Percent non-exchangeable hydrogen as reactivity-stability proxy of biochar

The long-term stability of biochar is of great importance for its capacity to serve as a carbon dioxide removal (CDR) method. However, not all pyrolysis techniques result in thermochemically stable biochar with a large fraction of condensed carbon. Moreover, for some applications the best biochars are those made at moderate highest treatment temperatures (HTT) but with highest fractions of functional groups on the carbon surface. Methodologies to quantify the relative amount of ‘stable’ and ‘labile’ biochar in one sample are currently either complex or unprecise. At present, the bulk H/C ratio is used as an easy-to-measure proxy for thermochemical, and by inference biological reactivity/stability of biochar, where a decreasing H/C ratio reflects the loss of functional groups and subsequent aromatization and condensation upon increasing pyrolysis temperature. However, different feedstocks and pyrolysis techniques may result in a large spread of H/C irrespective of the actual carbon structures, introducing significant uncertainty.

This study investigates the potential of quantifying exchangeable (‘labile’) versus non-exchangeable (‘stable’) hydrogen as a complementary proxy for biochar reactivity/stability. Hydrogen in organic substances like biochar is either bound weakly onto functional groups forming a pool of exchangeable hydrogen (H_ex), or is directly and strongly bound to carbon atoms, constituting a non-exchangeable hydrogen pool. The relative amount of H_ex (%H_ex) has previously been found to decrease with increasing maturity of coals, together with decreasing H/C ratio, and increasing vitrinite reflectance (R₀) that reflects an increase in condensed structures. Given the similarities between biochar and coal, %H_ex may be an informative and quantitative proxy of biochar reactivity and stability in conjunction with bulk H/C. %H_ex is determined by means of dual equilibration with H₂O with two different hydrogen isotopic compositions (deuterium:hydrogen ratio, expressed as δ²H) and subsequent comparison of the two resulting δ²H values of the treated material. The analysis is performed using thermal conversion / element analysis - isotope ratio mass spectrometry (TC/EA-IRMS), a common technique in isotope-related research and applied sciences, for instance for food authentication and environmental research. The %H_ex of several series of biochars pyrolyzed with HTT between 250°C - 1000°C were compared with other data on the carbon composition of biochars, in particular the degree of aromatization and condensation. %H_ex was found to respond sensitively to ongoing aromatization in the lower half of the HTT gradient, reaching lowest values once condensation starts to become more prevalent at higher pyrolysis temperatures. The method also allows for the determination of the δ²H value of the non-exchangeable hydrogen (δ²H_n), which was found to become higher with decreasing %H_ex, most likely due to isotope fractionation during pyrolysis. %H_ex and potentially δ²H_n may thus serve as a method to determine the relative amount of reactive and stable biochar, especially in biochars made at lower pyrolysis temperatures.