Grantee Research Project Results
2022 Progress Report: The BOHP/UV Process for Destruction of PFAS in Leachate and Groundwater: Tandem mechanistic advancement and pilot demonstration
EPA Grant Number: R839630Title: The BOHP/UV Process for Destruction of PFAS in Leachate and Groundwater: Tandem mechanistic advancement and pilot demonstration
Investigators: Cates, Ezra L
Institution: Clemson University
EPA Project Officer: Hahn, Intaek
Project Period: August 1, 2019 through July 31, 2022 (Extended to July 31, 2023)
Project Period Covered by this Report: August 1, 2021 through July 31,2022
Project Amount: $458,469
RFA: Practical Methods to Analyze and Treat Emerging Contaminants (PFAS) in Solid Waste, Landfills, Wastewater/Leachates, Soils, and Groundwater to Protect Human Health and the Environment (2018) RFA Text | Recipients Lists
Research Category: Human Health , Water Quality , Drinking Water , Water , PFAS Treatment
Objective:
This project seeks to apply a photochemical treatment method and reactor system to the destruction of poly-/perfluoroalkyl substances (PFAS) present in landfill leachate. The original objectives of the 3-year project revolved around application of a photocatalytic approach to degrade PFAS involving semiconductor particles and ultraviolet C (UVC) irradiation. Due to critical interference with photocatalytic efficiency by the high concentrations of inorganic anions in leachate, a “no-go” decision was made with respect to pursuing the photocatalytic approach at the end of Year 1. Instead, the project pivoted toward using cost effective pretreatment via coagulation and ceramic membrane filtration, to reduce dissolved organic carbon content of the leachate, followed vacuum UV (VUV) irradiation to further reduce organic content and degrade PFAS via direct photolysis. Results thus far suggest that this treatment approach is more efficient and effective in achieving PFAS destruction in leachate. The overall project structure remains unchanged and comprises tasks for gauging PFAS degradation energy efficiency in controlled matrices, optimization of pilot-scale photoreactor and process conditions, and pilot studies of real leachate treatment. Lastly, a no-cost extension has been granted, allowing the project to proceed for a third year, through July 2023.
Excluding work done prior to the no-go decision on photocatalysis, the current project objectives are as follows:
- Develop cost-effective physicochemical pretreatment method to achieve maximum removal of dissolved organic substances in order to improve efficiency of upstream photochemical process targeting PFAS.
- Assess degradation efficiency of leachate-relevant PFAS compounds in raw and pretreated leachate by VUV irradiation.
- Conduct pilot tests in a scaled-up photoreactor.
Project tasks associated with the revised objectives are:
Tasks | 2021 | 2022 | 2022 | 2023 |
Degradation of leachate-relevant FTCAs and PFOS in controlled water matrices by VUV photolysis | Completed | |||
Leachate sample collection and compositional analysis | Completed | |||
Optimization of FeCl3 coagulant dose and assessment of coagulation/filtration pretreatment performance (organics removal) | Completed | |||
Assessment of organics and PFAS degradation in raw and pretreated leachate by VUV | In progress | In progress | ||
Pilot demonstration in 7.5 L photoreactor | (Reactor construction complete) |
Table 1. Project Tasks
Progress Summary:
Task 1: Degradation of leachate-relevant fluorotelomer carboxylic acids (FTCAs) and perfluorooctane sulfonate (PFOS) in controlled water matrices by VUV photolysis. We have previously demonstrated that irradiation by VUV (185 nm) in photoreactors equipped with “ozone generating” low pressure mercury lamps (254/185 nm) provides effective destruction of both long and short-chain perfluorocarboxylic acids (PFCAs) and GenX. (Qanbarzadeh et al., ACS EST Eng. 2021, 1, 2, 239–248) Fluorotelomer carboxylic acids (FTCAs), however, are often the dominant PFAS in landfill leachate and photocatalytic degradation of these compounds was studied during year one. Following the switch to a VUV photolytic approach, degradation of these compounds in pure water was reexamined. As seen in Figure 1, both 5,3-FTCA and 7,3-FTCA were rapidly degraded in the bench scale photoreactor, resulting in evolution of their respective PFCA photolysis byproducts. As expected, degradation kinetics were faster for the FTCAs than observed previously for long-chain PFCAs, due to the weaker bond energies of the non-fluorinated carbons.
Figure 1. Photolytic degradation of leachate-relevant FTCAs in DI water using a low pressure mercury lamp photoreactor (254/185 nm).
