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Retention of Nickel in Soils: Sorption-Desorption and Extended X-ray Absorption Fine Structure Experiments
Citation:
Liao, L., A. Roy, K. Scheckel, G. Merchan, AND M. Selim. Retention of Nickel in Soils: Sorption-Desorption and Extended X-ray Absorption Fine Structure Experiments. Robert L. Tate III (ed.), Soil Science. Lippincott Williams & Wilkins, Philadelphia, PA, 178(5):215-221, (2013).
Impact/Purpose:
The fate of heavy metals such as nickel (Ni) in the vadose zone depends largely on their reactivities in the soil environment. The reactivity of heavy metals in soils is highly dependent on its affinity to different reactive surfaces in the soil matrix, and understanding of the various interactions is a prerequisite in the effort to predict their mobility, bioavailability, and toxicity behavior in soils. Sources of Ni in the environment include industrial metal processing, combustion of coal and oil, and application of sludge and phosphate fertilizers. The accumulation of Ni in the human body through chronic exposure can lead to lung fibrosis and cardiovascular and kidney diseases (Denkhaus and Salnikow, 2002). In this study, macroscopic and microscopic approaches were used to assess Ni behavior in soils having distinctly different properties. Specifically kinetic sorption-desorption experiments combined with sequential extraction and EXAFS spectroscopy were implemented to characterize kinetic Ni behavior in soils. One acidic and one neutral soil were selected where a wide-range Ni concentration was used. The speciation of Ni formed on soils was evaluated based on EXAFS and sequential extractions. Such information is necessary toward the understanding of the mechanisms governing Ni reactivities in the soil system and for the prediction of Ni behavior in the vadose zone.
Description:
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.
