## 1 Introduction
However, their main disadvantages are high energy requirements for their operation, high maintenance costs, and the production of a concentrated waste stream [2]. In recent years, increasing interest has been given to the treatment of landfill leachate using advanced oxidation processes (AOPs). The characteristic of these methods is that they produce hydroxyl radicals, which can break down a wide range of compounds. AOPs are particularly effective as they can oxidize many types of organic compounds into simpler ones that are biodegradable. Thus, they can be used as pretreatment methods for landfill leachate before further treatment in traditional biological treatment plants or natural systems (e.g., constructed wetlands) [20]. The main methods are the Fenton process, ozonation, photocatalysis, and electrochemical oxidation [21]. These methods remove ammonia and COD to a satisfactory degree. However, they present some disadvantages, such as large amounts of by-products (the Fenton process, electrochemical oxidation), a high energy consumption (ozonation, electrochemical oxidation), limited full-scale application, and potential electrode degradation over time (electrochemical oxidation) [2; 12]. Furthermore, although the aforementioned methods can be effective in treating landfill leachates, they often require sophisticated operational controls and maintenance, which can be challenging for some landfill operators, particularly in developing regions [22]. In response to these challenges, constructed wetlands (CWs) have gained attention as a sustainable and cost-effective alternative for effective pollutant removal. These engineered systems belong to biological treatment technologies, and they are similar in their mode of operation to natural wetlands, utilizing synergistic interactions between water, vegetation, microorganisms, and the substrate to remove pollutants from leachate [23; 24]. Constructed wetlands can remove a wide variety of pollutants through various processes occurring in their environment, primarily including sedimentation, adsorption, plant uptake, microbial degradation, and chemical precipitation. CWs used for wastewater treatment can be categorized into two main types: free-water surface (FWS) and subsurface flow (SF) wetlands. In FWS CWs, the water flows on the surface, supporting a variety of aquatic plants. SF CWs, on the other hand, have water flowing beneath the surface through a gravel or soil medium, which supports plant roots and provides a habitat for microorganisms. SF CWs are further divided into horizontal subsurface flow (HSF) and vertical flow (VF) systems, each with distinct design characteristics and treatment mechanisms [25; 26]. CWs offer numerous advantages over traditional treatment methods [27]. Their advantages are as follows: (a) They generally have lower construction and operating costs than conventional treatment systems [28; 29]. (b) They utilize natural processes, reducing the need for expensive and energy-intensive mechanical equipment. (c) These systems rely on natural energy sources, such as sunlight, wind, and plant growth, minimizing the need for external energy inputs. (d) CWs are characterized by simplicity and robustness as they are simple to design and operate, making them suitable for use in remote or underserved areas where advanced treatment technologies may be impractical. Additionally, CWs provide ancillary benefits such as habitat creation, biodiversity enhancement, and landscape aesthetics. They also have the potential to sequester carbon, thereby contributing to climate change mitigation efforts [30]. Despite their advantages, constructed wetlands also face several challenges and limitations. Their performance and efficiency can be influenced by climatic conditions, particularly in colder regions. Their proper design and operation and ensuring their optimum performance require the careful selection of hydraulic loading rates, hydraulic residence times, and vegetation.