Laserfiche WebLink
12 <br />. Combining the two steps gives the same as reaction 17. <br /> The hydrogen ions catalyze or speed up the dissolution reac- <br /> tion 17 without being consumed. Such secondary weathering <br /> reactions are of great importance in establishing the water <br /> quality. Iron dissolved in water is more of a nuisance and <br /> can be tolerated to a fairly high concentration as seen in <br /> the drinking water standards listed in Table I. In contrast, <br /> Cd, As, Pb and Ag are quite toxic and have low maximum per- <br /> missible concentrations in drinking water. <br /> High levels of iron will not remain dissolved in surface <br /> waters for very long because the pH has to be at an uncommonly <br /> low value of 3 or lower to keep mechanism 15 from proceeding <br /> to the precipitation of Fe(OH)3. Is this true for the other <br /> base metals dissolved in water? Unfortunately, some metal <br />• such as Zn, Cd and Mn can remain dissolved in water even <br /> when the pH is raised to values around 7. Boyles and co- <br /> workers (21) performed studies which show quite well <br /> the relation of pH to dissolved metal concentration. They <br /> increased the pH of Argo Tunnel mine drainage by controlled <br /> increments from its initial value of 2.8 to pH=8. At each <br />step an analysis was done of the metals still dissolved <br />in the water. Their results are reproduced in Figure 1. <br />The important point to note here is that Fe is precipitated <br />from the water quite early but the other base metals stay <br />dissolved. The conclusion is that precipitation of iron <br />from the water does not imply that mine effluents are of <br />9