PII S0016-7037(99)00010-1 Influence of sorbate-sorbent interactions on the crystallization kinetics of nickel- and lead-ferrihydrite coprecipitates ROBERT G. FORD, 1,2 * K. M. KEMNER, 3 and PAUL M. BERTSCH 1 1 Advanced Analytical Center for Environmental Sciences, Savannah River Ecology Laboratory, The University of Georgia, Aiken, South Carolina 29802 USA 2 Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware 19717 USA 3 Environmental Research Division, Argonne National Laboratory, Argonne, Illinois 60439 USA (Received April 21, 1998; accepted in revised form January 8, 1999) 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 logK 1,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. Copyright © 1999 Elsevier Science Ltd 1. INTRODUCTION Iron (hydr)oxides are often implicated in the control of trace metal concentrations in aqueous systems (Singh and Subrama- nian, 1984; McBride, 1989; Davis and Kent, 1990). Ferrihy- drite is commonly the iron (hydr)oxide initially formed in sediments or soils, or newly formed precipitates in treatment systems (Carlson and Schwertmann, 1987; Schwertmann and Taylor, 1989). This poorly ordered mineral is thermodynami- cally unstable and will ultimately transform to goethite or hematite, depending on the prevailing solution conditions (Cor- nell et al., 1989; Schwertmann and Taylor, 1989). Trace ions coprecipitated with ferrihydrite can delay these crystallization reactions by retarding the atomic rearrangements necessary to form the goethite or hematite structure. The degree to which these reactions are able to progress in the bulk solid phase (or sorbent) can significantly influence the reversibility of trace metal partitioning and mobility within environmental systems (Cornell, 1988; Ebinger and Schulze, 1990; Kumar et al., 1990; Cornell, 1991; Ford et al., 1997a). Retardation of ferrihydrite transformation has been corre- lated to the formation of a covalent bond between the copre- cipitate metal and hydroxy functional groups present in the iron (hydr)oxide structure (Giovanoli and Cornell, 1992). While the mechanism has not been precisely identified, the observed decrease in crystallization rate with increasing metal loading suggests that the sorbed metal inhibits the transformation path- way toward development of goethite or hematite. The pathway to goethite formation involves dissolution of ferrihydrite fol- lowed by nucleation and reprecipitation of the crystalline phase (Cornell and Schwertmann, 1996). Therefore, retardation of the structural transformation is linked to inhibition of ferrihydrite dissolution due to sorption of a foreign metal. This mechanism is supported by studies which have demonstrated the inhibitory effect of metal sorption on the proton-promoted dissolution of iron (hydr)oxides (Bondietti et al., 1993; Biber et al., 1994). The apparent pathway to hematite formation involves internal atomic rearrangement with less dependence on a dissolution step (Cornell and Schwertmann, 1996). In this case, retardation can be rationalized by proposing that the initial metal-Fe co- precipitate possesses a more energetically stable structure than pure ferrihydrite. For metals in the first transition series (Mn, Co, Ni, and Cu), crystallization rate reductions correlate with the (increasing) stability of the metal-ferrihydrite surface complex (Giovanoli and Cornell, 1992). This is consistent with the Irving-Williams order for the stabilities of metal-ligand complexes (Irving and *Author to whom correspondence should be addressed (rgford@ udel.edu). Pergamon Geochimica et Cosmochimica Acta, Vol. 63, No. 1, pp. 39 – 48, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/99 $20.00 + .00 39