Formation of layered single- and double-metal hydroxide precipitates at the mineral/water interface: A multiple-scattering XAFS analysis

Spectroscopic and microscopic studies have shown that Ni and Co sorption by clay minerals may proceed via formation of surface precipitates. Several s tudies employing X-ray absorption fine structure (XAFS) spectroscopy sugges ted the formation of turbostratic, alpha-type metal hydroxides, of layered double hydroxides (LDH) with Al-for-metal substitution, and of 1:1 or 2:1 p hyllosilicates. Distinction of these phases is difficult because they have low crystallinity and/or a small mass compared to the sorbents, and because they have similar metal-metal distances in their hydroxide layers/ sheets. Distinction of these phases is crucial, however, because they have substan tially differing solubilities. In this paper we show that an XAFS beat patt ern at about 8 Angstrom(-1) can be used as a fingerprint to unequivocally d istinguish LDH from the alpha-type hydroxides and phyllosilicates. Full mul tiple-scattering simulations and experimental spectra of model compounds in dicate that the beat pattern is due to focused multiple scattering at Me/Al ratios between 1 and 4 (Me = Ni, Co). By applying the fingerprint method t o new and to already published XAFS data on Ni and Co surface precipitates, we found that LDH preferentially forms in the presence of the Al-containin g sorbents pyrophyllite, illite, kaolinite, gibbsite, and alumina above pH 7.0. However, alpha-type metal hydroxides form in the presence of the Al-fr ee sorbents talc, silica, and rutile, and in the presence of the Al-contain ing clay minerals montmorillonite and vermiculite. We believe that the high permanent charge of these latter minerals prevents or retards the release of Al. When Al is available, the formation of LDH seems to be thermodynamic ally and/or kinetically favored over the formation of alpha-type hydroxides . (C) 2000 Academic Press.