Preparation of pickling-reheating activated alfalfa biochar with high adsorption efficiency for p-nitrophenol: characterization, adsorption behavior, and mechanism

The adsorption properties of alfalfa biochar, which is produced via high-temperature pyrolysis for 3 h, were improved by activating it with acid pickling and reheating for 2 h (named AB). The alfalfa biochar prepared under various conditions, such as ultrapure water washing (named AWB3), acid pickling (named APB3) without reheating and cracking, and pyrolyzing of alfalfa for 5 h before ultrapure water washing (named AWB5) or acid pickling (named APB5), were used as controls. The adsorption capacity of biochars was detected by using p-nitrophenol (PNP) as a model pollutant. The corresponding results showed that the specific surface area (SSA) of AB (119.99 m2 g−1) was substantially higher than those of AWB3 (0.030 m2 g−1), APB3 (2.58 m2 g−1), AWB5 (0.46 m2 g−1), and APB5 (2.10 m2 g−1). The enhancement was primarily a result of the following factor: acid pickling and reheating could effectively remove mineral salts and tars, respectively, thereby opening the inner pores. The removal efficiency for PNP was enhanced from 4.43% (AWB3) and 10.68% (APB3) to 98.35% (AB); further, the adsorption equilibrium data of AB followed the type II Langmuir isotherm well, with a high linear-regression value (R2 = 0.997), low chi-square statistic (χ2 = 0.0009), and RMSE (0.0031). X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) determination confirmed that hydrogen bonds and π–π EDA interactions participated in the adsorption process.