(20b) Bimetallic Nanoparticles for the Degradation of Haloamide Disinfection by-Products
Advanced analytical measurements have allowed the recent identification and quantification of many disinfection by-products (DBPs). While current regulations by the U.S. Environmental Protection Agency (EPA) focus on haloacetic acids and trihalomethanes, newly identified compounds families, including halonitromethanes and haloamides, have been shown to be significantly more toxic than the regulated compounds1. Toxicity studies reveal that these nitrogenous DBPs are consistently more toxic than their non-nitrogenous counterparts, and that DBPs containing the halogens bromide or iodide are more toxic than those containing chloride. The increased use of chloramines in place of chlorine as a water disinfectant in water treatment facilities has caused an increase in both nitrogenous and bromide/iodide substituted DBPs. Therefore, these newly identified nitrogenous DBPs are of particular interest when considering the development of novel treatment technologies for the degradation and removal of water micropollutants. The increased use of membranes in water treatment facilities, along with the development of novel membrane and membrane composite materials, indicates that membranes have grown in acceptance as an efficient physical barrier to a variety of water contaminants and will be used in the future as one of the final steps in water treatment before distribution.
The inclusion of nanoparticles in membranes has begun to be investigated as a way to improve the stability, structure, and separation properties of membrane materials, as well as make membranes an active system that both filters and reacts with water contaminants. Zero valent iron (ZVI) particles are one type of nanoparticle that are of particular interest due to their ability to catalyze both oxidation and reduction reactions and degrade recalcitrant organic contaminants. ZVI particles have been demonstrated to reduce halogenated organic compounds such as trichloroethylene2, but little work has been performed on the degradation of halogenated DBPs. In our research, modifications to ZVI nanoparticles are made to improve nanoparticle reactivity, including an investigation of different transition metal coatings and an investigation of the stabilization of ZVI nanoparticles during nanoparticle synthesis. Transition metals such as palladium or nickel are coated onto ZVI particles to enhance the degradation of halogenated organics; dichloroacetamide, an identified haloamide DBP, is used to test particles and compare differences in transition metals and the amount of metal coating. Palladium (Pd) has been demonstrated to significantly improve ZVI catalytic reactivity2, and other transition metals will be investigated and compared to ZVI alone and to ZVI-Pd bimetallic nanoparticles.
Several different types of organic polymers are investigated as stabilizers during ZVI nanoparticle synthesis. Glucose-based polymers with different degrees of substitution are evaluated and compared with organophosphates in their ability to prevent ZVI nanoparticle aggregation. Different ratios of stabilizer to ZVI are tested and used to control the particle size distribution. Based on results, other types of compounds may be considered, including organosulfate compounds and other nontoxic biological polymers. Research has shown that nanoparticles below an average diameter of 30 ? 40 nm tend to display enhanced or unique properties, while nanoparticles above this range have properties similar to micron-sized particles3. Based on this critical nanoparticle size threshold, a nanoparticle size range of 5 ? 50 nm is investigated to determine if catalytic reactivity and the extent of dichloroacetamide degradation change above and below the determined threshold.
ZVI nanoparticles are synthesized for each experiment by precipitating ferrous sulfate (FeSO4) with sodium borohydride (NaBH4); before the addition of borohydride, the iron solution is deoxygenated using argon gas and the precipitation is performed under vacuum. The stabilizer compound is added before borohydride addition and isolates nucleating particles during precipitation. The addition of the secondary transition metal is performed either before precipitation by borohydride or after the ZVI precipitation is completed. The reactivity of the two types of nanoparticles will be compared with dichloroacetamide. Nanoparticle size is evaluated through dynamic light scattering and scanning electron microscopy measurements, and surface charge is measured with a Zetasizer Nano (Malvern Instruments). The loss of dichloroacetamide is measured using gas chromatography with electron capture detection (GC-ECD); aqueous samples are extracted to ethyl acetate with no derivatization, and a standard curve is obtained from a set of known concentrations of dichloroacetamide extracted by the same procedure.
1. M. J. Plewa, M. G. Muellner, S. D. Richardson, F. Fasanot, K. M. Buettner, Y. T. Woo, A. B. McKague, E. D. Wagner, Occurrence, synthesis, and mammalian cell cytotoxicity and genotoxicity of haloacetamides: An emerging class of nitrogenous drinking water disinfection byproducts. Environmental Science & Technology 2008, 42. 955-961.
2. F. He, D. Y. Zhao, Hydrodechlorination of trichloroethene using stabilized Fe-Pd nanoparticles: Reaction mechanism and effects of stabilizers, catalysts and reaction conditions. Applied Catalysis B-Environmental 2008, 84. 533-540, DOI: 10.1016/j.apcatb.2008.05.008.
3. M. Auffan, J. Rose, J. Y. Bottero, G. V. Lowry, J. P. Jolivet, M. R. Wiesner, Towards a definition of inorganic nanoparticles from an environmental, health and safety perspective. Nature Nanotechnology 2009, 4. 634-641, DOI: 10.1038/nnano.2009.242.