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

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.