Abstract:

To elucidate the response characteristics of biochar’s physical and chemical adsorption capacities to pyrolysis temperature and to clarify the quantitative relationship between biochar’s adsorption capabilities and pyrolysis temperature, we carried out the following experiment. In this experiment, biochar samples with different properties were prepared from agricultural and forestry waste branches at five pyrolysis temperatures (250, 350, 450, 550 and 650℃). Each treatment was named T250, T350, T450, T550 and T650, respectively. In this experiment, the physical and chemical adsorption capacity of biochar was reflected by measuring the iodine adsorption capacity and methylene blue adsorption capacity of biochar. Additionally, the aromaticity and polarity of biochar were determined, and the apparent morphology and functional group richness of biochar were analyzed by scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR). The results indicated that the increase of pyrolysis temperature enhanced the biochar’s iodine adsorption capacity while decreased its methylene blue adsorption capacity. The T250 and T350 treatment had abundant oxygen-containing functional groups, while the biochar treated with T450, T550 and T650 had stronger aromaticity, lower polarity and richer pore structure. The relationship between biochar’s iodine adsorption capacity (Q₁) and pyrolysis temperature (x) conformed to the growth “S” curve, specifically: Q₁=180.78+(447.96-180.78)/[1+10^{(484.61-x)0.02}], R²=0.9991. The minimum, median and maximum values of biochar’s iodine adsorption capacity correspond to pyrolysis temperatures was 318.1, 484.6 and 616.4°C, respectively. The relationship between methylene blue adsorption capacity (Q₂) and pyrolysis temperature (x) of biochar conformed to the decreasing “S” curve, specifically (P<0.01): Q₂=5.78+(12.45-5.78)/[1+10^{(386.78-x)-0.02}], R²=0.9920. The minimum, median and maximum values of biochar’s methylene blue adsorption capacity correspond to pyrolysis temperatures was 511.3, 386.8 and 284.6°C, respectively. For every 50°C increase in pyrolysis temperature, the average iodine adsorption capacity of biochar increased by 16.7%, while the average methylene blue adsorption capacity decreased by 21.8%. Considering the adsorption principles and characteristic parameters, in different practical production scenarios, if the main application was chemical adsorption-based biochar, the recommended pyrolysis temperature should not exceed 386.8°C. If the focus was on biochar’s physical adsorption function, the recommended pyrolysis temperature should not be lower than 484.6°C. When the pyrolysis temperature is between 386.8 and 484.6°C, there was no significant distinction between the physical and chemical adsorption capabilities of biochar.

Key words: biochar, pyrolysis temperature, physical adsorption, chemical adsorption, iodine adsorption capacity, methylene blue adsorption, quantitative relation