关键词: 生物炭, 人工湿地, 植物, 微生物, 脱氮

Abstract:

Constructed wetlands are an environmentally friendly sewage treatment technology that is widely applied in the treatment of various types of sewage, including urban sewage, reclaimed water from farmland, and wastewater from aquaculture. Pollutants were removed by simulating the operational processes of the natural wetlands. However, the nitrogen removal efficiency of constructed wetlands is easily affected by environmental and operational conditions. Especially with the change of seasons, in winter, the operation of constructed wetlands is suppressed by low temperatures, resulting in a decrease in nitrogen removal capacity. To address this limitation, many researchers have proposed filling constructed wetlands with biochar substrates. Biochar is a porous, carbon-rich material produced via high-temperature pyrolysis of biomass under oxygen-limited conditions. This can effectively enhance nitrogen removal in constructed wetlands. This paper reviews the recent research progress on biochar-enhanced nitrogen removal in constructed wetlands in recent years and finds that the addition of biochar can generally improve the nitrogen removal efficiency of constructed wetlands. For instance, after adding bamboo biochar produced by pyrolysis at 700 ℃ to vertical-flow constructed wetlands, the TN removal efficiency increased by 53%. This indicates that the addition of biochar to constructed wetlands has a promising future. The enhanced nitrogen removal effect of biochar is influenced by multiple factors, including hydraulic loading, hydraulic retention time, C/N ratio, and the dosage and location of biochar. Some studies have tested wastewater with different carbon-nitrogen (C꞉N) ratios to explore the impact of the C꞉N ratio on the enhanced denitrification effect of biochar. The results showed that the strengthening effect of biochar was more pronounced under conditions of a low C꞉N ratio. Some studies have also shown that for specific types of biochar, both dosage and burial location can impact its denitrification effects. Higher dosages usually enhance denitrification effects, but excessive dosages can lead to pore blockage and inhibit microbial activity. The results showed that the strengthening effect of biochar was more obvious under conditions of a low C꞉N ratio. Some studies have also shown that for specific types of biochar, both dosage and burial location impact nitrogen removal. Higher dosages usually enhance the denitrification effect, but excessive dosages can lead to pore blockage and inhibit microbial activity, resulting in decreased denitrification efficiency. In addition, the burial location of biochar in constructed wetlands affects its contact with wastewater and microbial communities, thereby influencing its nitrogen removal effect. The enhanced nitrogen removal effect of biochar in constructed wetlands mainly occurs through the following three mechanisms: synergistic mechanisms between biochar and plants, synergistic mechanisms with microorganisms, and the direct and indirect influence of the physicochemical properties of biochar itself on key denitrification processes. Based on previous research, this study sorted and analyzed the internal mechanisms of the biochar denitrification enhancement. First, the carbon in biochar mainly exists as an aromatic structure. When added to constructed wetlands, biochar is slowly released into the environment as dissolved organic carbon (DOC), which promotes plant growth. The ash elements are gradually released into the constructed wetland as the wetland environment changes, providing soluble nutrients for wetland plants. This nutrient supply promotes the metabolic activities of plants, thereby increasing their height, root length, and biomass. Healthy plants are more conducive to nitrogen absorption and assimilate nitrogen efficiently. Biochar can also promote plant physiological processes by increasing the concentration of photosynthetic pigments and stimulating the activity of key enzymes in nitrogen metabolism, thereby further enhancing the ability of plants to absorb and remove nitrogen. Second, the large specific surface area of biochar provides a habitat for microorganism growth and promotes biofilm formation on its surface. The addition of biochar can also improve the microbial community structure of constructed wetlands and increase the abundance of bacterial communities related to nitrification and denitrification processes, such as Proteobacteria and actinomycetes, thereby promoting nitrogen removal in constructed wetlands. Simultaneously, the abundance of functional genes related to denitrification, such as narG (nitrate reductase) and nirK (nitrite reductase), which play key roles in the nitrogen conversion process, also increased. Therefore, biochar further enhances the nitrogen removal rate of constructed wetlands by promoting microbial reproduction and optimizing the community structure. Third, the unique physicochemical properties of biochar endow it with a good adsorption capacity for nitrogen-containing pollutants, which are determined by the type of raw materials and pyrolysis conditions. Although an increase in the pyrolysis temperature results in a larger specific surface area, which is conducive to the adsorption of nitrogen-containing compounds (including ammonium salts and nitric acid), it may simultaneously destroy the surface-active functional groups, thereby weakening the adsorption capacity for certain nitrogen-containing pollutants. In addition, owing to the limited adsorption capacity of the functional groups on the surface of biochar (such as hydroxyl, carboxyl, and phenolic groups) for nitrates, the adsorption capacity of biochar for different nitrogen pollutants varies. These functional groups tend to adsorb positively charged ions (such as NH₄⁺-N). This adsorption process not only helps remove nitrogen compounds from water but also reduces nitrogen loss in environment. Although biochar, as a new type of negative carbon biomaterial, can effectively enhance the nitrogen removal capacity of constructed wetlands, some urgent problems must be solved for its large-scale application. The long-term stability of biochar is one of the issues that people are concerned about. During the operation of constructed wetlands using biochar, problems such as pore blockage, functional group oxidation, and adsorption saturation may occur, leading to a decline in their performance. The potential environmental risks associated with the use of biochar in constructed wetlands may pose challenges to its application. Contaminated biochar in constructed wetlands gradually releases heavy metals and polycyclic aromatic hydrocarbons, causing secondary environmental pollution. The continuous operation of constructed wetlands and gradual saturation of biochar adsorption have made the regeneration or treatment of biochar has a new challenge. Appropriate biochar regeneration technologies have not yet been fully developed, and the feasibility of their application in engineering has not been verified to date. Further research on the stability and long-term performance of biochar is needed to assess potential leaching risks and ensure that these risks can be mitigated under practical conditions.

Key words: biochar, constructed wetlands, plant, microorganisms, nitrogen removal