Qingguo (Jack) Huang*
College of Agricultural and Environmental Sciences
University of Georgia – Griffin Campus
Griffin, Georgia, USA
Editor’s note: For 2021, the NASF-AESF Foundation Research Council has selected a project on addressing the problem of PFAS and associated chemicals in plating wastewater streams. This report covers the second quarter of work (April-June 2021). A printable PDF version of this report is available by clicking on HERE.
This project started in January 2021 with the aim of developing applicable electrochemical approaches to remove per- and polyfluoroalkyl substances (PFAS) from plating wastewater, including electrooxidation (EO) and electrocoagulation (EC) . This project comprises three research tasks designed to investigate the EC, EO, and EC-EO processing train, respectively, designed to probe three hypotheses specified below:
- EC generates amorphous metal hydroxide flocs which can effectively adsorb PFAS in plating wastewater, which with proper treatment can release PFAS into a concentrated solution.
- EO activated by a Magnéli Ti phase4Oseven anode can be used to effectively destroy PFAS in plating wastewater.
- The electrochemical treatment chain composed of EC and EO by Ti4Oseven the anode can remove and degrade PFAS in plating wastewater more efficiently than either process operated individually.
This report describes part of our ongoing efforts in Task 1, evaluating the isothermal-like sorption behavior of PFAS on EC-generated zinc hydroxide flocs.
The electrocoagulation (EC) process has shown the potential to remove PFAS from water by some recent studies.1,2,3. EC involves the dissolution of charged cations (for example., Zn2+, Al3+, Fe3+) formed at the sacrificial anode with simultaneous formation of monomeric and polymeric hydroxyl complex species, which can strongly sorb some pollutants and remove them from contaminated water.4 Linen, et al.1 evaluated PFAS removal using various sacrificial anodes, including aluminum, iron, zinc, and magnesium, and found that PFAS can be rapidly adsorbed onto zinc hydroxide flocs, generated on the spot during EC with zinc anode, mainly via hydrophobic interaction. Here in this report, we have systematically investigated the adsorption behavior of PFAS on flocs generated during CE with a sacrificial zinc anode.
The configuration of the EC reactor and the procedure of the EC experiments were described in detail in our first report of this project. In order to further evaluate the adsorption behavior of PFAS on the flocs generated from the zinc anode during EC, a series of EC experiments were conducted using a PFAS solution composed of ten PFAS, including (1) perfluorononanoic acid (PFNA), (2) perfluorooctanoic acid (PFOA), (3) perfluoroheptanoic acid (PFHpA), (4) perfluorohexanoic acid (PFHxA), (5) perfluorooctanesulfonic acid (PFOS), (6) perfluorohexanesulfonic acid (PFHxS), (7) perfluorobutanesulfonic acid (PFBS), (8) fluorotelomer sulfonic acid 8:2 (8:2 FtS), (9) fluorotelomer sulfonic acid 6:2 (6: 2 FtS) and (10) 4:2 fluorotelomer sulfonic acid (4:2 FtS). The initial concentrations were at different levels ranging from 0.001 to 0.1 μm (that is to say., 0.001, 0.002, 0.005, 0.01, 0.02, 0.05 and 0.1 μm), and the reaction time was 120 min using a low current density of 0.3 mA/cm2 to avoid foam formation. The supernatant was removed from each test cell and analyzed for the ten PFASs. The floc was collected and weighed after freeze-drying once the reactions were complete. The data was used to assess the sorption capacity of each of the PFAS constituents and the sorbate-sorbent interactions. The PFAS sorption data was best fitted using Langmuir’s isothermal model, equation 1.
or Qand is the quantity (μmol/g) of PFAS adsorbed at equilibrium; VSand is the equilibrium PFAS concentration (μM) in the solution; Qm represents the adsorption capacity; and kL is the adsorption affinity constant.
Results and discussion
Sorption data of each PFAS obtained by data fitting are shown in Fig. 1 (A, B, C) and in Table 1. As seen, the adsorption capacity of the ten PFASs followed the order PFOS > PFNA > 8:2 FtS > PFOA > PFHxS > 6:2 FtS > Acid 4:2 sulfonic fluorotelomer (4:2 FtS) > Perfluoroheptanoic acid (PFHpA) > Perfluorohexanoic acid (PFHxA) > PFBS. It conforms to the order of carbon chain length for each category, whereas for similar carbon chain length it is perfluoroalkanesulfonic acids (PFSAs, including PFOS, PFHxS, and PFBS) > acids sulfonic fluorotelomers (FTSA, including 8:2 FtS, 6:2 FtS and 4:2 FtS) > perfluoroalkyl carboxylic acids (PFCA, including PFNA, PFOA, PFHpA and PFHxA). PFAS with a longer carbon chain tend to be more hydrophobic. This result confirmed that the hydrophobic interaction plays a key role in the sorption capacity of PFAS on zinc hydroxide flocs (Lin, et al. 2015), while charge interactions may also have an impact, as sulfonate head groups tend to have a higher charge density than carboxylates in PFASs. According to the adsorption affinity constant (kL) shown in Table 1 (smaller value indicates higher sorption affinity), the sorption affinity of the ten PFASs followed the order PFHxS > PFOS > PFNA > PFOA > 6:2 FtS > PFHpA > 8:2 FtS > 4:2 FtS > PFHxA > PFBS. The order differs somewhat from that of the sorption capacity, with the larger molecules (PFNA, 8:2 FtS) being shifted down in order. It appears that larger molecules may be at a disadvantage in terms of sorption affinity, while charge interactions play an important role in the intensity of sorption interactions.
