A Review on Anatomical and Physiological Traits of Aquatic Macrophytes Coupled to a Bioelectrochemical System: Comparative Wastewater Treatment Performance

Abstract

Anthropogenic activities, such as agricultural, industrial, and domestic, generate wastewater, leading to environmental concerns. Wastewater constituents (organic matter, pathogens, pharmaceuticals, heavy metals, and nutrients) have a negative impact if not treated, harming ecosystems and human health. Phytoremediator plants are a good option for domestic wastewater treatment since they help remove pollutants through their physiological activities, which are highly related to anatomical adaptations due to their growth in humid habitats. Macrophytes are a useful tool when coupled with a bioelectrochemical constructed wetland and MFC (CW-MFC), which can enhance the removal efficiency of organic matter present in wastewater and promote higher bioelectricity due to the root activity of plants. This review aims to compare different aquatic macrophyte types in wastewater treatment efficiency and provide useful information for plant selection.

1. Introduction

With current intensive agricultural, industrial, and domestic activities, wastewater has become a major environmental issue. In recent years, global concern over freshwater pollution has increased due to inadequate treatment or, in the worst scenario, the absence of infrastructure to treat such wastewater. The treatment process is intrinsically complex due to the nature of wastewater composition; its constituents can vary greatly depending on the origin. It can range from organic matter, pathogens, pharmaceuticals, radionuclides [1], antibiotics, pesticides, heavy metals, and nutrients, which negatively affect the ecosystems and human health [2,3,4]. Studies have shown that water pollution mitigation can be performed using living plants, whose physicochemical and metabolic activities immobilize, uptake, reduce toxicity, and stabilize those pollutants [5].

Phytoremediation uses different types of plants to clean contaminated soil, air, or water, and in the case of polluted water, macrophytes are used. Phytoremediation requires identifying plants that can resist and degrade or adsorb pollutants [6,7,8]. Plant species considered ideal phytoremediation agents have a high biomass, a good root system, are fast-growing, and are easy to grow [7,9,10,11]. Phytoremediation treatment with Eichhornia crassipes (water hyacinth) resulted in a reduction in chemical oxygen demand (COD) in the range of 55–59% and total nitrogen (TN) of 85% in four weeks. In the same study, Pistia stratiotes (water lettuce) also showed similar results, with a COD removal in the range of 53–57% and TN removal in the range of 85–93% (four weeks), attributing this reduction to microbial activities and increased carbon dioxide (CO₂), lowering the pH level in wastewater [10].

Phytoremediation of wastewater involves the removal of water and soil pollutants through processes inside the plant, such as rhizodegradation, phytodegradation, phytoextraction, rhizofiltration, phytovolatilization, and phytostabilization [5]. The plant organs involved are roots, stems, and leaves, through their main tissues, namely, aerenchyma, xylem, and phloem, and leaf structures like epidermal cuticle and stomata [12]. Figure 1 shows internal processes that take place during phytoremediation, which include phytoextraction, phytoaccumulation, enhanced rhizosphere biodegradation, phytostabilization, and phytodegradation, as well as other adaptive physiological processes, which provide phytoremediator plants with sensory abilities, i.e., radial oxygen loss (ROL), toxicant concentration, anatomical structural changes, and production of secondary metabolism compounds [3,13].

The main indicators of a plant’s assessment ability for its use as a phytoremediator are if anatomical changes are observed and the presence of secondary metabolites in tissues, as these factors can help the plant to withstand environmental stress [14]. Plant roots play a key role in the phytoremediation of wastewater by providing a living yet highly specific surface area for developing biofilms that contain diverse microorganism communities responsible for filtering and entrapping fine suspended particles [15,16,17].

