Final Report - Strategic Environmental Research and Development ...

5.3.2.4 Results
5.3.2.4.1 Speciation in aqueous solutions . Speciation of Zn in dilute aqueous sodium chloride
solutions was calculated using the thermodynamic simulation program, MEDUSA TM (Make
Equilibrium Diagrams Using Sophisticated Algorithms) developed at Inorganic Chemistry,
Royal Institute of Technology (KTH), Stockholm, Sweden. MEDUSA has an embedded
software named HYDRA that contains the database of chemical equilibrium constants with log
K values at 25˚C. The starting concentrations of cations, anions and/or partial pressures of gases
defining the aqueous solution chemistry are given as inputs in HYDRA. The pH-dependent
solubility of Zn 2+ considering hydroxide, chloride and carbonate anions are shown.
Figure 3.8a shows the speciation diagram of 5 mM concentrations of Zn 2+ cation in an aqueous
solution containing OH - anions only. The concentration of different ionic species was plotted on
a logarithmic scale as a function of solution pH. The red dashed line represents the total
solubility and is approximately the sum of the concentrations of the dissolved species in the
solution. The solubility remained high with soluble Zn 2+ cations predominant in the low pH
regions up to pH 7. The solubility reduced by about three orders of magnitude by the hydrolysis
of Zn 2+ cations leading to precipitation of Zn oxide (ZnO) at pH 7. The overall solubility
remained low until pH 11.8 due to the sparingly soluble Zn(OH) 2 . Further hydrolysis above pH
11.8 increased the solubility by forming Zn(OH) 3 - and Zn(OH) 4 2- anions.
Figure 3.8b shows the pH-dependent solubility diagram obtained by 100 mM NaCl additions in
the aqueous system containing 5 mM Zn 2+ . The black dotted line represents the total solubility
for Zn 2+ with Cl - additions. The change in the total solubility of Zn 2+ due to the addition of Cl -
anions is shown in the diagram by superimposing the total solubility of Zn 2+ in the absence of Cl -
adopted from Figure 3.8a (red dashed line). Cl - additions caused the total solubility to reduce at
pH 6.7 instead of pH 7 as observed with no Cl - additions (Figure 3.8a). The first drop in the
solubility occurred due to the precipitation of insoluble Zn 5 (OH) 8 Cl 2 at pH 6.7 and the solubility
further reduced at pH 8.7 due to the precipitation of sparingly soluble Zn(OH) 2 . The limiting
solubility of dissolved Zn(OH) 2 was a little less in the Cl - containing solutions when compared to
that of the solution with no Cl - anion. The solubility starts to rise at pH 11.8 by forming
Zn(OH) 3 - and Zn(OH) 4 2- anions.
Figure 3.8c shows the speciation diagram of 5 mM Zn 2+ in aqueous solutions containing 100 mM
NaCl with carbonate anions in the solution due to atmospheric CO 2 (partial pressure of CO 2 in
air is 10 -3.5 atm). The natural pH of the solution was slightly less than 7 and is shown in Figure
3.8c. The thick blue line represents the total solubility for Zn 2+ with hydroxide, chloride and
carbonate anions in the solution. The total solubility for Zn 2+ with OH - anions only (red dashed
line) and both OH - and Cl - anions (black dotted line) were superimposed in Figure 3.8c. CO 3
2-
anions reduced the total solubility above pH 6.7 by the precipitation of insoluble
Zn 5 (OH) 6 (CO 3 ) 2 . The solubility started to increase at pH 8.25 by further complexation with
carbonates to form Zn(CO 3 ) 2 2- anion . Overall, the carbonate additions reduced the pH range of
solubility minimum compared to that observed in the absence of carbonates (Table 3.1).
However, carbonates reduced the solubility by more than a half order of magnitude compared to
the solubility profiles without carbonate additions.
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