Understanding the formation of colloidal mercury in acidic wastewater with high concentration of chloride ions by electrocapillary curves.

Acidic wastewater with high concentration of chloride ions was generated from washing elemental mercury (Hg(0)) existed in the roast flue gas by water. This process was simulated by mercury drops entering the electrolytes with its composition changed according to the characteristics of acidic wastewater. Electrocapillary curves of different electrolytes were determined by dropping mercury electrode to explore the formation mechanism of colloidal mercury in acidic wastewater. The changes of zeta (ζ) potentials were also obtained. The results indicate that chloride ions have a great impact on the formation of colloidal mercury. Thermodynamic calculation demonstrated that the main mercury species in acidic wastewater were HgCl2 (aq), HgCl3 (-), and HgCl4 (2-). Moreover, the model of colloidal mercury structure in acidic wastewater was established. Based on the changes of Gibbs free energy for ions passing through stern layer and metallic bond theory, it can be inferred that HgCl4 (2-) was preferentially over-adsorbed on the mercury interface by the weak π chemical bond, and then positive charge ions such as heavy metal ions and H(+) were adsorbed due to the electrostatic force; thus, the colloidal mercury was formed.