Production and characterization of biochars from agricultural by-products for use in soil quality enhancement

Byproducts are produced in significant amounts from crop residues such as pecan shells (PC), peanut shells (PS), and cotton gin (CG) trash. These residues can be used to produce biochar suitable for use in agricultural soil to sequester carbon and enhance plant growth by supplying and retaining nutrients while improving soil physical and biological properties. The objectives of this study were to produce biochars from different byproducts [PC, PS, CG, and switchgrass (Panicum virgatum L.)] at different pyrolysis temperatures and residence times, and to evaluate the resulting biocharʹs physico-chemical properties [yield, ash, pH, total surface area (TSA), surface charge (SC), and electrical conductivity (EC)] and elemental composition. Feedstocks were pyrolyzed under N2 at 3 temperatures (300, 500, and 750 °C) and residence times each (8, 16, and 24 h), (4, 8, and 12 h), and (1, 2, and 3 h), respectively, depending on the nature of the feedstock. Higher pyrolysis temperatures resulted in lower biochar recovery, greater TSA, higher pH, minimal SC, and higher ash contents. Among the eight biochars, switchgrass-derived biochar produced at 750 °C had the highest TSA (276 m2 g−1) followed by PC biochar (185 m2 g−1). Substantial increase in biochar pH (up to 9.8) occurred at the higher temperatures. Biochars produced at lower temperatures (350 °C) had measurable SC with PS biochar having the highest value (3.16 mmol H+ eq g−1 C). The highest ash content was observed in CG (up to 34%) compared to other biochars which contained <10% ash. These soil-related properties suggest that different biochars types can be produced to selectively improve physicochemical properties of soil through selection of specific feedstocks and pyrolysis conditions.