Superfund Research Program
Chloro-Organic Degradation by Polymer Membrane Immobilized Iron-Based Particle Systems
Project Leader: Dibakar Bhattacharyya
Grant Number: P42ES007380
Funding Period: 2000-2019
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Studies and Results
The relative chemical stability and ubiquitous nature of chlorinated organic compounds such as, polychlorinated biphenyls (PCBs) and trichloroethylene (TCE) continue to pose both remediation challenges and human health risks. PCBs have been shown to induce vascular endothelial cell dysfunction and inflammation as well as increasing the oxidative stress in vivo (Superfund Chemicals, Nutrition, and Endothelial Cell Dysfunction Project, PI: Dr. Bernhard Hennig). Through the in-situ synthesis of iron particles (50 to 100 nm range) within functionalized polymer membrane platform, this team of researchers demonstrated the treatability of chloro-organics by chemical remediation by the reductive and oxidative treatment. A second metal catalyst like Pd is necessary for PCBs hydrodechlorination, but not for TCE. The team has developed a common membrane platform functionalized with polyacrylic acid (PAA) for direct synthesis of iron particles to eliminate particle agglomeration and leaching problems associated with conventional systems. Similar platform has also been applied to remove toxic inorganics (e.g., selenate) from water.
In their integrated membrane-particle platform, commercial polyvinylidene fluoride (PVDF) membrane was selected as a supporting material due to its excellent chemical and thermal stability. In PAA functionalized membranes, the dissolved iron salt was immobilized to make NPs (nanoparticles) with the addition of environmentally benign reducing agents. Iron (Fe0) NPs with a small amount of Pd (0.9 wt% of Fe) in membranes exhibited high reactivity (surface area normalized reaction rate, kSA=0.32 L/m2 h) during the dechlorination of 2-chlorobiphenyl under the hydrodynamic flow condition. In batch mode, the immobilized NPs (kSA=0.26 L/m2 h) also shows higher dechlorination rate than the non-immobilized NPs (kSA=0.18 L/m2 h) because of elimination of particle aggregation (Gui, Ormsbee, Bhattacharyya, Industrial & Engineering Chemistry Research, May 2013).
The same platform used for NP synthesis can also be used for oxidative degradation of chloro-organics involving the free radical generation. Dilute hydrogen peroxide (H2O2) was added as the precursor of hydroxyl radicals. The oxidative pathway converted PCBs into the hydroxylated PCBs (hydroxylation), followed by the ring scission. However, the chlorinated degradation products such as chloro-organic acids can still be toxic and water soluble. On the other hand, the reductive pathway leads to the complete removal of chlorine atoms without ring scission (e.g., PCBs?biphenyl). Therefore, a combined pathway based on the functionalized membranes was established in terms of first converting PCBs into biphenyl, followed by an oxidative treatment to degrade the biphenyl to non-chlorinated degradation products. Iron/iron oxide (core-shell), ferrihydrite and magnetite NPs have been synthesized through the controlled oxidation of iron NPs (with air or H2O2) for oxidative pathway. To better understand the surface transformation and fate of NPs, iron NPs were analyzed by elemental mapping (Fe, O) of electron energy loss spectroscopy (EELS). The fresh iron NPs showed iron oxide shell of 3-5 nm. With H2O2 treatment, the thickness of oxide shell increased, and finally the iron core disappeared, forming mainly the magnetite. Both soluble iron and magnetite promoted the free radical generation, which enhanced the oxidation to non-toxic products.
Another achievement of the research group is the synthesis of hydrogel functionalized membranes through the redox polymerization of PAA in membrane pores for TCE dechlorination. Instead of using the thermal PAA synthesis, dissolved iron salt was added as an accelerant for the initiation of polymerization at the room temperature, and subsequently used it for NP synthesis. With Fe/Pd bimetallic NP treatment (450 mg Fe/g PAA) at pH 6, excellent TCE degradation to nontoxic chloride was observed (Hernandez, Papp, Bhattacharyya, Industrial & Engineering Chemistry Research, Nov. 2013).
In the team's previous studies, they have shown the use of temperature and pH responsive PNIPAAm (poly(N-isopropylacrylamide)-PAA hydrogels to determine how fast or slow the PCB/TCE degradation reaction can be achieved. With the responsive polymers incorporated in the membrane platform, the membrane pore structure can be altered resulting in the tunable control of pollutants destruction by altering the pollutant partitioning and water content around reactive NPs. The effective membrane pore size is controlled by the reversible swelling and shrinking of PNIPAAm around its lower critical solution temperature (LCST). The pure water flux was varied by up to 15 times for temperature changes with only 4oC, which makes it possible to utilize the membrane as a valve to regulate the separation (Journal of Membrane Science, in press, 2014). The rate of chloro-organics transport through the membrane can be easily controlled by temperature induced swelling (hydrophilic) and deswelling (hydrophobic). The hydrogel functionalized membrane was also used for the direct synthesis of Fe/Pd NPs for TCE dechlorinaion (kSA=0.15 L/m2 h).
To prove the feasibility of using their nanostructured metals and oxidative techniques for chloro-organic on-site remediation, the team of researchers do collaborative work with a membrane company (Sepro Membranes, Oceanside, CA) to synthesize larger-scale membrane modules with an active area of 5 ft2. With the repeated immobilization, they can obtain high loading (0.65 gm) after three cycles, showing the successful scale-up from bench-scale to larger-scale. Therefore, in addition to the direct injection of chopped membranes, the pump and treat above ground application through the iron functionalized membrane modules can be an alternative remediation solution. The researchers also tested the selenate removal efficiency using the same membranes (project funded by a company). Depending on the iron NP loading, toxic selenate can be reduced to selenium (Se0) within a short residence (contact) time. It is important to understand the reaction mechanism of inorganics with iron NPs, since the contaminated water at the Paducah Gaseous Diffusion Plant may contain low concentration of dissolved technetium oxyanions.
Through the collaboration with industry partners such as Sepro Membrane Inc., the large-scale iron functionalized membranes (flat-sheet and spiral-wound module) have been developed for toxic chloro-organic degradation and for inorganics. With these achievements, the team's NIEHS-SRP-based technologies should create new commercial applications in water purification and remediation.
Significance
The overall accomplishment is the ability to go from bench-scale to larger-scale membrane filter module containing iron/iron oxide nanoparticle and establishing the effectiveness of PCB/TCE remediation through fundamental understanding of particle surface characteristics. Some of the significant accomplishments are: 1) synthesized the highly reactive iron/iron oxide NPs in a common membrane platform for the oxidative degradation of toxic chloro-organics; 2) completely degraded PCBs in water by a combined pathway with no toxic chlorinated product formation; 3) identified the hydroxylation and ring scission products in the free radical oxidation of biphenyl; 4) studied the iron/iron oxide surface transformation with hydrogen peroxide; 5) joint work with a membrane company for the development of larger-scale functionalized membrane module for the above ground remediation application; 6) used the temperature and pH responsive polymers to allow the tunable control of PCB and TCE detoxification.