A microbial fuel cell (MFC) is a bioelectrochemical system (BES) that uses microorganisms as catalysts to convert chemical compounds present in the medium into electricity and is considered a promising platform for waste management and low energy output [18,19]. The search for new technology platforms to treat wastewater has led to the integration of a BES with constructed wetlands (CWs), and it has become a cutting-edge hybrid technology [20,21,22,23,24,25,26,27]. In the case of CWs, stratified redox potentials form along the wetland bed, where aerobic conditions prevail in the upper region while anaerobic microenvironments can be formed at the bottom. MFCs are also structured similarly to CWs, where the anode and cathode can be strategically placed in anaerobic and aerobic conditions [28]. Macrophytes live in aquatic ecosystems and humid soils, making them an ideal alternative for wastewater treatment, promoting the removal of pollutants and producing bioelectricity, constituting a feasible, low-cost, and sustainable tool for generating clean energy [21,29,30].

This present review aims to give an insight into the use of macrophytes in the wastewater treatment process. The performance in the removal efficiency of organic matter by macrophytes used in CWs coupled with a BES (mainly MFC), focusing on their anatomical and physiological traits, can enhance removal efficiency and bioelectricity generation.

2. Plant Microbial Fuel Cells (PMFCs)

PMFCs consist of living plants, microorganisms, substratum, and electrodes (an anode usually placed at the bottom and a cathode placed on the surface) [31]. The organic matter present in wastewater is converted to electrons, protons, and CO₂ during oxidation by the electroactive bacteria (EAB), and the anode collects the electrons which travel through an external circuit to the cathode; meanwhile, the protons travel through the medium to the cathode (Figure 2). At the cathode, oxygen or other chemicals are reduced together with protons and electrons to water, generating electricity during the process [32,33]. The plant-based microbial fuel cell (PMFC) is a product of the MFC, which uses the exudates released by plant roots to nourish electrochemically active bacteria (EAB) found at the anode and generate bioelectricity. These metabolites (root exudates) act as the electron donor for bacteria that colonize the anodic region (rhizospheric zone) of the PMFC, and the cathodic region, which is located at the surface and exposed to air (oxygen), acts as the electron acceptor, thus converting these root exudates to bioelectricity [34].

Incorporating plants into bioelectrochemical systems (PMFCs) for wastewater treatment may increase organic matter removal efficiency and energy output [29]. In recent years, PMFC technology has been implemented in diverse applications, such as green rooftops [35] and water bodies (marshy wetlands and paddy fields) [36]. Constructed wetlands, coupled with MFC systems (CW-MFC), can be an attractive option for wastewater treatment due to low operation and maintenance costs and high efficiency.

In CW-MFCs, macrophytes promote a high oxidation-reduction potential between the upper and lower layers, mainly due to exudation of oxygen by the root system; this also provides a carbon source and energy (comprising diverse compounds, such as sugars, amino acids, organic acids, phenolic compounds, and secondary metabolites). This rich-carbon environment promotes a highly diverse microbial community that benefits the plant [37]. It has been mentioned that rhizodeposits in a PMFC will be used as the organic matter source first instead of the organic matter in the wastewater [38]. In a PMFC, the roots are located in the anodic zone, where the secreted rhizodeposits provide bacteria as electron donors [39]. When coupled with growing plants, a seven times higher power output was observed with the sedimentary MFC (SMFC) [40]. Different reports mention that Ipomoea aquatica, Typha latifolia, or Canna indica-based wetland-MFC systems showed increased removal efficiency and bioelectricity production [41]. PMFCs with different plant species, i.e., Glyceria maxima, Spartina anglica, Lemna minuta, and Arundo donax, also showed enhanced electricity production using the rhizodeposits as the substrate [29,42,43,44]. Lu et al. [45] reported the maximum current when electrogenic active bacteria (EAB) used only rhizodeposits as a substrate was ~105 mA m⁻². Figure 2 shows the structure of a CW-MFC coupled to T. latifolia, an emergent aquatic macrophyte.

3. Anatomical Traits

3.1. Aerenchyma

Aerenchyma is a very efficient parenchymatic tissue that facilitates oxygen transportation, allowing for the flotation of specific organs and the maintenance of plant structural firmness [14]. Most aquatic plants have constitutive aerenchyma in the root, stem, petioles, and leaves in response to hypoxic conditions, presenting various adaptations among species [14,46].

