EGU General Assembly 2020
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Do chemical characteristics affect the potential of biochars to adsorb cations?

Heike Knicker1, Jose Maria Garcia Castro de Barragan1,2, Elena Elena Fernandez-Boy2, Michael Knicker3, Regina León-Ovelar4, and Marta Velasco-Molina1
Heike Knicker et al.
  • 1IRNAS-CSIC, Sevilla, Spain (
  • 2Departamento de Cristalografía, Mineralogía y Química Agrícola, Universidad de Sevilla, Sevilla, Spain
  • 3Consult Michael Knicker (CMK), München, Germany
  • 4Laboratorio de Química, Universidad Nacional de Asunción, San Lorenzo, Paraguay

Since peatlands are valuable habitats and provide important environmental services, the policy of several European countries is to decrease the use of peat in potting mixtures to preserve peat bogs as nature areas. As a consequence, alternative growing media are needed. Therefore, the impact of biochar addition to gardening soil on tomato plant growth has been investigated previously (García de Castro Barragán, 2018). Those studies revealed a positive effect on seed germination and plant development during the first growing stage. However after three months, leaf discoloration was observed and associated to the lack of macro or micronutrients. It was hypothized that adsorption of nutrients onto the biochar may have decreased their availability for plants. For a first evaluation of this hypothesis, we tested the adsorption of Cu2+ to three biochars derived from feedstocks with different chemical composition, aromaticity and content of polar groups.  We produced biochar from shrimp chitin which was highly aromatic and contained considerable amounts of N-heterocyclic aromatic structures. The biochar of shells of the oil seed of Acrocomia aculeata derived from a woody feedstock with high contribution of cellulose, but had a low charring degree. The peat biochar was prepared at a pyrolysis temperature of 500°C which resulted in a highly aromatic material.  The difference in the organic matter (OM) quality of the biochars went along with differences in their pH and electrical conductivity (EC); elemental composition and ash content. Concomitantly, different specific surface areas were measured using the BET method.

For the absorption test, copper nitrate solutions were used at increasing concentration, brought into contact with the biochar for 24 hours at 25 °C. In the equilibrium solution, the Cu2+ content was analyzed. The solid biochar was separated from the solution and dried. Due to the paramagnetic nature of Cu2+, solid-state NMR relaxometry was used to identify preferential adsorption sites within the organic network of the biochars.

Our results showed low Cu2+ adsorption for all three biochars. Neither biochar porosity, nor polarity could be identified as a responsible for Cu-adsorption. As revealed by NMR relaxation times (T1H, T1C, T1rohH and T1rohC), all organic C and H groups were affected by the interactions between OM and Cu2+, although no preferential adsorption site was revealed. We found indications that adsorbed Cu2+ act as bridging agent, lowering the mobility of aromatic domains. Based on our preliminary results, we suggest that in our biochars, metals are mainly adsorbed via bonding to π-orbitals of the aromatic rings. Based on the low adsorption potential of the studied cation, we conclude further that our biochars do not sequester Cu2+ (or other metals with comparable characteristics) sufficiently strong for preventing their uptake by growing plants. However, to which extend our findings may be generalized, has to be unveiled by ongoing studies.

How to cite: Knicker, H., Garcia Castro de Barragan, J. M., Elena Fernandez-Boy, E., Knicker, M., León-Ovelar, R., and Velasco-Molina, M.: Do chemical characteristics affect the potential of biochars to adsorb cations?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3075,, 2020


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  • CC1: Comment on EGU2020-3075, Tom Sizmur, 05 May 2020

    Dear Heike,

    Thank you for answering my questions in the live chat. This is just a message to say that this really helped to inspire my research. We have found in the past that feedstock C/N ratio governs biocahr adsorption capacity of Cu and Zn and now we have lots of sorption isotherms for Pb and are starting to target feedstocks with a high N content. It seems that the N-heterocyclic compounds created during pyrolysis may be responsible for the high adsorption capacity of these biochars. 

    Because we are using Pb (diamagnetic) we won't be able to use the NMR relaxometry technique that you applied, but I guess that we can use spectroscopy to look for the N-heterocyclic compounds in the biochars that we have made with a range of feedstocks and pyrolysis conditions.

    Thanks again, and please get in touch if you would like to consider any opportunity to collaborate in the future.


    • AC1: Reply to CC1, Heike Knicker, 05 May 2020

      Hi Tom, yes to do this experiments with Pb would be difficult. However, you may use different chars with different C/N and use Cu2+ as a probe. The next think. Yes 15N NMR is possible but implies a lot of sensitivity problems. You will need a lot of measurement time.  Don`t misunderstand me, I don`t want to desencourage you.... It is possible but it will take time.

      And actually, the Cu did not preferentially bind on the N or O site (phenol 160 to 140 ppm in the spectra) but affected the whole aromatic C ring. This was the reason why I concluded the pi-ring binding.

      On the other hand,  N-functional groups may also affect this system. To be honest, I think there is still a lot to do.

      Good luck

      • CC2: Reply to AC1, Tom Sizmur, 05 May 2020


        We may use Cu as a probe, but this might not fit well will with the PhD thesis which is focused on Pb. We may look at spectroscopic approaches to see if we can identify nitrogen heterocyclic compounds and correlate this with the adsorption capacity.

        I also found a paper that impregnated feedstocks with ammonia: 

        It might be that we could select a feedstock that creates a high surface area biochar and then impregnate it with an N source to optimise the adsorption capacity.


        • AC2: Reply to CC2, Heike Knicker, 05 May 2020

          Well, if the ammonia stays and incorporates itself into the aromatic network, it may work.

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