This session welcomes contributions leveraging cutting-edge approaches making use of expertise in organic petrography techniques to elucidate biochar’s role as a mean of carbon storage and a tool for sustainable development. The objective is to draw from the legacy of organic carbon characterization to create a robust backbone for the understanding of biochar’s formation mechanisms, properties and implications for innovative applications. By integrating different analytical methods through the lenses of organic petrography, we aim to foster deeper insight into biochar dynamics advancing its use according to a scientific approach.
Therefore, our goal is to provide a comprehensive and up-to-date toolbox for biochar characterization in geosciences through contributions concerning:
Innovative multi-proxy biochar characterization
Effects and optimization of biochar properties
Novel, data driven, insights on biochar’s stability and permanence for carbon storage
The pilot-scale furnace experiment was designed to assess the influence of biocoke on ferromanganese production processes. The investigation focused on the transformation behaviour of organic carbonaceous materials within a pilot ferroalloy furnace, employing a comprehensive, multi-analytical approach that included micro-computed tomography (μCT), micro-Raman spectroscopy, and organic petrology. The results indicate that the degradation pathways of biocoke and its biogenic component (biochar) differ markedly across individual furnace zones and are strongly governed by temperature gradients as well as the material’s position within the charge bed.
Distinct signs of partial graphitisation were detected in both the conventional coke matrix and the biochar fraction, as evidenced by Raman spectroscopic analyses. While biochar is generally classified as a non-graphitizable carbon, localised development of semi-graphitic structures was identified, suggesting that catalytic graphitisation may occur under specific furnace conditions. This transformation is most likely promoted by the presence of molten and/or vaporised transition metals—particularly iron and manganese—at temperatures exceeding 1500 °C, which may act as effective catalysts for the reorganisation of amorphous carbon into more ordered, graphite-like structures.
Catalytic graphitisation proceeds via interaction between amorphous carbon and metallic nanoparticles, leading to the formation of a metastable carbide phase that subsequently decomposes into graphitic carbon. In contrast to conventional graphitisation routes, this mechanism represents a single-step process that does not require a distinct carbonisation stage. The migration of metal vapours and molten droplets through the porous charge bed likely generates localised microenvironments that are especially favourable for such transformations, particularly in the lower regions of the furnace.
Complementary Raman analyses, including comparisons with established reference materials, confirmed the anisotropic and graphitic nature of selected carbon domains. Importantly, organic petrology plays a key role in the assessment of the biochar graphitisation process, enabling the identification, spatial characterisation, and textural interpretation of carbon structural evolution at the microscale.
These observations not only advance the understanding of the thermal and chemical behaviour of biogenic carbon in metallurgical systems, but also highlight a promising pathway for enhancing carbon structure through in-situ catalytic mechanisms. Research on efficient catalytic graphitisation of biochar, together with its systematic evaluation using petrographic techniques, is currently being actively conducted and further developed at ITPE (Institute of Energy and Fuel Processing Technology).
Fig. 1 The exemplary photomicrograph presenting a locally graphitized biochar particle (polarized light + Lambda plate,oil immersion, x500 mag.)
How to cite: Wojtaszek-Kalaitzidi, M., Rejdak, M., Książek, M., and Larsen, S. Y.: From Amorphous to Semi-Graphitic Carbon: Catalytic Transformations of Biochar in a Pilot Ferroalloy Furnace, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7137, https://doi.org/10.5194/egusphere-egu26-7137, 2026.
As biochar gains momentum as a carbon dioxide removal (CDR) strategy, robust, reproducible, and comparable stability metrics are increasingly needed. A growing number of novel biochar stability assessment methods are being adopted by the carbon crediting industry, however, comparative assessments between these novel approaches and with more established standards, such as molar H:C and O:C ratios, remain limited. Furthermore, there is ongoing dissensus among researchers and producers regarding which proxies most reliably capture long-term biochar stability.
