Q. We use ion exchange treatment for wastewater from our nickel, hexavalent chrome and zinc plating rinses. We have been trying to reuse this “clean water” but have found that it is attacking the finish from our bright zinc plating. What kind of characterization should we do to identify the pollutants that are affecting our products, and what steps are needed to reuse this water? H.S.
A. The very likely issue is not that there is some kind of pollutant in your “clean water” that is causing the problem but that your ion exchange effluent is too clean and is very “aggressive” towards metals. Since your ion exchange system has essentially removed all ions, including metals, from the wastewater, the wastewater wants to restore its ion balance and will attack metals to replenish its ions. Also, if you check the ion exchange effluent’s pH, you will likely find it below 5 and perhaps as low as 3. If you are using this water in your nickel and chrome plating rinses and not experiencing the same issue, the reason is that these metals are able to resist the water’s aggressiveness during the relatively short rinsing time.
Here are several alternatives for you to consider: