Biochar Modification and Doping
Traditional biochar has a defined set of properties determined by feedstock and pyrolysis temperature. But those properties can be deliberately shifted by introducing metals, minerals, or chemical agents before or during pyrolysis. The institute's work on biochar modification began with a systematic investigation of how co-pyrolysis of woody feedstock with specific metals changes the resulting biochar's adsorption behaviour.
Dieguez-Alonso et al. (2018) blended biomass feedstock with iron, calcium, and other metals prior to pyrolysis and measured the impact on oxyanion sorption. The results showed that metal doping during pyrolysis — rather than post-production treatment — can fundamentally alter biochar's surface chemistry and create targeted sorption properties. The paper became a trending reference in the emerging field of biochar functionalisation, which has since grown into one of the most active areas of biochar materials science.
A related approach explored magnetic modification. Šafařík et al. (2016) produced magnetically responsive biochar by coating it with microwave-synthesised iron oxide nanoparticles. The resulting biocomposite could be separated from water using a simple permanent magnet after adsorbing contaminants — solving the practical problem of recovering fine biochar particles from liquid treatment systems. The magnetic biochar was tested as an inexpensive adsorbent for the removal of water-soluble organic dyes and demonstrated effective uptake across five xenobiotic compounds. The work opened a pathway toward magnetically recoverable biochar sorbents for water treatment, where repeated use and easy separation are prerequisites for economic viability.
A second line of investigation addressed a more practical question: what happens when wood ash — an abundant by-product of biomass energy production — is added to feedstock before pyrolysis? Grafmüller et al. (2022) showed that adding 9% wood ash to softwood feedstock increased biochar yield (dry and ash-free) by 26% and carbon conversion efficiency by 36%. Up to that concentration, the relationship was linear. The mechanism involves alkali and alkaline earth metals in the ash catalysing secondary char formation during pyrolysis, retaining carbon that would otherwise be lost as gas. The practical implication is direct: ash co-pyrolysis is an environmentally sound way to recycle nutrients while increasing the amount of atmospheric carbon locked into stable biochar — more C-sink per tonne of feedstock.
Most recently, Grafmüller et al. (2025) demonstrated that acidification of biochar with nitric acid dramatically increases its capacity to sorb and retain nitrate. At soil-relevant pH of 5, sorption capacities ranged from 700 to 6,200 mg NO₃⁻-N per kg, with higher pyrolysis temperatures (≥750°C) maximising capacity. The mechanism operates through protonation of aromatic structures in the biochar — not through surface functional groups as previously assumed. Combined HNO₃ and KNO₃ enrichment prevented full nitrate release in both aqueous suspension and soil columns, opening a pathway toward biochar-based slow-release nitrogen fertilisers that reduce leaching rather than contributing to it.
Together, these four lines of work — metal doping for targeted sorption, iron additives for magnetization, ash co-pyrolysis for carbon efficiency and nutrient recycling, and acidification for nitrate retention — represent the institute's contribution to moving biochar from a generic soil amendment toward an engineered material with application-specific properties.