Fate of silicon in tropical agricultural soil clays using XANES spectroscopy

Abstract

Silicon (Si) is an abundant element in terrestrial ecosystems and makes essential contributions to agricultural crops, environments, and industries; however, current knowledge of Si speciation in soil systems is based primarily on crystalline mineral types with limited information about the effect of Si-adsorbed species on the most chemically reactive clay fraction. Herein, we investigated Si speciation in the clay-sized fraction of tropical soils. Silicon K-edge X-ray absorption near edge structure (XANES) spectra of clay samples, together with a wide range of reference compounds varying in clay and Fe/Al oxide mineralogy, showed the predominance of Si in a beidellite structure and of Si adsorbed to montmorillonite and to kaolinite. Unique features, including a pre-white-line feature, white-line position and intensity, and post-edge numbers and positions of Si reference spectra, could differentiate most Si reference standard spectra. Silicon adsorbed to goethite, Si-ferrihydrite-natural organic matter complexes, and Si in the structure of biotite, illite-smectite, chlorite, quartz, and Si nanoparticle, variously occurred in different samples. The CaCl2 extraction data revealed that extracted Si was below the limits of Si deficiency in one-half of the total soil samples and accounted for only 0.02% of total soil Si. Soil extractable Si had a positive correlation with pH, organic matter, and clay. Our results present a novel application of Si K-edge XANES as a tool for identifying Si speciation in natural clays that are the most chemically reactive phase of Si retention and release in terrestrial and aquatic systems.