Aerenchyma is produced after the collapse and lysis of a few mature cortical cells, forming a tissue with wide aerial spaces and air spaces [47]. This tissue allows a faster oxygen diffusion from the shoots to the roots [47,48]; its formation is seemingly caused by ethylene gas in minimal amounts and different parts of the plant body in response to stress conditions (i.e., lack of oxygen) [49]. Aerenchyma cells are star-shaped, leaving very wide intercellular spaces produced by either schizogenous or lysigenous processes, and they constitute 60–70% of the volume of a plant organ (Figure 3) [48,49]. As an alternative strategy, the aerenchyma tissue is interconnected from the stem’s lower part to the leaves’ upper part to provide oxygen and other gases to the plant, thus forming an aeration system [48,49]. Aerenchyma provides an internal oxygenation system. Therefore, its formation is closely related to aquatic habitats and wet soils [47,48].

Macrophytes exhibit morphological changes in the presence of pollutants, which can be easily measured in situ (i.e., aerial elongation and leaf senescence) and used as a biosensor to keep close track of water status [50].

3.2. Root Morphological Features and Biomass

In macrophytes, the root functional traits (RFTs) are mainly responsible for cycling or removing pollutants in aquatic ecosystems, and it has been reported that emergent macrophytes differ from other plants in macronutrients (N and P) and heavy metals removal (Cd, Cr, Cu, Ni, Pb, V, Zn) due to the differences in the RFTs. However, it is still unclear what root type of emergent macrophytes and their RFTs play more significant roles in mineralizing nutrients and heavy metals removal in aquatic ecosystems [5,12,41,51].

Increases in the cross-sectional area of internal air spaces lead to increases in oxygen transport [52,53]. In the case of wetland plants, when low oxygen conditions prevail, the aerenchyma tissues in their root systems contribute to increasing the oxygen transfer [54]. Fibrous root plants with relatively thin roots (i.e., C. indica) had a higher ROL and higher photosynthetic and transpiration rates than plants with thicker roots [5,41,55].

Different root features (morphology, structure, and ecophysiology) of 35 emergent plants were assessed to understand better the two-root-type hypothesis of wetland plants [55]. Plants were divided into two blocks; the first consisted of plants with fibrous roots, and the other of plants with thick roots. The fibrous root plant block presented had a higher ROL (106 ng cm⁻² min⁻¹ on average) and removal rate of the parameters assessed [TN removal rate (82 vs. 77), TP removal rate (50 vs. 42), transpiration rate (3.5 vs. 1.6), photosynthetic rate (8.6 vs. 6), and root activity (79.2 vs. 50.8 ug g⁻¹)] than the thick root plants (54 ng cm⁻² min⁻¹ on average; The findings indicated differences (structural and morphological) among the groups of wetland plants related to their root system [55]. Another study reported a correlation between the ROL and the root porosity, also suggesting that root and photosynthetic features have an important effect on the removal efficiency of organic matter from wastewater by wetland plants, and these traits can be used to properly select the type of plants for their application on constructed wetlands [51]. In regard to nutrient uptake (removal) by plants (Sagittaria trifolia and Caldesia reniformis, C. indica, Iris pseudacorus, Pontederia cordata, Cyperus alternifolius, Vetiveria zizanioides, and Pennisetum purpureum) from wastewater, the root length (S. trifolia and C. reniformis, 36 and 34 cm, respectively) was a main factor, as higher root biomass led (P. cordata had the largest root biomass with 16.68 g) to higher oxygen release that promoted an increase in nutrient uptake or removal. In relation to photosynthetic rate, C. indica presented the highest (12.2 μ mol CO₂ m⁻² s⁻¹) [51]. Phragmites australis is another species widely studied for its use in constructed wetlands [56].

It can also be mentioned that pollutants cause anatomic and morphologic changes in plants compared to those growing in non-polluted environments. The chlorophyll content in plants growing in polluted environments is lower than in plants that grow in non-polluted environments. Also, the chlorophyll in the leaf mesophyll has an irregular molecular structure, forming cavities inside the organelles that make chloroplasts increase their surface area and, consequently, increase the exchange of substances between the chloroplasts and the cytoplasm [57]. In the case of heavy metals, they produce cell damage, also causing a decrease in the chlorophyll content [58,59].

