It is well established in the literature that green systems can reduce pollutants from urban runoff. However, they may have a negative impact on wastewater quality due to chemical leaching of nutrients from the substrate, with nitrogen and phosphorous being the key elements of concern. The process governing the release or retention of nutrients in porous media is adsorption. It is a surface phenomenon driven by molecular attraction occurring when a solid phase (the adsorbent) comes into contact with a liquid or gaseous phase (the adsorbate). This mechanism is typically investigated through batch experiments. Nevertheless, before conducting batch tests, we performed an aqueous extraction to quantify the amount of nitrogen and phosphorus already present in a green-system substrate.

The experiment was carried out by mixing distilled water and soil at well-defined ratios using a magnetic stirrer, with the mixture sealed in a beaker. An S/L (Solid/Liquid) ratio of 1:10 and a stirring time of 24 h were considered to ensure proper dilution of the medium. The sample was then centrifuged for 15 minutes at 4500 rpm to ensure proper separation of the solid and liquid phases. Afterwards, the supernatant was filtered (90 μm) under vacuum, and the extracted liquid phase was analyzed for reactive phosphorus, nitrate nitrogen, and ammonium nitrogen using a UV-VIS spectrophotometer. Results from the aqueous extract showed very low concentrations of reactive phosphorus, nitric nitrogen, and ammonium nitrogen with values of 1,24 mg L^-1, 2,83 mg L^-1, and 0,084 mg L^-1, respectively. These findings indicate that the substrate does not contribute significantly to nutrient loads through initial leaching. In addition, the results provide an essential reference point for the design and interpretation of subsequent adsorption batch experiments, and they indicate that the substrate has a negligible initial nutrient release.