# A Review of Treatment Techniques for Short-Chain Perfluoroalkyl Substances
## 5 Conclusions and Future Research Recommended  
This paper showed that the adsorption, electrochemical oxidation, and photocatalytic degradation have certain removal effects on short-chain PFASs by comparisons on various treatment techniques. Considering the removal efficiency, treatment time, energy consumption, and cost, adsorption was the most widely applied technique for the effective removal of short-chain PFASs, which was suitable for a wide concentration range of pollution and to meet the low-carbon policy. Whereas, long adsorption period and unsatisfied regeneration ability were the main problems. The advanced oxidation techniques of electrochemical and photocatalytic activity could degrade short-chain PFASs, but low mineralization efficiency contributed to intermediates of short-chain PFASs, as well as abundant organic matter and CO2 and they were especially inappropriate to eliminate trace short-chain PFASs. Therefore, it was desirable to choose suitable techniques according to PFAS properties, as well as the advantages and disadvantages of various techniques.  
The contaminations of short-chain PFASs have attracted much attention at present, while most studies still focused on the laboratory-scale treatment of long-chain PFASs, including PFOA and PFOS, and data on the treatment of short-chain PFASs is still absent. Therefore, future studies need to focus on the following topics: (1) Targeting long duration period and poor material regeneration ability for the adsorption technique; the functional groups of adsorbent materials need to be modified for improving electrostatic attraction and hydrophobic effects and enhancing the adsorption efficiency and elution of short-chain PFASs. (2) Innovative design for short-chain PFAS treatment by electrode and catalyst modification to develop advanced oxidation techniques, with high degradation efficiency in low-carbon and low-energy consumption, as well as adaptability for low-concentration PFASs. (3) With the increasing pollution of short-chain PFASs in drinking water, extensive emphasis should be placed on the development of advanced treatment techniques for actual groundwater and surface water, along with exploration of new adsorption materials, electrode materials, and catalysts, which could remove low concentrations of short-chain PFAS under the background of multiple substances co-existing in the actual waters. (4) Short-chain PFASs were more resistant to degradation than long-chain congeners, and PFASs of C1-C3 were barely degradable. Therefore, concentration/recycling of short-chain PFASs in water bodies should be considered rather than degradation. In addition, combined techniques might be developed based on concentration/recycling-degradation, such as adsorption and advanced oxidation, for the efficient removal of short-chain PFASs from actual waters.  
Conceptualization, Y.L. and J.B.; methodology, Y.L. and T.L.; software, M.L.; validation, X.Z., C.L. and L.S.; formal analysis, T.L.; investigation, T.L; resources, Y.L. and J.B.; data curation, T.L. and X.H.; writing--original draft preparation, Y.L. and J.B.; writing--review and editing, Y.L. and X.H.; visualization, T.L.; supervision, Y.L. and T.L.; project administration, Y.L. and J.B.; funding acquisition, J.B. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (No. 21976124 and No. 21507092), the Natural Science Foundation of Liaoning Province of China (No. 2019-ZD-0217), and the Liaoning Revitalization Talents Program (No. XLYC2007195).  
Thanks for the financial support from the National Natural Science Foundation of China (No. 21976124 and No. 21507092), the Natural Science Foundation of Liaoning Province of China (No. 2019-ZD-0217), and the Liaoning Revitalization Talents Program (No. XLYC2007195).  
The authors declare no conflict of interest.