How to Get the Heavy Metal Out of the Soil for Analysis - BCR Sequential Extraction

Some time ago I have shared information about heavy metal analysis (simple introduction & more detailed one). Basically the samples we use for those machines are in liquid form so that they can be vaporised followed by being atomised or ionised, then the machines will do the rest. How about solid sample such as soil? Sometimes spillage happens or maybe the natural content of heavy metal in the soil is much higher that it can be harmful to human health. There are a few digestion methods (yes it is called digestion, like how we digest the food to obtain the nutrients) can be used, and this one, this 3-stage BCR sequential extraction method is one of the most basic ones.

(photo taken from Bonnie Plant website)

To make it simple, the same soil will go through 3 times of digestions using different solvent respectively to target heavy metals in different forms for several hours on a shaker to mix well(or overnight, depending on the stage), then it will be centrifuged (spun with high speed to separate the soil from the solvent) so that we can obtain the solvent which will be analysed later on. After we are done with each stage, the soil will be 'washed' with distilled water before the following stage can be carried out to remove the solvent in previous stage to avoid contamination (but remember not to loss any of the soil during the process!). Even though we have carried out the 3 stages of digestion, there still will be some residual heavy metal that is bound to the soil and is not removable with the above methods. Since they are tightly bound, we can ignore them as they can't really move around in the soil and will not bring much impact to us. For this whole process, only 1 gram of oven-dried soil is needed!



Stages:
A) 0.11M acetic acid (for exchangable metals which are soluble in acid)
1) Add 40mL of acetic acid to the sample in the centrifuge tube.
2) Cap the centrifuge tube (not too tight to avoid explosion due to pressure built up) and put it on shaker for 16 hours at 400 rpm.
3) Centrifuge it at 3800 rpm for 20 minutes.
4) Carefully retrieve the acetic acid using a syringe without disturbing the soil.
5) Transfer the acetic acid into a 100mL volumetric flask and add distilled water until the calibration mark.
6) The acetic acid is ready to be analysed.
7) Add 20mL of distilled water to the centrifuge tube, shake for 15 minutes then centrifuge, discard the distilled water.

B) 0.1M hydroxylamine hydrochloride (for metals bound to iron and manganese oxide, the oxides are reduced to release the metals)
1) Add 40mL of hydroxylamine hydrochloride (it can be replaced with hydroxylammonium chloride which is less harmful) which has been adjusted to pH 2 using nitric acid to the sample from stage A.
2) Follow all the following steps in stage A just that the solvent is no longer acetic acid but hydroxylamine hydrochloride.

C) 8.8M hydrogen peroxide followed by 1M ammonium acetate (for metals bound to organic matter and sulphides, they are oxidised to release the metals)
1) Add 10mL of hydrogen peroxide to the sample from stage B and leave it for 1 hour with occasional manual shaking (without cap).
2) Place the centrifuge tube in  85 ◦C water bath until the hydrogen peroxide is evaporated to a few mL left.
3) Add 10 mL of hydrogen peroxide again and repeat the previous steps to make sure all the organic matter and sulphides are fully digested. The hydrogen peroxide should be evaporated to near dryness.
4) Add 50mL of ammonium acetate which has been adjusted to pH 2 using nitric acid to the centrifuge tube.
5) Follow all the following steps in stage A just that the solvent is no longer acetic acid but ammonium acetate.

Mechanical shaker for centrifuge tube.
(photo taken from AliExpress website)

Centrifuge tube with centrifuge machine.
(photo taken from Scixchange website)

The electrical water bath equipment.
(photo taken from Scientific Supplies & Technology website)

So, after the 3 stages, you will obtain 3 different solvents namely acetic acid, hydroxylamine hydrochloride and ammonium acetate with heavy metals dissolved in them. Analysis can be carried out respectively and the total concentration of mobile heavy metals can be obtained. These 3 stages are really time consuming and should be carried out with great patient and careful-ness because spillage is costly, you will need to repeat the whole thing if it happens at any point of the extraction. Information provided in this post can be used only as reference and it is advisable to refer to journals if you want to carry out BCR sequential extraction.