不同老化方法对生物炭表面特征及镉吸附能力的影响

为明确不同自然环境过程（氧化还原、降雨、光照）对生物炭的老化作用及其对重金属吸附能力的影响，该研究以不同温度（200、500 °C）和气氛（O₂、N₂）热解的小麦秸秆生物炭为研究对象，采用化学氧化、干湿交替、紫外光照氧化3种人工老化方法模拟生物炭在自然环境中的老化过程，并分析老化作用对生物炭理化性质及镉（Cd）吸附能力的影响。结果表明：与初始生物炭相比，老化作用使生物炭表面破碎，孔隙结构增多，提高了生物炭比表面积。干湿交替老化使低温生物炭的比表面积增大0.85倍，而经过化学氧化后的低温生物炭、高温生物炭比表面积分别增大8.81、0.37倍。老化过程使生物炭的官能团种类减少，且含氧官能团数量发生不同程度的变化，其中化学氧化使羧基、内酯基等含氧官能团增多，而干湿交替及紫外光照老化主要引起含氧官能团数量的减少。此外，热重分析结果表明化学氧化使低温生物炭热稳定性降低，而所有老化后的高温生物炭热稳定性均增强。化学氧化、紫外光照、干湿交替3种老化处理均可提高两种生物炭的吸附能力，Cd²⁺吸附量分别提高498.95%~799.36%、436.10%~768.43%、35.53%~128.10%。因此，生物炭实际应用时需综合考虑其环境过程、特性变化以及目标污染物种类，以促进生物炭环境应用的长远发展。

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°C-O₂ and 500°C-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.