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

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 (Ca2 +) 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 (kd1) of labile Hg-HA complexes were found to vary from 4.3 × 10− 3 s− 1 to 9.2 × 10− 3 s− 1 with increasing Hg loading from 1.0 × 10− 8 M to 1.42 × 10− 7 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 kd1 values of ~ 2 × 10− 3 and 1.7 × 10− 3 s− 1, respectively). The complexation of Hg with HA was found to increase with increasing pH from 4 to 7. The concentration of inert complexes (c2) with dissociation rate constant (kd2) gradually increased with increasing pH. Increasing concentration of Ca2 + 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 Ca2+ ions screen out Hg2+ ions from undergoing complexation reaction with HA.