Final Report: Development of 'Smart' Polymer Nanofiber Mats for Selective and Efficient Removal of PFAS from WastewaterEPA Contract Number: 68HERC20C0057
Title: Development of 'Smart' Polymer Nanofiber Mats for Selective and Efficient Removal of PFAS from Wastewater
Investigators: Feng, Maoqi M
Small Business: Polykala Technologies, LLC
EPA Contact: Richards, April
Project Period: June 1, 2020 through May 31, 2022
Project Amount: $292,848
RFA: Small Business Innovation Research (SBIR) - Phase II (2020) Recipients Lists
Research Category: Small Business Innovation Research (SBIR) , SBIR - Water and Wastewater
This project developed novel nanofiber mats fabricated by electrospun polymer nanofibers (PFAS ShieldTM) for the "smart" (selective) removal of per- and polyfluoroalkyl substances (PFAS) from wastewater. In this project, both PFAS model compounds and real-world samples were tested to quantify the effectiveness of the materials in various scales.
Membrane substrate. Two different polyimide membranes manufactured by electrospinning were used as the substrate. The first is a polyimide nanofibers film in 58 mm thickness and 100 meters length by 300 mm width. The total area is 30 square meters (323 square feet), as shown in Figure 1. The second is a polyimide nanofibers film in 18 mm thickness and 100 meters length by 600 mm width. The total area is 50 square meters (538 square feet), as shown in Figure 2. A 12-inch ruler was placed at the bottom for comparison. Both membranes were further functionalized with a special PFAS-binding and cost-effective organic coating.
Figure 1. The polyimide nanofibers film of 58 mm thickness, 100 meters length by 300 mm width.
Figure 2. The polyimide nanofibers film of 18 mm thickness, 100 meters length by 500 mm width.
PFAS-selective organic coating on polyimide membranes. The PFAs-selective novel organic material (patent application pending) was coated to the polyimide membrane surface by dip coating in THF-water 50/50 (v/v) mixture. After the coating step was completed, the membrane was rinsed with DI water twice and soaked in 25% (v/v) isopropanol solution for 10 minutes to remove unattached or weakly-bound or) organic coating from the membrane surface. Finally, the membrane is rinsed thoroughly with DI water to remove the alcohol and dried at 40°C for 2 hours.
Previously we observed that in each batch after organic coating, there is color difference because of unevenly coated organic material. This can be improved by air bubbling and stirring during the coating process.
Membrane characterization. The functionalized membranes prepared was characterized using a Bruker nanoIR instrument, which combines an atomic force microscope with infrared lasers to perform infrared spectroscopy and scattering type scanning near field optical microscopy with a spatial resolution of 10-50 nm. The lasers cover the region from 850 cm-1 to 2250 cm-1. The results are shown in Figure 3. There are two characteristic peaks: carbonyl stretching modes (C=O) between 1600 cm-1and 1800 cm-1, and the imide peak (C-N-C) at 1378 cm−1.
Figure 3. The NanoIR analysis results.
Figure 4 shows the surface of the polyimide membrane before and after organic coating under a low resolution microscope.
Figure 4. Left: Polyimide membrane without organic coating; Right: Polyimide membrane after organic coating on the surface. Image taken by Amoeba digital microscope, 60X.
Sprayable organic coating formulation. Organic coating provides the PFAS removal selectivity in our technology. The organic coating support includes electrospun polyimide membrane, granular activated carbon (GAC), carbon foam, silica, and alumina pellets.
Since membrane coating in large scale is challenging, a sprayable gel formulation for organic coating on membrane and other supports was developed in this period. The gel contains environment-friendly gelling material, oxidizing agent, and pH adjust agent. Generally, the organic coating thickness is 18 - 24 nm, the amount of organic material was calculated based on the membrane size (area) before adding the organic material to the gel. After the organic material was added, the gel was used immediately by spraying to the surface of the membrane or sorbent support. The gelling material is spread evenly on the surface, after 3 hours exposing to the air, it is washed with water. Organic coating can be seen on the surface by dark color change.
Stability of organic coating on polyimide membranes. Continuous test of stability of organic coating on polyimide in water showed that there was no obvious color change (deterioration), and UV-Vis comparison indicated that the organic coating on polyimide was stable for at least three months.
The organic coating treated polyimide nanofiber membranes were immersed in de-ionized water for 3 months at room temperature, no swelling or decoloring was observed. UV-Vis measurement of the immersed water at different times showed that the materials are very stable in water.
Stability test at three temperatures, 30°C, 40°C, and 50°C showed that the coated polyimide membrane is stable.
The effect of pH was tested at 5, 7 and 9 in water adjusted by buffer solutions
PFAS removal efficiency for the polyimide membrane substrate without functionalization of the organic coating. Testing of the polyimide membrane substrate without functionalization of the organic coating for PFOA and PFOS removal showed that the PFOA concentration was reduced from 1,185 ng/L to 1,100 ng/L, and the PFOS concentration was down from 1,461 ng/L to 1,000 ng/L. The removal efficiency for the PFOA was only 7.2%, and the PFOS removal efficiency was 31.6%. PFAS concentrations were analyzed by the EPA 537.1 method.
