Influence of Carbonization Methods on the Aromaticity of Pyrogenic Dissolved Organic Carbon

Dissolved organic carbon (DOC) components of soil amendments, such as biochar, will influence the fundamental soil chemistry, including the metal speciation, nutrient availability, and microbial activity. Quantitative correlation is necessary between (i) pyrogenic DOC components of varying aromaticity and ionizable (carboxyl and hydroxyl) substituents and (ii) bulk and solution properties of biochars. This study employed fluorescence excitation–emission (EEM) spectrophotometry with parallel factor analysis (PARAFAC) to understand the influence of the pyrolysis platform (flash and high-yield carbonization, slow pyrolysis, and fast pyrolysis) and solution pH on the DOC structure of carbonaceous materials. The PARAFAC fingerprint representative of conjugated, polyaromatic DOC correlated (Pearson’s r ≥ 0.6; p < 0.005) with (i) volatile matter content and (ii) total organic carbon and nitrogen concentrations in water and base (50–100 mM NaOH) extracts. Electric conductivity of the extracts correlated with S (indicative of labile sulfate species) and Na + K concentrations ( r > 0.9; p < 0.0005). The pH-dependent changes in fluorescence peak position and intensity suggested (i) protonation of carboxylate/phenolic functionalities and (ii) acid-induced aggregation of colloidal particles for ≤350 °C slow-pyrolysis biochars; DOC of high-yield/flash carbonization charcoals and ≥500 °C slow-pyrolysis biochars were less sensitive to pH. Solid-state 13 C cross-polarization and magic angle spinning nuclear magnetic resonance analysis of bulk aromaticity (−C═C– peak at 110–160 ppm) suggested that both recalcitrant and labile fluorescence DOC fingerprints are composed of polyaromatic structures that begin to form near 350 °C. These biochar-borne DOC components of varying aromaticity and carboxyl substituents will participate in hydrophobic and hydrogen-bonding interactions with soil components that will ultimately impact the biogeochemical cycles.