Reaction Condition Effects on Nickel Sorption Mechanisms in Illite–Water Suspensions

to this solution, the edge-situated interlattice sites become occupied by ions other than K, presumably leading Nickel sorption in illite suspensions was studied as a function of to partial opening of the interlattice space (Bolt et al., pH (4.5–8.0), reaction time (3 h, 24 h, and 1 wk) and ionic strength (I 5 0.1 and 0.003 M ) using x-ray absorption spectroscopy (XAS) to 1979). characterize the Ni sorption complexes formed. The formation of The different sites available for metal uptake by illite Ni-Al layered double hydroxide (LDH) phases was observed at pH may lead to different Ni retention mechanisms occurring values .6.25, with increasing formation of these phases with time, in Ni–illite systems. The planar sites constitute permaand a faster formation rate with increasing pH. Comparison among nent negative charge. Metal interactions with these sites samples with the same Ni loading, but different reaction times and are electrostatic in nature and lead to the formation of pH values, showed larger second neighbor scattering for the samples outer-sphere metal complexes; that is, the metal ions reacted at higher pH, which had a faster Ni sorption rate. Most likely do not lose their primary hydration spheres upon interthis was due to a greater importance of Ni-Al LDH precipitation action with the clay mineral surface (Sposito, 1989; relative to other (mononuclear) Ni sorption mechanisms at higher Sparks, 1995). At the illite edge sites, both the formation pH, and/or a higher Al content in the Ni-Al LDH phase formed at lower pH (slower Ni sorption rate). Lowering the ionic strength reof outer-sphere complexes and chemisorption may ocsulted in increased Ni sorption for the entire pH range studied. Our cur. Chemisorption leads to the formation of innerextended x-ray absorption fine structure spectroscopy (EXAFS) data sphere metal complexes through a ligand exchange indicated that this was due to significant outer-sphere Ni sorption process, where the metal ions form chemical bonds with occurring at the planar sites at low I, leading to a reduced importance the clay mineral surface by coordination to surface hyof Ni-Al LDH formation in the overall Ni sorption process at droxy ligands (Sposito, 1989). Since illite is an Al-conpH . 6.50. taining clay mineral, the formation of Ni-Al LDH phases may also be expected. The importance of the formation of these precipitates has recently been demonstrated in R of heavy metal ions by soil minerals is spectroscopic studies of Ni sorption to Al-bearing clay a crucial process for maintaining environmental minerals and oxides (d’Espinose de la Caillerie et al., quality in contaminated areas. Sorption reactions at 1995; Scheidegger et al., 1998; Scheinost et al., 1999). solid–water interfaces decrease solute mobility and ofThese phases are typically observed in a pH range below ten control the fate, bioavailability, and transport of the pH where the solubility of b-Ni(OH)2 precipitates trace metal ions such as Ni in aquatic and soil environis exceeded. In the case of Ni sorption to pyrophyllite ments. Correctly determining the mechanisms of metal and other Al-bearing minerals described by Scheinost sorption to clay minerals such as illite is therefore of et al. (1999), the Ni phase that formed was unequivocally great importance for understanding the fate of metals identified as a mixed Ni-Al LDH sheet, where the Al in contaminated soils and sediments, and may help in originated from the sorbent structure. optimizing environmental remediation procedures by In the following the term sorption is used to describe accounting for the metal speciation in soils. the surface-mediated removal of Ni cations from soluIllite is a 2:1 phyllosilicate mineral with tightly held, tion. This includes adsorption processes at the illite clay nonhydrated, interlayer K cations balancing a high layer sites and planar sites (i.e., formation of mononuclear charge (Fanning et al., 1989). Illitic clays are often an innerand outer-sphere Ni sorption complexes), as well important constituent of the solid phase in alluvial soils as formation of Ni-Al LDH precipitates, which incorpoand in arid zone soils (Bolt et al., 1979; Fanning et al., rate Al dissolved from the illite structure. Studies em1989). As with many clay minerals, illite contains both ploying transmission electron microscopy showed that planar and edge sites available for metal uptake. Planar Co-Al LDH phases do not form coatings on Al2O3 and sites are due to a net negative structural charge resulting kaolinite surfaces but rather form three-dimensional from isomorphic substitution in the octahedral and tetprecipitate clusters that lack a preferred orientation with rahedral sheets, and edge sites are due to broken Al-OH respect to the surfaces of these minerals, and in some and Si-OH bonds at the edges of the clay crystallite cases are detached from the surface (Towle et al., 1997; (McBride, 1994). The presence of planar sites in illitic Thompson et al., 1999). These results indicate that forclays presumably results in part from interstratified vermation of Co-Al LDH is not a result of surface crowdmiculite and smectite layers (Srodon and Eberl, 1984), ing; that is, the precipitates do not consist of sorbed which form if the clay is brought in contact with a solumetal ions nucleated with metal ions on adjacent surface tion of low K concentration. Upon prolonged exposure sites, since this would result in precipitate coatings on the mineral surface. For this reason, Ni-Al LDH formaDep. of Plant and Soil Sciences, Univ. of Delaware, Newark, DE 19717-1303; E.J. Elzinga, Dep. of Geosciences, State Univ. of New Abbreviations: BET, Brunauer–Emmett–Teller; EGME, ethylene York, Stony Brook, NY 11794-2100. Received 24 Aug. 1999. *Correglycol mono-ethyl ether; EXAFS, extended x-ray absorption fine sponding author ([email protected]). structure spectroscopy; LDH, layered double hydroxide; RSF, radial structure functions. Published in Soil Sci. Soc. Am. J. 65:94–101 (2001).