INFLUENCE OF SORBATE-SORBENT INTERACTIONS ON THE CRYSTALLIZATION KINETICS OF NICKEL- AND LEAD-FERRIHYDRITE COPRECIPITATES - EVIDENCE FOR A TWO-STEP MECHANISM

Abstract

Metals sorbed to or coprecipitated with ferrihydrite can significantly inhibit transformation to more crystalline endproducts. We hypothesized that metals with a higher stability constant for a metal-ferrihydrite surface complex would retard the transformation process to a greater extent. To test this hypothesis, we examined the influence of Ni or Pb sorption on the kinetics of ferrihydrite crystallization to goethite/hematite. Reported surface stability constants for Ni and Pb sorbed to ferrihydrite are logK1,int = 0.37 and 4.0, respectively (Dzombak and Morel, 1990). The structural evolution of nickel- and lead-ferrihydrite coprecipitates was studied for various metal loadings during aging at pH 6 or 11 and 70°C. Results of aging studies demonstrated that the influence on transformation kinetics was not related to the magnitude of the stability constant of the Ni- or Pb-ferrihydrite surface complex. At pH 11, crystallization was retarded more significantly in the presence of Ni and rates decreased with increasing Ni/Pb surface loading. At pH 6, crystallization rates were accelerated in the presence of Pb, and this was also true for systems at the lowest Ni loading. However, crystallization rates in the presence of Ni were always slower relative to systems containing Pb. Characterization of crystalline iron (hydr)oxide endproducts by x-ray diffraction and high-resolution thermogravimetric analysis showed that hematite was formed to a greater extent than goethite in the presence of Ni. X-ray absorption fine structure spectroscopy suggested that the majority of sorbed Pb was present as an inner-sphere surface complex. The distribution of coprecipitated Ni or Pb on aged solids, as assessed via continuous dissolution with oxalic acid, suggested that a significant fraction of Ni was partitioned into the structure of a crystalline iron (hydr)oxide. In contrast, Pb desorption/dissolution behavior confirmed that this metal was primarily associated with surface sites or poorly ordered iron (hydr)oxide phases. The relative metal-specific influence on crystallization rate and endproduct, and the apparent Ni and Pb distribution in aged solids suggest that Pb forms a more kinetically labile sorption complex than Ni with iron (hydr)oxides.