The reason for high cation exchange capacity on biochars is twofold: Firstly, there is an increase in the surface area after pyrolysis. Secondly there is an increase in charge density on the surface. Generally, the higher the pyrolysis temperature the higher the biochar surface area. This high surface area is in the form of micro/nanopores as you correctly suggest.

However, it is not true to say that the ions (e.g. NH4+) are tightly trapped so that they cannot be flushed out. For the ammonium ethanoate method of measuring CEC to work the NH4+ ions must be flushed out with NaCl.If the ions are not exchanged with Na+ then they are not included in the CEC measurement. I see no reason why a physical trapping of ions that prevents flushing with alcohol or water would allow flushing with NaCl. Therefore, any elevated CEC measurement on a biochar must be due to adsorption on the biochar surface.

Biochars produced at low pyrolysis temperatures have a high number of oxygenated functional groups (carboxyl, hydroxyl, phenol etc) and dorption primarily occurs by chemisorption. However, biochars produced at higher temperatures have a higher C:O ratio and a higher degree of aromaticity. Sorption of metal cations to high temp biochars occurs primarily due to electrostatic physisorption (carion-pi interactions).

For further reading see:

Gomez-Eyles, J., L. Beesley, E. Moreno-Jiménez, U. Ghosh and T. Sizmur. 2013. The potential of biochar amendments to remediate contaminated soils. In: N. Ladygina and F. Rineau, editors, Biochar and Soil Biota. CRC Press, Boca Raton, FL

Dr Nguyen Hue, do you have another suggestion about the probable source of CEC. Indeed, the exchange process of biochar is a adsorption process, then generally, the surface, oxidized carboxylate and phenolate groups may be suggested as the probable source of CEC. Unless, this process is not a adsorption process.

I suspect that common CEC determination methods that are used for soils may not be appropriate for biochar or biochar-amended soils. That is because biochars, especially those derived from wood, have many micro/nanopores that can tightly trap ions (e.g., NH4+) and water or alcohol cannot flush them out. This is a physical adsorption as opposed to electrostatic attraction or chemi-sortion.

Yes, more micropores were obtained from biochar. We had published paper 'Biomass-based pyrolytic polygeneration system on cotton stalk pyrolysis: influence of temperature.' , and the result shown that the major pore of biochar is mciropores whose pore center is about 0.5-1.2 nm. I am not a research on the soil, but I am interest on the influence of biochar on soil. In your suspection, that the micropores can tightly trap ions and water or alcohol cannot flush them out, need be investigated. Do you have plan on this work?

The reason for high cation exchange capacity on biochars is twofold: Firstly, there is an increase in the surface area after pyrolysis. Secondly there is an increase in charge density on the surface. Generally, the higher the pyrolysis temperature the higher the biochar surface area. This high surface area is in the form of micro/nanopores as you correctly suggest.

However, it is not true to say that the ions (e.g. NH4+) are tightly trapped so that they cannot be flushed out. For the ammonium ethanoate method of measuring CEC to work the NH4+ ions must be flushed out with NaCl.If the ions are not exchanged with Na+ then they are not included in the CEC measurement. I see no reason why a physical trapping of ions that prevents flushing with alcohol or water would allow flushing with NaCl. Therefore, any elevated CEC measurement on a biochar must be due to adsorption on the biochar surface.

Biochars produced at low pyrolysis temperatures have a high number of oxygenated functional groups (carboxyl, hydroxyl, phenol etc) and dorption primarily occurs by chemisorption. However, biochars produced at higher temperatures have a higher C:O ratio and a higher degree of aromaticity. Sorption of metal cations to high temp biochars occurs primarily due to electrostatic physisorption (carion-pi interactions).

For further reading see:

Gomez-Eyles, J., L. Beesley, E. Moreno-Jiménez, U. Ghosh and T. Sizmur. 2013. The potential of biochar amendments to remediate contaminated soils. In: N. Ladygina and F. Rineau, editors, Biochar and Soil Biota. CRC Press, Boca Raton, FL

Dear Folks,

Very interesting discussion. First I think that many publications shows that not all biochar has high CEC. In some Biochar forum  functionaization of biochar is a hot topic. Functionalization here is meaning - creating CEC in them (by boiling with acid, enhance temperature). In my experiments using the traditional making char (wood chared at 400 C). It doesnt have high CEC what seens to create by aging.  That is a high diferrence of Cpyr from the old Amazonian Dark Earths that have seens to have a very large densisty of charges very resistant to degradation and new biochar (at least of many of them).

Biochar made at low temperatures (slow pyrolysis) also can bring a considerable amount of "not-drained-tar" which can carry ionic charges.

Couldn't the difference in CEC between pristine and ageing biochar be a result of the development of biofilm on the large internal surface of biochar? That could be tested by measuring the changes in CEC when disturbing the biological life of the surface.

His biochar quality determined by feed: soft wood vs hard wood?

I use Robinia trees for upgrading soil and control erosion (nitrogen fixation), fast growing and coppice.

I use Robinia as fire wood. It produce a lot of charcoal of very good quality, if quench. Robinia his harder than Red Oak!! At the end the flame is bleu (CO), if left burning (high temperature).

Biochar could be used to sequestrate Carbone (long persistence). Ex. Use as an amendment in potting soil, etc.…

Please suggest if there would be any other method to measure CEC of biochar besides NH4+  ions detection by AAS or ICP...??? Is it possible to measure CEC by some spectroscopic method or other method......???

Good morning. I am not an expert in soil science but I work on biochar from the production and process optimization point of view and I have also tried many feedstocks and reactor conditions in order to achieve different biochar qualities. Is the CEC maybe  relevant to the feedstock as well?  I mean, would there be some influence of the salts initially present on the biochar on its CEC value compared to a biochar made from "clean" wood?  I may be totally wrong here though.

Dear Lydia:

You may have a point here. Biochar made from manure or biosolids may contain bases such as CaO, CaCO3, K2O, Na2O, etc. If proper precautions are not taken to eliminate these bases, CEC measurement would be in error.

Nguyen, any chance you've run an experiment to look at your hypothesis concerning physical adsorption versus electrostatic attraction of ammonium ions?  I don't know that I'd say what you're proposing is impossible, but I wonder how likely it is.  The reason is that I would speculate most physical based adsorption in biochar is going to be based on hydrophobic domains on the adsorbate.  I would think most ions don't have these domains.

Other than functional groups conservation in low temperature biochar mentioned by Tom and Claudia, oxidation of surfaces during production and afterwards is a source of CEC.

This have been shown by the flame curtain pyrolysis reactors that produce hi CEC biochar while working at hi temps (650-750°C), by biochar incubation studies that incubated biochar at 30 and 70 °C for a few months and by characterization studies of historical charcoal sites.

A third mechanism is the adsorption of organic molecules in the biochar surfaces, this have been shown in composting studies in which biochar was included in the composting process.

As mentioned, the presence of ash (salts) in biochar also has an effect on CEC. This was shown in flame curtain pyrolysis experiments where they took CEC measurements before and after washing the biochar.

Finally, microbial colonization is bound to have an effect as Folke mentioned. Microbial colonization may also help the exchange of nutrients deep inside biochar particles micropores that plants would not be able to access otherwise.