4. Physiological Features

4.1. Oxygen Release

The oxygen released from the plant roots treats the wastewater’s chemical and biological oxygen demand (BOD). Root activity determines its oxidizing ability and encompasses two components: oxygen release and enzymatic oxidation; enzymes are catalysts and mediators for rhizospheric processes, which increase the organic matter degradation in aquatic sediments. Furthermore, with a higher oxidizing potential, the root system strongly influences microbial processes to unlock N, P, and heavy metals [60,61]. Additionally, compared to well-developed plants, the oxygen release rate of roots was significantly higher during the plants’ growth stages. The plant’s ability to release oxygen increases in submerged macrophytes, Potamogeton perfoliatus L. [62,63]. On the contrary, in certain floating plants, such as Nuphar lutea (L.), oxygen was released through the plant body that had diffused directly from the atmosphere and reached the rhizosphere [64]. In such plant types, oxygen is transported to the plant shoot after diffusion into leaves by a gas pressure gradient. Additionally, it can be mentioned that adding air from artificial sources (potentially powered by bioelectricity generated by the system) can enhance the removal of organic matter by 10–12%.

Liu et al. [30] reported the use of a coupled CW-MFC system to treat swine wastewater, comparing different wetland plant performances (C. indica, Acorus calamus, and I. aquatica) and mentioning that removal efficiencies of the parameters studied (COD, electricity generation, and ammonium nitrogen) varied depending on the root type.

4.2. Radial Oxygen Loss

Wetland plants can transport large amounts of oxygen to their roots, and this oxygen (ROL) is used by the plant or rhizosphere microorganisms [65]. The ROL of wetland plants promotes the degradation of organic matter by providing suitable habitats for aerobic, anaerobic, and facultative microorganisms to colonize around the rhizosphere [66]. According to Dong et al. [67], one-third (1/3) of the oxygen released during degradation of organic matter is used by the roots, approximately one-tenth (1/10) is used in the nitrification process, and one-half is used for root respiration. The amount of oxygen released will be influenced more by the plant surface size than the root system size [68,69].

Yang et al. [70] mentioned a relation between the iron plaque developed in the root system and the porosity and ROL in wetland plants, giving them a higher capacity to immobilize heavy metals in their roots. In a report by Rehman et al. [71], it was also mentioned that removal rates of TN and TP in wastewater were correlated with ROL and photosynthetic rate.

4.3. Photosynthetic Rate

The photosynthetic rate significantly influences oxygen release in the case of wetland plants, thus directly influencing the removal efficiency of organic matter (COD, TN, and TP, among others) in the wastewater treatment process in CWs; therefore, photosynthetic rate, ROL, and plant biomass are highly related [12,51,72]. During photosynthesis, the oxygen produced is diffused via aerenchyma tissue, and, except for oxygen used in root respiration, approximately one-third of the oxygen in roots is released into the rhizosphere via ROL, favoring the nitrification process that will directly benefit the plant [73,74].

In the case of P. perfoliatus, it has been reported that the oxygen release capability of the plant was related to its photosynthesis rate, showing an increase in oxygen release due to an increase in the photosynthesis rate [63]. Photosynthesis, oxygen released, and organic compounds excreted by roots were the main factors that promote organic matter removal [30].

In the wastewater treatment process, submerged and floating plants in CWs contribute significantly to enhancing the removal efficiency by supplying oxygen through their roots into the system; E. crassipes has been reported to provide a remarkable surface area in its root system for aerobic bacteria to form biofilm that contributes substantially to organic matter removal. This type of plant possesses aerenchyma and thick leaves that produce high amounts of photosynthetic oxygen that is further transported to the rhizosphere, benefiting aerobic bacteria [10,71,75,76,77].

