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

摘要：

人工湿地作为一种环境友好的水处理技术在处理城乡生活污水，农田退水以及养殖废水等领域得到了广泛的应用，但普遍存在低温条件下脱氮效率下降的问题。通过添加生物炭来强化人工湿地的脱氮效率，是当前的研究热点之一。该文综述了生物炭对人工湿地强化脱氮作用的研究进展。最新研究表明，在人工湿地中添加各类生物炭可显著提升人工湿地的脱氮效能，例如在垂直流人工湿地中添加经700 ℃制成的竹子生物炭，其脱氮效率提升了53%，展示了良好的应用前景。生物炭对人工湿地的强化脱氮作用主要有以下3种机制：首先，生物炭是一种缓释碳源并含有K、Ca、Mg等养分，作为基质可改善人工湿地植物根际营养环境，促进植物生长并增加对氮的吸收同化；其次，生物炭是一种多孔结构材料，可以为微生物提供良好的附着空间，增加根际脱氮微生物数量，提高氮转化效率；再次，生物炭本身所含的官能团对不同含氮化合物具有一定的吸附能力，可以通过理化途径进一步提升脱氮效率。生物炭作为一种新型负碳生物材料，潜力巨大，为推动生物炭强化人工湿地技术的工程应用，未来亟需开展不同来源生物炭理化特性的研究，并深入探究其在人工湿地脱氮过程中的具体作用机制。

关键词: 生物炭, 人工湿地, 植物, 微生物, 脱氮