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Abstract
Nickel adsorption on 5 different solid phases: amorphous-MnO2, amorphous-Fe(OH)3, CaCO3, humic acid and montmorillonite were studied. Comparative studies at pH: 6.7–8.4, using a conditional stability constant, showed that the order of surface affinity for Ni, followed the sequence: MnO2 > Fe(OH)3 > humic acid ∼ montmorillonite > CaCO3. MnO2 and Fe(OH)3 are therefore proposed as the major sinks for Ni in fresh water systems. Adsorption on montmorillonite and humic acid were described as linear. Adsorption on MnO2 was described by a Langmuir isotherm, whereas adsorption on Fe(OH)3 was described by the Redlich-Peterson model. Increasing the ionic strength decreases Ni adsorption on MnO2 and montmorillonite, whereas adsorption on Fe(OH)3 was unaffected. Adsorption on Fe(OH)3 was strongly effected by pH, in the region of 7.0 – 8.0, adsorption on MnO2 was unaffected, in the same region. The speciation calculation of Ni(II) in the pH region of 7.0 – 8.0, indicates that Ni2+ dominates at pH < 7.7, and NiCO3 0 dominates at pH > 7.7. The adsorption capacity of MnO2 and Fe(OH)3 was found to be large enough to keep the Ni concentration well below the threshold value for Danish drinking water (50 µg L−1).