Reduction of Polyhalogenated Methanes by Surface-Bound Fe(II) in Aqueous Suspensions of Iron Oxides

• Published in: Environmental science & technology Vol. 36; no. 8; pp. 1734 - 1741
• Main Authors: Pecher, Klaus, Haderlein, Stefan B, Schwarzenbach, René P
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 15.04.2002
• Subjects: Applied sciences; Biological and physicochemical properties of pollutants. Interaction in the soil; Chemistry; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Ferric Compounds - chemistry; Ferrous Compounds - analysis; Groundwaters; Halogens - analysis; Halogens - chemistry; Iron; Kinetics; Methane - analogs & derivatives; Methane - analysis; Methane - chemistry; Natural water pollution; Oxidation; Pollution; Pollution, environment geology; Soil and sediments pollution; Water Pollutants - analysis; Water treatment and pollution; Laboratory study; Iron oxide; Dehalogenation; Pollutant behavior; Iron II; Geochemistry; Particle suspension; Soil pollution; Iron Hydroxides oxides; Chemical reduction; Sorption; Organobrominated compounds; Selfpurification; Medium effect; Organochlorine compounds; Chlorine Organic compounds; Bromine Organic compounds; Water pollution; Aqueous solution; Chemical reactivity; Ground water
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es011191o

Uptake of ferrous iron from aqueous solution by iron oxides results in the formation of a variety of reactive surface species capable of reducing polyhalogenated methanes (PHMs). Pseudo-first-order reaction rate constants, k obs, of PHMs increased in the order CHBrCl2 < CHBr2Cl < CHBr3 < CCl4 < CFBr3 < CBrCl3 < CBr2Cl2. The k obs values increased with the exposure time, t eq, of Fe(II) to suspended iron oxides which was attributed to the rearrangement of initially sorbed Fe(II) species to more reactive surface species with time. At pH 7.2, the k obs values of PHMs also increased with the concentration of surface-bound ferrous iron, Fe(II)sorb, particularly when Fe(II)tot was increased to concentrations where surface precipitation becomes likely. At fixed total Fe(II) concentrations, k obs values increased exponentially with pH. The highest reactivities were associated with pH conditions where surface precipitation of Fe(II) is expected. Fe(II)sorb and pH, however, had opposite effects on the product formation of PHMs. At pH 7.2, the formation of formate from CX4 (X = Cl, Br) increased with Fe(II)sorb, whereas increasing pH favored the formation of CHX3. The ratio of halogenated products and formate formed is indicative of the relative importance of initial one- or two-electron-transfer processes, respectively, and was found to depend on the type of iron oxide mineral also. Our data form a basis to assess the importance of chemical reactions in natural attenuation processes of PHMs in environmental systems under iron-reducing conditions.