Dechlorination of CDCl3 on Fe(111): Implications for disinfection by-product degradation

Abstract

Disinfection by-products are produced in the water supply by reactions of organic matter with halogenated disinfectants. Remediation of disinfection by-products can be accomplished by decomposition through reductive dehalogenation or corrosion reactions. Iron is ubiquitous in soils and pipeline systems, which comes into contact with municipal water sources. This study investigated the decomposition of chloroform, a model disinfection by-product, with an Fe(111) surface to measure the dechlorination mechanism using surface vibrational spectroscopy at the gas/solid interface. It was found that chloroform physisorbs on Fe(111) at -118°C and undergoes dissociative adsorption above -50°C. Vibrational modes are compared with density functional theory cluster models to identify the molecular orientation and indicate a difference in the C-Cl stretching modes that is coverage dependent. The desorption energy of chloroform from Fe(111) was measured as 67.5 (+/- 0.018) kJ/mol for the monolayer and 52.7 (+/- 2.2) kJ/mol for the multilayer. At 25 °C, d-chloroform is shown to dissociate as observed by Auger electron spectroscopy. The co-adsorption of water vapor, provide OH sites on the iron surface that appears to block a fraction of surface sites, allowing for less adsorption and chlorine deposition on Fe(111). The results show that Fe0 sites catalyze the dechlorination of chloroform, while OH sites oxidize the surface, suggesting that in aqueous solutions, metallic Fe is needed for dehalogenation.