I agree with other comments ...

I agree with other comments about looking more to the source of this one-off contamination.  Make sure that contaminants are kept out of the welding process, as these vapors can be harmful to personnel.  Furthermore, chlorine with a specified contact time is very effective at eliminating cyanide from wastewater, but this requires super-chlorination above any other chlorine demand.  You may have to then remove the chlorine from the wastewater afterwards.  GAC is one of several methods for that, including sulfite addition.  It seems this would be too much trouble for a problem that has only appeared this one time, and with some minor investigation, should be prevented in the future from appearing.