The chemistry and nature of biochars are still far from being well understood. In the present work, solid-state 2D HETCOR 1H-13C NMR spectroscopy is introduced for an improved characterization of the aromatic network in biochars. To that end, a pyrochar obtained from the pyrolysis of cellulose at 350 °C for 1 h was used as an example. Variation of the contact time during cross polarization from 50 µs, to 200 µs and 1000 µs gave information about the protonation degree of the different C groups and their interactions. We demonstrated that carbohydrates did not survive the used pyrolysis conditions. Therefore, O-alkyl C was assigned to ethers. Phenols were not identified to a higher extent suggesting that furan and benzofuran-type units determine the O-functionality of the aromatic domains. The latter are directly connected to alkyl chains. Those features are expected to affect chemical but also physical properties of the biochar. Based on our results, we developed a new concept describing the nature of the aromatic network in the studied cellulose-based pyrochars. The latter contrasts common views about the chemical nature of biochar, possibly because pyrolysis temperatures > 350 °C are required for achieving advanced condensation of the aromatic domains.
<p>Biochar has become an accepted soil amendment due to its potential to improve soil properties and as a tool to increase carbon sequestration. The latter is based on its relatively high biochemical recalcitrance augmenting the slow C pool after its addition to soils. However, newer studies indicated that the longevity of biochar and naturally produced pyrogenic organic matter (PyOM) in soils is lower than commonly assumed. Many of those studies are based on the determination of CO2 production changes or on the recovery of their isotopic labels in the soil after amendment of biochar or PyC incorporation. Most probably because of the lack of appropriate techniques to differentiate between the natural soil organic matter fraction and the added black carbon, only few reports are available which relate turn-over data with chemical alterations of biochar during aging or the impact of the latter on the quality of the total SOM pool. &#160;In order to fill this gap, we applied virtual fractionation of SOM into different organic matter pools by different solid-state NMR techniques. Whereas the most common combines the determination of turnover rates via stable isotope techniques, an alternative approach takes advantage of different relaxation behavior of biochar and humified SOM. In both cases spectra can be calculated that show either the added biochar or the respective SOM.&#160; In the frame of the present work, the concept and the potential of the two approaches will be explained by using examples studied in our laboratory. &#160;With this, we intend to provide a further powerful tool which can lead to a better understanding of the biochemistry related to the transformation of PyC and biochar during aging and their subsequent integration into the soil organic matter fraction.</p><p>&#160;</p><p>Acknowledgement: Financial support has been provided by the European Institute of Innovation and Technology (EIT), a body of the European Union, under Horizon2020, the EU Framework Programme for Research and Innovation (Project 21217 Black to the future - biochar and compost as soil amendment)</p>
<p>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 (Garc&#237;a de Castro Barrag&#225;n, 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 Cu<sup>2+ </sup>to three biochars derived from feedstocks with different chemical composition, aromaticity and content of polar groups. &#160;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&#176;C which resulted in a highly aromatic material.&#160; 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.</p> <p>For the absorption test, copper nitrate solutions were used at increasing concentration, brought into contact with the biochar for 24 hours at 25 &#176;C. In the equilibrium solution, the Cu<sup>2+ </sup>content was analyzed. The solid biochar was separated from the solution and dried. Due to the paramagnetic nature of Cu<sup>2+</sup>, solid-state NMR relaxometry was used to identify preferential adsorption sites within the organic network of the biochars.</p> <p>Our results showed low Cu<sup>2+ </sup>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 (T<sub>1H, </sub>T<sub>1C</sub>, T<sub>1rohH</sub> and T<sub>1rohC</sub>), all organic C and H groups were affected by the interactions between OM and Cu<sup>2+</sup>, although no preferential adsorption site was revealed. We found indications that adsorbed Cu<sup>2+</sup> 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 &#960;-orbitals of the aromatic rings. Based on the low adsorption potential of the studied cation, we conclude further that our biochars do not sequester Cu<sup>2+</sup> (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.</p>
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