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I follow the soil sequential leaching procedure listed in this paper with a soil sample from Mauritania. The following is the basic steps.The chemicals sequence are 1M Ammonium acetate => 1M Acid acetic => 1M Nitric acid => Burn in oven at 500 degree in 8 hours => Milli Q => Total digestion.
Samples are equilibrated for 24 hours. Supernatants are collected after centrifugation then the samples are washed with MQ to get rid of the chemicals. The wash solution are also collected after vortex and centrifugation. The samples are then resuspend in MQ and dry in oven overnight at 60 degree.
Nothing comes out in the first 3 steps. However, in the final Milli Q steps (after burning in oven at 500 degree), even after centrifugation, the supernatant is red and turbid. I let it sit for a few days but it didn't change. When I added concentrated Nitric acid (15M) to reach 1% concentration, the supernatants quickly flocculated and settled down as pic attached.
I tried adding 2mL HCl 32% into 0.5mL sludge then vortex but it doesn't dissolve at all. So what happens here?
I will try to test the sludge on XRF or SEM to know the composition of the sludge but I am interested in knowing what is the mechanism after this phenomenon. If there's any info I can provide/clarify please ask.
Thank you!
You have probably formed a ferric oxide product during the furnacing step.
In these cases we usually digest the ferric complexes in a 50% solution of conc HCl. The reaction is slow and requires heat to go to completion, be careful not to dry out the sample during this stage.
The form of ferric complex formed in the furnace is dependant on the temperature and the water availability during the furnacing step. We usually run the furnace at 650C to ensure only one type of complex is formed no matter what the moisture presence level is.
Dean Butler Thank you so much for the information! I will try to put the sample on heat.. What is the temperature you would recommend and for how long should I leave it be?
I will take note of the temperature of the furnace for the next experiment too.
We usually run these leaches at 80C but if you are using small samples then 50C is a safer option to avoid drying out of the leach.
Leaching time is totally dependant on the sample matrix, it can be from hours to weeks, usually ours are around 1 to 3 days. We run the leach until visual observation confirms removal of the coatings on the particles.
Dean Butler Thank you for your answer, by "visual observation", did you mean that the slurry substances becomes all white instead of the orange color before leaching?
Depends on the sample matrix, the colour can range from white to various greys.
It is rare for furnaced samples to not have colours in this range when cleaned fully.
Thank you that's exactly what happened to the samples now! I'm glad that the problem is solved. Thank you very mcuh ^^
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