater (Cas").
If the DBL thickness is negligibly small, the mass measured in a given time by DGT should be inversely proportional to the thickness of the diffusive gel layer (eq 20). When holder I1 was
M = Dc@/
(&
+ 6)
(20)
used with a stirring rate of lo00 rpm, this was indeed the case (Figure 5). Moreover, the measured mass was theoretically predicted from the known concentration in solution, further indicating that the DBL was negligibly small. Similar results were obtained when holder I was deployed directly in seawater,' indicating that in this natural aquatic environment there is suf6cient natural convection to allow the use of eq (4) for calculating the bulk concentration. If 6 is significant compared to Ag, a plot of M versus l/Ag will be nonlinear. In this case, 1/M can be plotted against Ag providing the flow velocity is constant. The intercept of this l i e can be used to calculate 6 according to eq 21.
The Capacity of DGT. The DGT technique has the potential to be used for long-term (weeks or months) deployment to obtain an integrated record of trace metal concentrations. For such application, the capacity of the resin layer may be a limitingfactor. It is controlled by the capacity of the Chelex-100 resin and the amount of Chelex-100 used in the resin layer. In practice, the amount of Chelex-100is limited to the quantity necessary to form a monolayer adjacent to the diffusive gel layer. The capacity of Gel holder I1 was measured by immersing gel assemblies with a 0.4 mm diffusive gel layer for 10 h in CdClz solutions at various concentrationsof Cd up to 0.01 M. The mass of Cd measured in the resin layer increased linearly with increasing Cd concentration in the bulk solution up to 5 x M (Figure 10). At higher concentrations, the measured mass of Cd plateaued at a maximum value of 0.63 mg of Cd. This must represent the maximum capacity of this gel holder, which had a surface area of 2.3 cmz. Using the manufacturer's stated capacity, in terms of equivalents
6 T
.e
Table 2. in Situ Mean Concentrations and Standard Devlations ( n = 6) of Zn, Mn, and Fe (in pg L-l) Measured by M I T In the Menai Straits (UK) (Saiinlty 32x,Temperature 14 'C).
.e
e
5 t e
zn Mn
Fe
time series
thickness series
0.77 f 0.04 6.76 & 0.50 3.74 i 0.21
0.73 f 0.03 6.27 f 0.42 3.60 f 0.23
Data are presented for a series of measurements using different deployment times (time series) and a series using different gel layer thickness (thickness series). (I
O J -8
r = 0.980
I
-6
4
-2
0
Log [Cdin solution(M)]
Figure 10. Capacity of DGT presented as a log-log plot of the mass of Cd measured by DGT versus the solution concentration.
0.15
--
0.1
--
9 v
a per milliliter of Chelex, it was possible to predict the total capacity of the holder. The resulting value of 0.56 mg of Cd was in excellent agreement with the measurement. The linear relationship between mass of Cd measured and concentration in solution (Figure 10) shows that the principles of DGT, which allow calculation of bulk concentrations, are independent of the loading of metal on the resin providing saturation is not reached. From a practical point of view, it is important to know the maximum immersion time of a DGT device before its capacity is exceeded. The effective DGT maximum accumulation time is independent of surface area but dependent on the other conditions contributing to the flux (eq 3). For a 1 mm thick diffusion layer and an average diffusion coefficient of metal ions of 1 x cmz s-l, its maximum accumulation time can be estimated using eq 3. In a simple solution of metal ions with a total concentration of 1x N, the maximum accumulation time of DGT would be 15 months. Using the typical concentration of Chelex-exchangeable metals in ocean the maximum accumulation time of a DGT device in the ocean was estimated to be -2.5 years. Assuming the average concentration of metal ions in coastal seawater is 10 times higher than those in ocean water, DGT can be expected to provide a 3 month integrated record of trace metal concentrations in coastal environments. Similar maximum accumulation times are likely to apply to unpolluted freshwaters. These calculations do not consider binding of high concentrations of Ca and Mg ions, which trace metals are expected to replace. In practice, the maximum deployment time in natural waters is likely to be limited by microbial attack and biofouling. Trials in synthetic and natural waters will be required to test these effects. The minimum deployment time where eqs 3 and 4 can be used to interpret the results is 1 hS3 Shorter times could be used if allowance is made for the non-steady-stateconditions that apply during the lirst few minutes of deployment. Field Measurements. The DGT technique has been used to measure trace metal concentrations in situ in the seawater of the Menai Straits WK) and the North Atlantic Ocean. In the Menai Straits coastal water, six gel assemblies with a diffusion layer thickness of 0.5 mm were deployed for different time periods (1-6 h) and six gel assemblies with various diffusion layer thicknesses (0.5-2.5 mm) were deployed for 5.33 h. A linear relationship between the mass of diffused ions and the deployment
D z
0
0.5
1
1.5
2
IlAg (Ilmm)
Figure 11. Measured mass of Mn in the resin layer as an inverse function of the diffusion layer thickness (combined gel and filter layer) for gel assemblies exposed to seawater (Menai Straits, UK, salinity 32%0,14 "C)for 5.33 h.
