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 risk associated with the presence of chlorinated organics in contaminated ground water sources, soil, and sediments has generated immense interest in developing low temperature (~ 25oC) processes for the dechlorination of various toxic compounds. Our project deals with the continued development of new and modified approaches for dechlorination by oxidative and reductive techniques. These activities fall in three areas:: (1) dehalogenation with modified Fenton reaction with chelates for near neutral pH reaction, (2) reduction with zero-valent metals, and with Pd + hydrogen gas , (3) Vitamin B12 reduction and the use electropolymerized conductive polymer films for dehalogenation. Trichloroethylene (TCE), chlorobenzenes, and chlorophenols have been used as model compounds.
Oxidative Technique: Modified Fenton Reaction for Near Neutral pH Operation
Oxidation (hydroxyl free radical reaction) of contaminants by Fenton's reagent is a promising, in-situ remediation technology because of the powerful oxidizing potential of the Fenton system, the fact that the reagent is relatively inexpensive, and because byproducts of the reaction are benign. The role (Figure 1) of the chelating agent is: (1) control delivery of Fe2+ leading to reduced pressure and heat evolution (by reaction with peroxide) near the injection point, (2) operation at near neutral groundwater pH without precipitating iron (III) hydroxide, and (3) delay the Fenton reaction over a longer period of time and thus leading to enhanced H202 utilization for organic oxidation. The use of citrate over citric acid allows for better pH control. The chelate modified Fenton reaction has been successfully evaluated in the laboratory with many chlorinated organics, such as, TCE, CCL4, chloroethyl ether, and trichlorophenol. Based on extensive fundamental research results we have demonstrated the technique for an actual industrial site (involving a large chemical company), and the company plans to go full-scale remediation with our Iron/Chelate/H2O2 procedure.
For 2,4,6 trichlorophenol (TCP), we carried out both standard Fenton reaction (pH = 2.0) and chelate modified Fenton reaction (pH = 7.0). In Figure 2, it is clear that the reaction rate of the modified Fenton reaction is much slower than the standard Fenton reaction. The conversions of chloride ion for both methods after 48 hours are > 90%. Direct TCP analysis by UV showed 100% parent compound destruction. Furthermore, we calculated the mole H2O2 consumed /mole of Cl- formed in the reaction. From these calculations we demonstrated that the reaction with chelate has significantly higher H2O2 utilization efficiency than the reaction without chelate. For in-situ remediation, slower reaction rate (and reduction of subsurface pressures) resulted from the use of chelate is important in terms of enhancing the radius of influence and longer dechlorination effectiveness. Our Fenton reaction approach has also been used for above ground operation involving the treatment of a complex toxic waste (chlorophenols, and various organics) system at a Dow Chemical site.
We have extensive ongoing research activities on reductive dechlorination of organics by membrane immobilized nano-sized zero-valent metals (U.S. EPA STAR program), aromatics (chlorophenols, chlorobenzenes) and chlorinated ethenes by the use of Pd + H2 and by vitamin B12. Pd + in-situ hydrogen generation system provides excellent opportunities for dechlorination of various aromatic organics. Our results indicated highly significant dechlorination (50 – 100% in less than 30 min) of 1,2- dichlorobenzene, and 2,4,6-TCP with low levels of immobilized Pd in high surface area supports. The main goal will be to form nanosized Pd in polymer matrix for fast Dechlorination at room temperature. With vitamin B12 on the other hand, the reported literature results indicated slow rate (days rather than minutes for Pd system) of dechlorination of aromatics (such as, PCP, TCP).
Vitamin B12 – Based Reductive system: Conventional and Electrochemical Methods
Various bio-molecules have metal centers and can be used for organic transformations. In particular, vitamin B12 molecule consists of a central cobalt atom surrounded by porphyrin ring. Stable oxidation state of cobalt in vitamin B12 is +3. When Co (III) is transformed to Co (I), it becomes a very strong nucleophile, and thus can dechlorinate chlorinated organic compounds reductively. Ideally, one should be able to convert compounds such as trichloroethylene (TCE) to ethylene and Pentachlorophenol (PCP) to phenol. Of course, these reactions go through various intermediate reductive steps. Unfortunately, another strong reducing agent (such as Ti (III) citrate) is required in excess to convert Co (III) ----à Co(I). Mechanism of the process is being investigated by various researchers. Lliterature studies (Burris et al, Glod et al, etc.) have reported the dechlorination reaction (particularly TCE) using Vitamin B12. These studies indicate a very slow dechlorination rate of the intermediates (for example, cis-DCE). Our studies are focused on higher concentrations of chlorinated organics. In order to establish the rate of dechlorination and intermediate formations, studies were conducted with 200 mg/L (1.5mM) TCE, 0.5 mM Vitamin B12 and 15 mM Ti (III) citrate at an initial pH of 8. Figure 3 shows that indeed TCE degradation is quite rapid (75% degradation in half an hour, in contrast to days requirement with zerovalent metals). But, as literature indicated our results also show that the intermediate dechlorination of cis-DCE is very slow and thus, future studies will concentrate on enhancing the intermediate reaction rates by new immobilization approaches and by the use of mixed catalyst approach.
Another approach we have taken is to convert the Co (III) to Co (I) of vitamin B12 center electrochemically. This requires preparation of electropolymerized polypyrrole (conductive) film on graphite working electrode. This was done by starting with pyrrole solution in water and polymerizing by applying a voltage of +1.1 V. Our hypothesis was that if Co (III) can be reconverted back to Co (I) electrochemically (which requires a potential of –2 V), then one should see continuous chloride formation of a saturated chlorinated organic solution (TCE) in water. This was indeed the case as shown by chloride formation results in Figure 4.
The second hypothesis was that if the voltage application is stopped, then further chloride formation will cease, and this was also proved to be true. Dechlorination was conducted over a period of 2 days, with a continuous application of voltage and periodic addition of the chlorinated compound, to make sure the solution is saturated.
Another experiment carried out electrochemically involved the use of Ti (III) citrate, which allowed a continuous maintenance of Ti (III) formed by converting Ti (IV) to Ti (III) with the application of voltage (-0.4V). The goal of this experiment was to reduce the use of the bulk reductant significantly. This allows the possibility of carrying out the dechlorination continuously for a long time. Preliminary results indicate higher rate of intermediate formation with the application of voltage. It was also established that the electrochemical approach indeed resulted in the maintenance of Ti (III) form in the solution phase.
Our future studies on enhancement of dechlorination rates will also involve: (1) Synthesis of negatively charged Vitamin B12, (2) Synthesis of pyrrole-derivatized Vitamin B12 for electropolymerization. The main goal will be to incorporate controlled amount of Vitamin B12 into the polypyrrole film, and thus enhancing the electron transfer capability for organic reduction. The first step of the synthesis of Cobalamin – 3-(Pyrrol-1-yl)propylamine conjugate involves the formation of a lactone on the c side chain of pyrroline ring B of Cyanocobalamin. A subsequent conjugation with 3-(Pyrrol-1-yl)propylamine (3) will lead us to the desired product needed for electrochemical studies. The first step of the synthesis step has been completed. The main studies of the use of modified vitamin B12 will be in the area of enhancement of dehalogenation rate of intermediates (such as, cis-DCE).