Preparation of high adsorption performance and stable biochar granules by FeCl3-catalyzed fast pyrolysis

Abstract

As a low-cost and readily available adsorbent for removal of pollutants, biochar has been intensively studied for adsorption performance and its mechanism. However, it is still far from its practical applications due to difficulties of separation after adsorbing pollutants. In this study, we significantly improved mechanical stability and adsorption performance of biochar by fast co-pyrolysis of a mixture of sawdust, FeCl₃, and kaolin. Results indicated that dosing FeCl₃ during biomass pyrolysis can drastically increase the yield of biochar, and also increase the strength of granular biochar (GBC). The GBC prepared at 650 °C with 5 mmol g⁻¹ of FeCl₃ dosage (Fe₅-GBC₆₅₀) exhibited a 4-chlorophenol adsorption capacity of 35.71 mg g⁻¹, which was twice the adsorption capacity without FeCl₃ (Fe₀-GBC₆₅₀), although pure biochars in both Fe₀-GBC₆₅₀ and Fe₅-GBC₆₅₀ have a maximum 4-CP adsorption capacity of 250 mg g⁻¹. The compressive strength of Fe₅-GBC₆₅₀ was 4.86 MPa, which was four times higher than that of Fe₀-GBC₆₅₀ (1.12 MPa). Moreover, the scatter ratio of Fe₅-GBC₆₅₀ was only 2.58%, which was significantly lower than that of Fe₀-GBC₆₅₀ (45.61%). Multiple characterization techniques including SEM, FTIR, XPS, and scanning acoustic microscope imaging were conducted to explain the underlying mechanism. The preferable adsorption capacity of Fe₅-GBC₆₅₀ may be attributed to catalytic decomposition of the biomass and the reductive deposition of carbon (e.g., CH₄, C₂H₄, and C₂H₂) by FeCl₃ that increases the yield of biochar and the specific surface area of GBC. The high stability may have resulted from the binding interaction of FeCl₃ products. This work may facilitate the replacement of granular activated carbon by low cost biochar.