The main removal mechanism of nitrogen in CWs is through the microbial nitrification and denitrification process, which converts nitrogen to N₂O or nitrogen (N₂). The plant uptake, adsorption, filtration, and ion exchange by filler can also play a role [33]. In recent years, Gupta et al. [34] found that using BC as CWs filler could effectively adsorb and remove organic matter, total nitrogen (TN), nitrate nitrogen (NO₃⁻-N), and ammonia nitrogen (NH₄⁺-N) from wastewater compared to gravel filler, with removal rates of 91%, 58.3%, and 92%. Similarly, Rozari et al. [35] investigated the pollutants removal efficiency of CWs with and without BC added to the gravel filler, and the results showed that the removal rates of TN, NH₄⁺-N, and NO₃⁻-N were significantly higher in CWs with BC addition. Cheng et al. [36] found that the efficient removal of nitrogen was basically completed within 12 hours of adding BC to CWs. On the one hand, the BC has a larger specific surface area, total pore volume, and micropore volume compared to gravel, which enhances its ability to fix and transfer oxygen. The dissolved oxygen (DO) level is a key factor in the smooth progress of nitrification reactions. In addition, the surface of BC is conducive to biofilm formation, promoting the enrichment of different microorganisms, and thereby improving nitrogen removal [37]. The CWs supplemented with BC can release additional dissolved organic matter (DOM) during operation to promote the denitrification process within the systems and enhance nitrogen removal [38]. Moreover, the plant-source carbon from BC-addition CWs is considered more important for denitrification in CWs than the direct release of carbon from BC [39]. On the other hand, the adsorption performance of gravel is limited and significantly lower than that of BC, and it is more likely to reach saturation under the same concentration conditions [40].

Zhou et al. [38] found that combining BC with aeration vertical flow CWs had excellent intermittent aeration treatment effects on low carbon/nitrogen ratio wastewater. Abedi et al. [41] found that the chemical oxygen demand (COD) and TN removal of CWs with BC addition reached 99.9%, which was much higher than that of CWs without BC addition (64.6%, 71.1%). It has shown that aerobic degradation of COD in CWs systems generally occurs before nitrification reactions, and DO in CWs is prioritized for COD degradation. The BC filler not only enables an efficient transfer of DO but also absorbs organic matter, improving the water purification effect of CWs [18]. In contrast, too much BC filler may slightly reduce the COD removal performance of the CWs, because the release of DOM increases the COD concentration in the CWs with BC addition [42]. The DOM released from BC is influenced by the pyrolysis temperature during preparation. BC with pyrolysis temperature above 650°C shows a substantial reduction in DOM concentration [42, 43]. In addition, factors such as the pH of the water bodies, raw material of biochar, and microbial community all have different degrees of influence on DOM and should be further explored [44].

The BC also enhances phosphorus uptake in CWs. The removal rates of nitrogen and phosphorus by plants were 15.4% and 14.2% in CWs with BC addition, respectively, compared to 8.00% and 8.30% in CWs without BC addition [45]. Guo et al. [32] found that removal rates of COD, TN, NH₄⁺-N, and total phosphorus (TP) reached 96.34%, 100%, 80.02%, and 81.19%, a significant increase compared to the control group (96.13%, 69.67%, 62.27%, and 67.51%). The adsorption and precipitation of phosphorus by CWs filler are the main pathways for removing phosphorus in CWs, the rational selection of CWs filler is of great significance for phosphorus removal [46, 47]. The TP removal efficiency is continuously improved with the increase of the BC addition ratio [48, 49]. Although most studies indicate that the BC addition has a positive impact on phosphorus removal, several reports have shown BC addition in gravel-filled CWs does not promote phosphorus removal [50], and that mixed media of sand and BC are even less effective than pure sand in phosphorus removal [50, 51]. More research and applications related to phosphorus removal using BC filler are still needed. Table 2. records in detail the removal of pollutants in different types of CWs.

