Comparison of the Physical and Chemical Properties of Laboratory and Field‐Aged Biochars

Abstract

The long-term impact of biochar on soil properties and agronomic outcomes is influenced by changes in the physical and chemical properties of biochars that occur with time (aging) in soil environments. Fresh biochars, however, are often used in studies because aged biochars are generally unavailable. Therefore, a need exists to develop a method for rapid aging of biochars in the laboratory. The objectives of this study were to compare the physicochemical properties of fresh, laboratory-aged (LA), and field-aged (FA) (≥3 yr) biochars and to assess the appropriateness of a laboratory aging procedure that combines acidification, oxidation, and incubations as a mimic to field aging in neutral or acidic soil environments. Twenty-two biochars produced by fast and slow pyrolysis, and gasification techniques from five different biomass feedstocks (hardwood, corn stover, soybean stover, macadamia nut shells, and switchgrass) were studied. In general, both laboratory and field aging caused similar increases in ash-free volatile matter (% w/w), cation and anion exchange capacities, specific surface area, and modifications in oxygen-containing surface functional groups of the biochars. However, ash content increased for FA (18-195%) and decreased for LA (22-74%) biochars, and pH decreased to a greater extent for LA (2.8-6.7 units) than for FA (1.6-3.8 units) biochars. The results demonstrate that the proposed laboratory aging procedure is effective for predicting the direction of changes in biochar properties on field aging. However, in the future we recommend using a less aggressive acid treatment.

Modified method for proximate analysis of biochars

“…The source and abbreviations for the 22 biochars used in this study are listed in supporting materials (Table S1). Details of the production, collection, weathering and corresponding chemical and physical properties of the biochars are described elsewhere [7].…”

“…Secondary sieving to separate smaller biochar particles occurred by wet sieving (0.045 mm) and hand picking. All field weathered biochar was subsequently ground using a mortar and pestle to a particle size of <1 mm and stored in sealed containers [7]. H and C org content of the various biochars was determined using a combustion analyzer (Vario Microcube, Elementar Analysensysteme GmbH) after the samples had been ball milled to reduce particle size and treated for 24 h with 0.05 mol L -1 HCl to remove carbonates [7].…”

“…Higher apparent moisture levels by the ASTM method than the Modified method indicates mass loss as either CO 2 or other volatile compounds. Both the LW and FW biochars had an opportunity to adsorb biogenic dissolved organic compounds (DOC) during aging [7], hence it is possible that differences in apparent moisture content determined by the ASTM and Modified methods are related to the presence of thermally labile biogenic organic compounds adsorbed on the surfaces of the LW and FW biochars.…”

“…Brewer et al [5] conducted an extensive biochar characterization study and reported that the chemical and physical properties of biochar differ depending on both production technique and biomass feedstock. Other studies indicate that biochar properties change over time (weathering or aging) in soil environments [6,7] and thus both fresh and aged biochars must be studied [8,9,10]. Lastly, Bradbury and Shafidazeh [11] reported weight gain by biochar during moisture determination using standard oven drying techniques, implying O 2 chemisorption on biochar surfaces.…”

Impact of Pyrolysis Temperature and Feedstock on Surface Charge and Functional Group Chemistry of Biochars

“…Other studies suggest that biochar can substitute for activated C in some applications such as the production of potable water (Azargohar and Dalai, 2006; Park et al, 2013). These applications depend on the ability of biochar surfaces to adsorb various cationic, anionic, and organic contaminants; however, the capacity of biochars to adsorb contaminants varies substantially depending on biomass feedstock characteristics, pyrolysis conditions, and various pre‐ or post‐pyrolysis treatments (Jindo et al, 2014; Bakshi et al, 2016; Lawrinenko et al, 2017a; Li et al, 2018). Hence, it is important to select the appropriate type of biochar for a specific application.…”