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

Abstract

Formation of surface‐induced precipitates may play an important role in the immobilization of Ni and other metals in nonacidic soils. To investigate the influence of commonly present organic ligands on precipitate formation, we monitored the uptake of Ni by gibbsite and pyrophyllite in the presence of citrate and salicylate for 4 wk and identified the Ni hydroxide precipitates with diffuse reflectance spectroscopy (DRS). In the absence of organic ligands, Ni uptake proceeded by formation of Ni–Al layered double hydroxide (LDH) precipitates. Citrate and salicylate generally decreased both the Ni removal from solution and the precipitate formation. The suppression by citrate was more pronounced than that by salicylate due to the stronger complexation of Ni by citrate. In the presence of citrate and salicylate, the precipitate phase was Ni–Al LDH on pyrophyllite, but predominately α‐Ni hydroxide on gibbsite. This difference can be explained by the differing Al solubilities of the two minerals. Pyrophyllite is relatively soluble, causing the rapid formation of amorphous Al hydroxide, which, in turn, is a necessary precursor for the formation of Ni–Al LDH. In spite of the complexation of Al by organic ligands, sufficient amorphous Al hydroxide was available to promote the formation of Ni–Al LDH. Gibbsite, on the other hand, is much less soluble, and the smaller amount of initially released Al may be fully complexed by citrate and salicylate. The subsequent lack of amorphous Al hydroxide prevented the formation of Ni–Al LDH, and, instead, α‐Ni hydroxide formed. Only after a longer period of 30 d and at a low citrate concentration did enough Al become available to transform α‐Ni hydroxide into the thermodynamically more stable Ni–Al LDH.