NASF/AESF Foundation Research Project #122: Electrochemical Approaches to the Treatment of PFAS in Plating Wastewater – 4th Quarterly Report

by
Qingguo (Jack) Huang* and Yuqing Ji
College of Agricultural and Environmental Sciences
University of Georgia
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 fourth quarter of work (October-December 2021). A printable PDF version of this report is available by clicking on HERE.

Introduction

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 includes three research tasks designed to investigate the EC, EO, and EC-EO processing train, respectively, designed to probe three hypotheses specified below:

1) 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.

2) EO activated by a Magnéli Ti phase4O7 anode can be used to effectively destroy PFAS in plating wastewater.

3) The electrochemical treatment chain composed of EC and EO by Ti4O7 the anode can remove and degrade PFAS in plating wastewater more efficiently than either process operated individually.

Results reported in previous reports from this project have demonstrated the feasibility of a novel treatment chain that combines electrocoagulation (EC) with electrooxidation (EO) treatment to remove and degrade per- and polyfluoroalkyl substances (PFAS) plating wastewater. Electrocoagulation with a zinc anode can effectively remove PFAS from water, especially long-chain PFAS (C7 -VSten) present in the plating wastewater, concentrating them on the flocs or in the foams generated during EC. Flocs and foams can be dissolved by acid to recover and concentrate PFAS in controlled volumes. Concentrated PFAS in acidic solutions were effectively destroyed using EO treatment with a Ti4O7 anode at 10 mA/cm2, and no additional electrolyte was needed for the flocs dissolved in the solution. This EC-EO electrochemical treatment train can probably separate, concentrate and destroy PFAS in plating wastewater economically.

This report describes our ongoing efforts in Task 3. First, we calculated the energy consumption of the EO treatment process in terms of EE/O which is defined as the electrical energy required to reduce the concentration of ‘a pollutant of an order of magnitude (kWh /m3).1 Second, we evaluated ways to remove residual zinc ions that may exist after EO treatment of the dissolved acid solution of zinc flocs.

Experimental

The calculation of EE/O was based on the results of the EO treatment experiment reported earlier, which is shown in Fig. 2 in 3rd report.2 In this experiment, three different concentrated solutions prepared by the electrocoagulation (EC) method were subjected to electrooxidation (EO) treatment using the Magnéli Ti phase.4O7 anodes with current density of 10 mA/cm2. Solution I was the dissolved acid solution of PFAS-loaded floc generated using a low current density condition after 120 min (0.3 mA/cm2, 0.005 μM each of 10 PFAS). Solution II was the acid-dissolved PFAS-loaded floc solution obtained by EC treatment under high current density conditions after 60 min (5 mA/cm2, 0.5 μM each of 10 PFAS). The foam collected during this EC process was supplemented with 20 mM Na2SO4 to a final volume of 10 ml as solution III.

An experiment was performed to evaluate methods for removing zinc ions from solution produced by the acidic dissolution of zinc hydroxide flocs generated during the EC process. Specifically, the removal of zinc ions was achieved by precipitation with the addition of Na2S or Na2CO3. In this experiment, CE was first performed in a 20 mM Na2SO4 solution with PFAS at 0.3 mA/cm2 for 120 min or at 5 mA/cm2 for 60 mins. The entire solution, including the flocs, was then collected and filtered through a 0.22 μm acetate membrane filter. EC flocs from both current density conditions were then collected and dissolved in 10 mL of 4.0 MH2SO4, respectively. N / A2S or Na2CO3 was then added to the solution at different dosages. Zn concentration2+ in the solution was determined using an ICP-MS (Perkin Elmer Elan 9000 inductivelycoupled plasma equipped with a mass spectrometer),3 with a detection limit of 0.05 mg/L.

Results and discussion

EE/O (kWh/m3) of the PFAS degradation in the three solutions described above was calculated by equation 1,1

(1)

or youcell is the average cell voltage during EO treatment (V), I is the applied current (A), V is the volume of the reaction solution (L). you90% is the time (h) for the removal of 90% of the PFAS which was calculated by equation 2:

$t_{90%}=ln\left ( \frac{C}{C_{0}} \right )/60k$ (2)

or VS/VS0 is 10% and k (min-1) is the pseudo-first order rate constant for the degradation of the different PFASs in the three solutions which was obtained by fitting the PFAS degradation data, shown in Fig. 1 of 3rd report,2 to the pseudo-first-order rate model, which are listed in Table 1.

Calculated EE/O values ​​of PFAS degradation in concentrated solution are presented in Table 2. EE/O ranges from 0.34 to 15.7 kWh/m3 for different PFAS in different solutions. It appears that the EE/O was lower for the long chain PFAS, for example., PFNA, PFOA, PFOS, than the shortest, for example., PFBS and PFHxA. It should be noted that the EE/O for PFAS frequently present in plating wastewater is particularly low, for example, it was 0.66 (kWh/m3) for 6:2 FTS, 0.55 (kWh/m3) for PFOS, and 0.95 (kWh/m3) for PFOA in Solution I. Such EE/O levels are considered favorable for wastewater treatment applications.

The result of the experiment to evaluate the methods of removing zinc ions from solution by precipitation with the addition of Na2S or Na2CO3 is shown in Figure 1. It is evident that the concentration of zinc remaining in the solution decreased dramatically as the added Na2S or Na2CO3 increased, due to precipitation of ZnS or ZnCO3. Almost all dissolved Zn2+ was precipitated when enough salts were added. This proves that chemical precipitation can be used as an effective way to remove residual zinc in the final effluent of the proposed EC-EO treatment chain.

Table 1 – The pseudo-first order rate constant (min-1) of PFAS in concentrated solution in the EO process.

Table 2 – EE/O (kWh/m3) for the degradation of PFAS in concentrated solution during the EO process.

Figure 1 – Concentration of zinc ions in the solution at different current densities and with Na2S or Na2CO3 added at different doses.

References

1. K. Yang, H. Lin, S. Liang, R. Xie, S. Lv, J. Niu, J. Chen and Y. Hu, “A Reactive Electrochemical Filter System with Excellent Porous Penetration Flux Ti/ SnO2–Sb filter for effective removal of contaminants from the water,” RSC Adv., 8 (25), 13933-13944 (2018).

2. Q. Huang, “NASF/AESF Foundation Research Project No. 122: Electrochemical Approaches to Treating PFAS in Plating Wastewater – 3rd Quarterly report,” NASF Surface Technology White Papers, 86 (6), 11-14 (2022); http://short.pfonline.com/NASF21Dec2.

3. Y. Shu, N. Zheng, A. Zheng, T. Guo, Y. Yu and J. Wang, “Intracellular quantification of zinc by fluorescence imaging with a FRET system, Anal. Chemistry., 91 (6), 4157-4163 (2019).

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); Full article: 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); Full article: http://short.pfonline.com/NASF21Sep1.

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

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

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 160 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 Science
University of Georgia
1109 Experiment Street.
Griffin, Georgia 30215, USA.

Telephone: (770) 229-3302 Fax: (770) 412-4734
Email: [email protected]