Filtered zinc higher than total!

[Andy Barnes]

On a number of occasions it has been found the filtered concentrations of zinc in some mine waters have been consistently higher than total concentrations. The difference has been greater than the estimated analytical error and also checks of trip blanks have been clear. Analytical error has therefore been discounted.

A colleague of mine has suggested that the cause may be due to a high suspended clay particulate load in the mine water that is at disequilibrium with the solution. On filtration the clay particulates are removed and the filtrate represents the dissolved zinc fraction at that time. Now the total fraction is acidified to pH 2 which under normal circumstances inhibits any further adsorption on to mineral surfaces. However, this does not prevent zinc adsorption to the clay surfaces and this continues in the acidified solution. Because the clay particles are resistant to the digestion process they are then removed during filtration prior to analysis. Hence the clay surfaces mop up zinc decreasing the total zinc concentration.

Has anyone experienced similar behaviour?

[Chris Wolkersdorfer]

Yes, I find that quite often with iron. Filtered iron sometimes has a significantly higher concentration than unfiltered (with significantly I mean that the concentrations is higher than the analytical error of the method).

I discussed that matter with a colleague who has a very good chemical background and his answer was: the answer is not simple, as there might be many reasons and it might be different from time to time. Most important: it can happen and it is neither a sampling nor an analytical error. The reason are interferences with other constituents in the mine water or it might be due to the acidification of the sample. In case of iron, according to the colleague, argon and oxygen cause interferences. And if you used colorimetric methods you might also encounter interferences (see Hach literature). And, finally, the acidification might cause unpredictable effects in some cases. You correctly discribe such a case.

We, for example, noticed that also the type of filter you use influences the result of the chemical analyses. With type of filter I mean cellulose nitrate or cellulose acetate for example.

Here some literature that helped me to understand the matter:

Hall, G. E. M., Bonham-Carter, G. F., Horowitz, A. J., Lum, K., Lemieux, C., Quemerais, B. & Garbarino, J. R. (1996): The effect of using different 0.45 µm filter membranes on "dissolved" element concentrations in natural waters. – Appl. Geochem., 11 (1 & 2): 243-249, 1 Abb., 7 Tab.; Oxford.

Ball, J. W., Nordstrom, D. K. & Alpers, C. N. (1998): Comparison of filtration procedures and analytical procedures on iron (II/III); results from upper Animas, Summitville, and Iron Mountain. – Open-File Report – US Geological Survey, OF 98-0297: 22; Reston.



Shiller, A. M. (2003): Syringe Filtration Methods for Examining Dissolved and Colloidal Trace Element Distributions in Remote Field Locations. – Environ. Sci. Technol., 37 (17): 3953-3957, 5 Abb., 1 Tab.; Washington.

[Andy Barnes]

Thanks Chris. I will chase up this literature.

[romy]

Hi Andy,

just for clarification. Which method did you use for analysis ? Was it indeed a colorimetric based method ?

thanks

romy

[Andy Barnes]

Filtration followed ICP analysis!

[kzamzow]

I had this happen with zinc, and only zinc, in very pure natural waters in Alaska. The difference was not much (there wasn't much zinc in the water, either total or filtered), but the percent difference was higher than would normally be acceptable.

Kendra Zamzow