Measuring PFAS in wastewater biosolids: Improving analytical methods

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Introduction

The presence of per- and polyfluoroalkyl substances (PFAS) in wastewater sludges and biosolids is gaining increasing attention in the water quality sector.  In many parts of North America, biosolids are used as a soil amendment to increase organic matter content and as a substitute for mineral fertilizers.  The presence of PFAS in biosolids represents a potential threat to the beneficial use of biosolids.

Policies that address the use of biosolids containing low concentrations of PFAS are evolving. A challenge with policy development is the lack of consistent and reliable data on the quantities of PFAS that are present in biosolids.  A variety of methods for measuring PFAS in biosolids have been reported, including the recent United States Environmental Protection Agency (USEPA) Draft Method 1633, but there is no widely accepted standard method for analysis.

The objective of this study was to evaluate the performance of existing methods of PFAS analysis in sludges and biosolids, focusing specifically on assessing the cleanup of extracts by four commonly used adsorbents prior to liquid chromatography – tandem mass spectrometry (LC-MS/MS) analysis. Based on the results, a new cleanup method was devised and evaluated.
 

Methodology

The study measured 24 PFAS, including perfluoroctane sulfonate (PFOS) and perfluorooctanoate (PFOA), which are the two most problematic PFAS for whom the proposed regulatory limits in drinking water are less than 5 ppt or 5 ng/L. Six biosolid samples were used: samples 1–3 were dewatered anaerobically digested sludges collected from three different wastewater treatment facilities, samples 4 and 5 were generated from the alkaline treatment of biosolids 1 and 2, and sample 6 was collected from an aerobic composting facility. In order to isolate and quantify PFAS in these samples, the extraction procedure shown in Figure 1 was used.

Figure 1: Analytical workflow for the quantification of PFAS in biosolids. Various adsorbent types and quantities were assessed in step 2 to clean up the extract generated in step 1. Photo A presents the physical appearance of the biosolid 1 extract cleaned with ENVI-Carb. The yellow precipitates could be removed by centrifugation followed by filtration (Photo B). (A) presents the chromatogram of a PFOS standard. (B and C) and (D and E) present the chromatograms of PFOS in the extracts of biosolid 1 and biosolid 2, respectively; these extracts were cleaned with either 40 mg or 1000 mg ENVI-Carb.

Recovery values were estimated by comparing the peak area of the spiked isotopically labeled PFAS with the peak area of the respective isotopically labeled PFAS in the calibration standards. A separate set of experiments were conducted to confirm that the recovery values of isotopically labeled PFAS were representative of the recovery values of native PFAS.

PFAS in the studied biosolid samples were also measured following the procedure described in USEPA Draft Method 1633 (Figure 2), which included cleaning up the extract by graphitized non-porous carbon (also known as ENVI-Carb) and Weak Anion Exchange resin (WAX).  

Figure 2: Analytical workflow of the ENVI-Carb and WAX cleanup steps as described in US EPA Method 1633. The photos present the physical appearance of the biosolid 1 extract after cleanup (Photo A) followed by centrifugation and filtration (Photo B). Panels show comparison of the chromatograms of various PFAS standards and PFAS in the filtered biosolid 1 extract.

Outcomes

ENVI-Carb has been widely used to clean up biosolid extracts (Step 2 in Figure 1). However, it was found in this study that the formation of precipitates occurred with all six tested biosolids (Photo A, Figure 1), which prevented direct extract analysis by LC-MS/MS. While the precipitates could be effectively separated by centrifugation followed by filtration (Photo B, Figure 1), certain soluble organic compounds were still present in the extract, which interfered with PFAS analysis. When ENVI-Carb was used in combination with WAX, as described in US EPA Draft Method 1633, precipitates were also formed (Photo A, Figure 2). Given the physical appearance of the extracts, they were not deemed appropriate for LC-MS/MS analysis and were further pretreated by centrifugation followed by filtration (Photo B, Figure 2). Analysis of the filtered/centrifuged extracts revealed that, as in the case when the extracts were cleaned with only ENVI-Carb, the chromatograms of PFOS and PFHxA contained a noisy baseline, broad analyte peaks, and unidentified peaks that overlapped with the analyte peak (Figure 2). Overall, the chromatograms of other perfluoro sulfonate compounds were not of acceptable quality (Figure 3 top). It was concluded that PFAS in the studied samples could not be measured using US EPA Draft Method 1633.

Figure 3: Chromatograms of PFBS, PFPeS and PFHxS in the biosolid 1 extract cleaned with ENVI-Carb and WAX (top) and with a blend of ENVI-Carb, PSA, and C18 (bottom). These compounds were not present in biosolid 1 and were added to the cleaned-up extract prior to LC/MS/MS analysis.

To improve the analytical procedure, various blends of adsorbents with different adsorptive properties were tested. A blend consisting of ENVI-Carb, PSA, and C18 was found to be capable of removing a wide range of interfering compounds, resulting in chromatograms that consisted of well-resolved analyte peaks and low baseline noise levels (Figure 3 bottom). When integrated into the overall analytical method, the recoveries of PFAS were between 40% and 160% except for some precursors when six different biosolids were analyzed.

The newly developed cleanup method was used to analyze PFAS in the six studied biosolid samples. It was found that the total perfluoroalkyl acid (PFAA) concentrations were generally comparable to those reported in previous studies and it appeared that alkaline treatment of the biosolid resulted in a decrease in PFAS concentration (Figure 5).

Figure 4:  Chromatograms of various PFAS in the biosolid 1 extract cleaned with blends of ENVI-Carb, PSA, and C18.

Figure 5: The concentrations of various PFAS in the five biosolids investigated in this study

Conclusions

Several analytical methods for the quantification of PFAS in wastewater sludges and biosolids have been proposed, including the recently published US EPA Draft Method 1633. In all methods, the cleanup of extract to eliminate interfering organic and inorganic compounds prior to LC-MS/MS analysis is critical to obtaining acceptable chromatogram quality and recoveries of PFAS. While a variety of adsorbents have been used in the cleanup step, the study argues that a “one-size-fits-all” method may not exist. Specifically, the study observed that none of the existing methods could prevent precipitate formation and that centrifugation and filtration of the reconstituted extracts did not sufficiently eliminate interferences to allow PFAS quantification. To overcome this issue, the use of blends of adsorbents (ENVI-Carb, PSA, C18) with different adsorptive properties was evaluated and used to successfully analyze PFAS in the six studied biosolids.

While the method developed in this study has the potential to facilitate the analysis of PFAS in a broad range of sludge and biosolid matrices, the optimum adsorbent formulation could be sample-specific given that sludge and biosolids characteristics vary widely. The adsorbent formulation should therefore be modified to obtain an optimal trade-off between chromatogram quality and PFAS recovery. Additional research is needed to assess the robustness of the cleanup method developed in this study against a broader range of sludge and biosolid matrices.

Can Ozelcaglayan, Ali, Parker, Wayne, Pham, Anh. The analysis of per- and polyfluoroalkyl substances in wastewater sludges and biosolids: which adsorbents should be used for the cleanup of extracts? Environmental Science: Water Research & Technology, January 2023. https://doi.org/10.1039/D2EW00617K

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