Chemical speciation of environmentally significant metals with inorganic ligands. Part 5: The Zn2+ + OH-, Cl-, CO32-, SO42-, and PO43-systems (IUPAC Technical Report)

The numerical modeling of Zn-II speciation amongst the environmental inorganic ligands Cl-, OH-, CO32-, SO42-, and PO43- requires reliable values for the relevant stability (formation) constants. This paper compiles and provides a critical review of these constants and related thermodynamic data. It recommends values of log(10) beta(p,q,r)degrees valid at I-m = 0 mol.kg(-1) and 25 degrees C (298.15 K), and reports the empirical reaction ion interaction coefficients,Delta epsilon, required to calculate log(10) beta(p,q,r) values at higher ionic strengths using the Bronsted-Guggenheim-Scatchard specific ion interaction theory (SIT). Values for the corresponding reaction enthalpies Delta H-r, are reported where available. There is scope for additional high-quality measurements for the Zn2+ + H+ + CO32- system and for the Zn2+ + OH- and Zn2+ + SO42- systems at I > 0. In acidic and weakly alkaline fresh water systems (pH < 8), in the absence of organic ligands (e. g., humic substances), ZnII speciation is dominated by Zn2+(aq). In this respect, ZnII contrasts with Cu-II and Pb-II (the subjects of earlier reviews in this series) for which carbonato- and hydroxido- complex formation become important at pH > 7. The speciation of Zn-II is dominated by ZnCO3(aq) only at pH > 8.4. In seawater systems, the speciation at pH = 8.2 is dominated by Zn2+(aq) with ZnCl+, Zn(Cl)(2)(aq), ZnCO3(aq), and ZnSO4(aq) as minor species. This behaviour contrasts with that for CuII and PbII for which at the pH of seawater in equilibrium with the atmosphere at 25 degrees C (log(10) {[H+]/c degrees} approximate to 8.2) the MCO3(aq) complex dominates over the MCln(2-n)+ species. The lower stability of the different complexes of ZnII compared with those of CuII, PbII, and Cd-II is also illustrated by the percentage of uncomplexed M2+ in seawater, which is ca. 55, 3, 2, and 3.3 % of [M-II](T), respectively.