Surface-Induced Nickel Hydroxide Precipitation in the Presence of Citrate and Salicylate

X-ray absorption spectroscopy (XAS) studies revealed that Ni(II), Co(II), and Zn(II) formed LDH preFormation of surface-induced precipitates may play an important cipitates on Al-bearing minerals and in soil at pH $ 7 role in the immobilization of Ni and other metals in nonacidic soils. (d’Espinose de la Caillerie et al., 1995; Towle et al., To investigate the influence of commonly present organic ligands on 1997; Scheidegger et al., 1997; Scheidegger et al., 1998; precipitate formation, we monitored the uptake of Ni by gibbsite and pyrophyllite in the presence of citrate and salicylate for 4 wk and Roberts et al., 1999; Ford and Sparks, 2000). These identified the Ni hydroxide precipitates with diffuse reflectance specLDH phases consist of brucite-type mixed-metal hytroscopy (DRS). In the absence of organic ligands, Ni uptake prodroxide sheets, which are separated from each other ceeded by formation of Ni–Al layered double hydroxide (LDH) preby water and charge-balancing anions. Their formula is cipitates. Citrate and salicylate generally decreased both the Ni [M21 12xAl x (OH)2] (x/n)A2 mH2O, where M21 repreremoval from solution and the precipitate formation. The suppression sents a range of transition metals. The net positive layer by citrate was more pronounced than that by salicylate due to the charge is balanced by anions such as NO 3 , Cl, CO22 3 , stronger complexation of Ni by citrate. In the presence of citrate and and ClO 4 (A2 ) (Hashi et al., 1983; Génin et al., 1991). salicylate, the precipitate phase was Ni–Al LDH on pyrophyllite, but In the presence of Al-free minerals, structurally very predominately a-Ni hydroxide on gibbsite. This difference can be similar but thermodynamically less stable a-type metal explained by the differing Al solubilities of the two minerals. Pyrophylhydroxides with the formula M(OH)22x (x/n)AmH2O lite is relatively soluble, causing the rapid formation of amorphous Al hydroxide, which, in turn, is a necessary precursor for the formation have been identified by DRS and XAS (Scheinost et of Ni–Al LDH. In spite of the complexation of Al by organic ligands, al., 1999; Scheinost and Sparks, 2000). Both types of sufficient amorphous Al hydroxide was available to promote the forprecipitates create a sink for Ni and are more stable mation of Ni–Al LDH. Gibbsite, on the other hand, is much less than Ni bound as outer-sphere or inner-sphere sorption soluble, and the smaller amount of initially released Al may be fully complexes (Bradbury and Baeyens, 1997). However, complexed by citrate and salicylate. The subsequent lack of amorNi–Al LDH is more resistant to dissolution than a-Ni phous Al hydroxide prevented the formation of Ni–Al LDH, and, hydroxide. (Scheckel et al., 2000). Therefore, to accuinstead, a-Ni hydroxide formed. Only after a longer period of 30 d rately predict the fate of Ni in soils and sediments, it is and at a low citrate concentration did enough Al become available important to understand the controls for the formation to transform a-Ni hydroxide into the thermodynamically more stable of specific precipitates. Ni–Al LDH. Scheidegger et al. (1998) suggested that the rate-limiting step for the formation of Ni–Al LDH is Al dissoluN contamination of soils is a serious problem tion from the mineral surface. This is in consistent with as a result of industrial and mining activities. Since the observation that Ni–Al LDH formed after only 5 Ni is highly toxic to plants and animals, its fate and min in the presence of the relatively soluble pyrophylmobility in soils are of great concern. The sorption of lite, but only after 24 h in the presence of the more Ni onto soil surfaces controls the Ni distribution in soil stable gibbsite (Scheinost et al., 1999). In both cases, and aquatic system. Therefore, identification of sorption dissolution of the mineral surfaces may be enhanced by mechanisms is a prerequisite to establish risk assessment Ni-promoted dissolution (d’Espinose de la Caillerie et and remediation strategies for Ni contaminated soils. al., 1995). Evidence for pyrophyllite dissolution was sugMany attempts have been made towards that goal using gested by increasing Si concentrations in solution. Howbinary sorbent–sorbate systems and spectroscopic techever, the Al concentrations in gibbsite and pyrophyllite niques. However, the fate of metals in soils may differ systems remained below 1 mmol L21, most likely due to from that predicted by such relatively simple laboratory precipitation of amorphous Al hydroxide (Thompson experiments because of the presence of a variety of et al., 1999). Together with an initial Ni hydroxide phase, inorganic and organic ions in soil solution. Organic lithe Al hydroxide is a necessary precursor for the formagands are of special interest since they are exuded into tion of Ni–Al LDH (Boclair and Braterman, 1999; Taythe soil solution by plants, fungi, and microorganisms lor, 1984). The progression of the dissolution explains (Strom, 1997), and since they may enhance the solubility the constant growth of Ni–Al LDH, which has been oband mobility of metals (Gadd, 1999; Jones, 1998). served as long as sufficient Ni was in solution (Scheinost et al., 1999). Organic ligands form complexes with both surfaceN.U. Yamaguchi, Dep. of Biological and Environmental Engineering, bound and aqueous cations. Depending on pH, and type Graduate School of Agricultural and Life Sciences, The University of and concentration of organic ligands, mineral dissoluTokyo, 1-1-1 Yayoi, Bunkyo, Tokyo, 113-8657, Japan; A.C. Scheinost, Inst. of Terrestrial Ecology, ETHZ, CH-8952 Schlieren, Switzerland; and D.L. Sparks, Dep. of Plant and Soil Sciences, University of DelaAbbreviations: DRS, diffuse reflectance spectroscopy; HS-gibbsite, ware, Newark, DE 19717-1303. Received 3 Feb. 2000. *Corresponding high surface area gibbsite; LDH, layered double hydroxide; LS-gibbsauthor ([email protected]). ite, low surface area gibbsite; PZSE, point of zero salt effect; XAS, x-ray absorption spectroscopy. Published in Soil Sci. Soc. Am. J. 65:729–736 (2001).