This study aims to evaluate the relationships, strengths, and limitations of multiple novel and traditional biochar stability proxies. Biochars were produced from two common feedstocks, barley straw and chestnut wood, pyrolyzed at 400, 600, and 700 °C under an inert N₂ atmosphere. Biotic incubations were conducted using a soil microbial inoculum to quantify mineralizable carbon using gas chromatography. Borrowing from organic petrography, random reflectance microscopy was conducted as a measure of thermal maturity and Rock-Eval 6 was used to determine thermal stability and organic matter transformation. Lastly, chemical oxidation methods were explored as rapid proxies for biochar reactivity, including comparison of different oxidizing agents: bleach, nitric acid, and hydrogen peroxide (Edinburgh stability tool). Elemental analysis was used to calculate H:C and O:C molar ratios in order to benchmark results against established stability criteria.
Correlation and statistical analysis are used to explore how these techniques relate to each other, as well as the influence of feedstock and production temperature. Preliminary results suggest that while many stability metrics correlate well, some exhibit greater reproducibility across replicates and others demonstrated more sensitivity to methodological or user-related variability. The comparison highlights the importance of method standardization, particularly for emerging stability assessment techniques such as reflectance microscopy. Overall, the results provide insight into how stability metrics align and diverge, informing biochar producers and researchers on selecting tools or multi-proxy approaches to accurately evaluate biochar stability.
How to cite: McCall, M., Hennissen, J., Vane, C., and Sephton, M.: Comparative assessment of novel and traditional biochar stability proxies, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7696, https://doi.org/10.5194/egusphere-egu26-7696, 2026.
Biochar is a porous, carbonaceous material derived from biomass pyrolysis with diverse applications. It holds significant potential in the agricultural sector (as a soil conditioner or fertilizer carrier), for environmental purposes (e.g., carbon sequestration), and industrial applications (e.g., plastics, paper and textile industries). However, its effective application depends on a clear understanding of biochar's long-term stability and the application of reliable methods to assess its durability and reactivity.
This study aims to characterize the microscopic properties of ground coffee-derived biochar using incident white light microscopy. Furthermore, it investigates the correlations between reflectance, surface area measurements, and elemental composition. A total of 16 samples were examined, including one sample of raw ground coffee and 15 coffee biochar samples produced at three pyrolysis temperatures (300, 600, and 850 °C) and five residence times (1, 3, 6, 12, and 24 hours) at laboratory conditions. For all samples, proximate and ultimate analyses were conducted. Reflectance and surface area measurements were determined for a comprehensive characterization.
For the examined coffee biochars, the random reflectance increased with rising pyrolysis temperature and residence time. Photomicrographs of coffee biochars pyrolyzed at 300 °C, 600 °C and 850 °C illustrate a color change. At 300 °C for 1, 3, and 6 hours, the biochar appears predominantly grey, indicating partial pyrolysis. With extended pyrolysis at 300 °C for 12 hours, as well as at higher temperatures of 600 °C and 850 °C, the biochar exhibits a bright white coloration. The results demonstrate a strong correlation between coffee biochar reflectance and both pyrolysis temperature and residence time, as reasonably expected. The coffee biochar samples, which formed at temperatures of 600 °C and 850 °C, exhibit Ro ranges (2.58 – 5.10%) well above the inertinite benchmark (IBR 2%). BET analysis reveals that specific surface area values range from 0.07 to 1020 m²/g, micropore area from 3 to 641 m²/g, and total pore volume from 0.0008 to 0.56 mL/g, all positively correlated with increasing pyrolysis temperature and residence time. Furthermore, the carbon and oxygen contents of the samples exhibit expected trends, characterized by an increase in carbon (C) and a decrease in oxygen (O).
Coffee biochars exhibit higher reflectance, surface area, and carbon content with increasing pyrolysis temperature and residence time, as expected, indicating greater stability, microporosity, and overall quality. These characteristics support their potential use in environmental applications. Finally, this study also demonstrates that reflectance measurement is the most qualified quality parameter.
