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
Iron-based systems are highly useful for oxidation/reduction of various chlorinated hazardous organics. Reductive decomposition of organic compounds by zero valent iron, developed as a groundwater remediation technology over the last decade, has been the subject of much research and has seen wide application in the field. Similarly, iron-mediated oxidation of contaminants via Fenton's reagent has been demonstrated effective at decomposing a wide range of organic contaminants, including BTEX, PCE and phenols, etc.. These two approaches have the potential to be complementary. This report summarizes activities in four areas: (1) Dehalogenation with oxidative Fenton reaction near neutral pH, (2) Formation of nano-iron particles in membranes and subsequent dehalogenation behavior, (3) Multiwall, cabon nanotubes for iron supports and dehalogenation reaction, (4) Vitamin B12 derivative based dehalogenation. Trichloroethylene (TCE) has been used as the model compound. Future studies will include aromatic PCB fragments.
Iron-Based Reactions Oxidative Technique
Fenton's oxidation reaction by iron-H2O2 based chemistry has been applied to degradation of various organic compounds. The requirements of low pH operation and iron precipitation have limited direct in-situ application. On the other hand, the use of chelates allow the use of operation even at pH 6 without any iron precipitation. Project investigators have demonstrated through fundamental studies that the addition of a simple chelate like citric acid allowed excellent degradations of TCE without using excessive peroxide. The parent compound degradation was found to be excellent. These types of chelates are non-toxic and inexpensive. Oxidative decomposition also may complement chemical reduction of contaminants, such as that achieved by zero valent iron, serving to destroy compounds that are refractory to reductive decomposition.
In the presence of chelates (e.g. citric acid), Fe2+ release can be controlled (thus minimizing unwanted H2O2 consumption) and ferric hydroxide precipitation can be eliminated. The OH· free radicals produced by the reaction (Fe2+ + H2O2 ® Fe3+ + OH- + OH· ) can oxidize various organics and the complexation reaction of citric acid with Fe2+ and Fe3+ provides the condition for higher pH operation in groundwater remediation conditions. Having established (experimentally) the ability of citric acid to solubilize iron in the presence of H2O2 at near-neutral pH, trials were conducted to determine the potential for Fenton-like destruction of 0.0028M TCE in the presence of chelate at near-neutral pH. Runs were conducted using 0.0025M Fe-L at H2O2 concentrations varying from 0.0050 to 0.025M (165 to 825 mg/L). The pH of solutions was adjusted to approximately 6.4. Effectiveness of TCE destruction was evaluated by GC-MS measurement of TCE concentrations, and by chloride ion measurement using an ion specific electrode. TCE results demonstrated that virtually 100% destruction of TCE was obtained in less than 10 hours. Chloride formation was about 58% of the possible maximum. Chloride formation can be enhanced by adjustment of H2O2 / Fe2+ ratio.
Organic degradation at zero-valent metal surfaces is a well-documented phenomenon whose effectiveness has been linked to metal surface area and active sites. These studies are based on the assertion that if metal ions can be reduced in place to form very nano-sized iron particles, their effectiveness in the aforementioned reactions could be dramatically increased over previous work with 20 - 100 micrometer iron particles. Project investigators have taken a novel approach to form nanosized Fe0 particles and this was done in two steps. The two steps involved attachment of multi-functional ion exchange ligands (polyamino acid) to aldehyde functionalized cellulose-fiber membrane surface to capture Fe2+, followed by subsequent reduction with borohydride to form immobilized nano Fe particles. With 200 mg/l TCE solution (at pH 6.2) two thousand-fold increases in the initial reaction rate of membrane-based iron nanostructures were observed in comparison with that of larger mesh (150 micron) iron particles in the dechlorination reaction. This result is due to the fact that the nanostructures that we have developed in membrane pores possess much greater surface area per unit mass and have completely accessible reactive iron sites. In the next phase, the researchers plan to study the particle characteristics, and dechlorination for other aromatic organics.
Multiwall, cabon nanotubes
A CVD method for the production of MWNT's has been developed at the Center for Applied Energy Research under funding from the National Science Foundation (MRSEC on Advanced Carbon Materials). These MWNTs are readily dispersed in liquids, and their only impurity is the iron catalyst particles, which can be removed by washing with mild acid. The tubes average 50 microns in length and 30-50 nanometers in outside diameter. This material is electrically conductive, and would be an excellent choice as a support for plating iron or other metal catalysts of dehalogenation reactions. The concept is similar to that proposed for activated carbon fibers. The MWNTs can be attached to each other to form a high surface area solid. The attachment can be done by using a polymer solution, and removing the solvent so that a thin coating of polymer covers each tube. This solid has already be fabricated. In phase two of the work, researchers will plate iron to this MWNT matrix and test it in dehalogenation reactions.
Vitamin B12 derivative based dehalogenation
Vitamin B12 contains a cobalt-centered corrin ring system that can reductively dehalogenate chlorinated organics in the presence of a bulk electron donor like titanium (III) citrate. However, titanium citrate is an expensive soluble reagent that would need to be recovered and recycled for economic and environmental reasons. One of the goals is to polymerize appropriate vitamin B12 derivatives on electrodes, which can provide the needed reducing power electrochemically, thus avoiding the need for reducing agents such as Ti(III). Project investigators have initiated the synthesis of the vitamin B12 derivative that incorporates a polymerizable pyrrole unit at the periphery of the corrin ring. This will allow electrochemical polymerization of the derivative on electrode surfaces that can be subsequently used to degrade the target hazardous compounds. The B12 compound is being prepared by first synthesizing dicyanocobalt(III)-hexamethylcobester-c-lactone by oxidative esterification of vitamin B12. The lactone can be reduced with zinc in acetic acid to yield a corrin that contains a single carboxylic group, which then reacts with N-[3-aminopropyl]pyrrole (prepared by reduction of N-[2-cyanoethyl]pyrrole with LiAlH4) to yield the desired compound.