The above results and those already published confirm that 185 nm photons are capable of photolyzing all major leachate-relevant PFAS at relatively high efficiency, with the notable exception of perfluorooctane sulfonate (PFOS). Earlier in this project, we attempted to enhance PFOS photolysis by using reducing conditions, as well as by using ferric iron-PFOS complexation under oxidizing conditions, based on a literature report. Both attempts were unsuccessful; however, we recently reexamined the Fe-complexation approach using a corona discharge Xenon excimer lamp, which emits higher energy photons of 172 nm. As seen in Figure 2A, significant PFOS degradation was observed when 100 µM of FeCl3 was added and pH adjusted to 3. The calculated electrical energy per order destruction (EE/O) was 74 kWh·m- 3·order-1, indicating efficient PFOS destruction. Liberation of fluoride was clearly observed as well (Figure 2B), however the recovery was low (<5%). This was expected, as the low pH of 3 was below the pKA of HF (3.8); fluoride was thus primarily in a volatile protonated form, much of which likely escaped to the atmosphere.
Figure 2. (A) Degradation of PFOS via Fe3+ complexation and direct photolysis by a Xenon dimer lamp (172 nm); (B) associated fluoride release in the FeCl3 augmented solution.
Task 2: Leachate sample collection and compositional analysis. The original plan called for use of synthetic leachate, which proved to have chemical properties dissimilar to real leachate. The plan was thus modified to involve real leachate only and samples were collected from Twin Chimneys Landfill, which serves Greenville County, SC. A suite of characterization analyses was performed on the leachate, including common parameters (e.g. COD, Total nitrogen etc.) as well as analysis of initial PFAS concentrations for a number of analytes. For conciseness, this data will be included in the final report, but not shown here. Overall, the leachate was characteristic of a relatively mature landfill, with high COD (3,600 mg/L) and pH of ~8.
Task 3: Optimization of FeCl3 coagulant dose and assessment of coagulation/filtration pretreatment performance. The high COD of leachate poses a significant challenge for degrading target contaminants efficiently via an oxidative approach. We chose to implement an pretreatment scheme to reduce the organic content of the leachate via coagulation, followed by ceramic membrane ultrafiltration. Ferric chloride was selected as the coagulant based on literature reports that it is most effective for landfill leachate. Dosage was optimized and ultimately a high dose of 10 g/L (as FeCl3•6H2O) was determined to result in the best coagulation, depicted in Figure 3b. Figure 3d shows the pretreated leachate and Figure 4 shows the results of pretreatment, with respect to COD reduction, dissolved organic carbon (DOC), and UV absorbance (UVA254). Therein, pretreatment resulted in a nearly 50% decrease in COD and DOC, while UVC transmittance improved by over 12 orders of magnitude (i.e. 12 absorbance units lower).
Figure 3. (a) Raw leachate; (b) rapid mix following addition of FeCl3; (c) flocculated leachate in filtration cell; (d) final pretreated leachate (membrane permeate).
Figure 4. Impact of coagulation and ultrafitration pretreatment and VUV irradiation on leachate organic content.
Figure 5. Bleaching effect on raw leachate of VUV irradiation with air bubbling.
Task 4: Assessment of organics and PFAS degradation in raw and pretreated leachate by VUV. Task 4 is currently in progress, though some results have already been obtained. Included in Figure 4 are results of VUV irradiation experiments with respect to further reduction in leachate organic content. Both COD and DOC remained unchanged following irradiation, however, UVA254 decreased. This likely indicates that dissolved complex organics were fragmented and oxidized via direct photolysis and reaction with reactive oxygen species. In order to amplify this effect, air bubbling was added to add dissolved O2 and scavenge reducing species produced by water photolysis (e.g. H•); this resulted in further decrease in UVA (Figure 4 and 5).
To assess PFAS degradation, we attempted to first characterize the concentration polyfluorinated precursors that could be transformed into PFCAs during initial stages of treatment via the Total Oxidizable Precursors assay. Ultimately, this method was found to be unsuitable for use with raw or pretreated leachate. Preliminary results of PFOA degradation are shown in Figure 6a. Therein, PFOA concentrations increased when the pretreated leachate is irradiated without bubbling, suggesting conversion of precursors to PFCAs. When bubbling was used, PFOA did not increase, which may suggest either bubbling results in much faster conversion and degradation, such that no accumulation in evident. To confirm, leachate was spiked with isotopically labeled C13 PFOA. As seen in Figure 6b, bubbling and irradiation indeed results in rapid disappearance of PFOA in both raw and pretreated leaching. These experiments are being repeated due to the low recovery of the spiked PFOA concentration (100 ppb).
Figure 6. Generation and degradation of PFOA in leachate by VUV irradiation with and bubbling, including (a) normal PFOA, and (b) Spiked C13-PFOA.
Future Activities:
Based on results from Year 3, the projective objectives have been revised to include the following in Year 4:
- Develop more robust PFAS analysis methods for leachate. Modifications such as replacing syringe filtering with centrifuging, and diluting in methanol have been incorporated into the LC-MS methods, with improved results.
- Quantify VUV photolytic degradation of PFAS in pretreated leachate and identify key operational parameters
- Conduct pilot tests in 7.5 L reactor
Journal Articles:
No journal articles submitted with this report: View all 9 publications for this projectSupplemental Keywords:
hazardous waste remediation; water purification technologies; photocatalytic reduction, groundwater remediation; photocatalyst water disinfectionProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.