Effective Adsorption of Chlorinated Polyfluoroalkyl Ether Sulfonates from Wastewater by Nano-Activated Carbon: Performance and Mechanisms
The adsorption capacity of adsorbents is closely related to the specific surface area and pore structure of the adsorbents [32]. In order to investigate the effect of different activation temperatures on the pore structure of coconut shell-activated carbon, N2 adsorption and desorption experiments were carried out on the prepared coconut shell-based activated carbons at 77 K. As shown in Figure 2a, the N2 adsorption-desorption isotherms of three coconut shell activated carbons are all type I, and when the relative pressure P/P0 is less than 0.1, the adsorption amount of N2 increases rapidly, indicating that three coconut shell activated carbons all have a rich microporous structure. The P and P0 represent the actual pressure and saturated vapor pressure of N2 at the measured temperature, respectively. The relative pressure P/P0 can be increased by increasing the input of N2, which enhances the adsorption of N2 on adsorbents. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, there are six types of N2 adsorption-desorption isotherm curves. The N2 adsorption-desorption isotherms of activated carbons usually exhibit type I, suggesting that they are microporous [33,34]. The results of pore size distribution further confirmed that the three kinds of coconut shell activated carbons were mainly micropores, which were mainly distributed at 0.8 nm, 1.2 nm, and 1.5 nm, and had a small number of mesoporous pores (>2 nm) (Figure 2b). According to the parameters of pore structure (Table 1), the specific surface area and total pore volume of coconut shell activated carbon increase with the increase of activation temperature, which can be attributed to the fact that the increase of activation temperature promotes the corrosion of KOH to the coconut shell carbon and promotes the formation of nanopores. During the corrosion process, potassium-containing compounds such as K2CO3, K2O, and K were formed. When the activation temperature further reaches the boiling point of metal potassium (762 °C), the potassium vapor escapes and diffuses into the carbon layer, resulting in an enlarged carbon lattice and richer nanopore structure.
The elemental analysis results of coconut shell activated carbons are shown in Table 2. As the activation temperature increased from 600 °C to 800 °C, the carbon content of the coconut shell activated carbon increased, indicating that the activation temperature increased the carbonization degree of the coconut shell carbon [35]. The decrease in the content of hydrogen and oxygen elements may be due to the escape of gases (such as water vapor (H2O) and potassium oxide (K2O) gas) produced in the process of high-temperature pyrolysis [36,37]. The escape of gases creased a complex porous network and increased the specific surface area of coconut shell activated carbons (Figure 2 and Table 1).