time was found for the measured Zn, Mn, and Fe. The mean concentrations from these time-dependent measurements were very reproducible (Table 2). The mass of ions measured in the resin layer was inversely proportional to the diffusion layer thickness for Mn (Figure 11). As the tidal current varied from 0 to -4 knots during the deployment, the thickness of the DBL is expected to vary as well. Consequently good linearity in Figure 11 indicates that the gel thickness is dominating the control of mass transport and that the DBL thickness is negligibly small. In other words, there is sufficient natural convection in this natural water to support the use of DGT. Mean concentrations obtained from the diffusion layer-dependent measurements were in excellent agreement with those from the timedependent measurements (Table 2). The total dissolved concentration of Zn was also measured by ASV after acidification and UV irradiation of filtered samples collected at hourly intervals during the DGT deployment. Its value of 1.72 f 0.18 pg L-' was higher than the DGT measured concentration of 0.77 f 0.04 pg L-l. The difference is to be expected as DGT only measures labile species and therefore excludes kinetically inert organic species and large colloids. The concentration of Cu in the North Atlantic water at 40 m depth was measured by DGT by deploying four assemblies with different gel thicknesses. Again plots of measured mass against l/Ag are linear, showing that there was adequate natural convection for the use of DGT. The mean value of 0.15 f 0.01 pg L-l Analytical Chemistty, Vol. 67, No. 79, October 7, 7995
3399
Cu is comparable with typical concentrations in ocean water.33 CONCLUSIONS
DGT has been shown to be a robust technique for in situ measurements. By simply controlling the mass transport to a binding agent, such as Chelex resin, it is possible to quantify the accumulated metal in terms of a measured flux or concentration. The equations which predict that the mass of accumulated metal ions per unit area is proportional to deployment time and inversely proportional to the thickness of the diffusion layer have been verified. The well-defined diffusion layer is a major factor in determining the measured species. Only those species are measured which react directly with the binding agent or, within an effective measurement time determined by the diffusion layer thickness, are in labile equilibrium with the reacting species. Above a threshold level of flow, the rate of accumulation of ions is independent of flow. The measurement of Cd by DGT using Chelex as a resin binding agent is independent of ionic strength (10 nM-1 M). Within the limits of the Chelex resin, which binds less effectively below pH 5, it is also independent of pH. The temperature dependence can be predicted from the known temperature dependence of diffusion coefficients and viscosity. Because metal ions are preconcentrated in situ in the resin layer, contamination problems associated with trace metal sampling and handling are greatly reduced, the final analysis is less demanding, and very low detection limits (pM) are possible. Metals are quantified according to their kinetics of uptake, so deployment times can vary from as little as 1h to several months. The capacity of the resin is such that the device should still provide quantitative results when it is deployed for 3 months in coastal seawater. This work briefly demonstrates the ability of DGT to make in situ speciation measurements of trace metals in coastal and open (33) Bmland, K. W. In Chemical Oceanography; Riley, J. P., Chester, R, Eds.; Academic Press: London, 1983: Vol. 8, p 157.
3400 Analytical Chemistry, Vol. 67, No. 19,October 1, 1995
seawater. It has also been applied to the measurement of trace metals in fresh waters and the pore waters of sediment^.^,^ The measurements in pore waters can in some cases be interpreted as concentrations and in others they provide a direct in situ measurement of the flux of metal from solid phase to s~lution.~ Further applications can easily be envisaged. So far it has only been applied to the measurement of trace metals, but in principle it could be applied to any solution species that can be rapidly bound. Work in progress has shown phosphate can be quantitatively measured by DGT when an iron oxide binding agent is used. When concentrations vary during deployment, DGT provides a time-averaged mean concentration. It could therefore be used in streams and effluents to provide an integrated measurement of mean element concentrationsover selected periods from days to months. Fully quantitative interpretation is only possible, however, if temperature is relatively constant ( f 2 "C), the pH remains within the working range of the resin, and there is always adequate solution flow. DGT always measures a flux which is then interpreted as a concentration. In some cases, it is the flux which is required. The example of solid phase to solution fluxes in pore waters has already been mentioned. When the bioavailability of a nutrient or trace element is being assessed, the local or in situ flux is important. Therefore, DGT may be a useful tool for assessing bioavailability in situ in soils and sediments where the resupply from solid to solution phase must be considered. ACKNOWLEWYENT
We are grateful to Dr. Peter Statham for deploying the assemblies in the Atlantic water and to the NERC for providing financial support. Received for review May 15, 1995. Accepted August 14, 1995.B AC9504676 Abstract published in Advance ACS Abstracts, September 1, 1995.
