The formation and transformation of schwertmannite: kinetics and the effects of toxic trace contaminants

Schwertmannite; a poorly-ordered ferric (oxy)hydroxy-sulphate nano-phase that is common to acid mine drainage (AMD) is known to effectively sequester toxic trace contaminants, such as - ' arsenate, upon formation. Schwertmannite is metastable and will transform to crystalline iron ' (hydr)oxides over time. This thesis presents results on the kinetics and mechanisms ofthe formation and transformation of schwertmannite and schwertmannite co-precipitated with arsenate (Asschwertmannite). The rates and mechanisms of schwertmannite nucleation and growth from solution were dete~ined using synchrotron-based small angle X~ray scattering (SAXS). Schwertmannite was precipitated at temperatures ranging from 4 to 50 °C and at pH 3 and 4. As-schwehmannite was precipitated in solutions containing 0.25,0.5, and 1.25 mM arsenate at 21 °C and pR3. The timeresolved and normalised radius ofgyration (Rg) data were fitted to the JMAK and Thetadash kinetic models. The kinetic and mechanistic information obtained indicate that pure- and Asschwertmannite nanoparticles form via classical nucleation and growth. Critical nuclei radii ranged from 4.8 Aat 4 °C and 9.1 Aat 50 °C. Instantaneous homogeneous nucleation was followed by surface-controlled growth (activation energy = 47 kJ morl ) with a first order ~ependence on reactant supersatura~ion.The primary particles were approximately equant but became elongated over time, with growth predominantly occurring in the c-axis direction due to adsorption of sulphate to the particle surface. Schwertmannite precipitated from solutions containing Asaq ~ 1.25 mM resulted in a) reduced particle groWth rates, and b) a reduced dimensionality ofgrowth, relative to pure and low- As schwerlmannite samples. The changes resulted from arsenate adsorpti,!n whichinhibited the attachment of ferric (oxy)hydroxide octahedra to the surface of the growing particles. Schwertmannite nanoparticles grew to maximum diameters of 4.4 nm (by 1020 s) at 4 °C and 7.6 nm (by 110 s) at 50 °C and remained in their nanoparticulate state for at least 60 min. However, aggregation was observed in the dynamic light scattering (DLS) data much earlier (e.g., 340 s). The high-As-schwertinannite aggregates were significantly smaHer than the pureschwertmannite aggregates (e.g., 100 to 200 nm and 500 nm, respectively) due to inhibition of orientated aggregation by a) reduced schwertmannite crystallinity, and b) blocking of the Fe-9-Fe linkages due to surface adsorbed arsenate. The. transformation of schwertmannite to goethite and/or hematite and. of high-Asschwertmannite to hematite only in high pH solutions (1 MNaOH) was studied between 60 and 240 °C using synchrotron-based energy-dispersive X-ray diffraction (EDXRD). The reactions proceeded via a ferrihydrite intermediate. Crystallisation ofpure-schwertmannite proceeded via a primary .stage of goethite formation (T ~ 80'C) and goethite and hematite formation (T::: 90 'C), followed by a secondary crystallisation stage where hematite formed at the expense of goet'hite (T::: 150 'c). The time-resolved and normalised data representing the growth in the area under specific goethite and hematite diffraction peaks were fitted to the JMAK. kinetic model. The kinetic and energetic information obtained suggest that goethite formed via a dissolution / re-precipitation mechanism. Sulphate from the initial schwertmannite inhibited the dissolution of ferrihydrite, and increased the . induction periods for goethite crystallisation, relative to the pure ferrihydrite system. Goethite crystallised via a first order reaction, and was limited by the dissolution of the ferrihydrite intermediate, as evidenced by a sigmoidal peak growth curve and an increased activation energy of nucleation (26.9 ± 1 kJ morl ) relative to pure ferrihydrite (7 ± 1 kJ morl ). However, the presence of sulphate did not significantly affect the goethite growth rates. The activation energy of growth (33.0 ± 1 kJ morl ) supported a surface-controHed growth mechanism. . At high te~peratures (::: 90°C) the presence of sulphate favoured the crystallisation of hematite rela!ive to the'pure ferrlhydrite system where goethite was the sole reaction product. This was a result of the stabilisation of the ferrihydrite intermediate against dissolution. Hematite crystallisation foHowed an aqueous-aided diffusion~ontrolled first-order mechanism with activation energies ofnucleation andgrowth (25.2 ± 1 and 28.4 ± 1 kJ morI, respectively). The activation energy of crystallisation for the slower secondary stage of crystallisation of . . goethite to hematite was 100.3 ± 3 1<1 morland this indicated a first-order diffusion-controlled mechanism, which describes the migration of hydroxyIs and protons through the structure du~ng dehydration of goethite. The presence of arsenate during the transformation of As-schwertmannite blocked goethite formation due to the stronger affinity arsenate has for the iron (oxy)hydroxide octahedra than sulphate. This caused an increased stability of the ferrihydrite intermediate, relative to pureschwertmannite, and favoured the solid-state transformation to hematite. The presence of arsenate significantly affected hematite growth, which was apparent from the deviation from simple first . . order reaction kinetics and slower crystallisation rates. This resulted from the strong arsenate-iron .'\. interaction, which hindered the Fe-O-Fe linkages required for aggregation offerrihydrite in the nucleation step and inhibited ion diffusion within the lattice in the crystallisation step. This was further evidenced by elevated nucleation (40 1<1 morl ) and crystallisation (51 1