Mineral surface reactivity and mass transfer in environmental mineralogy

Abstract

New concepts and analytical techniques pertinent to environmental mineralogy are reviewed. Environmental mineralogy is defined as: “the application of mineral sciences to understanding low temperature earth surface processes, especially processes which directly or indirectly involve the biosphere”. Because most low-temperature reactions involving minerals are surface-controlled, a key focus in Environmental mineralogy must be mineral surface chemistry and structure. Case studies are used in each to illustrate recent progress in three areas of interest: 1) mineral surface structure; 2) organic surface complexation reactions; and 3) microbiological biofilm growth at mineral surfaces. All three interfacial processes play a critical role in controlling mass transfer processes. In the first section glancing incidence X-ray methods and fractal models of mineral surface evolution are discussed. Dissolving silicate surfaces may best be categorized as falling into the directed percolation depinning (DPD) class based on the fact that in the equation which relates surface roughness (σ) to time ( t ): σ = t β , the exponent β has been constrained in several scattering experiments to be equal to approximately 0.2. Glancing incidence X-ray diffraction patterns for uranium precipitates on feldspar are also presented which show that the micro-precipitates are most likely becquerelite (Ca(UO 2 ) 6 O 4 (OH) 6 .8H 2 O) and thus have incorporated calcium from the mineral surface. Infrared and X-ray spectroscopy as applied to organic surface complexation are presented next. Multiple-internal reflection FTIR spectroscopy is used to show that an organic acid, phthalate, deprotonates and complexes with the fluorite surface. X-ray absorption spectroscopy (XAS) is then used to constrain how polyvinylsulfonate scale inhibitors function on the barite surface: direct binding between the sulfonate group and surface barium is demonstrated. The use of XAS to determine soil sulfur speciation is also presented. Finally, Environmental SEM (ESEM) and other techniques are applied to conditioning film and biofilm growth. Upcoming challenges to Environmental mineralogy and several other promising new techniques are also discussed.