A HIGH RESOLUTION TEM-AEM, EXAFS, SORPTION EDGE MEASUREMENT, AND MODELING STUDY OF PB2+ COPRECIPITATION WITH FERRIHYDRITE

Many environmental, geological, and analytical chemical processes involve the simultaneous removal of a trace constituent, together with a carrier constituent from a homogeneous aqueous solution. However, the term coprecipitation is loosely used in a phenomenological sense, and the mechanism of coprecipitation is poorly understood. To understand coprecipitation mechanism and develop quantitative models, we studied Pb²⁺ and Fe³⁺ coprecipitation by measuring the sorption edges at ambient temperature, pressure, and various Fe:Pb ratios, with High-resolution Transmission and Analytical Electron Microscopy (HR TEM-AEM), Extended X-ray Absorption Fine Structure (EXAFS) analysis, and geochemical modeling of solid solution and surface complex formation. The coprecipitates have an X-ray diffraction pattern typical of 2-line ferrihydrite. HRTEM shows that the freshly formed coprecipitates are single crystalline spheres with ~5 nanometer diameter and with clear lattice images showing no massive defects. Coprecipitation of Pb²⁺ with ferrihydrite begins at about pH 4, and the sorption edges are located at a lower pH than published sorption edges from adsorption experiments under otherwise similar experimental conditions. In other words, coprecipitation removes the same amount of Pb²⁺ with about half the amount of the carrier constituent Fe³⁺, clearly indicating that coprecipitation is more efficient than adsorption in removing Pb²⁺ from aqueous solutions. HRTEM-AEM analysis confirms that Pb²⁺ is associated with the ferrihydrite. Both HRTEM-AEM and EXAFS did not detect any Pb-concentrated phases in the coprecipitates cluster or at single crystal edges. Hence, occlusion and surface precipitation are excluded as major coprecipitation mechanisms. EXAFS shows distinct spectrum patterns different from those of adsorbed Pb²⁺ under otherwise similar conditions, indicating different Pb²⁺ incorporation patterns. After ageing for one year, Pb²⁺-ferrihydrite with a Pb/Pb+Fe ratio of 10% transformed into a mixture of lepidocrocite, goethite, and 2-line ferrihydrite. In contrast, ferrihydrite with no co-precipitated Pb²⁺ transformed to goethite needles, and ferrihydrite with 7% Zn/Fe+Zn remained as spherical ferrihydrite. Thus, co-precipitated divalent metals altered the rate of ferrihydrite transformation to more ordered iron oxyhydroxides.