A study by Li et al. [12] focused on the performance of different species that are commonly used in CWs (C. indica, I. pseudacorus, P. cordata, Cyperus alternifolius, V. zizanioides, P. purpureum, A. calamus, A. donax var. versicolor, Cyperus flabelliformis, C. indica, Iris tectorum, and Scirpus validus). The study showed that the difference in the porosity of their root system was correlated with the ROL, and the ROL rate was significantly related to the plant’s capacity to excel in wastewater. Of all species studied, C. indica had significantly higher ROL and greater domestic wastewater tolerance, obtaining a higher TN, TP, and COD removal.

Oxygen released by the root system could influence the redox potential [78], potentially enhancing the nitrification process and supporting heavy-metal sedimentation; besides releasing oxygen, the root system of emergent plants will release carbon compounds. The root exudates may act as a carbon source for denitrifiers, improving nitrate removal; however, several studies reported that plant uptake is inappreciable [78].

Plants and associated microorganisms break down organic pollutants into harmless substances, either mineralizing them to CO₂ and H₂O or metabolizing them within plant tissue with dehalogenase and oxygenase as degradation catalysts [79].

A report by Saadi et al. [80] mentioned that aquatic plants were ideal for heavy metal accumulation, making them suitable for heavy metal removal present in wastewater; although heavy metals harmed the plant, leading to a decrease in its chlorophyll content in the leaves, the plants assessed had a good capability to accumulate heavy metals.

5. Bioelectricity Generation

A CW-MFC configuration will typically consist of a submerged anode and a surface cathode, separated by a layer of substratum, and the anode and cathode will be connected to an external circuit [81]. In a report by Liu et al. [21], they coupled a constructed wetland with a microbial fuel cell to enhance power generation by utilizing the root exudates of I. aquatic, obtaining a maximum power density of 12.42 mW m⁻². Additionally, compared to the system without plants, the CW-MFC system demonstrated improved nutrient removal efficiencies for COD and TN, demonstrating potential use for wastewater treatment and bioenergy recovery through the CW-MFC approach [21].

Another report of using a CW-MFC focused on the role of macrophytes, specifically Cyperus alternifolius, by analyzing different root placements relative to the cathode. Results showed that planted systems significantly enhance bioelectricity generation and pollutant removal compared to non-planted systems.

The optimal performance was observed when roots were directly on the air-cathode layer, promoting microbial diversity and activity. These findings contribute to understanding the interaction between plant roots and cathodes, providing insights for improving CW-MFC design and efficiency [82].

Another report [30] using a CW-MFC (vertical flow CW) assessed three species of macrophytes (C. indica, A. calamus, and I. aquatica) for treating swine wastewater. It was mentioned that C. indica showed the best organic matter removal efficiency and highest electricity generation, followed by I. aquatica and A. calamus. The study focused on the role of root exudates in promoting microbial biodiversity and enhancing contaminant removal and electricity generation, also suggesting, as mentioned earlier, that macrophyte selection significantly impacts the efficiency of wastewater treatment and energy production in constructed wetlands [30].

Saz et al. [83] reported that the use of vegetated CW-MFC modules, particularly with Typha angustifolia, showed higher treatment efficiencies and bioelectricity production compared to unplanted modules, enhancing microbial activity and electron flow, thus improving overall performance. Gonzalez et al. [84] assessed an integrated system of an MFC and a CW, comparing planted and unplanted systems, mentioning that the planted system had a higher removal efficiency and that plants enhanced bioelectricity generation in the system. Both reports [83,84] emphasize the importance of selecting appropriate vegetation types in CW-MFC systems to enhance their effectiveness in treating wastewater and producing bioelectricity.

Shen et al. [85] reported the use of a CW-MFC (surface flow CW with Hydrilla verticillata); this study had the particularity that the anodes were “enclosed” (densely packed and tied with titanium wire). The study highlighted the benefits of integrating plants and enclosed anodes, emphasizing that enclosing the anodes can enhance the flow of matter through them, boosting electricity generation. Ultimately, enclosure played a crucial role in shaping microbial communities, promoting an increase in EAB.

