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
The contamination of groundwater aquifers and soil in various Superfund sites by toxic chloro-organic compounds (such as, polychlorinated biphenyls PCBs, trichloroethylene,TCE, etc.) is a widespread problem that prevents these potentially potable sources from being used for drinking water. By using "green" approaches, researchers have demonstrated that through the integration of nanostructured metals and metal oxides within functionalized polymer membrane platforms, one can conduct environmentally important reductive and oxidative reactions for toxic organic degradation and detoxification from water without the addition of harmful chemicals. To eliminate the concerns of direct nanoparticles (NPs) injection in sites, researchers used different platforms to support metallic NPs due to their open structure and to prevent agglomeration. These platforms included membranes (polyvinylidene fluoride, PVDF), silica (two types of sulfonated silica particles), and responsive hydrogel.
For example, PVDF membranes were functionalized with non-toxic polyacrylic acid (PAA) via green synthesis (water-based chemistry) to immobilize dissolved iron salt, followed by the reduction to NPs with green and biodegradable materials polyphenols from green tea extract and other non-toxic agents. The polyphenols reduced Fe NPs (20-30 nm) maintained excellent activity and great longevity. Another green support is silica particles with ascorbic acid as a "green" reducing agent to synthesize bimetallic Fe/palladium (Pd) NPs. These NPs have been successfully applied for the dechlorination of TCE (kSA, surface-area normalized reaction rate, = 8.1 × 10−4 L/m2 h) (Industrial & Engineering Chemistry Research, 2012). Iron can also be deposited inside membrane pores by using a diffusion cell to synthesize metal nanotubes (after polymer dissolution) (400 nm diameter, 20 nm Fe core thickness, 2-5 micrometer long) with a highly reactive surface area (50 m2/g) (Analytical and Bioanalytical Chemistry, 2012). About 95% PCB 77 (with four chlorines) was dechlorinated in 1 hour mainly to biphenyl, by Fe doped with 2.5 wt% Pd.
In an earlier study, researchers have shown that the same membrane platform used for NP synthesis can also be used for oxidative pathway involving free radical generation by adding dilute hydrogen peroxide (used in some mouthwash). Researchers successfully synthesized the highly reactive iron oxide (like rust) NPs within the membrane platform via air oxidation of Fe NPs. TCE and PCB oxidation from water was investigated with these materials. Depending on the NP loading, hydrogen peroxide concentration and contact time, complete TCE oxidation could be achieved (J. Nanoparticle Research, 2012).
If one just uses a hydroxyl radical-based oxidative degradation pathway the intermediates may still contain highly water soluble and toxic chloro-organic acids. On the other hand, the use of reductive pathways (such as, bimetallic nanoparticles) leads to complete dechlorination without ring rupture (for example, PCBs, biphenyl). To eliminate intermediate chlorogenic formation problems, a combined technology was established in terms of converting PCBs to biphenyl (reductive step) by highly active Fe/Pd nanostructured materials in polymer/hydrogel, followed by an oxidative pathway with polymer immobilized iron/iron oxide NPs for hydroxyl radical generation. These indeed show partial degradation to non-toxic and biodegradable products (such as benzoic acid).
Another achievement of our group is the use of temperature and pH responsive polymers and hydrogels to determine how fast or slow the degradation reaction can be achieved. With the responsive polymers, the pore structure and properties of supports with NPs can be altered resulting in the tunable control of pollutants destruction by altering pollutant partitioning and water content around reactive NPs. Our results with green and non-toxic temperature responsive hydrogel (poly(N-isopropylacrylamide-co-acrylic acid, PNIPAAm-PAA) (widely used in the medical field) immobilized nanoparticles show three-fold increase of trichloroethylene (TCE) degradation reactivity with only 4oC temperature increase (J. Applied Polymer Science, 2012), and also decreased the complete degradation time from 6 h to 2h for PCB4 (with two chlorines).
Researchers are also working with the Commonwealth of Kentucky and DOE for remediation applications of these nanostructured metals and oxidative techniques to the Paducah Superfund site involving chlorinated organic remediation. In addition, to establish the extent of degradation and product toxicity and health effects, researchers collaborated with Project 1 (PI: Bernhard Hennig) to determine how the resulting biological activities differ from the final dechlorination products (biphenyl) vs parent compound (PCBs) and intermediates. Using data from various reaction times containing mixtures of several intermediate PCBs, researchers did observe the decrease of pro-inflammatory potential to the extent of dechlorination. This shows the evidence that the degradation indeed decreased the toxicity on vascular tissue. Similar patterns were observed in expression of downstream inflammatory parameters. Furthermore, all dechlorination mixtures except for final product, biphenyl contributed to increased reactive oxygen species (ROS, the main cause of aging) production in endothelial cells. Our remediation technologies indeed show potentially decreased biomedical risks for human health.
Based on these research advances, three full utility patents were filed in 2012 on green and functional membrane technologies relating to water purification and remediation. Recently researchers have successfully made full-scale functionalization of membranes (40 inches wide and 300 feet long) at Sepro Membrane, Inc., manufacturing facility in Oceanside, CA (via the licensing of patents). Through this, full-scale responsive membrane supports (along with compact filter module development) with these NIEHS-SRP-based technologies should create new and advanced commercial applications in water purification and remediation technologies.
The most significant accomplishments are:
- Greener way making functionalized membrane support (water-based chemistry) and greener NPs synthesis by green tea and Vitamin C
- Synthesized the iron oxide NPs in the membrane platform for the oxidation of TCE in the presence of hydrogen peroxide at non-acidic conditions
- In-situ generation of hydrogen peroxide by enzyme to regulate the generation of free radicals and eliminate unnecessary waste
- Completely degraded PCBs in water by using combined pathway with no toxic product formation
- Studied the products and mechanism of oxidative degradation of biphenyl in the presence of Fe NPs and hydrogen peroxide
- Use the temperature and pH responsive polymers and hydrogels to allow the tunable control of PCB and TCE detoxification by altering pollutant partitioning and water content around reactive NPs
- Determined potential remediation strategies for groundwater remediation via in-situ and above ground treatment at Paducah Gaseous Diffusion Plant Superfund site in Paducah, KY
- Diminished health effects of the final dechlorination product (biphenyl) vs. parent compound (PCBs) and intermediates through toxicity data in conjunction with Project 1
- Potential application for broader green remediation, such as for Triclosan (widely used in antibacterial agents), reported to have adverse health effects
- Joint work with Sepro Membrane company in California resulted in the successful production of full-scale functionalized membrane supports needed for site remediation activities.