Retention of nickel in soils: Sorption-desorption and extended x-ray absorption fine structure experiments

Adsorption and desorption of heavy metals in soils are primary factors that influence their bioavailability and mobility in the soil profile. To examine the characteristics of nickel (Ni) adsorption-desorption in soils, kinetic batch experiments were carried out followed by Ni release using successive dilutions. Two soils of distinctly different properties were used: one acidic (Olivier) and one neutral (Webster) soil, where a wide-range Ni concentration was implemented. Adsorption of Ni by both soils was kinetic and increased with increasing initial (input) Ni concentration. The rate of sorption was initially rapid and was followed by gradual retention over time. A sequential extraction procedure and extended X-ray absorption fine structure (EXAFS) spectroscopy were implemented to characterize Ni kinetic sorption behavior. Five sequential extractions were quantified: exchangeable, bound to carbonates, bound to Fe-Mn oxides, bound to organic matter, and residual fraction. The exchangeable fraction showed a slight increase as initial Ni concentration increased, indicating weakly bound Ni adsorption complexes. Extended X-ray absorption fine structure analyses indicated that Ni hydroxide precipitate formed over time for the neutral Webster soil. This precipitate was likely bound to the Fe/Al oxide fraction. We conclude, based on EXAFS analyses and sequential extractions, formation of Ni hydroxide precipitate depends on soil pH and the amount of Ni sorbed. No Ni hydroxide precipitate was formed on the acidic Olivier soil. Copyright © 2013 by Lippincott Williams & Wilkins