Structure and stability of the Fe(II)-Fe(III) green rust "fougerite" mineral and its potential for reducing pollutants in soil solutions

Fe(II)-Fe(III) layered double hydroxysalt green rusts, GRs, are very reactive compounds with the general formula, [Fe-(1-x)((II)) Fe-x(III) (OH)(2)](x+).[(x/n) A(n-). (m/n) H2O](x-), where x is the ratio Fe-III/Fe-tot, and reflects the structure in which brucite-like layers alternate with interlayers of anions A(n-) and water molecules. Two types of crystal structure for GRs, GR1 and GR2, represented by the hydroxychloride GR1(Cl-) and the hydroxysulphate GR2(SO42-) are distinguished by X-ray diffraction due to different stacking. By analogy with GR1(Cl-) the structure of the fougerite GR mineral, [Fe-(1-x)(II) Fe-x(III) (OH)(2)](x+). [x OH-. (1-x) H2O](x-) = Fe(OH)((2+x)). (1-x) H2O, is proposed displaying interlayers made of OH- ions and water molecules tin situ deprotonation of water molecules is necessary for explaining the flexibility of its composition). The space group of mineral GR1(OH-) would be Rim, with lattice parameters a congruent to 0.32 and c congruent to 2.25 nm. Stability conditions and the E-h-pH diagram of Fe(OH)((2+x)) (the water molecules are omitted) are determined from hydromorphic soil solution equilibria with GR mineral in Brittany (France). Computed Gibbs free energies of formation from soil solution/mineral equilibrium fit well with a regular solid solution model: mu degrees [Fe(OH)((2+x))]=(1-x) mu degrees [Fe(OH)(2)]+x mu degrees [Fe(OH)(3)] + RT [(l-x) In (1-x)+x In x] + A(0) x (I-x), where mu degrees [Fe(OH)(2)] = -492.5 kJ mol(-1), mu degrees [Fe(OH)(3)]=-641 kJ mol(-1) and A(0) = -243.9 kJ mol(-1) at the average temperature of 9+/-1 degreesC. The upper limit of occurrence of GR mineral at x = 2/3, i.e. Fe-3(OH)(8), is explained by its unstability vs. alpha -FeOOH and/or magnetite; Fe(OH)(3) is thus a hypothetical compound with a GR structure which cannot be observed. These thermodynamic data and E-h-pH diagrams of Fe(OH)((2+x)) can be used most importantly to predict the possibility that GR minerals reduce some anions in contaminated soils. The cases of NO3-, Se(VI) or Cr(VI) are fully illustrated. (C) 2001 Elsevier Science Ltd. All rights reserved.