How to cite: Kalaitzidis, S., Georgaki, M., Wojtaszek-Kalaitzidi, M., Vakros, J., and Manariotis, I.: Impact of pyrolysis temperature and residence time on biochar reflectance: A case study of ground coffee biochar, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14241, https://doi.org/10.5194/egusphere-egu26-14241, 2026.
Biochar is a carbon-rich residue of biomass pyrolysis or pyro-gasification that mimics natural coal macerals, transforming biomass into inertinite-like carbon for long-term storage. To directly assess nine commercial biochar’s stability, this study applies optical microscopies (organic petrography and reflectance measurements) combined with micro-Raman spectroscopy. These techniques, commonly used to evaluate the thermal maturity of geological organic matter, are here adopted to identify differences in degree of carbonization. Bulk chemical indicators, especially the H/C molar ratio, are also included for comparison. While molar ratios remain a useful proxy of overall biochar stability, spectroscopic and petrographic results provided the necessary resolution for study complex heterogeneous biochar.
Biochar reflectance (BCRo) emerged as key indicators of carbonization uniformity. Unimodal, narrow BCRo distributions reflect homogeneous thermal degradation, whereas bimodal or skewed patterns identify incomplete or uneven carbonization linked to inefficient heat transfer, short residence times and/or heterogeneous biomass traits. Using the value of 2% as inertinite benchmark (IBRo2%), reflectance data effectively discriminated incompletely carbonized domains from fully stabilized aromatic structures. Furthermore, Raman spectra showed systematic evolution of the D1 and G bands, with D1-G separation, intensity (iD1/iG), area (aD1/aG), and width (wD1/wG) ratios increasing with BCRo. These parameters defined two carbonization stages across the dataset. Biochar with BCRo > 3% show inertinite-like signatures consistent with high thermal maturity. In contrast, samples dominated by low-reflectance fractions (BCRo < 2%) are characterized by Raman spectral features typical of poorly carbonized, labile material.
By integrating micro-Raman spectroscopy with reflectance measurements, this study introduces a set of rapid diagnostic parameters for evaluating the carbonization efficiency and long-term stability of commercial biochar. The approach enables rapid discrimination between poorly carbonized and fully inertinite-like materials, offering practical benchmarks for CDR applications and for optimizing conditions in real production scenarios. For geoscientists, this multi-proxy framework provides a comprehensive guideline to characterize biochar’s formation, properties, and long-term stability, aligning its evaluation with established concepts of inert organic carbon in the geological record.
How to cite: Mitillo, N., Animali, L., Mattei, M., and Corrado, S.: Guidelines for assessment of biochar’s stability through Organic Petrography and micro-Raman Spectroscopy, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17932, https://doi.org/10.5194/egusphere-egu26-17932, 2026.
Posters virtual: Mon, 4 May, 14:00–18:00 | vPoster spot 2
EGU26-10788 | ECS | Posters virtual | VPS16
Sewage sludge-derived biochar as a circular “waste-to-resource” strategy for wastewater treatmentMon, 04 May, 14:00–14:03 (CEST) vPoster spot 2
The increasing production of wastewater and sewage sludge (SS) is a major environmental challenge of the 21st century, while the need for sustainable waste management and resource recovery drives the development of innovative technologies for sludge utilization. The thermochemical conversion of SS through pyrolysis to biochar (BC) is a promising “waste-to-resource” strategy, as it allows both the reduction of sludge volume and the production of a functional, value-added material.
This study aims to examine sewage sludge-derived biochar (BCxSS), focusing on its characterization methods, the effect of pyrolysis conditions on its physicochemical properties, and its practical applications in water and wastewater treatment. By applying a structured PRISMA-based methodology, 170 studies concerning the production, modification, and environmental utilization of BCxSS were studied. The results showed that pyrolysis conditions, and particularly pyrolysis temperature, have a major influence on the properties of the BC, such as yield, ash content, pH, specific surface area, porous structure, and surface functional groups. Furthermore, BCxSS effectively removes heavy metals, dyes, phenolic compounds, and emerging organic micropollutants, such as pharmaceuticals and antibiotics. These removals occur through mechanisms such as physical adsorption, ion exchange, surface complexation, and π-π interactions. BCxSS is also attracting attention as a precursor for catalysts capable of degrading persistent pollutants. Despite these advances, the application of BCxSS for the adsorption and inactivation of pathogenic microorganisms and antibiotic resistance genes remains limited, revealing a critical research gap. Understanding BC-microorganism interactions is vital, given the significant public and environmental health risks posed by waterborne pathogens.
