Modification of biochar with high-energy ball milling: Development of porosity and surface acid functional groups

Biochar is widely used in many applications and its properties can be modified with various methods, including ball milling. In this work, six different biochar materials (made from pine, hemp, switchgrass, pulp sludge, wheat straw, and sugarcane bagasse) were ball milled and characterized. The pore volume and surface area measured by N2 adsorption increased after milling, but that measured by CO2 changed minimally suggesting that occluded pores were exposed. Ball milling also increased the total number of accessible carboxylic, phenolic, and lactonic groups on the surface proportionally to the initial oxygen content, but no new functional groups were detected with FTIR. To account for different milling conditions reported in the literature, the total applied energy was correlated to the developed properties. The methylene blue adsorption capacity of the biochar increased after milling up to 256 mg/g (20-fold increase), which was proportional to the increase in accessible oxygen functional groups.