Characterization of narrow micropores in almond shell biochars by nitrogen, carbon dioxide, and hydrogen adsorption

Authors: Klasson, K Thomas; Uchimiya, Sophie; Lima, Isabel

Submitted to: Industrial Crops and Products
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/8/2015
Publication Date: 5/1/2015
Publication URL: http://handle.nal.usda.gov/10113/5428915
Citation: Klasson, K.T., Uchimiya, M., Lima, I.M. 2015. Characterization of narrow micropores in almond shell biochars by nitrogen, carbon dioxide, and hydrogen adsorption. Industrial Crops and Products. 67:33-40.

Interpretive Summary: Characterization of charred material made from plant waste (also called biochar) usually includes measuring the pores (empty space) in the materials using technique called nitrogen adsorption. In this study, we show that there is a substantial pore volume in biochars created via slow heating of almond shells that cannot be measured in this fashion due to the tiny pore sizes. Thus, we used hydrogen and carbon dioxide instead of nitrogen to measure these tiny micropores. Based on the information we measured that the average pore size diameter was estimated to be 0.4-0.8 nanometer. When washed, the biochars’ pore volumes increased but the tiny pore-size structure remained. Over all, the results suggest that the nitrogen technique should be combined with the carbon dioxide technique for proper characterization of biochars. When both of them are used together we can better understand the pore structure.

Technical Abstract: Characterization of biochars usually includes surface area and pore volume determination by nitrogen adsorption. In this study, we show that there is a substantial pore volume in biochars created via slow pyrolysis from low- and high-ash almond shells that cannot be characterized in this fashion due to the narrow pore sizes. Hydrogen and carbon dioxide were used, in addition to nitrogen, to characterize these narrow micropores. All isotherms, when appropriately normalized, fell on the same characteristic curve which aids in the characterization and the understanding of the pore structure. Based on the carbon dioxide adsorption isotherm, the average pore size diameter was estimated to be 0.4-0.8 nm. When washed, the biochars’ pore volumes increased but the narrow micropore structure remained. While the biochars were alkaline, the pH of the wash solution had no or little impact on the adsorption of the acidic CO2. Over all, the results suggest that N2 isotherms should be complemented with CO2 isotherms for proper characterization of biochars. Alignment of such normalized isotherms to characteristic curves can assist in generating a more complete understanding of the pore structure over the entire region of pore diameters.