3.1 Preparation of polystyrene hollow nanospheres H-CPS
Firstly, core-shell polystyrene nanospheres were synthesized via modified emulsion polymerization. By delaying the addition of the cross-linking agent divinylbenzene during the process, core-shell polystyrene nanospheres (CPS) with a linear core and cross-linked shell was successfully produced. Considering that DMF can dissolve linear polystyrene but not cross-linked polystyrene, the obtained CPS was etched with DMF to produce hollow polystyrene nanospheres H-CPS, as shown in Fig. 1 (a) and (b). Both CPS and H-CPS exhibit uniform size and favorable monodispersity. The diameter of CPS is approximately 330 nm, which slightly decreased to approximately 260 nm after etching treatment to form H-CPS. Additionally, the shell thickness of the obtained hollow sphere is around 75 nm. Hence, polystyrene hollow nanospheres have been successfully prepared.
3.2 Preparation of sulfonated polystyrene hollow nanospheres H-CPS-SO3H
Subsequently, sulfonation treatment was performed on the as-synthesized H-CPS to obtain sulfonated polystyrene hollow spheres (H-CPS-SO3H). The effectiveness of this treatment was confirmed by analyzing the infrared spectrum, as depicted in Fig. 2. The spectrum showed new peaks at 1650, 1412, and 1389 cm− 1, which corresponded to the vibration of the sulfonated benzene ring. Furthermore, several strong IR peaks between 1000 and 1250 cm− 1 appeared, which were attributed to the symmetric and asymmetric vibrations of the S = O double bond on the introduced sulfonic acid group. These findings clearly demonstrate the successful preparation of H-CPS-SO3H. Furthermore, the N2 physisorption isotherms indicate that H-CPS-SO3H has BET surface areas of 37m2/g.
3.3 Study on urea adsorption performance of H-CPS-SO3H
The kinetics experiment of H-CPS-SO3H adsorbing urea was investigated to determine the rate and maximum adsorption capacity of urea. As depicted in Fig. 3, the adsorption of urea by H-CPS-SO3H was rapid in the first 10 minutes and the correlation between adsorption capacity and time was nearly linear. Afterward, the adsorption rate decreased and reached equilibrium after 30 minutes with a maximum urea adsorption capacity of approximately 0.9 mmol/g. The kinetics experiment indicates that the rate of urea adsorption by H-CPS-SO3H is exceptionally fast, reaching equilibrium within approximately 30 minutes and remaining substantially stable in the following 1.5 hours. To ensure a thorough examination of the adsorption ability of H-CPS-SO3H, the adsorption time was set to 2 hours in the subsequent experiments.
In the introduction, we mentioned that acid can react with urea in water. To examine the impact of acid concentration, specifically pH, we conducted the following experiments. Initially, urea aqueous solutions with pH values of 3, 4, 5, and 6 was obtained by adjusting with HCl solution. Then H-CPS-SO3H was added to the above solutions and incubated them at 37℃ for 2 h. Finally, the filtrate was collected to determine the urea concentration. As depicted in Fig. 4a, in the pH range of 3–6, the adsorption capacity of urea marginally increased as pH decreased, peaked at pH 4 with a maximum adsorption capacity of 0.85 mmol/g, and then slightly declined. Prior studies have indicated that H+ can protonate urea to create (NH2)2COH+ cation [17, 18]. In this experimental system, (NH2)2COH+ interacted with the negatively charged sulfonic acid groups on H-CPS-SO3H via electrostatic attraction, leading to the adsorption of urea onto H-CPS-SO3H. Therefore, we hypothesize that the decrease in pH results in the formation of more (NH2)2COH+ and subsequently adsorption onto H-CPS-SO3H, causing the increase in urea uptake capacity. However, as the pH decreases further, the ionic strength of the entire system increases, resulting in a decrease in electrostatic interaction between (NH2)2COH+ and sulfonic acid groups, hence leading to a decrease in urea adsorption capacity.
To confirm our hypothesis regarding the effect of ionic strength on the urea uptake ability of H-CPS-SO3H, we conducted further experiments. Firstly, urea aqueous solutions with different ionic strength values ranging from 5 to 500 mM were prepared by adjusting the amount of sodium chloride. Subsequently, H-CPS-SO3H were added to aforementioned solutions. The samples were shaken at 37℃ for 2 h, after which the filtrate was collected by centrifuging and the urea concentration was measured. As shown in Fig. 4b, the results clearly demonstrate that the ionic strength of the solution has a significant impact on the urea binding capacity of H-CPS-SO3H. Specifically, as the ionic strength increased, the urea binding capacity of H-CPS-SO3H decreased sharply, reaching a minimum of 0.2 mmol/g at 500 mM of ionic strength. These findings are in agreement with our previous hypothesis. Notably, although the concentration of (NH2)2COH+ remained stable due to the constant pH value, the increasing ionic strength weakened the electrostatic interaction between (NH2)2COH+ and sulfonic acid groups, ultimately resulting in the decrease of urea binding capacity.
The adsorption performance of H-CPS-SO3H towards urea is not only affected by parameters such as pH and ionic strength, but also by the volume of the urea aqueous solution. Figure 5a shows that as the volume of the solution increases, the adsorption capacity of H-CPS-SO3H to urea first increases, reaches a maximum value of about 1 mmol/g at a volume of 8 mL, and then slowly decreases. We hypothesize that this is due to the increase in the total amount of urea in the system with increasing volume, leading to an increase in the content of (NH2)2COH+ and ultimately an increase in urea adsorption capacity. However, when the volume of the urea solution exceeds a certain threshold, the (NH2)2COH+ generated saturates the sulfonic acid groups on H-CPS-SO3H, and the increase in urea solution volume dilutes the H+ concentration, resulting in a slight increase in pH. This leads to a reduction in urea adsorption capacity of H-CPS-SO3H when the solution volume is further increased.
Reusability would be an important feature of a urea sorbent, lowering its ecological footprint. Thereto, we investigated the regenerability of the urea sorbent H-CPS-SO3H. To regenerate H-CPS-SO3H, the urea-bonded H-CPS-SO3H was first treated with 500 mM sodium chloride, followed by diluted HCl treatment. As shown in Fig. 5b, after 5 regeneration cycles, up 90% of the adsorption capacity was still retained, which indicates that H-CPS-SO3H had excellent reusability.