Yang et al. [86] used a CW-MFC with three different plant species (I. pseudacorus, Hyacinth pink, and P. australis), obtaining the highest power density achieved with I. pseudacorus; this system also presented the highest growth, emphasizing the importance of plant growth in the performance (COD and N removal) of the CW-MFC system, also mentioning that dead plants in the system lowered the energy output. A report by Zang et al. [87] using C. indica mentions how dead aquatic plants are decomposed in an MFC during electricity generation using lignocellulosic biomass for energy production.

6. Macrophyte Biotypes and Wastewater Treatment Efficiency

As mentioned earlier, macrophytes possess unique properties that allow them to grow in water-saturated soils, extensive lacunar systems with constrictions at regular intervals for maintaining structural integrity, a high growth rate, and the ability to incorporate into biomass. It has been mentioned that oxygen release plays an important role in CWs, which can help significantly reduce the hydraulic retention time (HRT). Additionally, temperature and light intensity can enhance DO release, improving CW removal efficiency [71].

Aquatic plants are classified into four main groups according to their growth pattern, as seen in Figure 4. Group I, called emergent macrophytes, is a group of plants with roots in the soil growing considerably above the water, examples of these are C. indica and T. latifolia; Group II, called floating-rooted macrophytes, which are found mainly in waterlogged sediments and are found at water depths of approximately 0.5 to 3.0 m, and include angiosperm plants, examples of these are Potamogeton pectinatus and Nymphaea mexicana; Group III consists of submerged macrophytes, which grow mainly below the water surface; it includes mosses, angiosperms, charophytes, and pteridophytes, examples of these are H. verticillata and Vallisneria americana; and Group IV includes free-floating plants, these plants float without being rooted, examples of these are E. crassipes and L. minuta. The latter group is highly diversified in its habitats and characteristics [6].

As mentioned earlier, RFTs of macrophytes play a central role in the cycling of aquatic pollutants, and there is evidence that emergent macrophytes differ in the reduction in macronutrients (N and P) and heavy metals (Cd, Cr, Cu, Ni, Pb, V, and Zn) due to differences in RFTs [41]. However, it remains ambiguous which root type of emergent macrophytes and their RFTs play a more significant role in the mineralization and removal of nutrients and heavy metals in aquatic systems [41]. It is still unclear whether ROL is an active or passive process in wetland plants, as is the relationship between ROL and photosynthesis, and how ROL varies among plant species and relates to wastewater treatment efficiency [12].

In a report by González et al. [84], in which a CW-MFC (vs. non-planted CW-MFC and a conventional CW with no electrodes) was assessed compared to other systems, it was mentioned that plants enhanced bioelectricity production in the CW-MFC system. The study focused on the removal of organic matter and nitrogen transformation; the results showed a COD removal efficiency of 74–87% (planted CW-MFC), 69–81% (non-planted CW-MFC), and 62–72% (CW); the maximum power density achieved was 8.6 mW m⁻² at an organic loading rate of 7.9 g COD m⁻³ d⁻¹, approximately 1.5 times higher than that of the non-planted system; the planted CW-MFC also had the highest NH₄⁺ removal efficiency (98%). The authors also mentioned that plants were most likely responsible for the decrease in internal resistance in the CW-MFC system that led to increased electricity production.

Yang et al., 2020 [86], determined the role of plants in pollutant removal and bioelectricity production in treating municipal wastewater with a CW-MFC compared to a traditional CW. Multi-anode unplanted and planted CW-MFCs with I. pseudacorus, P. australis, floating, and emergent types were established in fed-batch mode. Compared to the unplanted CW, CW-MFC modules with established vegetation had high treatment efficiencies (COD, NO₃⁻, NH₄⁺, and PO₄³⁻). The CW-MFC planted with I. pseudacorus achieved the highest maximum power density. One important remark stated by the authors was that the plants play a more significant role in electricity production compared to their role in the wastewater treatment process.

In CW-MFC, the presence of macrophytes becomes even more relevant. On the one hand, oxygen released by roots growing around the cathode reduces the internal resistance and improves electrical performance [82]. On the other hand, rhizodeposits and root exudates have been used as fuel (carbon sources), contributing positively to power generation if they are in contact with the anode [32] (

Table 1. Different macrophyte biotypes’ performance in the wastewater treatment process and power generation.

