## 1 Introduction
Reports indicate that CWs based on gravel remove only about 45% and 35% for NH\({}_{4}\)-N and orthophosphate, respectively [19; 12]. In CW systems, choosing suitable substrates is essential for maximizing treatment effectiveness since these substrates are important for the production of biofilms and adsorption process [16; 20]. Zeolite substrates for CWs have a broad variety of applications and a good removal efficacy. Zeolites occur in a wide variety and are available worldwide. There are many types of zeolites, including synthetic and natural zeolite. In addition to expanding the range of applications for zeolite in wastewater treatment, using it as a CW substrate increases the material's usefulness as a green absorbent material [21]. It possesses a high ion exchange capacity, a large specific surface area, a robust adsorption capacity, and the ability to support microbial colonization. Its incorporation has been associated with enhanced nitrogen removal and biofilm formation [22; 23; 24; 25], while its impact on plant and microbial interactions remains a subject of active investigation. Also, zeolite may compensate for the shortcomings of a single substrate and greatly enhance removal efficiency when combined with other materials, such as the CW substrate [26]. The removal of ammonium nitrogen is one of the most important uses of zeolite. Research shows that CWs with zeolite substrates outperform traditional substrates like sand or gravel, achieving ammonium removal rates of over 95% [27]. Zeolite is essential for retaining phosphorus and removing heavy metals in addition to nitrogen. Hexavalent chromium Cr(VI) removal through microbial-mediated processes was greatly enhanced with modified zeolite coatings (e.g., Zn-layered double hydroxides, Zn-LDHs), whereas zeolite-filled tidal flow wetlands in hybrid CW systems achieved 46-64% phosphate removal. Zn-LDH-zeolite substrates enhanced the synthesis of extracellular polymeric substances (EPSs) and microbial activity (e.g., Novosphingobium, Brevundimonas), both of which are essential for the reduction of Cr(VI). Zeolite is also exceptionally effective at removing pesticides and pharmaceuticals. Zeolite substrates removed \(>\)98% of antibiotics, such as ciprofloxacin and erythromycincin, from tidal flow CWs treated with pharmaceutical wastewater, mostly due to microbial decomposition (61% contribution) [28]. In a comparable way, zeolite-enhanced CWs outperformed gravel-only systems in removing fluopyram from pesticide-laden rinsing water by 72%, with bioaugmentation further improving results [29]. A major problem that occurs in CWs with an impact on their sustainable development is clogging. This occurs through various mechanisms, such as the sedimentation of suspended solids, the accumulation of organic matter in the substrate, biofilm development, and plant root system development. The factors that influence and have the greatest impact on CW clogging are the porosity of the substrate and the organic load. The smaller the porosity of the substrate, the easier it is to clog. Porosity depends on the type, grain size, and unevenness coefficient of the substrate. When the grain size is 3-4 mm, there is a risk of clogging, while substrates with larger grain sizes effectively prevent clogging [30,31]. While various substrates have been investigated for their ability to enhance TKN, NH\({}_{4}\)-N, and TP removal in CW systems, a significant gap remains regarding natural zeolite as a substrate. Especially for natural zeolites, the quantification of the adsorption capacity and a comprehensive description of the adsorption mechanisms have not been investigated, although the zeolite substrate in CWs guarantees effective nitrogen and phosphorus removal [32]. In the literature, there are studies describing the ability of zeolites to remove ammonia and orthophosphates through adsorption, but they refer to zeolite that is either in powder or granular form with a very small grain size (i.e., \(<\)4 mm) [33, 34, 35, 36].