# A Review of Treatment Techniques for Short-Chain Perfluoroalkyl Substances
## 3 Treatment Techniques for Short-Chain PFASs
### Adsorption Technique
#### 3.1.1 Adsorption by Carbon-Based Adsorbents  
AC adsorption. AC is widely used to remove contaminants in wastewater due to low cost, high efficiency, and convenient operation [4]. [PERSON] et al. adopted granular activated carbon (GAC) to adsorb PFBS, with an adsorption capacity of 98.7 mg/g [23]. [PERSON] et al. used GAC and powdered activated carbon (PAC) to carry out batch adsorption experiments for short-chain PFASs; PAC achieved higher removal rates within 10 min (20-40% for GAC, 60-90% for PAC) due to shorter internal diffusion distance and higher BET surface area (\(\mathbb{S}_{\text{BET}}\)) of PAC [24]. This phenomenon demonstrated that the particle size of AC was a significant factor affecting the removal efficiency of short-chain PFASs, and smaller particle sizes presented superior removal efficiency. The effect of pore size on the removal efficiency of bamboo-derived AC (BdAC) and coal-based AC (microporous type) for PFHxA were investigated, revealing that the BdAC adsorption capacity was 13-fold lower than microporous ACs [25,26]. In addition, the GAC was explored to eliminate different carbon chain length PFASs [27], the removal efficiency of short-chain PFCASs was lower than congeners PFSAs, in which the removal rates of PFBA, PFPeA, PFHxA, and PFHpA were all below 19%, especially 10% for PFBS. Furthermore, the regeneration capacity of adsorbed AC was also inferior when eluting. Therefore, it is necessary to explore new catalysts to modify AC for promoting adsorption and regeneration capacity [1]. Biocarbon adsorption. Biocarbon is a pollution-free solid biofuel produced by pyrolysis of biomass under aerobic or anaerobic conditions, which contains abundant voids, high carbon content, and high calorific value. The nature of biocarbon is different due to various raw materials and parameters in the production process [28]. [PERSON] and [PERSON] [29] explored the adsorption capacity of hardwood biochar (HWC) and pinewood biochar (PWC) for PFBA and PFOA, demonstrating that the PFBA adsorption capacity was 3-4 times lower than PFOA. Meanwhile, the removal efficiency of HWC vaporized at 900 \({}^{\circ}\)C could be improved due to high S\({}_{\text{BET}}\). However, the batch adsorption kinetics experiments showed that the removal efficiency of GAC for PFBA was superior to HWC, indicating a low adsorption capacity of biocarbon for short-chain PFASs. Carbon nanotube (CNTs) adsorption. CNTs have the advantages of easy reaction process control, convenient operation, and low cost of raw material. [PERSON] et al. [30] used single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT) to remove short-chain PFASs (PFBA, PFHxA, PFHpA, PFBA, and PFBS) and long-chain PFASs (PFOA, PFHxS, and PFOS) under neutral conditions. It showed that 95% of PFOS and PFOA were removed by SWCNT within 5 h, but only 7.5% of PFBA was eliminated by SWCNT within 48 h, and PFSAs were more easily adsorbed than PFCASs. Moreover, in the comparison study of MWCNT functional groups, owing to the deprotonation that occurred on the carboxyl and hydroxyl functional groups, contributing to lower hydrophobicity and more negative surface potential, the adsorption efficiency of non-functional MWCNT for short-chain PFASs was improved. Therefore, based upon the above studies, the adsorption efficiency and technical maturity of CNTs were similar to ACs. The main adsorption mechanism of carbon materials was hydrophobic and electrostatic effects, as well as possible hydrogen bonds and covalent bonds effects [4]. The hydrophobic effect would be improved with an increased C-F chain length; thus, long-chain PFASs were better adsorbed than short-chain congeners. Meanwhile, the removal efficiency of adsorbents for PFASs was also depended on the terminal functional groups; for instance, the removal efficiency of PFSAs was better than PFCASs. In addition, the electrostatic attraction could occur between the anionic PFASs and positive charge adsorbents.