NASF/AESF Foundation Research Project #122: Electrochemical Approaches to Treatment of PFAS in Plating Wastewater - 11th Quarterly Report

Editor’s Note: The NASF-AESF Foundation Research Board selected a project addressing the problem of PFAS and related chemicals in plating wastewater streams. This report covers the eleventh quarter of work (July-September 2023). A printable PDF version of this report is available by clicking HERE.

1. Introduction

This project started in January 2021 with the goal of developing applicable electrochemical approaches to remove per- and polyfluoroalkyl substances (PFASs) present in plating wastewaters, including electrooxidation (EO) and electrocoagulation (EC). This project includes three research tasks that are designed to investigate EC, EO and EC-EO treatment train, respectively, designed to probe three hypotheses specified follows:  

1)  EC generates amorphous metal hydroxide flocs that can effectively adsorb PFASs in plating wastewater, which, through an appropriate treatment, can release PFASs into a concentrated solution.

2)  EO enabled by a Magnéli phase Ti₄O₇ anode can be used to effectively destruct PFASs in plating wastewater.

3)  The electrochemical treatment train comprised of EC and EO by Ti₄O₇ anode can remove and degrade PFASs in plating wastewater more efficiently than either process operated individually.

In our last report, we described the performance and effect of surface fluorinated Ti₄O₇ anodes on PFAS degradation in a batch reactor operation mode. We discovered that surface fluorination of Ti₄O₇ anodes slightly inhibited PFAS degradation, while it significantly suppressed the formation of chlorate and perchlorate formation during EO treatment of PFOA, PFOS and 6:2 FTS, three representative PFASs. In this report, we will discuss the effect of surface fluorinated Ti₄O₇ anodes on PFAS degradation in a reactive electrochemical membrane (REM) operation mode.

2. Experiments

Electrooxidation (EO) experiments were performed in a reactive electrochemical membrane (REM) operation mode. The porous T_i4O₇ plate was used in the REM reactor simultaneously as the anode and a membrane through which the feed solution was filtered in crossflow mode (Figure 1). Both the pristine and F-18.6 Ti₄O₇ anodes were tested for comparison, and two current densities, 10 mA/cm² and 40 mA/cm², were applied, corresponding respectively to 2.79 V and 4.05 V_SHE anodic potentials. The feed solution contained PFOA, PFOS and 6:2 FTS (2.0 μM each), 100 mM Na₂SO₄, and 1 mM Cl^—.

3. Results and discussion

The concentrations of PFOA, PFOS and 6:2 FTS in the permeate dropped immediately and were maintained at low level when the electric current was applied (Figure 2). The PFAS removal was greater at the higher current density, while it was slightly lower on the F-18.6 anode than on the pristine one. At 40 mA/cm², the removal of PFOA, PFOS and 6:2 FTS reached 70.5%, 88.3% and 58.9% on a pristine Ti₄O₇ anode, and 68.6%, 86.7% and 56.3% on the F-18.6 anode, respectively.

The concentrations of chlorate and perchlorate in the permeate flow during REM treatment were measured, and the results for the treatment at 10 mA/cm² with 1 mM Cl^— in the feed solution are presented in Figure 3(A) and that with 10 mM Cl^— in Figure 3(B). Both chlorate and perchlorate were detected when a pristine anode was used.  The concentrations of chlorate and perchlorate were 95.3 μM and 48.0 μM, respectively with 10 mM Cl^— in the feed solution, 2.1-3.5 times greater than those with 1 mM Cl^—. Interestingly, chlorate and perchlorate were below detection limits when the F-18.6 anode was used at 10 mA/cm² with either initial Cl^— concentration, indicating effective inhibition of chlorate and perchlorate formation in REM treatment at a practical current density.  This observation has profound implications, as the formation of chlorinated byproducts is a major hurdle to overcome for water/wastewater treatment technologies involving EO and other advanced oxidation processes.^1,2 

The concentrations of chlorate and perchlorate in the permeate flow during REM treatment at 40 mA/cm² with 1 mM Cl^— and 10 mM Cl^— in feed solution are shown in Figure 3(C) and 3(D), respectively.  With 1-mM Cl^— in the feed solution, 223 μM chlorate and 134 μM perchlorate were formed for the REM with a pristine Ti₄O₇ anode, while their formation dropped to 90.3 μM and 35.4 μM on the F-18.6 anode. With 10-mM Cl^— in the feed solution, the formation of chlorate and perchlorate on a pristine anode was 688 μM and 314 μM, respectively, while those on the F-18.6 anode were significantly reduced to 266 μM and 75.4 μM. It should be noted that the background Cl^— is 12 – 46 mg/L in groundwater,³ close to 1 mM.  Thus, REM tests were performed with 1-mM Cl^— in the feed solution at 15 mA/cm² (2.99 V), 20 mA/cm² (3.28 V), 25 mA/cm² (3.59 V), 30 mA/cm² (3.77 V) and 35 mA/cm² (3.98 V), in addition to 10 mA/cm² (2.86 V) and 40 mA/cm² (4.19 V) described above, and the concentrations of chlorate and perchlorate in the permeate flow are compared in Figure 4. The results in Figure 4 suggests that the formation of chlorate and perchlorate in REM can be avoided by using fluorinated Ti₄O₇ anode under a proper anodic potential range (< 3.0 V_SHE). 

