Uncovering surface area and micropores in almond shell biochars by rainwater wash

- Surface area of almond shell biochar developed at 800 °C at times >120 min.
- Biochar's performance is related to surface area and rainwater alters that property.
- Contacting biochar with rainwater changed the surface area, revealing more pores.

Biochars have been considered for adsorption of contaminants in soil and water, as well as conditioning and improving soil quality. Pore surface area is an important property of biochar. Biochars were created from shells of two almond varieties with different ash content. The pyrolysis was performed at 650 and 800 °C for 40-240 min. Significant surface areas developed at the higher temperature and at pyrolysis times of 120 min and longer. Washing the materials in synthetic rainwater removed ash and exposed additional surface area, particularly in small-diameter pores. When results from low-ash almond shell biochars were compared with high-ash almond shell biochars, it was found that the pore distribution was more uniform for the high-ash starting material and almost independent of pyrolysis time or washing. The result from the washing study is important as it suggested that adsorptive properties may change once biochars are exposed to rainwater.