In recent years, due to its characteristics such as large specific surface area, high degree of aromatization, good stability, and rich microporous structure, BC has been proven a green adsorbent for the removal of hard-to-degrade pollutants in water, such as herbicides, pesticides, and heavy metals, in many studies [52]. Chang et al. found [53] that CWs filled with walnut shell BC pyrolyzed at 450°C were more effective in removing Hg than the gravel CWs, which was mainly attributed to Hg adsorption and precipitation on the CWs filler. Cheng et al. [36] found that adding 20% BC to CWs significantly improved the removal efficiency of Microcystins (MC-LR) and reduced the potential risk of MC-LR over three days. Uchimiya et al. [54] found that chicken manure BC prepared at high temperatures had good adsorption efficiency for atrazine. A study by Ahmad et al. [55] also noted that BC prepared with tofu residue and peanut shells at 700°C was much more effective in the adsorption of trichloroethylene than BC prepared at 300°C. In contrast, a study by Sun et al. [56] showed that BC prepared at low temperatures (400°C) showed better adsorption of Norflurazon and Nluridone. Abedi et al. [41] found that the addition of BC approximately doubled the removal rates of Phenols, Pb, and Mn, reaching 99%. Guo et al. [32] confirmed that the addition of BC increased the removal of Zn from 8.15% to 22.38%, and As from 35.58% to 74.09%. Tang et al. [57] had reached similar conclusions in degradation experiments with Chlorpyrifos, Endosulfan, Fenvalerate, and Diuron.

Based on the analysis of various pollutants using BC-based CWs, hard-to-degrade organic matter, and heavy metals are mainly removed by BC adsorption through mechanisms such as electrostatic attraction, hydrophobic effects, hydrogen bonding, pore filling, ion/ligand exchange, complexation, precipitation, and π-binding interactions [58]. On the other hand, the growth and reproduction of microorganisms related to the degradation of these pollutants may be stimulated by BC filler. Mittal et al. [59], conducting antibacterial testing on BC, observed substantial microbial growth without any obvious inhibition zones, and found bacterial colonies inhibiting ciprofloxacin. The BC, which is added to CWs, has a dual role of adsorbing pollutants and enhancing biodegradation. The above expressions were summarized in Table 3.

However, CWs dealing with heavy metal contaminants (As, Zi, Ni, Cr, Cu) and organic pollutants (polycyclic aromatic hydrocarbons, chlorinated hydrocarbons) may accumulate these contaminants in CWs plants [60]. The use of BC prepared from contaminated plant residues may pose some environmental risks due to the fact that heavy metals and polycyclic aromatic hydrocarbons are generally difficult to volatilize during high-temperature carbonization and remain in biochar [61]. Moreover, BC does not always play a positive role in the treatment of complex contamination. Zheng et al [62]. found that the addition of BC to rice soils contaminated with As-Pb-Cr resulted in a 98% and 752% reduction in Pb and Cr content in rice stems, respectively, but As content in stems and soil pore water increased by 3.3- and 14.2-fold, respectively.

The CWs rely mainly on the action of microbial communities for the purification of sewage. Due to its large surface area and porous structure, BC is considered a carrier for microbial growth in bioreactors [63]. Antibacterial testing of BC shows [59] significant microbial growth around the BC, indicating that BC can serve as a supporting substrate for microbial adhesion and growth. Kizito et al. [64] found that adding BC to vertical flow CWs can enhance sustainable wastewater treatment performance and increase microbial diversity. Deng et al. [65] studied the microbial community structure and metabolic characteristics of CWs systems with different proportions of BC, the results showed that the addition of BC enhanced the diversity of wetland microbial communities, promoted the biodegradation of extracellular polymeric substance (EPS), and improved the removal efficiency of nitrogen and organic matter in the CWs. Cheng et al. [36] found that the addition of BC improved the microbial community and increased the relative abundance of a variety of functional degrading bacteria. In addition, a lot of studies have shown that microorganisms can be stably attached to BC and are less likely to be lost due to sudden changes in hydraulic conditions or by predation, which maintains microbial diversity to a certain extent. It has also been suggested that BC can promote the structural diversity of nitrifying bacterial communities, promote the abundance and activity of ammonia-oxidizing bacteria, etc., and increase the nitrification rate [66, 67]. BC also has a rich source of carbon and a porous structure, providing a good habitat for microbial growth [68].

