Nickel sorption to goethite and montmorillonite in presence of citrate

Mobility and bioavailability of nickel (Ni) in soil strongly depends on the interaction between Ni(II), ligands, and sorbents like organic matter and minerals. Sorption of Ni(II) and Ni(II)-citrate complexes to goethite and montmorillonite was examined in batch experiments with and without citrate as ligand in the pH range pH 4-7.5. Without citrate, montmorillonite shows higher Ni sorption than goethite. Citrate strongly decreases Ni sorption to montmorillonite; in presence of 100 μM citrate goethite becomes a stronger Ni sorbent than montmorillonite. Ni and citrate sorption was modeled successfully using the diffuse double layer model with the following reactions: Goethite: 3≡FeOH + Citrate ^3- + 3H ⁺ ⇄ ≡ Fe3Citrate + 3H ₂O, ⇄ FeOH + Ni ²⁺ ⇄ ≡FeONi ⁺ + H ⁺ and 2≡FeOH + Citrate ^3- + Ni ²⁺ ⇄ ≡FeONiCitrate ^2- + H ⁺. Montmorillonite: 2X ^- + Ni ²⁺ ⇄ X2Ni and ≡AIOH + Ni ²⁺ ⇄ ≡ AIONi ⁺ + H+. Sorption of Ni to a mixture of goethite and montmorillonite could be calculated by use of reactions and constants for the monomineral systems. Without citrate, the sorbed amount of Ni per mass unit in the mixture can be found as a simple average of sorption to the two single sorbents, while in presence of citrate Ni sorption to montmorillonite is strongly influenced by citrate sorption to goethite.