Kinetics of Nickel Adsorption – Desorption in Different Soils: Evidence From XAFS.

Recent evidence using XAFS (X-ray absorption fine structure spectra) suggests that sorption mechanism and retention of nickel (Ni) in alkaline soils may involved new solid phase precipitation which is best regarded as an immobile phase. In this study, we quantified sorption characteristics of nickel using kinetic retention experiments for two acidic soils (Windsor and Olivier) and one neutral (Webster) soil. Ni sorption exhibited strong nonlinear retention behavior for all soils with initially rapid reaction and was followed by gradual or slow retention behavior with reaction time. Desorption of nickel was hysteretic in nature indicative of lack of equilibrium retention and/or irreversible or slowly reversible processes. Soil samples with highest Ni concentrations were analyzed using EXAFS.  The EXAFS results provided evidence that a significant increase of Ni sorption with time was irreversibly adsorbed on our non-acidic soil. 

A multireaction model (MRM) with nonlinear equilibrium and kinetic sorption with consecutive irreversible reaction successfully described time-dependent sorption for the two acidic soils.  However, for the neutral (Webster) soil, which exhibited strongest affinity for Ni, an additional irreversible reaction phase was necessary to predict Ni desorption or release kinetics which is supportive of EXAFS findings.