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 3h, were improved by activating it with acid pickling and reheating for 2h (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 5h 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 (119.99m(2)g(-1)) was substantially higher than those of AWB3 (0.030m(2)g(-1)), APB3 (2.58m(2)g(-1)), AWB5 (0.46m(2)g(-1)), and APB5 (2.10m(2)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 (R-2=0.997), low chi-square statistic ((2)=0.0009), and RMSE (0.0031). X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) determination confirmed that hydrogen bonds and - EDA interactions participated in the adsorption process.