DIVISION S-2—SOIL CHEMISTRY Dissolution Kinetics of Nickel Surface Precipitates on Clay Mineral and Oxide Surfaces Kirk G. Scheckel* and Donald L. Sparks ABSTRACT M 3+ are divalent and trivalent metal cations, respec- tively, and A - is an interlayer anion that may include The formation of Ni surface precipitates on natural soil materials NO 3 , silicate, or water] in which the metals are aligned may occur during sorption under ambient environmental conditions. in brucite-like octahedral sheets with anions in the inter- In this study, we examined proton- and ligand-promoted dissolution of Ni surface precipitates on pyrophyllite, talc, gibbsite, amorphous layer for charge balance (O’Day et al., 1994; Scheinost silica, and a mixture of gibbsite and amorphous silica aged from 1 h et al., 1999; Scheinost and Sparks, 2000). If the sorbent to 2 yr, by employing an array of dissolution agents (ethylenediamine- possesses Al within its structure, the resulting precipi- tetraacetic acid [EDTA], oxalate, acetylacetone, and HNO 3 ). Ligand- tate phase is a mixed metal–Al LDH in which the Al promoted dissolution was more effective in removing Ni than the has substituted into the octahedral sheet for the metal protolysis by HNO 3 . In all cases, as residence time increased from (d’Espinose de la Caillerie et al., 1995; Scheidegger et 1 h to 2 yr, the amount of Ni released from the precipitates decreased al., 1996, 1997, 1998; Scheidegger and Sparks, 1996; from 98 to 0%, indicating an increase in stability with aging time Towle et al., 1997; Scheinost et al., 1999; Thompson et regardless of sorbent and dissolution agent. For example, as residence al., 1999). Likewise, if Al is not available in solution time increased from 1 h to 2 yr, Ni release from pyrophyllite, as a during sorption, brucite-like metal hydroxide precipi- percentage of total Ni sorption, decreased from 96 to 30% and 23 to tates [M 2+ A - (OH) 2 ] are formed (Scheinost et al., 1999). 0%, respectively, when EDTA (pH 4.0) and HNO 3 (pH 6.0) were employed as dissolution agents for 14 d. Dissolution via oxalate of Formation of surface precipitate phases drastically 1-yr-aged Ni–Al layered double hydroxide (LDH) on pyrophyllite reduces metal concentration in soil and sediment solu- saw 19% Ni removal, in comparison with 52% Ni release from tions (Elzinga and Sparks, 1999). However, only a few -Ni(OH) 2 precipitates on talc, suggesting that -Ni(OH) 2 is less stable investigations have assessed the stability of the surface than Ni–Al LDH. The increase in stability of the Ni surface precipi- precipitates. Scheidegger and Sparks examined the dis- tates in this study with residence time was attributed to three aging solution of short-aged Ni–Al LDH precipitates formed mechanisms: (i) Al-for-Ni substitution in the octahedral sheets of the on pyrophyllite using HNO 3 at pH 4 and 6. Nickel de- brucite-like hydroxide layers, (ii) Si-for-NO 3 exchange in the interlay- tachment was initially rapid at both pH values (with ers of the precipitates, and (iii) Ostwald ripening of the precipitate 10% of total Ni released) and was attributable to phases. It appeared that the second factor, Si-for-NO 3 exchange in desorption of specifically adsorbed, mononuclear Ni. the interlayers, was a major mechanism for the increase in stability Dissolution then slowed due to the gradual dissolution of the precipitates. of the precipitates. In comparison with a -Ni(OH) 2 reference compound, the Ni–Al LDH surface precipi- N ickel is a heavy metal of concern in many parts of tates were much more stable. the world. The concentration of Ni in soil averages Ford et al. investigated the dissolution of Ni–Al LDH 5 to 500 mg Ni kg -1 soil, with a range up to 53 000 mg surface precipitates on pyrophyllite using an EDTA so- kg -1 Ni in contaminated soil near metal refineries and in lution at pH 7.5. Detachable Ni drastically decreased dried sludges (USEPA, 1990). Agricultural soils contain when the age of the precipitate increased from 1 h to approximately 3 to 1000 mg kg -1 Ni (WHO, 1991). 1 yr. By employing high-resolution thermogravimetric Nickel sorption on soil minerals can result in both ad- analysis (HRTGA), which is sensitive to changes in the sorbed (outer- and inner-sphere complexes) and precip- interlayer composition of LDH, and by comparing the itated phases (Scheidegger et al., 1997). With increasing results of the surface precipitates with those of synthe- awareness of the formation of metal surface precipitates sized reference compounds, Ford et al. (1999) showed on clay mineral and oxide surfaces (Chisholm-Brause that a substantial part of the aging effect was due to et al., 1990; O’Day et al., 1994; Sheidegger et al., 1996; replacement of interlayer NO 3 by silicate, which trans- Towle et al., 1997), as well as soils and clay fractions formed the initial Ni–Al LDH into a Ni–Al phyllosili- (Roberts et al., 1999), understanding the potential long- cate precursor. The source of the silicate was the dissolu- term fate of the solid-state metal is necessary. The basic tion of the pyrophyllite surface during Ni sorption. structure of these surface precipitates is a hydrotalcite- Scheckel et al. (2000) and Scheckel and Sparks (2000), like structure [(M 2+ ) 6 (M 3+ ) 2 A - (OH) 16 , where M 2+ and employing EDTA (pH 7.5) and HNO 3 (pH 4.0) for sorption aging times ranging from 1 h to 1 yr, investi- Kirk G. Scheckel, National Risk Management Research Lab., US EPA, 5995 Center Hill Ave., Cincinnati, OH 45268; D.L. Sparks, Dep. Abbreviations: A, interlayer anion; DRS, diffuse reflectance spectro- of Plant and Soil Sciences, Univ. of Delaware, Newark, DE 19717- scopies; EDTA, ethylenediaminetetraacetic acid; HRTGA, high-reso- 1303. Received 16 June 2000. *Corresponding author (Scheckel.Kir- lution thermogravimetric analysis; LDH, layered double hydroxide; k@epamail.epa.gov). M, metal cation; XAFS, x-ray absorption fine structure; XRD, x-ray dif- fraction. Published in Soil Sci. Soc. Am. J. 65:685–694 (2001). 685