Alkali-modified biochar as a sustainable adsorbent for the low-temperature uptake of nitric oxide

Nitric oxide (NO) as the major constituent of nitrogen oxides (NOx), which is mainly emitted from combustion processes, is responsible for several environmental and health issues, and the control of its emission is a global concern. In this regard, this study investigates low-temperature oxidative uptake of NO on alkali-modified biochar. Rubber seed shell was utilized as a locally available lignocellulosic waste to develop biochar through slow pyrolysis. The porosity attributes and surface chemistry of the developed biochar were then improved through alkali modification. The NO capture capacity of KOH, NaOH, K2CO3 and Na2CO3-activated biochars was 63.0, 59.3, 59.0 and 58.5 mg/g, respectively, which was considerably higher than that of the pristine biochar, 17.8 mg/g. This considerable improvement was mainly attributed to the introduction of more oxygen functionalities, improvement of microporosity and creation of more carbon defects in the biochar, after alkali modification. With some investigations on the alkali agent/biochar impregnation ratio, the adsorption capacity of the KOH-activated biochar (at a ratio of 2:1 (w/w)) could be improved to 87.0 mg/g at 30 degrees C. Characterization studies using FTIR and XPS suggested that the adsorbed species on the biochar surface was in the form of monodentate nitrito (-O-N=O), which was formed from the oxidation of NO on the surface-oxidized groups. Kinetic studies using pseudo-first, pseudo-second and Elovich models indicated that the adsorption of NO on KOH-activated biochar surface was mainly controlled by a physisorption or a weak chemisorption process, with a low activation energy of - 29 kJ/mol.

Automated summary

This study investigated the low-temperature oxidative uptake of NO on alkali-modified biochar, showing that the NO capture capacity was significantly improved after modification. The adsorption capacity of KOH-activated biochar reached 87.0 mg/g at 30 degrees C, mainly attributed to factors like oxygen functionalities and carbon defects.