Physicochemical characteristics of biochars can be beneficially manipulated using post-pyrolyzed particle size modification

Post-processing of biochars to enhance plant growth responses: a review and meta-analysis

“…2) suggests that it is not simply the case that "smaller is better"; rather, there appears to be an optimal particle size at a linear dimension of somewhat less than 1 mm. Although the prior literature on biochar particle size effects mentions the possibility of an optimal particle size (e.g., Liao and Thomas 2019), papers have generally emphasized the agronomic advantages of small particle sizes, such as enhanced mixing, increased liming effects and more rapid pH equilibration (e.g., Rees et al 2014;Chen et al 2017), increased CEC (He et al 2018;Sangani et al 2020), and increased micro-porosity (Sun et al 2017). It is thus less clear what biochar properties would produce reduced growth responses at very low particle sizes.…”

“…Many physio-chemical properties of biochars are strongly dependent on particle size (Table 2). Sangani et al (2020) recently compared a wide variety of properties across size classes of sieved biochars derived from several feedstock sources and found that in general physical properties were strongly influenced by particle size, while feedstock effects were more important for chemical properties. Particle size has particularly strong effects on porosity characteristics, such as total porosity, mean pore diameter, and BET surface area (Sangani et al 2020).…”

“…Sangani et al (2020) recently compared a wide variety of properties across size classes of sieved biochars derived from several feedstock sources and found that in general physical properties were strongly influenced by particle size, while feedstock effects were more important for chemical properties. Particle size has particularly strong effects on porosity characteristics, such as total porosity, mean pore diameter, and BET surface area (Sangani et al 2020). Smaller particle size is also commonly associated with increased water-retention capacity (Lim et al 2017;Liao and Thomas 2019;Sangani et al 2020).…”

“…Particle size has particularly strong effects on porosity characteristics, such as total porosity, mean pore diameter, and BET surface area (Sangani et al 2020). Smaller particle size is also commonly associated with increased water-retention capacity (Lim et al 2017;Liao and Thomas 2019;Sangani et al 2020). However, in the soil matrix, small biochar particles can reduce water-retention capacity by filling soil inter-pores (Liu et al 2016;Lim et al 2017); hydraulic conductivity can likewise be reduced (Esmaeelnejad et al 2017;Ibrahim et al 2017).…”

“…Smaller particle size feedstock material will generally undergo more complete pyrolysis, associated with greater losses of volatile materials, loss of O-containing functional groups, and increased concentration of alkaline elements-which result in increased ash content and pH. There is also evidence for increased CEC and AEC in biochars derived from small particles (Sangani et al 2020), though data are surprisingly lacking on this point for biochar-soil mixtures. Smaller particles more readily allow escape of volatile organics during pyrolysis, potentially mitigating toxic effects (Dutta et al 2017).…”