4. References

1.  J. Radjenovic, N. Duinslaeger, S.S. Avval and B.P. Chaplin, “Facing the Challenge of Poly- and Perfluoroalkyl Substances in Water: Is Electrochemical Oxidation the Answer?” Environmental Science & Technology, 54 (23), 14815-14829 (2020); https://doi.org/10.1021/acs.est.0c06212 .

2.  J. Radjenovic, and D.L. Sedlak, “Challenges and Opportunities for Electrochemical Processes as Next-Generation Technologies for the Treatment of Contaminated Water,” Environmental Science & Technology, 49 (19), 11292-11302 (2015); https://doi.org/10.1021/acs.est.5b02414.

3.  J.R. Mullaney, D.L. Lorenz and A.D. Arntson, “Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States,” Scientific Investigations Report 2009-5086, United States Geological Survey, Connecticut Water Science Center, East Hartford, CT (2009); https://pubs.usgs.gov/sir/2009/5086/.

5. Past project reports

1.   Introduction to Project R-122: Summary: NASF Report in Products Finishing; NASF Surface Technology White Papers, 85 (6), 13 (March 2021); Full paper: http://short.pfonline.com/NASF21Mar1.

2.   Quarter 1 (January-March 2021): Summary: NASF Report in Products Finishing; NASF Surface Technology White Papers, 85 (12), 13 (September 2021); Full paper: http://short.pfonline.com/NASF21Sep1.

3.   Quarter 2 (April-June 2021): Summary: NASF Report in Products Finishing; NASF Surface Technology White Papers, 86 (3), 18 (December 2021); Full paper: http://short.pfonline.com/NASF21Dec2.

4.  Quarter 3 (July-September 2021): Summary: NASF Report in Products Finishing; NASF Surface Technology White Papers, 86 (6), 16 (March 2022); Full paper: http://short.pfonline.com/NASF22Mar2.

5.  Quarter 4 (October-December 2021): Summary: NASF Report in Products Finishing; NASF Surface Technology White Papers, 86 (9), 21 (June 2022); Full paper: http://short.pfonline.com/NASF22Jun2.

6.  Quarter 5 (January-March 2022): Summary: NASF Report in Products Finishing; NASF Surface Technology White Papers, 86 (12), 22 (September 2022); Full paper: http://short.pfonline.com/NASF22Sep2.

7.   Quarter 6 (April-June 2022): Summary: NASF Report in Products Finishing; NASF Surface Technology White Papers, 87 (3), 17 (December 2022); Full paper: http://short.pfonline.com/NASF22Dec1.

8.   Quarter 7 (July-September 2022): Summary: NASF Report in Products Finishing; NASF Surface Technology White Papers, 87 (6), 19 (March 2023); Full paper: http://short.pfonline.com/NASF23Mar2.

9.   Quarter 8 (October-December 2022): Summary: NASF Report in Products Finishing; NASF Surface Technology White Papers, 87 (9), 19 (June 2023); Full paper: http://short.pfonline.com/NASF23Jun1.

10. Quarter 9 (January-March 2023): Summary: NASF Report in Products Finishing; NASF Surface Technology White Papers, 87 (12), 17 (September 2023); Full paper: http://short.pfonline.com/NASF23Sep2

11. Quarter 10 (April-June 2023): Summary: NASF Report in Products Finishing; NASF Surface Technology White Papers, 88 (5), 19 (February 2024); Full paper: http://short.pfonline.com/NASF24Feb2

6. About the principal investigator

Qingguo (Jack) Huang is professor in the Department of Crop and Soil Sciences, University of Georgia, Griffin Campus. He holds a bachelor’s degree in environmental science (1990) and a Ph.D. 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 interest focuses on catalysis involved in the environmental transformation of organic pollutants, and 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
• Electrochemical and reactive electrochemical membrane processes in wastewater treatment
• Catalysis in biofuel production and agro-ecosystem management
• Environmental fate and destructive treatment methods of PFASs
• Environmental application and implication of nanomaterials

Huang has published over 170 peer-reviewed journal articles, five book chapters and had six patents awarded. He has taught three courses at the University Georgia: Introduction to Water Quality, Environmental Measurement and Advanced Instrumental Analysis in Environmental Studies.