Retention of Nickel in Soils: Sorption-Desorption and Extended X-ray Absorption Fine Structure Experiments

Adsorption and desorption of heavy metals in soils are pri- mary 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. Adsorp- tion of Ni by both soils was kinetic and increased with increasing ini- tial (input) Ni concentration. The rate of sorption was initially rapid and was followed by gradual retention over time. A sequential extrac- tion 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 in- crease 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 se- quential 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.