The plants biomass in CWs is an important component of the carbon sinks role played by CWs [69]. The addition of BC affects the physical and chemical conditions of CWs, such as conductivity, DO, and pH, improving nutrient uptake by plant roots [39]. To some extent, the ability of plant roots can be assisted by BC [70]. BC can improve the CWs substrates environment and make the soil fertile [71]. The BC filler can provide rich nutrients to surface plants and increase plants biomass production [68]. BC also released substances that directly affect plants growth, increasing the biomass of above-ground and below-ground parts by 1.9 and 1.5 times, respectively, compared to control systems without BC [45]. In the study by Li et al. [72], it was found to be 1.73 times and 1.37 times. These benefits are mainly attributed to the various nutrients released by BC (such as K, P, N, Mg, and Ca) and the increased porosity of the cultivation medium due to the oxygen conditions created by BC, which is extremely beneficial for plants growth [73]. The addition of BC filler can promote nutrient-using efficiency in CWs plants by enhancing plant growth through nutrient uptake, oxygen release, organic matter secretion, and providing more attachment sites for root-associated microbes [57]. In addition, studies have shown that the addition of BC increases nitrogen content in plant biomass, which improves the ability to remove nitrogen from plants [72]. The addition of woody BC significantly increased the nitrogen content of reed from 1.40 mg/g to 1.95 mg/g [39]. A study by Kasak et al. found that the addition of BC increased plants nitrogen uptake from 40.7 g/m² to 78.1 g/m² [45]. Plants can thrive in the CWs of BC filler.

With the increasing use of fossil fuels worldwide, reducing GHG emissions has become a global challenge [74]. As an important component of the global carbon cycle, plants can fix and convert atmospheric CO₂ into organic compounds through photosynthesis [75]. Hence, plants are highly regarded for sequestering carbon. However, once the plants life cycle is complete, burning or decay of the plants material releases the previously fixed CO₂ back into the atmosphere, contradicting the goal of carbon sequestration.

Kong et al. discovered [76] that charring plants into BC can be stable without rapid decomposition, thus promising to development of an effective carbon sequestration technology. At present, it is generally believed that BC has excellent thermal stability, chemical stability, and microbial stability, which derives from the tight structure of aromatic rings and alkyl groups in BC. This results in higher resistance to biological and chemical degradation than other organic carbon. On the other hand, the organic components on the surface of BC can form new agglomerate structures with minerals and organic matter in the CWs, reducing the risk of microbial degradation through physical protection [77]. Guo et al. [78] further pointed out that the stability of BC not only depends on the degree of aromatic carbon aggregation but also on the integrity of the aggregated aromatic carbon and the silica-carbon complex, which is an important protective mechanism. A study extrapolated experimentally and through modeling that the amount of carbon lost from BC due to mineralization and degradation (including DOM) is between 3 ~ 26% at the annual scale, and further extrapolated that the half-life of BC may be between 10² ~ 10⁷ years, confirming the high stability of BC [79].

Global agricultural waste can produce 373 Mt of BC per year and have the potential to sequester 0.55 Pg CO₂ [80]. The potential for carbon sequestration by BC is enormous. Compared to biomass entering the soil directly, sustaining complete degradation to CO₂ and returning to the atmosphere, the stabilization of carbon in the form of BC is considered a “carbon-negative” pathway, which can effectively achieve GHG emissions reduction and biomass carbon sequestration [81, 82].

The addition of BC can reduce the mineralization of CWs substrates organic carbon. The soil carbon pool is the largest carbon pool in terrestrial ecosystems and an important part of the global carbon cycle, dominating the global carbon balance. A small change in the soil carbon pool can affect the global carbon balance. Studies have shown that CWs substrates organic carbon mineralization is the main source of CO₂ release from CWs substrates, and controlling the mineralization rate of CWs substrates organic carbon plays an important role in reducing atmospheric CO₂ [83]. The BC can achieve high carbon sinks to the soil carbon pool through a mechanism of sorption of CWs substrates organic carbon and aggregates. Researchers conducted a 532-day incubation experiment using soils collected from central Amazonia (Brazil) and showed that the total mineralization rate of soil organic carbon containing BC was 25.5% lower than that of the without BC control, demonstrating the inhibitory effect of BC on organic carbon mineralization [84].

