Monochloramine (NH2Cl) at low concentrations is frequently used as a secondary disinfectant to treat pathogens in the municipal water supply. This disinfectant has the potential to react with the surface of the pipelines and dissolved species in complex environments resulting in the dissolution of unwanted by-products. Iron (Fe) is the 4th most prevalent earth-abundant element in pipelines and the natural environment in soils, minerals, and dust. In these complex environments, iron undergoes spontaneous redox reactions producing Fe2+ and Fe3+ species to form various corrosion products. The surface reaction pathway of NH2Cl with Fe surfaces has the potential to decompose, thus contributing to additional disinfectant by-products. The reaction of NH2Cl and oxidation on Fe surfaces was investigated at the air/solution/Fe interface using in-situ polarized modulated-infrared reflection absorption spectroscopy and surface analysis techniques. The Fe surface exposed to NH2Cl solutions was compared to that of Fe exposed to the reactants used to synthesize NH2Cl, such as NH4Cl(aq), NaOCl(aq), and NaOH(aq.), to exclude the effect of the solutions and explicitly define the NH2Cl mechanism on Fe corrosion. The Fe surface immersed in each NaOH(aq), NH4Cl(aq), and NaOCl(aq) solution show different surface corrosion products under the same conditions. In NH2Cl(aq), the Fe surface was found to oxidize and corrode while simultaneously absorbing atmospheric CO2 at the interface region. In the submerged region, heterogenous corrosion was observed, where the presence of amine and carbonate species were produced as the corrosion products as identified by attenuated total-reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS). To further investigate the corrosion products in the submerged region, the interaction between solid ferrous and ferric chloride (FeCl2 or FeCl3) with NH2Cl(aq) was measured using ultraviolet-visible spectroscopy, confirming the interaction of monochloramine with Fe2+ and Fe3+ species. This work is a crucial step towards fully understanding the reactions at complex interfaces between Fe and monochloramine and its degradations effect on pipelines and other Fe materials in the environment.

Investigation of monochloramine disinfectant effects on the surface reaction and corrosion of iron interfaces

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