Overall, BCxSS provides a tangible example of circular economy in practice, transforming wastewater treatment byproducts into valuable resources that reduce waste and mitigate pollution.
How to cite: Gkogkou, E. V., Isari, E. A., Grilla, E., Manariotis, I. D., Kalavrouziotis, I. K., and Kokkinos, P.: Sewage sludge-derived biochar as a circular “waste-to-resource” strategy for wastewater treatment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10788, https://doi.org/10.5194/egusphere-egu26-10788, 2026.
EGU26-19240 | Posters virtual | VPS16
Advanced descriptive statistics of random reflectance measurements on plant-based biochars-do they even matter?Mon, 04 May, 14:03–14:06 (CEST) vPoster spot 2
Numerous recent publications have demonstrated the relationship between random reflectance and the proportion of the fully carbonized fraction (equivalent to the fusinite maceral) contained in a biochar sample. These findings have motivated international frameworks and independent carbon registries to consider random reflectance among the core analytical proxies required to assess biochar permanence in soil. However, skepticism for the application of the proxy still persists, with main challenges revolving around the aspects of data acquisition and data interpretation. This is mostly attributed to the fact that the methods applied in the microscopic study of biochar were originally developed and standardized for the study of coal, where the calculation of the average and standard deviation of a 100 measurements on collotelinite, accompanied by a histogram showing the frequency distribution of the measured values, is often enough to describe and report this optical property.
Even though biochar samples are petrographically much simpler than coal and other sedimentary rocks, they have peculiarities that require a more careful consideration when applying standard petrographic techniques for their study. Biochar manufacturing conditions play a major role in the extent of the carbonization degree of the feedstock and this in turn has an impact on the heterogeneity of the formed biochar grains often resulting in complex distributions of the reflectance values, not always accurately captured in the basic descriptive statistics (mean and standard deviation, etc.), particularly in the case of polymodal distributions. For this reason a higher number of measurements, ranging between 300-500, on fields of view selected along a regular grid, is required to acquire meaningful average and standard deviation values, as opposed to coal samples, where a “run of the sample” on parallel traverses where collotelinite occurs is a common practice.
Advanced descriptive statistics have long been successfully used for the evaluation of grain size analysis of clastic sedimentary rocks for the assessment of reservoir properties and depositional environment. This study attempts to investigate the frequency and probability distributions and derived advanced descriptive statistics of random reflectance measurements acquired from 50 plant-based biochar samples, in order to characterize their heterogeneity with regards to the proportion of the fully carbonized fraction. The calculated advanced descriptive statistics include the coefficient of variation and confidence intervals, measures of central tendency (median and mode), measures of dispersion (variance and interquartile range), shape parameters (skewness and kurtosis), and probability-related measures (probability density function, cumulative distribution function, and percentiles, particularly those associated with the established “inertinite benchmark”-IBRo2). In addition to those, the study attempts a comparison between the IBRo fractions determined by the measurement of 3-4 points per field of view, against those determined by just measuring the point located at the crosshair of each field of view, together with the convergence of the acquired set of measurements to the mean and median of each sample. Findings are expected to contribute to a mathematically more robust characterization of the acquired datasets, providing greater rigor in how this data can be utilized with regards to the assessment of biochar carbon permanence.
How to cite: Siavalas, G., Alami Sounni, K., and Camps Arbestain, M.: Advanced descriptive statistics of random reflectance measurements on plant-based biochars-do they even matter?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19240, https://doi.org/10.5194/egusphere-egu26-19240, 2026.
