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?