SCALE REDUCTION AND SCALE MODIFICATION EFFECTS INDUCED BY ZN AND OTHER METAL SPECIES IN PHYSICAL WATER-TREATMENT

The effect of Zn and other metal ions, released by some physical, wate r treatment devices, on the nucleation rate and crystal morphology of calcium carbonate was investigated. Zn in particular caused a substant ial increase in induction time and induced the formation of calcium ca rbonate in the aragonite rather than the calcite form. These effects w ere quantified. A minimum Zn/Ca mass ratio of 0.06 x 10(-3) was requir ed for Zn to cause measurable effects. At [Ca] > 300 mg.dm(-3) additio n of Zn of up to 100 mu g.dm(-3) had a negligible effect on nucleation rate and crystal morphology. Cu was found to be only half as effectiv e as Zn while Mg required to be present at concentrations 1 000 times more than Zn to produce comparable effects. Colloidal Fe2O3 caused a d ecrease in induction time. The direct effect of Zn on scale reduction in laboratory tests amounted to about 77+/-6% and was achieved with 30 0 mg.dm(-3) Ca solutions to which 200 mu g.dm(-3) Zn was added.