The pore structure of biochar determines many biochar-induced environmental serves. In order to predict quantitatively, the environmental serves of biochar, it is very important to characterize the porosity and pore size distribution of biochar and to understand how biochar pore structure relates to the environmental serves. In this study, pore characteristics of biochars derived from different feedstocks were determined using nitrogen adsorption and the mercury intrusion porosimetry (MIP) methods. A great variation of pore characteristics in biochar was found, depending on feedstock material. The specific surface area (SSA) of biochars varied greatly, ranging from 1.06 to 70.22 m2/g. Total pore volume and porosity of biochars determined by the MIP method ranged from 1.28 to 3.68 cm3/g and from 57.8 to 79.7%, respectively. The pore size distribution of biochars had bimodal peaks in the range of 5-15 and 1.5-5 μm for the herbaceous plant and broad-leaf forest biochars, while coniferous forest biochar had two peaks at the pore sizes of 6-25 and 1.5-3 μm, respectively. Biochars had substantial storage pores (0.5-50 μm), accounting for about 85% of total pore volume, and small transmission and residual pores. The herbaceous plant biochars had larger volume of transmission pores (> 50 μm) than broad-leaf and coniferous forest biochar. Effects of pyrolysis conditions (temperature and residence time) on pore characteristics largely depended on feedstocks types. The difference in feedstocks would greatly affect pore characteristics of biochar, while the effect of pyrolysis conditions on biochar pore characteristics varied with biomass type. The detailed characterization of pore structure in biochars could effectively predict the potential impacts of biochars as soil amendment and pollutant sorbent.
Keywords: Biochar; Biomass; Mercury intrusion porosimeter; Pore size distribution (PSD); Porosity.