Nickel chemical transformation in polluted soils as affected by metal source and moisture regime

A laboratory incubation experiment was undertaken to study the periodic changes in the five sequentially extractable chemical fractions: water soluble and exchangeable (F(1)), NaOAc extractable (F(2)), organically bound (F(3)), Fe-Mn oxide bound or reducible (F(4)) and residual (F(5)) of nickel added through industrial sludge or inorganic salt in three soils (acidic, neutral and alkaline) under field capacity and flooding moisture regimes during 16 months. Most of soil Ni was present in the residual fraction (60-89%) in unamended soils. Sludge addition increased NaOAc extractable-, organically bound- and Fe-Mn oxide bound-Ni fraction but decreased the proportion of Ni in residual fraction. With inorganic salt as the Ni source, there was an increase in water-soluble and exchangeable-and NaOAc extractable-Ni fractions in acidic soil. In neutral soil, there was an increase in water-soluble and exchangeable-, NaOAc extractable-and Fe-Mn oxide bound-Ni fractions while NaOAc extractable-and Fe-Mn oxide bound-Ni fractions increased in alkaline soil. After 16-month incubation, Ni added as inorganic salt was transformed from the water soluble and exchangeable fraction into the NaOAc extractable fraction in acidic soil. In neutral soil, the water soluble and exchangeable fraction transformed into NaOAc extractable-and organically bound fractions while the water soluble and exchangeable Ni fraction transformed into Fe-Mn oxide bound fraction in alkaline soil under both moisture regimes. With sludge as the Ni source, added Ni was transformed from water-soluble and exchangeable-and residual fractions into the Fe-Mn oxide bound fraction in all soils irrespective of the moisture regime.