We have a problem filtering microorganisms from a water sample which is very rich in metals (5000mg/L Al, 2000mg/L Fe, 600mg/L Mg, in addition lots of SO4 and very acidic). We want to get the microorganisms. Filtering samples by vacuum filtration, we get a heavy brownish precipitate (from the clear water) on the filters. This probably interferes with DNA extraction. Any suggestions?
The metal load is large so the addition of a complexing agent to prevent precipitation would need to be commensurately large. This seems impractical. However, after filtration, a complexing agent like citrate may clear the precipitate from the filter. Citrate may not be the best reagent, but I hope to prompt input from others with more relevant knowledge
You have indicated the issue occurs post filtration, and that the water is otherwise clear prior to and during filtration. I suggest you're experiencing formation of metal oxy(hydr)oxides due to (a) rise in pH and (b) exposure to higher redox potential (oxygen) of filter entrained/deposited material once the acid aqueous phase is filtered. I suggest thoroughly rinsing the filter post-filtration with a low-redox potential, low-pH buffer. Three successive rinses of 20-50 ml buffer should do it (assuming 45 mm membrane filters), but avoid letting the filter dry between rinses. Avoid thio buffer compounds. Ascorbic acid is one reductant. You want to rinse away the residual metals and possibly metal-hydroxide colloids from the filter. The latter can form very quickly with rise in pH and/or redox potential. Also, are you using cellulose ester or polycarbonate membrane filters?
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