Reaction dynamics and chemical speciation of phosphorus and arsenic (III and V) at the metal oxide -water interface and in soils
Understanding the molecular scale reaction mechanisms and speciation of oxyanions at the soil mineral-water interface provides significant insights and predictions on their fate and transport and bioavailability in soil-water environments. In this research, phosphate (P), arsenite (As(III)), and arsenate (As(V)) adsorption and desorption mechanisms on ferrihydrite and bayerite surfaces were investigated as a function of reaction time, pH, and ionic strength. An array of techniques were employed including equilibrium and kinetic studies, electrophoretic mobility (EM) measurements, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray absorption near edge structure spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS). To expand on the knowledge gained from the reaction dynamics and the surface speciation in the model component systems, As solid-state speciation and reactivity were further investigated in natural materials. Traditional chemical extraction/digestion, novel microfocused in situ X-ray fluorescence (μ-SXRF) and X-ray absorption spectroscopies (μ-XAS), and electron microprobe spectroscopy were employed to understand the complex chemical speciation in the natural materials.
In the model component systems, reaction conditions (i.e., pH and ionic strength) highly influenced the As(III and V) and P adsorption behavior on metal oxides (bayerite and ferrihydrite, respectively). The ATR-FTIR, EXAFS and EM analyses showed that the formation of inner-sphere As(III and V) and P adsorption complexes. Reversibility of adsorbed As(V) and P decreased with increasing aging times, and an increase in higher energy bindings such bidentate binuclear adsorption complexes and/or Al-As(V) like surface precipitates might be responsible for aging effects as evidenced in XAS analyses. Taking further steps to expand our knowledge of more complex systems, arsenic speciation of subsurface landfill materials was investigated using novel in situ μ-SXRF, μ-XANENS and bulk EXAFS spectroscopies. Mixed As solid-state speciation was indeed found in the landfill materials suggesting the presence of complex speciation and reaction dynamics in this real world system.
Research findings in this dissertation suggest that reaction conditions relevant to natural systems as well as an inherent heterogeneity of natural materials must be carefully considered in environmental modeling programs for predicting the long term fate and transport of these oxyanions and designing the best remediation strategies.