You will be limited in the rate of plating by the conductivity of the water and your plate area, but mostly by the fact that the metal ion concentration is so low. As I wrote, even saltwater chlorine generator pools don't plate out metals fast enough and they have 3000 ppm salt (so higher conductivity) and enough plate area to be able to generate sufficient amounts of chlorine. What you are proposing is simply not practical. If you had strong solutions of metal ions, then plating is reasonable, but we're talking about levels in the 0.2 to 1 ppm range and that is VERY low concentration in terms of plating.
You absolutely would want the water to be moving, otherwise the metal ions in the water would only get to the plate by diffusion which is VERY slow. As for trying to "attract" the metal ions to the plate by charge, you are forgetting that there are much higher concentration of other positive ions in the water, namely sodium and calcium. Though these won't plate out due to their thermodynamics, they will migrate to balance charge. Likewise, hydroxyl ions generated at the plate will migrate away.
The cathode does not need to be of like metal in order to have plating occur, but it is true that different metals will have different over-voltages so some will be more likely to plate than others. However, all of this is swamped by the fact that primarily hydrogen gas will be produced since that it more thermodynamically favorable (at least for iron, as shown below).
CuLatorâ„¢ has had mixed reviews so far so the jury is out on this product. It works by essentially having a metal sequestrant chemical bound in a polymer so that it stays on the surface of plastic balls in the bag rather than being dissolved in the water (see
How It Works as well as
this patent). So as metal ions in the water flow through the bag they attach to the sequestrant in the bag. No mystery here and it's a reasonable approach, but as I wrote we don't have enough positive info to recommend this yet.
TECHNICAL DETAILS OF THERMODYNAMICS OF PLATING AT CATHODE:
The relevant half-reactions at the cathode are the following:
2H
+ + 2e
- --> H
2(g) ..... E
0 = 0.000V
Fe
3+ + 3e
- --> Fe(s) ..... E
0 = -0.037V
Cu
2+ + 2e
- --> Cu(s) ..... E
0 = +0.3419V
To figure out the actual reduction potential, one needs to use the Nernst Equation to account for concentrations of reactants and products:
E = E
0 - (RT/zF)*ln(a
red/a
ox) = E
0 - (8.314*300/(2*96485))*ln(a
red/a
ox) = E
0 - (0.0129)*ln(a
red/a
ox)
At a pH of 7.5, [H
+] = 3.47x10
-8 M and at 1 ppm (mg/L) [Fe
3+] is 1.79x10
-5 M and [Cu
2+] is 1.57x10
-5 M. Hydrogen gas has a Henry's constant of 7.8x10
-4 M/atm and a normal partial pressure in the atmosphere of 0.55 ppmv (5.5x10
-7 atm) so the normal concentration in water is 4.29x10
-10 M. So using the Nernst equation, the actual reduction potentials for the above are the following:
2H
+ + 2e
- --> H
2(g) ..... E
0 = +0.056V
Fe
3+ + 3e
- --> Fe(s) ..... E
0 = -0.178V
Cu
2+ + 2e
- --> Cu(s) ..... E
0 = +0.199V
The above ignores over-voltages, reaction rates (including ion diffusion rates) and the fact that hydrogen gas concentration in the water will rise especially near the plate so will be less favorable than indicated. Based on the above, it would seem that it is unlikely for iron to plate out, but possible for copper to do so, albeit slowly.
Perhaps the saltwater chlorine generator folks can comment on whether they see any plating of copper onto their cathodes in pools with copper ions, though the reversal of the salt cell will tend to minimize any long-term accumulation.
