*5.6. Vegetation*

Vegetation is another key component in CW systems as plants cannot only directly uptake pollutants, but also modify the surrounding environment, for example, by transporting oxygen into a rhizosphere to enhance the diversity and biomass of microorganisms, microbial degradation, and sequestration [104]. Several studies have compared treatment with plants versus without plants and most of them showed higher removal performance for herbicides [92,93,96] and pesticides [82,97] with plants as treatment, while one study presented no significant difference with plant treatment for veterinary antibiotics [39]. Research has also been performed to compare the removal performances of various plant species. For example, Lyu et al. (2018) compared five plant species (*Typha latifolia*, *Phragmites australis*, *Iris pseudacorus*, *Juncus effusus* and *Berula erecta*) for pesticide removal and found *Berula* to contribute to significantly higher removal efficiency compared to the rest four plant species [97]. Moreover, Tang et al. study (2019) indicated that *Canna indica*, *Cyperus alternifolius* and *Iris pseudacorus* had better removal performance for pesticides than *Juncus effusus* and *Typha orientalis* [82]. However, Souza et al.'s study (2017) showed no significant differences in pesticide removal among *Polygonum punctatum*, *Cynodon spp.* and *Mentha aquatica* [81].

Another study also confirmed that vegetation type impacts antibiotic removal efficiencies in surface flow CWs. The authors compared three varieties of ryegrass (*Dryan, Tachimasari* and *Waseyutaka*) to treat three antibiotics (sulfadiazine, sulfamethazine, and sulfamethoxazole) and found that *Dryan* outcompeted the other two types of plants due to its highest removal rates for both nutrients and antibiotics [62]. Gikas et al. (2018) compared treatments planted with *Phragmites australis* and *Typha latifolia*, with an unplanted control and the results showed that *Phragmites australis* had the highest removal capacity for both herbicide (S-metolachlor) [96] and pesticide (terbuthylazine) [92].

Besides the plant species, studies have also shown that various planting patterns may impact the removal performance. Huang et al. (2019) compared CW treatments with single plant species and mixed plant species and found that CWs with single plant type performed better in reducing antibiotic and ARG concentrations [88]. These findings imply that different plant species and planting patterns should be applied to achieve best performance depending on the target contaminant. Furthermore, studies indicated that after a certain time of exposure to the pollutant, the plant would uptake the pollutants with more concentrations in the root part than in the shoot part [62]. Harvesting the vegetations planted in the CW reduced the concentration of antibiotics in the soil, implying plant harvest as an effective procedure to maintain sustainable efficient removal performance [86].

#### **6. Research Bottlenecks and Prospects**

As stated in previous sections and presented in Table 1, numerous studies and reviews have been performed either on the topics of constructed wetlands pollutant removal performance or chemicals of emerging concern (such as pesticides, herbicides, veterinary antibiotics, etc.), but fewer studies have been focused on the overlapping research area of these two topics, which is using constructed wetlands to remove CECs. Among those studies, even fewer are related to agricultural runoff, since most of them studied treating domestic sewage or effluent from wastewater treatment plants. Even those on agricultural runoff, the studies are dominated by nutrients and sediment. Therefore, most future research needs to be performed on the application of CWs to remove CECs from agricultural runoffs. In addition, compared to livestock and poultry wastewater treatment applications, even fewer data were collected and reported from aquacultural wastewater and farm runoff either due to irrigation or precipitation. That is to say, more studies need to be conducted in these specific areas to safeguard our water resources, environmental, and human health.

From the scale's perspective, the majority of current studies are mainly in lab scale or pilot field scale, with only a few papers reporting the data from full-scale field studies. Small-scale studies in a controlled environment in the laboratory or greenhouse setting are valuable to serve as the first step attempt to address the research questions, but eventually large-scale studies fitting the real-world scenario are still needed for future applications. The designed CW system needs to be tested under real field conditions with fluctuating temperatures, flow rate, redox state, etc. to prove its durability. Nowadays, climate change has resulted in more extreme weather conditions happening more frequently; therefore, future research should also take consideration of the impacts of extreme weather, such as flooding and drought, on designed CW systems. To assist the optimization of design parameters, predication models coupled with remote sensing data could be built for simulating various conditions and potential extreme weather events. With the screening feedback from such models, suggestions could be provided for future application development.

In addition, after a period of operation, the CWs could accumulate the CECs within the system and lead to the development of ARGs into the local environment by self-developing and transferring to other microorganisms [101]. Especially for the down-flow SSVF CWs, the enrichment of pollutants and ARGs in the surface soil could become problematic in the long term [86,87]. Therefore, it is necessary to investigate methods to periodically remove and safely treat the accumulated contaminations from the CW system in order to maintain a sustainable high removal performance in the long term. Currently, very few papers have reported such operation and maintenance practices for CW applications.

For the theoretical investigation part, it is broadly accepted that the CW removes pollutants through a variety of processes, including adsorption to the substrate and soil, plant uptake, and biological degradation. The physiochemical sorption process has been well studied based on parameters, such as solubility (S), sorption coefficient (Kd), octanol–water coefficient (Kow), oxidation-reduction potential (ORP), pH and pKa, with a lot of studies reporting certain correlations between the above parameters and removal efficiencies. However, most of the current studies failed to provide detailed explanations on biological processes and their role in the pollutant removal [40,41,86,89]. Therefore, further research is also needed for understanding the mechanisms of microbial biodegradation and plant uptake of CECs within the CW systems. For example, more research can explore various microorganisms' functions under aerobic/anaerobic conditions and compare contaminant uptake at different plants parts (root/stem/leave/shoot/etc.). The identification of optimal conditions for biodegradation and extraction of plant tissues with highest accumulation could be beneficial for future CW system applications by providing suggestions of ideal set-up conditions as well as operation protocols, such as harvesting the plant parts with greatest pollutant accumulations to maintain a high removal rate throughout the entire treatment period.

Current studies have also reported contradictory results of ARG occurrence and removal within the CW systems as some of the studies showed significant removal of ARGs with CWs since they arrest and inhibit the growth of bacteria, while others reported increases of ARGs due to the exposure and adaptations to accumulated contaminants in the substrate/soil. Therefore, future research is also needed to determine the internal complicated processes and mechanisms underlying various conditions of ARG sequestration and removal within the CW system. Based on these results, application suggestions of CW could be provided to avoid ARG accumulation during operation. In addition, further studies could be performed on the evaluation of potential impacts of ARG accumulation within the CW system, such as whether accumulated ARGs are going to change the structure of microorganisms within the system and the system performance; or whether the accumulation may lead to increase in effluent ARG concentrations. If severe impacts are noticed from such accumulation, future research on appropriate approaches to prevent the ARG accumulations will be needed.

With successful CW design, we can treat wastewater containing various contaminants in an efficient and economical manner. However, there are several ways we can improve the performance removal of the pollutants by CWs. For example, finding ways to promote aeration in the CWs can enhance aerobic biodegradation. Additionally, selection of substrate medium is key to achieving better elimination of ARGs. Studies also showed hybrid setup to perform differently based on the order of SF or SSF. Moreover, plant species affect the performance of the CWs. Consequently, screening of plants and plant selection is important for improving the removal efficiency. Another potential method is to improve the design of CWs, for example, CWs in series to boost performance.

Because nature-based systems need time to establish and function, real field studies over longer period without spiking concentrations are needed. As short-term studies with spiked concentrations may not represent the true removal efficiency, real field studies conducted for a long time are required. In addition, sampling strategies, for example, before vs. after in long-term study rather than treatment vs. control, may be needed to represent the efficiency of removal.