Although the addition of BC to CWs may increase CH₄ emissions, CWs are still net carbon sinks in the context of the CWs ecosystems as a whole [11]. Overall, Landry et al. [85] reported that the carbon sequestered by CWs ecosystems may be 2 to 15 times greater than the carbon they emit. Luan et al. and Meng et al. [86, 87] suggested that the majority of wetlands are carbon sinks from an ecological perspective when they are not destroyed. Estimates of GHG fluxes between CWs and natural wetlands indicates that CWs have higher carbon sequestration potential than natural wetlands [88].

GHG is a significant factor in global climate change, and GHG reduction is crucial to mitigating the greenhouse effect [89]. This adverse environmental impact could undermine the benefits and sustainability of CWs [90]. GHGs, such as N₂O, CO_2, and CH₄, may be generated through the conversion of carbon and nitrogen by microorganisms and plants, released into the air [91]. Therein, CH₄ and N₂O have 28 and 298 times more global warming potential (GWP) than CO₂ [92]. GHG produced in CWs is correlated with water bodies composition, microbial species, filler, etc. The DO, pH, microbial community, plants growth, GHG emissions can be regulated by BC in CWs [93]. The BC applied to the CWs not only changes the native physicochemical properties of CWs substrates in several aspects such as bulk weight, porosity, pH, cation exchange, soil mineral elements, and organic matter but also has some effects on CWs microbial community, changing the emission of GHG [94]. GHG emissions strategies in CWs have received increasing attention in recent years [95].

Some studies have found that the N₂O emissions from BC-added CWs are lower than those from other CWs [96]. The N₂O emissions were reported to be reduced from 418.75 μg·m⁻²·h⁻¹ to 60.54 μg·m⁻²·h⁻¹ [38]. Abedi et al. [41] found that N₂O emissions were reduced to 1/6 of the original level after adding BC to CWs. The average N₂O fluxes in the BC-added and none-BC CWs were 253.8, 219.4, 183.9, 167.8 μg·m⁻²·h⁻¹ and 351.2, 300.3, 259.8, 238.9 μg·m⁻²·h⁻¹ at influent C/N ratios of 3, 6, 9, 12, respectively [97]. The addition of BC significantly reduces N₂O fluxes, and N₂O emissions decrease with increasing C/N ratio and oxygen supplementation in CWs [97]. BC effectively facilitates nitrification and denitrification processes in CWs, reducing the byproducts of nitrogen intermediate [98]. Spokas et al. [99] and Yanai et al. [100] reported that BC could be used as a means to reduce or suppress N₂O. The BC-rich CW can increase the activity of nitrifying and denitrifying bacteria, reducing N₂O emissions. There may be another explanation: BC as a potential carbon source can promote complete denitrification to reduce N₂O emissions and enhance the bio-transformation of N₂O to N₂ [101, 102]. The lime treatment of alkaline BC can promote the activity of N₂O reductase in denitrification and enhance the biological conversion of N₂O to N₂ [101]. In addition, BC can adsorb the generated N₂O for subsequent biological consumption [103] or abiotic reduction catalyzed by metal oxides on the biochar surface [102].

However, the BC filler may increase the emission of CH₄ compared to gravel-based CWs [104]. The BC has a high redox-active property and charging and discharging capacities that facilitates the direct interspecies electron transfer between methanogens and anaerobic bacteria, as well as providing labile organic carbon for CH₄ [105]. Overall, when considering multiple GHGs, BC-added CWs generally exhibit low GWP [106]. For instance, an 18 ~ 24% lower GWP in BC-added CWs compared to gravel-based CWs was reported by Guo et al [97].