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Positive saturation indices imply the potential to precipitate mineral phases. If the saturation index is negative, the mineral cannot precipitate. If the saturation index is positive, the mineral may precipitate. The precipitation is governed by a number of factors. The minerals likely to precipitate are presented in Tables 4 and 5. In all the three organic chelating substances used, the mineral assemblages were increasing saturation as the pH increased. The presence of Pb forms cerrusite, which further aggravates scaling (Table 4). The Zn saturation indices' simulations were all negative, indicating no precipitation of mineral phases. This agrees with experimental data that Ca is more coordinating to humic acid in the presence of Zn, hence the more precipitation experienced with Pb than Zn, relatively.

Table 4

PHREEQC model saturation indices of the main Pb/Ca mineral assemblages at basic pH values for humic acid, catechol and tannic acid

Mineral phasesSaturation indices
At pH = 4
At pH = 8
Humic acidCatecholTannic acidHumic acidCatecholTannic acid
Aragonite (CaCO3−9.05 −9.06 −9.05 −2.75 −2.87 −2.95 
Calcite (CaCO3−8.91 −8.91 −8.91 −2.60 −2.72 −2.80 
Cerrusite (PbCO3−5.38 −4.81 −5.81 0.36 0.77 0.87 
Mineral phasesSaturation indices
At pH = 4
At pH = 8
Humic acidCatecholTannic acidHumic acidCatecholTannic acid
Aragonite (CaCO3−9.05 −9.06 −9.05 −2.75 −2.87 −2.95 
Calcite (CaCO3−8.91 −8.91 −8.91 −2.60 −2.72 −2.80 
Cerrusite (PbCO3−5.38 −4.81 −5.81 0.36 0.77 0.87 
Table 5

PHREEQC model saturation indices of the main Zn/Ca mineral assemblages at basic pH values for humic acid, catechol and tannic acid

Mineral phasesSaturation indices
At pH = 4
At pH = 8
Humic acidCatecholTannic acidHumic acidCatecholTannic acid
Aragonite (CaCO3−8.89 −8.89 −8.89 −2.54 −2.54 −2.54 
Calcite (CaCO3−8.74 −8.74 −8.74 −2.39 −2.39 −2.42 
Smithsonite (PbCO3−3.3 −8.19 −8.34 −1.81 −1.71 −1.24 
Mineral phasesSaturation indices
At pH = 4
At pH = 8
Humic acidCatecholTannic acidHumic acidCatecholTannic acid
Aragonite (CaCO3−8.89 −8.89 −8.89 −2.54 −2.54 −2.54 
Calcite (CaCO3−8.74 −8.74 −8.74 −2.39 −2.39 −2.42 
Smithsonite (PbCO3−3.3 −8.19 −8.34 −1.81 −1.71 −1.24 

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