Kinetic speciation of mercury–humate complexes in aqueous solutions by using competing ligand exchange method
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.
Collections
- Scholarly literature [9069]