Influence of sorbate-sorbent interactions on the crystallization kinetics of nickel- and lead-ferrihydrite coprecipitates

Citation
Rg. Ford et al., Influence of sorbate-sorbent interactions on the crystallization kinetics of nickel- and lead-ferrihydrite coprecipitates, GEOCH COS A, 63(1), 1999, pp. 39-48
Citations number
58
Categorie Soggetti
Earth Sciences
Journal title
GEOCHIMICA ET COSMOCHIMICA ACTA
ISSN journal
00167037 → ACNP
Volume
63
Issue
1
Year of publication
1999
Pages
39 - 48
Database
ISI
SICI code
0016-7037(199901)63:1<39:IOSIOT>2.0.ZU;2-P
Abstract
Metals sorbed to or coprecipitated with ferrihydrite can significantly inhi bit transformation to more crystalline endproducts. We hypothesized that me tals with a higher stability constant for a metal-ferrihydrite surface comp lex would retard the transformation process to a greater extent. To test th is hypothesis, we examined the influence of Ni or Pb sorption on the kineti cs of ferrihydrite crystallization to goethite/hematite. Reported surface s tability constants for Ni and Pb sorbed to ferrihydrite are logK(1,int) = 0 .37 and 4.0, respectively (Dzombak and Morel, 1990). The structural evoluti on of nickel- and lead-ferrihydrite coprecipitates was studied for various metal loadings during aging at pH 6 or 11 and 70 degrees C. Results of agin g studies demonstrated that the influence on transformation kinetics was no t related to the magnitude of the stability constant of the Ni- or Pb-ferri hydrite surface complex. At pH 11, crystallization was retarded more signif icantly in the presence of Ni and rates decreased with increasing Ni/Pb sur face loading. At pH 6, crystallization rates were accelerated in the presen ce of Pb, and this was also true for systems at the lowest Ni loading. Howe ver, crystallization rates in the presence of Ni were always slower relativ e to systems containing Pb. Characterization of crystalline iron (hydr)oxid e endproducts by x-ray diffraction and high-resolution thermogravimetric an alysis 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 comp lex. The distribution of coprecipitated Ni or Pb on aged solids, as assesse d via continuous dissolution with oxalic acid, suggested that a significant fraction of Ni was partitioned into the structure of a crystalline iron (h ydr)oxide. In contrast, Ph desorption/dissolution behavior confirmed that t his metal was primarily associated with surface sites or poorly ordered iro n (hydr)oxide phases. The relative metal-specific influence on crystallizat ion rate and endproduct, and the apparent Ni and Pb distribution in aged so lids suggest that Pb forms a more kinetically labile sorption complex than Ni with iron (hydr)oxides. Copyright (C) 1999 Elsevier Science Ltd.