System	Species	Biotype	COD (%)	Nitrogen (%)	Phosphorus (%)	Maximum Power Density	Coulombic Efficiency (%)	References
PCW-MFC	Iris pseudacorus	emergent	46.9	NO3− 51.6 NH4+ 66.2	TP 57.6 PO43− 71.5	25.14 mWm−2	--	Yang et al. (2020) [86]
CW-MFC	Hydrilla verticillata	submerged	64.02	NO3− 77.9	TP 93.54	20.70 mWm−2	--	Oon et al. (2017) [41] Yang et al. (2020) [86] Xu et al. (2021) [88]
CW-MFC	Elodea nuttallii	floating	31.5	NH4+ 66.0	TP 97.5	6.37 mWm−2 41.46 mWm−2	10.28	Zhao et al. (2013) [81] Shen et al. (2018) [85] Yang et al. (2020) [86]
PCW-MFC SMFC	Phragmites australis	emergent	86.6	--	PO43− 81.0	0.856 mWm−3	0.6	Oon et al. (2017) [41] Doherty et al. (2015) [24] Villaseñor et al. (2017) [89] Zhao et al. (2013) [81] Aswad et al. (2020) [31]
CW-MFC	Ipomoea aquatica	floating rooted	94.8	TN 90.8 NH4+ 68.47	--	12.42 mWm−2	1.71	Liu et al. (2013) [21] Liu et al. (2020) [30] Liu et al. (2013) [21]
PMFC CW-MFC	Canna indica	emergent	88.67	NH4+ 73.02 NO3− 57.82	PO43− 88.81	320.8 mWm−2 0.4136 Wm−3 15.73 mVm−2 1.12 V	1.86	Yadav et al. (2012) [20] Wang et al. (2017) [37] Srivastava et al. (2020) [15] Liu et al. (2020) [30]
PCW-MFC	Schaenoplectis californicus	emergent	87.0	TN 98.0	--	8.6 mWm−2	2.4	González et al. (2021) [84]
CW-MFC	Eichhornia crassipes	floating	77.22	NO3− 45.23	--	80.08 mWm−2	2.15	Mohan et al. (2011) [9] Oon et al. (2017) [41]
CW-MFC	Diffenbachia seguine	emergent	94.00 ± 0.05	NO3− 50.02 NH4+ 64.31	PO43− 42.05	7.75 mWm−3	--	Kim et al. (2021) [27]
SP-MFC	Ceratophyllum demersum	submerged	81.16	TN 65.27	PO43− 79.10	24.5 mWm−2	--	Xu et al. (2021) [88]

).

7. Conclusions

Plants that act as phytoremediation agents must show strong growth properties, such as biomass, non-edible nutritional properties, complex root systems, ability to accumulate excess target pollutants, and specific mechanisms for stress tolerance; ROL is positively correlated with most indices measured, including total biomass, aboveground biomass, leaf biomass, root biomass, maximum root length, root porosity, photosynthetic rate, root activity, removal rates of N and P, COD, and electricity production. Root features and physiological characteristics of macrophyte biotypes and species are important determinants of good treatment efficiency and power generation performance. More studies should be carried out in this field, comparing species and biotypes. This information will be useful in selecting the wetland plant for treatment efficiency, and when coupled with BES, voltage can be an added-value product. Further research should focus on optimizing the operational parameters of CW-MFCs so that the capabilities of EAB and plants will allow for the development of a sustainable wastewater treatment process that can provide a solution to two of our society’s greatest challenges: wastewater cleaning and energy. Integrating these two processes will offer a significant advantage by enhancing wastewater treatment in anaerobic zones within the wetland. Harnessing the capabilities of electro-active bacteria, leveraging the beneficial effects of wetland plants, and optimizing various operational parameters, CW-MFCs can become a more efficient and sustainable technology. Finally, the complexities of interchain interactions, treatment of recalcitrant pollutants, and potential applications in biosensing should be carried out, thereby advancing the field and facilitating the practical implementation of CW-MFCs on larger scales.