Effect of the structural change of an iron–iron oxide mixture on the decomposition of trichloroethylene

Abstract

The effect of the structure of a mixture of industrially produced iron and iron oxide on the decomposition of trichloroethylene (TCE) was investigated by gas chromatography, scanning electron microscopy, Fourier transform infrared spectroscopy, energy dispersive X-ray analysis, X-ray diffractometry, and ⁵⁷Fe-Mössbauer spectroscopy. The concentration of 10 mg L⁻¹ TCE aqueous solution decreased to 0.41, 0.52, 0.26, and 0.09 mg L⁻¹ when stirred for 7 days with iron–iron oxide mixtures having mass ratios of 2:8, 3:7, 4:6, and 5:5, respectively. The Mössbauer spectra of the mixtures after leaching were composed of two sextets with respective isomer shifts (δ) and internal magnetic fields (H) of 0.29_±0.01 mm s⁻¹ and 48.8_±0.1 T, and 0.64_±0.01 mm s⁻¹ and 45.5_±0.1 T, attributed to the Fe³⁺ species in tetrahedral (T _d) and the Fe²⁺ and Fe³⁺ mixed species (Fe^2.5+) in octahedral (O _h) sites, respectively. Mössbauer spectra of a 3:7 mass ratio iron–iron oxide mixture showed a gradual decrease in the absorption area (A) of zero valent iron (Fe⁰) from 40.6. to 12.6, 13.2, 3.8 2.8, and 1.0_±0.5 % and an increase in A of Fe₃O₄ from 31.8 to 59.4, 71.4, 93.2, 95.6, and 98.0_±0.5 % after leaching with 10 mg L⁻¹ TCE aqueous solution for 1, 2, 3, 7, and 10 days, respectively. Consistent values of the first-order rate constant were calculated as 0.32 day⁻¹ for Fe⁰ oxidation, 0.34 day⁻¹ for Fe₃O₄ production, and 0.30 day⁻¹ for TCE decomposition, which indicates that the oxidation of Fe⁰ was the rate-controlling factor for Fe₃O₄ production and TCE decomposition. It is concluded from the experimental results that an iron–iron oxide mixture is very effective for the decomposition of TCE.