ESR06: Washing activated charcoal

Washing activated charcoal by Ángel Velasco Sánchez

Activated charcoal, never heard of it before arriving to France and it was not the most pleasant discovery during my PhD.

When measuring phosphorus (P) from soil many different extractions are available. Normally, soil is shaken together with a saline solution that will extract a certain amount of P. This amount of P is then linked with different fractions of P in the soil and thus with the availability of P to the belowground living creatures. 

One of the most common P extractions is the P Olsen test. 

This test dates back to the ‘50s and consists in of an extraction with a solution of sodium bicarbonate at a 0.5 N concentration. This test is most appropriate for alkaline soils, which are very common in the Mediterranean Basin. The pH of the solution with bicarbonate is adjusted to 8.5. 

Because of this, Olsen P test is the rule in countries like Spain, Italy, Greece or France. Yet, this is not restricted to these countries, and it is common to see Olsen P tests in other European countries.

Sterling Robertson Olsen (1916-2008)

After the short extraction with bicarbonate (30 minutes) it one is likely to encounter one of the biggest drawbacks, in my opinion,  of the Olsen P test. 

When the soils used are rich in soil organic matter, no matter how well you filter it nor ultra-centrifugate it, the supernatant is colored. This yellowish to brownish coloration can be a real issue when colorimetric methods are used to determine the amount of P in the extract. It becomes even aan even greater problem when the quantities of P are really small like the case of some soils. This is no longer an issue if other methods or equipment are used such as ICP. Yet, this is not an option in many laboratories and was certainly it was not my optionnot an option available for me.

Colorimetric methods for measuring P are also very wide-spread and are used in laboratories routinely. From what I know, there are two different approaches: the ascorbic acid molybdenum blue coloration and the malachite green one. I consider that the ascorbic acid molybdenum blue method is more common than the malachite green. 

There are many variations for the ascorbic blue coloration method, but I would say that the most famous one is the one by Murphy and Riley back in the 60s. Since then, many modifications were made. The protocol that I used in my experiments is based on the molybdenum ascorbic acid blue coloration and it is a normalized protocol used in my lab for many years.

But, getting back to the topic, how do we ensure this coloration does not impact our readings in the spectrophotometer? 

Well, I don’t have the perfect answer but in my lab – and I guess in many others – activated charcoal is used. Activated charcoal is a porous material with a very very high surface area that can efficiently bind to all these yellowish impurities and produce a crystal-clear solution. This is achieved by adding a small amount of activated charcoal to your soil sample. I tested it and indeed, the differences are huge:, activated charcoal is really efficient in removing these interferences.

Washing activated charcoal

I was a very happy PhD candidate using my activated charcoal until I had to prepare the activated charcoal myself – yes activated charcoal is not sold already prepared -. 

In the protocol I was using there was a short description in French on how to make your charcoal ready for P-Olsen determination. The only information I had was that I had to wash it twice with sodium bicarbonate and then rinse it with water until reaching a pH of 7 – the initial pH of the charcoal in a solution with water is around 9 – . 

Sounds easy right? 

But it took me around 3 weeks of rinsing just 100 grams of charcoal to reach a pH of around 7. 

As you can imagine this process was nothing anything but peaceful. Activated charcoal when dissolved in water is very dirty. It makes everything dirty, everything it touches becomes impregnated in black. And when it is dry it is extremely light so even the atmosphere of the room you work in looks darker. My lab coat at some point was more suitable for repairing cars than for working in a laboratory. People even started to joke around calling me Ángel the Black in a clear reference to “Gandalf the gray” and his dark coat.     

Small leakages can turn your laboratory into a dark hell.

But finally, I got it:, I had some grams of perfect charcoal at neutral pH. 

Unfortunately, when I started measuring, I discovered that my charcoal still contained like about ten times more P than my soil samples. This contamination obviously made my samples unreadable and around of 3 weeks of washing time went to the sink. Did I mentioned that my samples were fresh, and I had no chance to repeat the measurement? Anyway, I took a deep breath and I said to myself: you will learn from this error, let’s be imaginative.

Here is when the fun started. 

I started to look and try ways to increase the speed of reducing the pH of the charcoal. And of course, I got rid of the charcoal that contained such a high concentration of P. In fact, if you pretend to use charcoal for P determination the very first thing you should do is to ensure the concentration of P in the starting material is low, otherwise you will not be able to make it. From now on I used another one that came in particles of a larger size that I grounded myself into dust. 

