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DIVISION S-2 - SOIL CHEMISTRY

Dissolution Kinetics of Nickel Surface Precipitates on Clay Mineral and Oxide Surfaces

^a National Risk Management Research Lab., US EPA, 5995 Center Hill Ave., Cincinnati, OH 45268
^b Dep. of Plant and Soil Sciences, Univ. of Delaware, Newark, DE 19717-1303

Corresponding author (Scheckel.Kirk{at}epamail.epa.gov)

The formation of Ni surface precipitates on natural soil materials may occur during sorption under ambient environmental conditions. In this study, we examined proton- and ligand-promoted dissolution of Ni surface precipitates on pyrophyllite, talc, gibbsite, amorphous silica, and a mixture of gibbsite and amorphous silica aged from 1 h to 2 yr, by employing an array of dissolution agents (ethylenediaminetetraacetic acid [EDTA], oxalate, acetylacetone, and HNO₃). Ligand-promoted dissolution was more effective in removing Ni than the protolysis by HNO₃. In all cases, as residence time increased from 1 h to 2 yr, the amount of Ni released from the precipitates decreased from 98 to 0%, indicating an increase in stability with aging time regardless of sorbent and dissolution agent. For example, as residence time increased from 1 h to 2 yr, Ni release from pyrophyllite, as a percentage of total Ni sorption, decreased from 96 to 30% and 23 to 0%, respectively, when EDTA (pH 4.0) and HNO₃ (pH 6.0) were employed as dissolution agents for 14 d. Dissolution via oxalate of 1-yr-aged Ni–Al layered double hydroxide (LDH) on pyrophyllite saw 19% Ni removal, in comparison with 52% Ni release from -Ni(OH)₂ precipitates on talc, suggesting that -Ni(OH)₂ is less stable than Ni–Al LDH. The increase in stability of the Ni surface precipitates in this study with residence time was attributed to three aging mechanisms: (i) Al-for-Ni substitution in the octahedral sheets of the brucite-like hydroxide layers, (ii) Si-for-NO₃ exchange in the interlayers of the precipitates, and (iii) Ostwald ripening of the precipitate phases. It appeared that the second factor, Si-for-NO₃ exchange in the interlayers, was a major mechanism for the increase in stability of the precipitates.

Abbreviations: A, interlayer anion • DRS, diffuse reflectance spectroscopies • EDTA, ethylenediaminetetraacetic acid • HRTGA, high-resolution thermogravimetric analysis • LDH, layered double hydroxide • M, metal cation • XAFS, x-ray absorption fine structure • XRD, x-ray diffraction

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