PCB etchant solutions are by definition and purpose nasty. They eat metal. They give off toxic fumes. They can stain. They contain metal that can only be called toxic waste, and should be treated as such. That said, they are indespensible to the process, and must be mastered. There are three common PCB etchants available in electronics stores: ferric chloride [FeCl3], ammonium persulphate [(NH4)2S2O8], and sodium persulfate [Na2S2O8] Another popular copper etchant is cupric chloride [CuCl2], or copper ii chloride, which can be made at home.
In my area the hazardous waste must be in "...it's original packaging and have it's contents clearly marked...". That is not always possible, and the alternative is "business waste" which I have to pay to have removed.
According to the USA DOT ammonium persulphate, even the water used to dilute a spilled solution of it, must be treated as hazardous waste. USA OSHA says ammonium persulphate "decomposes into ammonia, oxides of sulfur, oxides of nitrogen, and oxygen that supports combustion". I would wait for the hazardous waste pickup to get rid of this.
According to the USA DOT sodium persulphate is hazardous waste because it promotes combustion. There are no known long term effects from overexposure. It should be disposed of in a hazardous waste disposal facility.
Cuppric chloride, CuCl2(H2O)2, is another PCB etchant that is actually made by etching copper in a mixture of HCl and H2O2. During it's first phase, it gives off chlorine gas in quantities that coulld be dangerous in small spaces - ventilate well. When exhausted it can be revived by adding hydrogen peroxide or by bubbling air through it. Adding hydrogen peroxide can release chlorine gas. When the H2O2 doesn't revive the mix (doesn't turn it green again) you add a little more acid. If you have a swimming pool, you may already have everything required to make it. The muriatic acid you use to lower the pH of the pool is HCl. If you use the Baquacil system, you already have the hydrogen peroxide. If not, run to the beauty supply store and pickup some peroxide. Handle with care.
To make a little less than a gallon, you need to add 5 cups HCl to 10 cups of water and 1/4 cup of 24% H2O2 from the pool supply store. If you are only able to get drugstore 3% H2O2, add 6 cups of HCl to 10 cups of H2O2. Wear appropriate face and hand protection. These chemicals are dangerous.
"When mixed with potassium or sodium , it produces a strong explosion on impact". Yet the recommended disposal method is to add sodium carbonate, pour off the resulting saltwater, and reduce the sediment to a cake. The cake contains the metal and should be disposed of as hazardous waste.