In order to identify whether competitive sorption occurred in the solution of ten PFASs, EC experiments were also performed to investigate the Langmuir sorption isotherms of three individual PFASs, including 4:2 FtS, PFOA, and PFOS, on the flocs using the same reaction conditions above. . These three PFAS were chosen to represent PFSAs, FTSAs and PFCAs respectively. As shown in Figure 1(D) and Table 1, the sorption capacity of PFOS and PFOA obtained in the individual solutions was much higher than that obtained in the mixture solutions, which apparently indicates competitive sorption effects in solutions containing several PFAS. The sorption capacity of 4:2 FtS was however similar for individual and mixed solutions. This is probably due to the fact that FtS 4:2 has a lower affinity on flocs, for which the competitive sorption effect may not be evident.
Figure 1 – (A, B, C) Langmuir isotherm of sorption of the ten PFAS on the flocs (VS0 = 0.001 – 0.1 μM, current density = 0.3 mA/cm2, Na 20 mM2SO4, EC time = 120 min); (D) Langmuir sorption isotherm of a single 4:2 FtS, PFOA and PFOS on the flocs, respectively. (VS0 = 0.002 – 5 μM, current density = 0.3 mA/cm2, Na 20 mM2SO4, EC time = 120 min).
Table 1 – Parameters obtained by fitting isothermal type sorption data using Langmuir’s equation.
1. H. Lin, et al., “Efficient sorption and removal of perfluoroalkyl acids (PFAAs) from aqueous solution by metal hydroxides generated in situ by electrocoagulation”, About. Science. Technology., 49 (17), 10562-10569 (2015).
2. Y. Wang, et al., “Perfluorooctanoate (PFOA) electrocoagulation mechanism on a zinc anode: influence of cathodes and anions”, Science. About total., 557-558, 542-550 (2016).
3. B.Yang, et al., “Efficient Removal of Perfluoroalkyl Acids (PFAAs) from an Aqueous Solution by Electrocoagulation Using an Iron Electrode,” Chem. Eng. J., 303, 384-390 (2016).
4. MYA Mullah, et al., “Fundamentals, current and future perspectives of electrocoagulation”, J. Hazard. Mater., 114 (1-3), 199-210 (2004).
Past Project Reports
1. Introduction to the R-122 project: Summary: NASF report in Products Finish; NASF Surface Technology White Papers, 85 (6), 13 (Mar 2021); Complete paper: http://short.pfonline.com/NASF21Mar1.
2. Quarter 1 (January-March 2021): Summary: NASF report in Products Finish; NASF Surface Technology White Papers, 85 (12), 13 (September 2021); Complete paper: http://short.pfonline.com/NASF21Sep1.
About the Author
Dr. Qingguo (Jack) Huang
Qingguo (Jack) Huang is a professor in the Department of Crop and Soil Sciences, University of Georgia, Griffin Campus. He holds a bachelor’s degree in environmental sciences (1990) and a doctorate. in Chemistry (1995) from Nanjing University, China, as well as a Ph.D. in Environmental Engineering from the University of Michigan, Ann Arbor, Michigan. Dr. Huang’s research interests focus on the catalysis involved in the environmental transformation of organic pollutants and the development of catalysis-based technology for pollution control and environmental remediation and management. His laboratory has been actively involved in several cutting-edge research topics:
- Enzyme-based technology for water/wastewater treatment and soil remediation
- Reactive Electrochemical and Electrochemical Membrane Processes in Wastewater Treatment
- Catalysis in the production of biofuels and the management of agro-ecosystems
- Environmental Fate and Destructive PFAS Treatment Methods
- Environmental application and implication of nanomaterials
He has published over 150 peer-reviewed journal articles, five book chapters, and four patents and three patents pending. He taught three courses at the University of Georgia: Introduction to Water Quality, Environmental Measurements, and Advanced Instrumental Analysis in Environmental Studies.
* Contact details of the principal researcher:
Qingguo Huang, Ph.D.,
Professor, Department of Crop and Soil Sciences,
University of Georgia,
1109 Experiment Street,
Griffin, Georgia 30215, USA.
Phone: (770) 229-3302
Fax: (770) 412-4734
Email: [email protected]