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!