Boron is technically a semi-metal, but that's really not relevant. If the metal doesn't form compounds that stain (such as oxides/hydroxides or carbonates) or scale (calcium carbonate), then whether or not it's a metal doesn't really matter.
As was noted, copper ions do not go away except from dilution (of from precipitation/staining). If there is an algae bloom, then copper taken up in algae could get caught along with that algae in the filter. So in general, one would not expect the copper level to drop much faster than other water parameters such as salt or borates (if used).
In
this post I give some references to copper kill times. Though it is true that
Legionella pneumophila is slow-to-kill with chlorine, it is also slow-to-kill with copper such that they are roughly comparable unless the copper level is higher. For most pathogens, including the more common fecal bacteria, chlorine (even with CYA) kills much faster than copper. Copper isn't very good at inactivating viruses. Basically, copper is best, in practical terms for pools, at preventing algae growth, but requires levels that make staining a risk unless you make sure your pH and copper levels don't get too high.
As for biofilms, it seems to be a mixed bag. As noted in
this link, biofilms have been looked at as copper ion filters to remove copper from wastewater.
This link describes how a combination of copper with a linear quat can kill bacteria in biofilms. However, the study was using concentrations of 8 mM Cu
2+ which is 0.5 ppm for the fastest kills, though the lowest concentration tested, 1 mM Cu
2+ which is 0.06 ppm still showed some killing effect. The Polycide concentration that was comparable was 200 ppm for the former and 25 ppm for the latter. Unfortunately this study didn't give detailed kill times so really just showed a synergestic relationship between copper and QAC (linear quat algaecide).
It should be noted, however, that if one kills bacteria quickly enough in the bulk pool water where introduced, then biofilms won't form in the first place and that is yet another reason why proper pool water chemistry should be maintained at all times.
If copper (or silver) didn't have staining and discoloration issues, it would be a fine persistent preventative and aid to keeping water more sanitary in conjunction with chlorine, but unfortunately it does have these side effects which make it risky to use. PolyQuat doesn't have such side effects (not even foaming as linear quats) so is one of only two supplements recommended on this forum, the other being borates whose only downside appears to be ingestion by dogs drinking cups of water every day from the pool. These supplements are optional, but if someone wants additional protection, then these are the ones with the least side effects.
Richard
