In situ and ex situ spectroscopic monitoring of biochar's surface functional groups

•Time courses of biomass slow pyrolysis was monitored in situ by infrared spectroscopy.•Quantitative correlation was obtained to predict the degree of carbonization.•Chemical properties of biochar control its ability to stabilize heavy metals in soil.

A number of studies concluded that peak pyrolysis temperature determines essential biochar properties: surface functional group, fixed carbon content, O/C, H/C, and BET surface area. Chemical (most importantly oxygen-containing surface functionalities), rather than physical (surface area) property of biochar dictates its ability to stabilize heavy metals in soil. Labile carbon fraction of biochar is rich in heavy metal coordinating functionalities, and recalcitrant carbon fraction is necessary for the long-term stability in soil. To produce desirable biochar properties with minimal time and cost, the slow pyrolysis platform must be optimized not only for the peak temperature, but additional parameters including the residence time and heating rate. In this study, time courses of biomass slow pyrolysis were investigated in the real time using a time-resolved (0.5 s) in situ Diffuse Reflectance Infrared Fourier Transform (DRIFTs) to continuously monitor the changes in surface functionality. The greatest change in surface functionality was observed at 200–500 °C from both in situ DRIFTs (during heating up at 10 °C min−1 to reach the peak temperature) as well as ex situ (after a set residence time at the peak temperature) proximate, ultimate, attenuated total reflectance (ATR)-FTIR, and 1H NMR analyses of slow/fast pyrolysis and activated biochars. The FTIR spectral features at respective temperatures during in situ pyrolysis of lignocellulosic feedstock matched the peak temperature of post-production analysis. The FTIR peaks attributable to the oxygen-containing functional groups dramatically diminished after 20 min residence time. Quantitative relationships were observed between parameters attributable to the aromatic characteristics: (1) baseline FTIR absorbance and (2) atomic H/C ratio with the fixed carbon content of biochar.