Chemical Speciation of Hg(ii) with Environmental Inorganic Ligands

Complex formation between Hg(ii) and the common environmental ligands Cl-, OH-, CO32-, SO42-, and PO43- can have profound effects on Hg(ii) speciation in natural waters with low concentrations of organic matter. Hg(ii) is labile, so its distribution among these inorganic ligands can be estimated by numerical modelling if reliable values for the relevant stability constants are available. A summary of critically reviewed constants and related thermodynamic data is presented. Recommended values of log10ßp,q,r° and the associated reaction enthalpies, ?rHm°, valid at Im = 0 mol kg-1 and 25°C, along with the equations and specific ion interaction coefficients required to calculate log10ßp,q,r values at higher ionic strengths and other temperatures are also presented. Under typical environmental conditions Hg(ii) speciation is dominated by the reactions Hg2+ + 2Cl- ? HgCl2(aq) (log10ß2° = 14.00 ± 0.07), Hg2+ + Cl- + H2O ? Hg(OH)Cl(aq) + H+ (log10ß° = 4.27 ± 0.35), and Hg2+ + 2H2O ? Hg(OH)2(aq) + 2H+ (log10*ß2° = -5.98 ± 0.06).