Surface-induced nickel hydroxide precipitation in the presence of citrate and salicylate

Formation of surface-induced precipitates may play an important role in the immobilization of Ni and other metals in nonacidic soils, To investigate t he 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 precipit ates 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 ci trate was more pronounced than that by salicylate due to the stronger compl exation of Ni by citrate. In the presence of citrate and salicylate, the pr ecipitate phase was Ni-Al LDH on pyrophyllite, but predominately alpha -Ni hydroxide on gibbsite. This difference fan be explained by the differing Al solubilities of the two minerals. Pyrophyllite is relatively soluble, caus ing the rapid formation of amorphous Al hydroxide, which, in turn, is a nec essary precursor for the formation of Ni-Al LDH. In spite of the complexati on of Al by organic ligands, sufficient amorphous Al hydroxide was availabl e to promote the formation of Ni-Al; LDH. Gibbsite, on the other hand, is m uch less soluble, and the smaller amount of initially released Al may be fu lly complexed by citrate and salicylate. The subsequent lack of amorphous A l hydroxide prevented the formation of Ni-Al LDH, and, instead, alpha -Ni h ydroxide formed, Only after a longer period of 30 d and at a low citrate co ncentration did enough Al become available to transform alpha -Ni hydroxide into the thermodynamically more stable Ni-Al LDH.