Effects of aging methods on surface characteristics and cadmium adsorption in biochar

Biochar can be expected to improve soil quality, environmental pollution stress, greenhouse effect, and soil remediation, due to its high stability and carbon content. A series of influencing factors can also be found in the properties of biochar, such as the raw materials, temperature, and atmosphere. Among them, the pyrolysis atmosphere and temperature have been two of the most important parameters to dominate the final yield, surface functional groups, and pore structure properties of biochar. The commonly-used high temperature and slow pyrolysis have limited the promotion and application in the process of biochar preparation. Fortunately, the low-temperature oxygen-limited pyrolysis has been developed for cost saving, high yield, and greenhouse gas reduction. The more complex structure and acidic groups can also provide more ion adsorption sites for the pollutants. In addition, the environmental application of biochar is also confined by oxidation, temperature, and humidity differences, and light factors, leading to the varying specific surface area, functional group content, and surface structure properties. The natural aging of biochar in the environment cannot fully meet the large-scale production in recent years. Particularly, the efficacy and possible chemical changes cannot be accurately predicted in a short time during long-term natural aging. This study aims to clarify the aging performance of biochar and its ability to absorb heavy metals under different natural environmental conditions (redox, rainfall, and sunlight). Two types of the original wheat biochars were pyrolyzed at 200 ℃−O₂ and 500 ℃−N₂ aged by chemical oxidation, dry-wet cycles, and finally the UV light oxidation to simulate the aging process of biochar in the natural environment. The physicochemical properties and cadmium (Cd) adsorption capacity of the aged biochar were characterized by scanning electron microscopy (SEM), specific surface area analysis (SSA), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TG). The results showed that there were significantly different properties of biochar that were prepared at different temperatures and atmospheres. Specifically, the low-temperature biochar contained the more oxygen-containing functional groups, whereas, the pore structure was developed to increase the specific surface area of the biochar after the high-temperature pyrolysis. The more seriously rupturing pore structures also increased the specific surface area of aged biochar, compared with the original ones. The specific surface area of low-temperature biochar after dry-wet cycles increased by 0.85 times. By contrast, the specific surface area of the low- and high-temperature biochar after chemical oxidation increased by 8.81 and 0.37 times, respectively. The aging process reduced the types of functional groups. There was also a variation in the number of oxygen-containing functional groups. Particularly, chemical oxidation promoted the number of oxygen-containing functional groups, such as the carboxyl and lactone groups, whereas, the wet-dry cycles and UV light aging reduced the number of oxygen-containing functional groups. In addition, TG analysis showed that the chemical oxidation decreased the thermal stability of low-temperature biochar, while the thermal stability of high-temperature biochar increased after all aging processes. The adsorption capacity for the Cd²⁺ of aged biochar increased by 498.95%-799.36%, 436.10%-768.43%, and 35.53%-128.10%, respectively, after chemical oxidation, UV light oxidation, and dry-wet cycles. Therefore, it is highly required to fully consider the environmental parameters, material properties, and targeted pollutants in the applications of biochar technology.