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    Kinetic speciation of mercury–humate complexes in aqueous solutions by using competing ligand exchange method

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    Microchem_J_126_551a.pdf (356.4Kb)
    Date
    2016
    Author
    Vudamala, K.
    Chakraborty, P.
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    Abstract
    Competing ligand exchange method (CLEM) in conjunction with direct mercury analyzer (DMA) was found to be a useful method for kinetic speciation study of (mercury–humic acid) Hg–HA complexes in aqueous systems. Ambersep GT74 (containing a thiol functional group) was used as a suitable competing ligand for this study. The CLEM/DMA was used to estimate lability and dissociation rate constant of Hg–HA complexes in aquatic systems. Influences of pH, metal loading and varying major cation (Ca<sup>2+</sup>) concentrations on Hg–HA complexes were investigated. Concentration of dynamic Hg–HA complexes gradually increased with increasing Hg loading in the system. The dissociation rate constant (k<sub>d1</sub>) of labile Hg–HA complexes were found to vary from 4.3 × 10<sup>-3</sup> s<sup>-1</sup> to 9.2 × 10s<sup>-3</sup> s<sup>-1</sup> with increasing Hg loading from 1.0 × 10<sup>-8</sup> M to 1.42 × 10<sup>-7</sup> M. The concentrations of Hg–HA complexes with low thermodynamic stability were found to be similar at pH 4 and 5 (~ 59% and 58% of the total Hg with k<sub>d1</sub> values of ~2 × 10<sup>-1</sup> and 1.7 × 10<sup>-3</sup> s<sup>-1</sup>, respectively). The complexation of Hg with HA was found to increase with increasing pH from 4 to 7. The concentration of inert complexes (c<sub>2</sub>) with dissociation rate constant (k<sub>d2</sub>) gradually increased with increasing pH. Increasing concentration of Ca<sup>2+</sup> in the model solutions increased the concentrations of Hg–HA dynamic complexes in the system. This finding suggests that the Hg–HA bond is predominantly ionic in nature. It suggests that Ca<sup>2+</sup> ions screen out Hg<sup>2+</sup> ions from undergoing complexation reaction with HA.
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    http://drs.nio.org/drs/handle/2264/4924
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