To remove organics and carbonates from lake sediment samples for diatom enumeration, I was taught to boil sediment samples in 40ml concentrated nitric acid and then add potassium dichromate. After that I dilute with DI, let the suspension settle overnight, decant and then repeat the dilution/settling/decanting for a total of six times. This methods yield wonderfully clear and easy to count diatom slides. However, I'd like to not have to use the potassium dichromate. (And I wouldn't mind a speedier prep that avoids the several days of dilution/settling/decanting!) Suggestions?
We generally use a 50:50 molar mixture of hot nitric and sulfuric acid for sediment diatom prep in Dr Hall's Lab. We remove carbonates by addition of 10% HCl followed several rise cycles prior to acid digestion to prevent CaSO4 precipitation on the diatom frustules. If sediment carbonates are low we skip that step.
You do have to be careful and use precautions (fume hood, gloves, etc) as hot nitric and sulfuric acid are still rather dangerous substances to work with. The creation of the acid blend has to be done slowly as just adding one acid to the other generates substantial heat.
Alternately 30% hydrogen peroxide (again hot) works and we have used that for both sediment and macrophyte digestion though it needs to be watched closely, especially after adding the digestion tubes to the hot water bath as it can get a bit effervescent with some materials and cause sample foam over and loss if the digestion is allowed to proceed too rapidly.
In the end if a digestion of sediment organic matter is desired some corrosive and dangerous chemicals are going to be involved but it's nice to have options.
To speed up settling/decanting time we use a centrifuge. The Supernatent is aspirated off (not poured) and distilled water is added. This is repeated until pH ~6 is reached. The centrifugation and aspiration cycles get a bit tedious but you can get the job done in less than a day at least.
Hope this helps a bit.
Cheers,
Johan