Turning charcoal into dust!!!

The first approach I triedy was the common pump assisted filtration in a funnel with some paper filter. I used two paper filters to avoid losing as much material as possible. At some point I was using 4 pumps at the same time, and it is not ideal. 

The speed of filtering is extremely low and if your material is very fine – like my first charcoal – it will easily clog the paper filter. Also, if you use too much pressure in your pump the paper filter may break causing spills in your lab and losing all the material and time. Don’t forget that the material makes everything dirty! Finally, you will need to be constantly checking the pumps, which is not great.

Pump-assisted filtration

Another method I tried was using ultracentrifugation to separate the charcoal from the water. This method is also not ideal because if the material is very light it will never be separated from the water. You will lose a lot of material in the process and more importantly, time, since you will be forced to be checking the centrifuge machine constantly. 

Then, I thought of the straightforward answer: What if we add some acid to the water to make the washing faster? 

Well, it also did not work. 

What I obtained after doing this was an incredibly fast drop of the solution pH – it went to levels below 1 -. Also, all the charcoal precipitated at the bottom of the beaker leaving the acid solution at the top. At this point, you may have noticed that I am not a chemist, because of it one of my biggest fears was to change the properties of the charcoal to become a sink for P. I still don’t know if that happens when you add an acid, but I am sure adding acids to it de-naturalize the material. So, if there is some chemist reading this, please share with me some ideas about what could have happened. Unfortunately I have no photo for this method.


I also read on the internet some comments in ResearchGate asking this question. Someone recommended to use a column to wash constantly the charcoal. I did not have any of these columns in my laboratory, but me and another colleague and myself borrowed one from another laboratory and we tried it out. 

This way was promising, it reduced the pH quickly and without altering the nature of the material. Yet it has some drawbacks: you must be constantly checking the water flow in the column – if you use a rudimentary system like we did – and removing the charcoal from inside the column can be tricky and dirty. We used some cotton to filtrate it and we added water from the top of the column with a small hose.

Column for constant charcoal washing

The last method I tried and, in my opinion, the more recommendable is also the simplest one. 

First, you will need to verify that your tap water is not a source of P. We don´t want the precious sample to beto contaminated ourselves the precious sample. Then, simply use a very big container. I used a large bucket of around 25 liters, but any large container will do the trick. It is important to clean it very well since the ccontainer might contain some soluble compounds that could also cause contamination. 

After, just add a small amount of charcoal – I used no more than 100 grams – and fill the bucket with water. Let it sit there for the night or even for 24 h. After that, almost all the charcoal should be at the bottom. You should repeat this process until you reach the pH of 7. Yet, it did not take me more than 3-4 days. 

In the meantime, you can be doing other things in the lab, and it is not a dirty process since you can keep the bucket right next to a sink. Also, the more you clean it the less time it will take for the charcoal to settle at the bottom. Then, when the pH is close to 7 you can pour the wet charcoal into a beaker and place it at 105 C to evaporate the water, this should not take longer than a day. If you can´t collect the charcoal dry enough you may filter it before drying it.

The bucket I used to wash my charcoal, or as I like to call it Charcoal Cleaner Device.

To conclude, I would say that this is my personal experience and perhaps it is not the ideal situation for everyone. Perhaps some readers found better and faster ways to clean charcoal. I believe I have learned many things about this product in the previous months but still I am not a chemist. What I can strongly recommend is to use charcoal only in very specific situations when you have no other alternative. 

A little yellowish coloration won’´t cause such a great disturbance in the reading! In fact, activated charcoal will always be a source of contamination no matter how well you clean it. A simple look in the available scientific literature will show you that it is very uncommon to use charcoal to reduce the color interference for P Olsen measurements. Moreover, the interferences of soil organic matter´s coloration at >800 nm wavelength should be very small. 

Also, you could use other chemicals such as polyacrylamide that efficiently removes the coloration, I did not use it because of the high toxicity. Or if your soil gives very high background colors, try to stay away from colorimetric methods when possible!


I hope my experience helps someone and it would be veryI would very much welcome it if you could share with me your experiences and tricks in charcoal washing.


Ángel Velasco Sánchez

PhD researcher 


Wageningen University

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