The hydrogen puzzle in rock-enhanced biochar: Pyrogenic coating, mineral redox and pore accessibility

Rock-enhanced (RE-)biochars, produced by co-pyrolysis of silicate rock powder with biomass, were proposed to combine pyrogenic carbon capture and storage (PyCCS) with enhanced rock weathering (ERW) as complementary carbon dioxide removal approaches. Catalytic effects of rock-derived AAEM on pyrolysis and carbon speciation were anticipated. Here we produced six RE-biochars (10-90% rock content) by co-pyrolysis of basanite rock powder with wood at contrasting highest treatment temperatures of 450 °C and 750 °C. To provide an in-depth physico-chemical characterization of the RE-biochars, focusing on the biochar-mineral interface region, analytical methods including random reflectance, fast field cycling proton nuclear magnetic resonance relaxometry and electron energy loss spectroscopy were employed. While most treatments had limited effect on the biochar carbon yield (yc) and aromaticity of the pyrogenic carbon, RE-biochar produced at 750 °C and containing 90% rock showed an increased hydrogen to carbon molar ratio (H:Corg), which we partly explained by i) contribution of geogenic hydrogen, ii) physical adsorption of hydrogen to minerals and iii) pyrogenic coating of rock particles with secondary char containing aliphatic- and carboxylic moieties. Still, we cannot fully identify the reasons for increased hydrogen content with constant aromaticity. A decreased reflectance (Ro) and thermal stability (BC1000C) for RE-biochars with 90% rock content, despite similar aromaticity as quantified by hydropyrolysis, highlight the limitations of analytical methods developed for carbonaceous materials in the assessment of high ash biochars. Co-pyrolysis can reduce metal oxides, such as hematite (Fe-III) to magnetite, producing (potentially) reactive metals species and can accumulate geogenic potassium (K) at micron scale at rock particle surfaces. This study provides evidence that co-pyrolysis of biomass and basanite does not increase yc but alters the mineral speciation and elemental distribution at the surface of rock particles. These findings may be further exploited in the design of functionalized RE-biochars and in ERW research.