MöSSBAUER STUDY OF MAGNETITE AS PART OF A CORROSION SCALE FOUND WITHIN A DRINKING WATER DISTRIBUTION SYSTEM

Corrosion scales containing the iron minerals; lepidocrocite, magnetite and goethite are commonly found lining the inner surface of cast iron pipes of drinking water distribution systems. Texturally there are three morphologic regions: the surface layer (SL), hard shell layer (HSL) and the core (C). The mineralogy of the SL is lepidocrocite, the HSL is composed of magnetite and the core is primarily goethite. In some scales, there are vein like structures (V) in the core made of a texturally different type of magnetite which is non-metallic in appearance. The purpose of the study was to investigate the origin and formation of low temperature magnetite (Fe₃O₄) found in the corrosion scales using Mössbauer spectroscopy. Mössbauer was chosen because the method is used with success to identify various iron oxides and the coordination number of Fe atoms.

The Mössbauer spectra of low temperature magnetite at room temperature indicate that the magnetite found in the corrosion scales has different atomic nuclei than high temperature magnetite. A doublet peak at 0 velocity is absent in high temperature magnetite, but found in magnetite from corrosion scales and in magnetite which was formed by iron and sulfate reducing bacteria in an amorphous hydroxide medium. The similarities in spectra may indicate the magnetite from the corrosion scales is bacterial in origin. The concentration ratio of octahedral to tetrahedral (O_h/T_d) sites is 1.24 which is lower than the high temperature reference at 2.02 and this value compares with a value of 1.9 from another sample of high temperature magnetite. Similarities between low and high temperature magnetite are that all samples exhibit hyperfine splitting indicating the presence of a magnetic field and the ratio of magnetic fields at both octahedral and tetrahedral sites of the low temperature magnetite was 1.04 and 1.06 for high temperature magnetite. An isomer or chemical shift is not seen between high and low temperature magnetite.