Structural and magnetic studies on heavy-metal-adsorbing iron sulphide nanoparticles produced by sulphate-reducing bacteria

In previous and in work to be published, it has been shown that iron sulphi de material, produced by sulphate-reducing bacteria (SRB), is an excellent adsorbent for a wide range of heavy metals. The material adsorbs between 10 0 and 400 mg g(-1) and residual levels in solutions can be of the order of pg per litre. Further, strongly magnetic forms of this material can now be produced which can be effectively and cheaply removed from suspension toget her with the adsorbate by magnetic separation. This paper examines the stru cture of weakly magnetic and strongly magnetic iron sulphide material produ ced by SRB with a view to increasing the understanding of its adsorbent and the magnetic properties. The structural properties have been examined usin g high-resolution imaging and electron diffraction in a transmission electr on microscope (TEM), the measurements of magnetisation versus field and tem perature, extended X-ray absorption fine-structure (EXAFS) spectroscopy, X- ray absorption near-edge structure (XANES) spectroscopy and neutron diffrac tion. Before drying the surface area of both the weakly magnetic and the st rongly magnetic iron sulphide is of the order of 400-500 m(2) g(-1) as reve aled by the magnetic properties, neutron scattering and the adsorption of a number of heavy metals. After freeze-drying the surface area falls to betw een 18 and 19 m(2) g(-1). The initial inocula came from a semi-saline sourc e and when fed with nutrient containing Fe2+ and Fe3+ produced a weakly mag netic iron sulphide (Watson et al., Minerals Eng. 8 (1995) 1097) and a few % of a more strongly magnetic material. Further work using a novel method ( Keller-Besrest, Collin, J. Solid State Chem. 84 (1990) 211) produced a stro ngly magnetic iron sulphide material. EXAFS and XANES spectroscopy revealed (Keller-Besrest and Collin, 1990) that the weakly magnetic iron sulphide m aterial had the Ni-As structure in which the Fe is tetrahedrally coordinate d with the composition Fe1-xS. The strongly magnetic iron sulphide was comp osed of some greigite (Fe3S4) and mackinawite (Fe1+xS), however, the bulk o f the material at room temperature probably consists of disordered greigite and mackinawite. The weakly magnetic and strongly magnetic iron sulphide a re good adsorbents for heavy metals and halogenated hydrocarbons. (C) 2000 Elsevier Science B.V. All rights reserved.