ReviewAdsorption behavior and mechanism of perfluorinated compounds on various adsorbents—A review
Introduction
Perfluorinated compounds (PFCs) are a class of anthropogenic organofluorine compounds with each hydrogen atom on alkyl chain replaced by fluorine atom [1]. Some PFCs contain a hydrophilic functional group head and a hydrophobic perfluorinated tail, and they have unique different amphiphilic properties from conventional hydrocarbons, enabling them widely used in various application areas such as fire fighting foams, chromium plating, photolithography, paper and fabric protection [1]. Although the C–F chains of typical PFCs are hydrophobic, their functional groups (such as sulfonate and carboxyl) are hydrophilic and make them high water solubility. Due to the high bond energy (approximately 110 kcal/mol) of C–F in fluorocarbon, PFCs have high chemical stability [2]. These properties make them persistent, bioaccumulative and ubiquitously distributed in aquatic environments [2], [3]. Their widespread application may produce long-term and extensive harm to animals and human beings, which has been confirmed in recent years [1].
Although two most typical PFCs, perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA), have been added to the limited/forbidden list of Stockholm Convention on Persistent Organic Pollutants or the relevant regulations in America, Canada and Germany, they are still allowed to be used in some areas including electroplating, polytetrafluoroethylene manufacturing, and optoelectronic industries [4], [5]. Some developing countries have not expressly restricted the use of PFCs, leading to the continued pollution, and it remains necessary to develop effective remediation technologies. Schroder et al. [6] compared the conventional physical and chemical treatments for PFOS and PFOA removal. They found that granular activated carbon (GAC) adsorption was significantly superior to other conventional removal techniques including reverse osmosis and some advanced oxidation processes. Recently, some novel advanced oxidation technologies [2], [7], [8], [9], [10], [11], [12] have been reported to be effective for PFC removal from water, but their inherent weaknesses such as high energy consumption, complicated operation and harsh reaction conditions hamper their large-scale application. Adsorption is considered as a suitable technique for PFC removal from water or wastewater [6], and the efficient adsorbents have high and selective adsorption for PFCs. In addition, adsorption on natural materials like sediments and minerals is also undoubtedly involved in the transport and transformation processes of PFCs, which determines their fate in aquatic environments.
Many literatures have reported the adsorption of typical PFCs on various adsorbents. Their adsorption behavior and mechanism on these adsorbents are different but comparable. Therefore, these different adsorbents for PFC removal will be first categorized briefly, and then their adsorption behavior on different adsorbents will be compared and overviewed in the following section. There are some contradictions and misunderstandings for the adsorption mechanism of PFCs, and thus all interactions between PFCs and adsorbents reported by previous publications will be finally summarized to make the adsorption mechanism clear.
Section snippets
Adsorbents for PFC removal
Besides conventional adsorbents, some effective adsorbents have been synthesized to remove PFCs from aqueous solution. Table 1 lists the adsorption of PFOS and PFOA onto different adsorbents reported in the literature, and their regeneration results are shown in Table 2. The adsorption and desorption properties of PFCs on various adsorbents are significantly different, which will be briefly introduced below.
PFC adsorption behavior
Adsorption kinetics and isotherms of typical PFCs on different adsorbents have been studied in detail, and adsorption behaviors are affected by solution pH and co-existing compounds as well as PFC and adsorbent properties. The adsorption behavior of typical PFCs on different adsorbents in these aspects will be fully reviewed below.
Adsorption mechanisms of PFCs
Many interactions, such as electrostatic interaction, hydrophobic effect, π–π bond, hydrogen bond, ion exchange, and van der Waals force, have been reported to be possibly involved in the adsorption of PFCs on various adsorbents. Among these interactions, the π–π bond is impossible to form in the adsorption of PFCs due to the absence of π electrons in their molecules, while the van der Waals force is also unimportant for the adsorption of PFCs because of the low polarizabilities and small
Conclusions and future perspectives
This review summarized the adsorption behavior and mechanism of different PFCs on various adsorbents under different conditions. The surface functional groups and pore structure of adsorbents are reported to play important roles in the adsorption of PFCs. The suitable pore sizes can accommodate the formed hemi-micelles/micelles of PFCs and prevent the blockage in adsorbents. The mine groups on the adsorbent surfaces could be protonated easily and are mainly responsible for anionic PFC
Acknowledgments
We thank the National Nature Science Foundation of China (Project no. 21177070), and the National High-Tech Research and Development Program of China (Project no. 2013AA06A3) for financial support. Additionally, the analytical work was supported by the Laboratory Fund of Tsinghua University.
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