Controlling the Reactivity of Chlorinated Ethylenes with Flavin Mononucleotide Hydroquinone CHRISTOPHER G. E. CIPTADJAYA, † WEN GUO, ‡ JAYNI M. ANGELI, † AND SHERINE O. OBARE* ,†,‡ Department of Chemistry, Western Michigan University, Kalamazoo, Michigan 49008, and Department of Chemistry and the Nanoscale Science Program, University of North Carolina at Charlotte, Charlotte, North Carolina 28223 Received August 03, 2008. Revised manuscript received January 5, 2009. Accepted January 6, 2009. Reduction rate constants of the chlorinated ethylenes cis-1,2- dichloroethylene ( cis-DCE), trichloroethylene (TCE), and tetrachloroethylene (PCE) reacted with flavin mononucleotide hydroquinone (FMNH 2 ) under anoxic conditions were investigated. FMNH 2 was produced in methanol solvent by the photoreduction of FMN. In aqueous solution, FMN was not fully reduced to FMNH 2 but instead yielded the semiquinone radical FMNH • . However, when FMN was anchored to nanocrystalline TiO 2 , band gap irradiation resulted in electron transfer from the TiO 2 conduction band to FMN, thus yielding FMNH 2 . The FMNH 2 generated in aqueous solution on the TiO 2 surface was a stronger reductant toward chlorinated ethylenes, relative to FMNH 2 in solution. Furthermore, by combining the reactivity of the TiO 2 conduction band electrons [TiO 2 (e - CB )] with FMNH 2 , reduction rate constants for the chlorinated ethylenes increased by 2 orders of magnitude relative to FMNH 2 alone. The results show how biological molecules such as FMNH 2 could have significant effects toward the remediation of organic pollutants. 1. Introduction For several decades chlorinated ethylenes have presented serious environmental concerns. One of the current tech- nologies used for treating groundwater contaminated with chlorinated ethylenes is zerovalent iron (1-11). One major concern with this technology is that many chlorinated ethylenes undergo incomplete degradation, thus forming products more toxic than the parent compound (8-13). In addition, there are concerns related to the limited lifetime of zerovalent iron due to its susceptibility to corrosion. Consequently, significant efforts have been devoted toward the remediation of chlorinated ethylenes (14, 15). In 2001, Totten and Roberts (16) demonstrated through theoretical calculations that the degradation of chlorinated ethylenes would undergo a favorable thermodynamic deg- radation pathway if two-electron transfer reductants were used relative to one-electron transfer pathways. On the basis of the calculations, two-electron reduction potentials were found to be more positive compared to one-electron reduc- tion potentials. Despite this prediction, very few reports have appeared in the literature that show the reduction of chlorinated ethylenes via a two-electron pathway (17). Identifying two-electron or multielectron transfer catalysts and understanding their reactivity could potentially yield viable methods for the remediation of organic pollutants under mild conditions. Here, we report the reactivity of the biological molecule flavin mononucleotide (FMN) in its reduced form, FMNH 2 , with the chlorinated ethylenes cis- 1,2-dichloroethylene (cis-DCE), trichloroethylene (TCE), and tetrachloroethylene (PCE). An added and significant advan- tage of this approach relative to others is the ability to run these degradation reactions in either organic solvents or aqueous solutions at ambient temperature and pH. FMN is a riboflavin and is a redox-active chromophore found in many enzymes and photoreceptors (18, 19). FMN can be reduced to form FMNH • or the well-known two-electron two-photon FMNH 2 , as shown in Scheme 1 (19). The redox-active behavior of FMN led us to investigate its reactivity with chlorinated ethylenes and to further identify methods to control and enhance its reactivity. The results provide insights on how various biological molecules could potentially be tuned toward the remediation of various environmental pollutants. 2. Experimental Section 2.1. Materials and Methods. All chemicals were used as received without further treatment. Methanol (MeOH; g99.9%, HPLC grade), acetonitrile (CH 3 CN; g99.9%, HPLC grade), titanium(IV) isopropoxide [Ti(iOPr) 4 ; 97%], cis-1,2- dichloroethylene (cis-DCE; g97%), trichloroethylene (TCE; g99.5%), tetrachloroethylene (PCE; 99.9%), riboflavin (g98%), and riboflavin 5′-phosphate sodium salt (flavin mononucle- otide or FMN; ∼85%, HPLC grade) were purchased from Aldrich Chemicals. Nitric acid and microscope glass slides were obtained from Fisher Scientific. Tetrabutylammonium hexafluorophosphate (TBAPF 6 ; g98.0%) was purchased from Fluka Co. Indium-doped tin oxide (ITO) coated sheets of glass were purchased from Hartford Glass Company, Inc. Deionized Milli-Q water at a pH of 7 was used where aqueous measurements are described. Colloidal TiO 2 nanoparticles were imaged on a JEOL scanning electron microscope (SEM). A custom-designed quartz cuvette with a 1 cm path length was used as the spectroelectrochemical cell. 2.2. Nanocrystalline TiO 2 Film Preparation and Func- tionalization. TiO 2 nanoparticles (∼12 nm) were synthesized by the hydrolysis of titanium(IV) isopropoxide [Ti(iOPr) 4 ] via a sol-gel technique and prepared as a paste, as described in the literature (20). The TiO 2 paste was cast as mesoporous thin films onto microscope glass slides. The attachment of FMN to the TiO 2 surface was achieved by soaking freshly prepared TiO 2 films for 6 h in a 1 × 10 -4 M FMN solution in MeOH. The nanocrystalline thin films were placed diagonally in a standard quartz cuvette. Absorption spectra and steady- state kinetic measurements were acquired on a Cary 50 UV-visible spectrophotometer. Irradiations of FMN/TiO 2 films or FMN in solution were carried out with a 1000-W Xe lamp with a KV 370 cutoff filter. In each case, samples were illuminated for 30 min. 2.3. Spectroelectrochemical Measurements. Spectro- electrochemical measurements were conducted on a CV 50-W potentiostat to apply the desired potentials in a standard three-electrode arrangement with a Pt counter electrode, Ag/AgCl (3 M KCl) reference electrode, and ITO/TiO 2 /FMN as a working electrode on alligator clips. A fresh solution of 0.1 M TBAPF 6 in acetonitrile was used as the supporting electrolyte. A Varian Cary 50 UV-visible absorbance spec- trophotometer was used to measure absorbance spectra. Each * Corresponding author. † Western Michigan University. ‡ University of North Carolina at Charlotte. Environ. Sci. Technol. 2009, 43, 1591–1597 10.1021/es8021792 CCC: $40.75  2009 American Chemical Society VOL. 43, NO. 5, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 1591 Published on Web 02/04/2009