Quantification and characterization of dissolved organic carbon from biochars

Dissolved organic carbon (DOC) in biochars is critical to carbon dynamics and contaminant transport in soils. This study aimed to develop a robust and easy method to characterize and quantify the biochar-DOC, using water-, acid-, and base-extractable DOC samples (WEOC, AEOC, and BEOC respectively) from 46 biochars produced from diverse feedstocks and pyrolysis conditions. BEOC concentrations were the highest (2.3-139 mgC/g-biochar), followed by WEOC (0.5-40 mg-C/g-biochar) and AEOC (0.2-23 mg-C/g-biochar). Fast-pyrolysis biochars generally had higher DOC concentrations than slow-pyrolysis biochars. DOC concentrations in slowpyrolysis biochars decreased exponentially with increasing pyrolysis temperature from 300 to 600 degrees C. The solidstate C-13 NMR showed that biochar-DOC had abundant small fused-ring aromatics, aliphatic C, and carboxyl C. Biochar-DOC included an acid-precipitated (AP) fraction of higher molecular weight and aromaticity and an acid-soluble (AS) fraction of lower molecular weight and aromaticity. BEOC generally had a greater AP fraction than WEOC and AEOC. Molecular weight, aromaticity and composition of AEOC and BEOC differed from those of more environmentally-relevant WEOC, suggesting that the acid- and base-extraction may not produce the DOC released in real soils. Finally, a quick, easy and robust UV-vis spectrometric method was developed to measure the composition and concentrations of WEOC in diverse biochar samples (R-2 = 0.96, n = 46).