## 3 Results and Discussion  
### Adsorption Kinetics  
The mass loading of fecal ash and the initial urine concentration are two critical parameters determining the maximum nutrient removal efficiency during adsorption experiments. As a result, it is essential to predict the range of values for the above mentioned factors, which would maximize nutrient recovery efficiency. As shown in Figure 1a, with fecal ash loading of 0.5 g-1.5 g, urea sorption was high, exceeding 660.37 mg.g\({}^{-1}\); this may be due to the existence of sorption sites on the adsorbent's surface. When the dose was raised from 1.5 to 2.0 g, a significant decrease in equilibrium adsorption capacity occurred (results decreased to 481.73 mg.g\({}^{-1}\)). This was mainly due to the adsorbent material's abundance of sorption sites compared to the solution's urea concentration. As the mass loading varied between 0.5 and 2.0 g, urea removal increased from 80.63% to 92.80%. Onegram of loading was shown to be the optimal dosage for 0.1 L of synthetic urine, with an increase in urea recovery of less than 5% for loading doses of 1.0-1.5 g.  
A set of experiments were performed at various urine concentrations to see how the initial urine concentration affects mass transfer resistance between the adsorbent and adsorbate. To achieve different initial concentrations, deionized water diluted the urine (from its original concentration), thereby achieving varying initial concentrations. Keeping the adsorbent loading of fecal ash at a constant 1.0 g, the effect of varying initial concentrations of urine (between 25% and 100%) was studied. It was observed that when the urea concentration was increased from 25% to 100%, the adsorption capacity increased from 547.42 to 1242.13 mg.g\({}^{-1}\). The likelihood of urea and fecal ash molecules colliding rises with concentration, allowing urea molecules to overcome the barrier seen between the solid and aqueous phases. The smooth and continuous curves obtained at all concentrations indicate that monolayer urea may be adsorbed onto the ash [24,40]. It took 180 min for the sorption to achieve equilibrium. Figure 1b, demonstrates that a significant quantity of urea was adsorbed during the first 120 min, following which stirring was used to reach equilibrium with very little urea being adsorbed. The large surface area of the fecal ash accounts for the initial phase's high adsorption capability. The increased surface area over the fecal ash is the increased uptake capacity during the initial phase. Until 120 min, a thick one molecular layer of urea is produced over the fecal ash, at which point the rate of urea transfer regulates the rate of sorption from the outside to the inside of the ash. Similar findings have been obtained in many investigations employing activated carbon to remove carboxylic acids, dyes and heavy metals [41, 42, 43]. Urea recovery was higher than 80% at all concentrations, implying that the starting concentration has no impact on the quantity of urea recovered at equilibrium. Figure 1: _Cont._  
The impact of temperature was investigated using a shaker speed set to a constant 150 rpm and a fixed adsorbate concentration of 13.375 g.L\({}^{-1}\), and a fixed adsorbent loading of 1.0 g. The experiments were carried out at 3 unique incubator temperatures, and  
Figure 1: The effect of process parameters on adsorption of urea. (**a**) Fecal ash loading (initial urine concentration—13.5 g.L\({}^{-1}\), temperature—30 \({}^{\circ}\)C); (**b**) initial urine concentration (fecal ash loading—1.0 g, temperature—30 \({}^{\circ}\)C); (**c**) temperature (fecal ash loading—1.0 g, initial urine concentration—13.5 g.L\({}^{-1}\)). the adsorption capacity was found to decrease from 92.8% at 30 \({}^{\circ}\)C to 75.13% at 40 \({}^{\circ}\)C (Figure 1c). As a result, the urea adsorption capacity was 1242.13 mg.g\({}^{-1}\), 1097.55 mg.g\({}^{-1}\), and 1013.85 mg.g\({}^{-1}\) at 30 \({}^{\circ}\)C, 35 \({}^{\circ}\)C, and 40 \({}^{\circ}\)C, respectively, consistent with an exothermic nature of adsorption.