(264b) Laser Diffraction Particle Size Measurement and the Influence of Structural Iron Reduction in a Ferruginous Smectite (Swa-1)

The modification of the physical and chemical properties of soils through redox processes in water-logged zones, such as flooded rice fields, has a great influence on the sustainability of such areas for food production [1]. The consumption of O2 during aerobic microbial respiration can lead to the reduction of Fe3+ species resulting in reductive dissolution of iron oxides and reduction of structural Fe in clay minerals. The modifications of the Fe structural redox status effects their swelling, hydraulic conductivity, surface area, cation exchange capacity and surface chemistry. The fate of pesticides and their toxicity on mammalian cells can be modified by the redox status of structural Fe. The modification of the clay properties may influence their dispersion behaviour and thus rheology and transport. Characterisation of the changes in clay or soil properties induced by redox processes are of great importance for the sustainability of these food producing areas. The reversibility of structural Fe reduction-reoxidation processes and its concomitant effects is a question that needs further attention. One very important aspect is to monitor the modification of the particle size distribution (PSD) which is not always easy to perform on materials that are both anisotropic in shape and very broad in size range (tens of nanometres to hundreds of microns). This paper looks at the application of laser diffraction to the characterization of PSDs of ferruginous smectite samples submitted to oxidation-reduction cycles in sodic or potassic media. PSD measurement using laser diffraction is a rapid and reliable method for particles in the size range of 1 to 1000 microns [2]. For finer particles, the lower scattering intensities associated with particles smaller than 0.5 microns leads to some uncertainties in the overall accuracy and detection limits. With respect to the clay particles investigated in the present study a major limitation arises from the fact the particles are plate-like and not the spheres assumed in all commercial algorithms that transform the diffraction pattern collected into a PSD. Previous work on platelets and clays has however shown that the particles align in the laser diffraction cell and a particle size distribution of the projected area of the platelets can be determined [3]. The PSDs for the soils were measured and normalized with the assumption that the particles were well aligned in the measuring cell and results were coherent with optical and electron microscopy observations. The modification of the particle size distributions from bi-modal to monomodal with respect to redox reactions and modification of the clays surface and physical properties will be discussed.[1] F. Favre, A. M. Jaunet, M. Pernes, M. Badraoui, P. Boivin and D. Tessier, Clay Minerals, (2004) 39, 123-134. [2] P. Bowen, J. Dispersion Science and Technology, 23(5) 631-662 (2002). [3] P. Bowen, J. Sheng, and N. Jongen, Powder Technology,128, 256-261 (2002).