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

Abstract

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 α‐Ni(OH)2 precipitates on talc, suggesting that α‐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.