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
Chlorinated organic compounds, such as polychlorinated biphenyls (PCBs), and trichloroethylene (TCE), constitute a large group of pollutants due to their high toxicity, persistence and various sources of distribution in the environment. The remediation of these recalcitrant, toxic chloro-organics often requires both reductive and oxidative treatment. The research group has attained significant achievements in the area of detoxification of these compounds by using common iron-based systems. The reductive platform included membrane and silica-based Fe/Pd nanoparticles (NPs) whereas the oxidative approach involved immobilized iron (II) – polymer membrane platform.
For the case of reductive pathway, it is well established that solution phase synthesis of iron and/or iron/palladium bimetallic nanoparticles leads to agglomeration and reduction in reactivity toward degradation of toxic organic chlorinated compounds. To overcome the problems, membranes and silica particles were used as platforms for iron nanoparticle synthesis, followed by palladium deposition. The research team pioneered an easy method of immobilization of iron (II) and iron (III) in a polymer membrane pores containing non-toxic poly-acrylic acid (obtained via green synthesis). The resulted iron was converted to NPs by reduction and then coated with a very small amount of Pd. The particle sizes ranged between 30 – 50 nm. Iron can also be deposited inside membrane pores forming nanotubes (after polymer dissolution) by using a diffusion cell. These nanotubes (200 nm diameter, 10 nm wall thickness, 3 micrometer long) have three times higher reactive surface area (50 m2/g) than typical 50 nm NPs. Another approach to production of bimetallic NPs for remediation is to disperse them on silica particles. Silica is an ideal "green" platform for the production of nanoparticles (without agglomeration) for groundwater applications. By functionalizing commercial silica particles (with sulfonic or polyaspartic acids) to capture iron and by post reduction researchers were able to make 50 nm Fe/Pd NPs.
The bimetallic nanoparticles showed a high reactivity towards two superfund chemicals, TCE and PCBs. For example, for a membrane immobilized with bimetallic particles (only 15 mg iron and 2.6% wt palladium), 75% conversion of PCB 4 (2 chloride groups) parent compound was attained in 1 hour of reaction time. Complete TCE degradation to non-toxic ethane is easily obtained. The research team’s recent innovation to enhance NP longevity included post sorption of iron (II) in the polymer pore matrix. The team’s recent studies showed that NPs can be repeatedly regenerated with sodium borohydride or potentially ascorbic acid. Nanotubes also showed a very good reactivity toward PCB77 (4 chloride groups) dechlorination, 85% of PCB 77 was converted to biphenyl and lower PCBs after two hours of reaction time.
The team has shown that the same membrane platform used for nanoparticle synthesis can also be used for hydroxy radical generation (by passing dilute hydrogen peroxide) within the membrane pores. The team’s research has shown that by immobilizing the iron ions (without converting to NPs) within the polymer/membrane domain, they are able to generate free radicals effectively and repeatedly at near-neutral pH. High effectiveness of the system was demonstrated through significant destruction of a model, persistent organic pollutant (pentachlorophenol). In order to increase the team’s understanding of the rate of free radical formation in these systems, they have utilized hydroxyl radical "trapping" compounds to quantify rate of free radical production. This information can help to determine effective remediation (minimizing chemical usage) strategies for various pollutants, such as PCBs.