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
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.