The result indicated that the polyimide substrate is more selective to PFOS than that of PFOA.
PFAS removal efficiency for the polyimide membrane after functionalization of the organic coating. De-ionized water spiked with 500 ng/L of PFOA + 500 ng/L of PFOS was used as the feed. The spiked water was fed to the membrane continuously using a MasterFlex pump at 750 mL/min flow rate. The test lasted for one hour.
The PFAS concentration of the treated water was analyzed by the EPA 537.1 method. The PFOA concentration was reduced to 4.6 ng/L, and the PFOS concentration was down to 5.3 ng/L. The removal efficiency for the PFOA was 99.1%, and the PFOS removal efficiency was 98.9%.
Wastewater treatment results. Wastewater samples were collected from landfill runaway water in San Antonio, TX. Analytical results showed that the PFAS level was down to less than 70 ng/L after the treatment.
Regeneration of polyimide membrane after PFAS removal. Regeneration of functionalized polyimide membrane was tested with acetone and 95% ethanol. For each piece (1 ft x 3 ft), 40 mL of solvent acetone was used to immerse the membrane for 15 minutes, the membrane was then removed, and the solvent was distilled for reuse. The residue contains the PFAS recovered from water.
Multi-functional organic coating functionalized sorbents development. Organic coating is PFAS selective for PFAS due to the strong H-bond capability of the functional groups. Since most membrane technology needs pretreatment support with sorbent-packed filters. In this project, we also developed PFAs-selective sorbents from commercially available GAC and alumina with organic coating using the sprayable gelling formulation we developed. The organic coating coated sorbents were then sintered at 600°C to form C-N nets (for GAC) and Al-N-O nets, which are capable of PFAS removal by forming H-bonds with the fluorine in PFAS as N-H-F and other contaminants removal.
Figure 5 shows the prepared alumina sorbent prepared in this project.
Figure 5. Functionalized Alumina Sorbent.
The functionalized alumina sorbent was tested for PFOA and PFOS removal. Ionized water with conductivity at 500 mS/cm spiked with 1,185 ng/L of PFOA and 1,461 ng/L of PFOS was used as the feed. The PFAS concentrations of the feed and the product were analyzed by the EPA 537.1 method. The PFOA concentration was reduced to 260 ng/L, and the PFOA removal efficiency for the functionalized alumina sorbent was 78.0%. The PFOS concentration was reduced to 0, and the PFOS removal efficiency was 100.0%.
Figure 6 shows the PFAS-Selective nitrified GAC (N-GAC) prepared in this project.
Figure 6. PFAS-Selective Nitrified Granular Activated Carbon (N-GAC).
The N-GAC sorbent was tested for PFOA (results reported last month) and PFOS removal. Ionized water with conductivity at 500 mS/cm spiked with 1,185 ng/L of PFOA and 1,461 ng/L of PFOS was used as the feed. The PFAS concentrations of the feed and the product were analyzed by the EPA 537.1 method.
For the GAC after nitrification, the PFOA concentration was reduced to 2.7 ng/L, and the PFOA removal efficiency was 99.8%. The PFOS concentration was reduced to 1.8 ng/L, and the PFOS removal efficiency was 99.9%.
Portable unit assembly with organic coating functionalized polyimide membranes. Two portable units were assembled using our functionalized polyimide membranes and commercially available 12" reverse osmosis cartridge. The treatment capacities were 100 and 125 gallons per day.
Portable unit assembly with functionalized sorbent. Two kinds of small portable units were assembled using different commercially available filter house: 10" clear housing with white flat cap filtration system, and countertop drinking water filtration system with carbon filter (2.5" x 10"). The first one was equipped with the functionalized polyimide membrane, and the latter was packed with functionalized sorbent.
Commercialization of the technology developed in this project is the key to the project success. Here are some of the achievements.
Under-sink PFAS removal water treatment unit. The under-sink device prototype was finalized with PFAS-selective, organic coating-functionalized nanospun polyimide membrane at 10-inch (width) by 20-inch (length), and wrapped into a 12-inch membrane housing, and the treatment capacity is 125-gallon per day. Currently we are preparing for producing 1,000 small portable PFAS removal units for household use.
PFAS-selective membranes as syringe filter media. The PFAS-selective membranes are available for sale as syringe filter media to produce PFAS-free water for lab R&D applications.
PFAS-selective alumina sorbent. The PFAS-selective alumina sorbent is available for sale as packing material to produce PFAS-free water.
PFAS-selective nitrified GAC sorbent. The PFAS-selective nitrified GAC (N-GAC) sorbent is also available for sale as packing material for PFAS removal, or PFAS pretreatment packing media.
All the products will be marketing under our PFAS ShieldTM portfolio.