## 2 Materials and Methods  
### Experimental Design of Adsorption Kinetics  
In order to evaluate the adsorption of ammonium cations (NH\({}_{4}\)+) and orthophosphate anions (PO\({}_{4}\)-3) on porous media, adsorption kinetics and isotherm experiments were conducted at the Laboratory of Ecological Engineering and Technology, Department of Environmental Engineering, Democitus University of Thrace (DUTh), Xanthi, Northern Greece. The selected porous media (or substrate) were (a) coarse-grained zeolite (codename: CG-Z, range 8-16 mm), (b) fine-grained zeolite (code name: FG-Z, range 1-2.5 mm), and (c) coarse-grained gravel (code name: CG-G, range 8-16 mm). The zeolite (provided by \"ZEOLIFE\", Thessaloniki, Greece) consists primarily of SiO\({}_{2}\) (-69.62%), Al\({}_{2}\)O\({}_{3}\) (-13.62%), and other minor components including CaO (-3.28%), K\({}_{2}\)O (-2.94%), MgO (-0.9%), Na\({}_{2}\)O (-0.55%), Fe\({}_{2}\)O\({}_{3}\) (-0.75%), and TiO\({}_{2}\) (-0.11%), with a loss of ignition (LOI at 1050 \({}^{\circ}\)C) of 8.23% and a clinopilolite content of at least 85%. The gravel was of igneous origin with the following composition: Si 28.50%, Al 7.95%, Fe 4.22%, Ca 3.62%, Mg 1.76%, and P 0.11% and was obtained from the bed of a local river. The chemical composition of the gravel was measured at the Laboratory of the Institute of Geological and Mining Research, Xanthi, and the granulometric analysis was performed at the Laboratory of Building Materials, Department of Civil Engineering, DUTh, Xanthi. The porous materials were washed with deionized water to remove dust and placed in an oven for 24 h at 105 \({}^{\circ}\)C for drying. One of the more important physical characteristics of an absorbent used to remove pollutants in CWs is surface area. The precise calculation of surface area using the Brunauer-Emmett-Teller (BET) isotherm analysis may help to explain the differences among the investigated materials. However, it was decided to perform batch adsorption studies in order to include the ions under consideration (i.e., ammonium and orthophosphate) and to obtain a comprehensive picture of the sorption capacity of the investigated materials for these ions. For the kinetic adsorption experiments on ammonium ions, approximately 100 g of adsorbent material (i.e., CG-Z, FG-Z and CG-G) and 500 mL of NH\({}_{4}\)Cl solution with an ammonium nitrogen concentration of 35 mg/L were placed in glass bottles. The pH of the solution was adjusted to be approximately 7.0-7.5 (natural value in constructed wetlands). For each adsorbent material (or porous medium), 3 series of 7 bottles each (a total of 63 bottles) were prepared. Furthermore, seven control bottles were prepared using only ammonia solution. The bottles were sealed airtight and placed in a thermostatic chamber at 20 \({}^{\circ}\)C and continuously stirred (start of experiment, time 0 h). At a certain time after the beginning of the experiment, i.e., 3, 6, 12, 24, 48, 96, and 144 h, one bottle of each adsorbent material was taken and the ammonium nitrogen (NH\({}_{4}\)-N) concentration in the solution was measured. For the orthophosphate adsorption kinetic experiments, the same procedure was followed as in the case of NH\({}_{4}\)-N. In this case, 500 mL of an aqueous solution with a PO\({}_{4}\)-P concentration of 20.0 mg/L was added to glass bottles containing approximately 100 g of adsorbent material each. This solution was prepared by adding KH\({}_{2}\)PO\({}_{4}\) and K\({}_{2}\)HPO\({}_{4}\) in an appropriate amount and ratio in distilled water so that the pH of the final solution was 7.2 \(\pm\) 0.2. A total of 72 bottles were prepared (i.e., for each adsorbent, 3 rows of 8 bottles each). Furthermore, eight control bottles were prepared using only orthophosphate solution. The bottles were placed in a thermostatic chamber at 20 \({}^{\circ}\)C and continuously stirred.