Electrochemical Destruction of Perfluorooctanesulfonate in Electroplating Wastewaters: AESF Research Project #R-120, 6th Quarterly Report

by Brian Chaplin*
Department of Chemical Engineering
University of Illinois at Chicago
Chicago, Illinois, USA

Editor’s Note: This NASF-AESF Foundation research project report covers the sixth quarter of project work (July-September 2021) at the University of Illinois at Chicago.  A printable PDF version of this report is available by clicking HERE.

Summary

Work this past quarter focused on the electrochemical oxidation of poly- and perfluoroalkyl substances (PFAS) using a tubular Magnéli phase titanium sub-oxide (Ti₄O₇) reactive electrochemical membrane (REM).  The oxidation experiments were performed in synthetic solutions, using a crossflow setup in full recycle mode.  The synthetic solutions were prepared by spiking perfluorooctane sulfonate (PFOS) to a background electrolyte with an ionic strength that was representative of industrial electroplating wastewaters (i.e., 240 mM NaClO₄).  The solution was subjected to electrochemical oxidation at a constant potential of 3.0 V_SHE and as a function of the permeate flux.  Results indicated that negligible removal was accomplished at a low permeate flux, but approximately 60% PFOS removal could be accomplished with less than a 2 min electrolysis time under high flux conditions.  A mathematical reactive transport model was used to investigate the effect of flux as a function of REM reactivity at a given electrode to solution volume ratio.  It was found that higher flux always outperformed lower flux conditions with regard to PFOS removal.  Simulations also indicated that further improvements in reaction kinetics for PFOS destruction would have minimal impacts on removal under low flux conditions, but significant increases in overall removal at high flux conditions.  Therefore, future work is focused on improving electrocatalytic activity for PFOS and other PFAS.

Electrochemical oxidation experiments

The operating conditions for the electrochemical oxidation experiments are shown in Table 1 and results are shown in Figure 1.  PFOS oxidation experiments were conducted with 100% recycle of both the permeate and feed solutions.  The applied anodic potential was 3.0 V_SHE and permeate fluxes (J) of 60 and 1800 L m^-2 hr^-1 (LMH) were tested.  Results show that negligible PFOS was removed at 60 LMH, while approximately 60% was removed at 1800 LMH.  These results were attributed to the small amount of solution volume that was processed at the low flux conditions relative to the high flux conditions.

 

 

To further explore the effect of J on PFOS removal, a mathematical reactive transport model was used to estimate PFOS removal under different reactivity conditions at J = 60 and 800 LMH (Fig. 2).  Results indicated that simulations at J = 800 LMH always resulted in more total PFOS removal than simulations at J = 60 LMH, even though single pass removals under J = 60 LMH were always higher (e.g., >90%).  Results also indicated that increasing the PFOS oxidation rate constant from 0.3 to 5.0 min^-1 did not significantly increase total PFOS removal in the recirculating feed tank at J = 60 LMH, due to the fact that conversion was already high under all conditions tested.  However, at J = 1800 LMH total PFOS removal increased significantly, as single pass removals increased from 16% to 95% with increasing rate constant of 0.3 to 5.0 min^-1, respectively.  Future work will thus focus on evaluating REM properties to enhance PFAS reaction rates.

About the author

Dr. Brian P. Chaplin is Associate Professor in the Department of  Chemical Engineering, at the University of Illinois at Chicago.  He holds a B. Civil Engineering (1999) and an M.S. (2003) in Civil Engineering from the University of Minnesota and a Ph.D. in Environmental Engineering (2007) from the University of Illinois at Urbana-Champaign.