### Adsorption Processes
Metal-organic frameworks have also been investigated and have shown promise in terms of PFAS adsorption. For instance, [PERSON] et al. [11] found that a mesoporous Cr-organic framework outperformed GAC in PFAS adsorption. Recently, [PERSON] et al. [39] presented findings on the potential of covalent organic frameworks (COFs) characterized by structural regularity and task-specific functionality as highly promising materials for efficiently removing PFASs. In their study, they synthesized hollow COF nanospheres grafted with cystamine through a hard-template method. Subsequently, they functionalized these nanospheres using the thiophene \"click\" reaction. The results demonstrated the exceptional efficacy of these functionalized nanospheres in removing various anionic PFASs that included carboxylic, sulfonic, and phosphoric groups, as observed through batch and column-mode experiments. Remarkably, the Langmuir maximum adsorption capacities of the functionalized material towards three PFASs were nearly double those of powdered activated carbon (AC). In summary, while AC remains widely used for PFAS removal, anion exchange resins and metal-organic frameworks offer promising alternatives with higher adsorptive capacities for specific PFAS compounds, particularly short-chain PFASs. GAC, though less effective in some cases, remains suitable for certain treatment processes, and its regeneration capability is advantageous in terms of cost-effectiveness. While adsorption stands as a well-established method for removing PFASs from water, it presents certain limitations requiring attention. One challenge lies in the concentration of PFASs from the solution phase to the solid phase during adsorption, necessitating appropriate treatment for handling spent adsorbents [31]. The effective destruction of adsorbed PFASs, either through regeneration or safe disposal of spent adsorbents, is imperative in  
Figure 2: Possible mechanisms of PFAS adsorption. (Green colour = PFAS; large porous ball in the centre = adsorbent) (modified from [36]). this context. Moreover, the relatively low removal efficiency of short-chain PFASs and mixtures of PFAS compounds and their precursors poses another obstacle, prompting research efforts to develop novel adsorbent materials, enhance the removal capacity of existing materials, and improve the regeneration potential of adsorbents [31]. The widespread application of ion exchange resins and carbon nanotubes (CNTs) is hindered by their cost despite their promising efficiency as adsorbents [35]. Conversely, abundant biosorbent materials like plant wastes, wood chips, compost, manures, and sewage sludge offer inexpensive, renewable, and environmentally friendly alternatives to activated carbon and resins [22; 34; 35]. However, their adsorption capacities are relatively modest, necessitating research to modify them by functionalizing surfaces with cationic groups to increase adsorption capacities and identifying cost-effective regeneration or disposal methods for used adsorbents. For example, [PERSON] et al. [40] crosslinked chitosan beads and utilized them as efficient biosorbents to remove PFOS from aqueous solutions. This modified chitosan biosorbent exhibited a sorption capacity of up to 5.5 mmol/g for PFOS at an equilibrium concentration of 0.33 mmol/L, surpassing the capabilities of some conventional adsorbents. The authors identified the mechanisms of adsorption, including electrostatic and hydrophobic interactions, along with the contribution of hemi-micelles and micelles formed in the porous structure, which led to the high adsorption capacity. In general, most of the research on PFAS adsorption has been conducted at the laboratory scale under ideal conditions and in short-term studies [21; 31; 34]. Therefore, it is crucial to validate the developed methodologies in real-world, large-scale applications through longer-term studies involving actual wastewater containing organic matter and other contaminants.