Ferric Chloride Etchant Disposal
Both Sodium Hydroxide and Sodium Carbonate (washing soda) can be used to neutralise Ferric Chloride. Once the pH has risen above 7 the etchant is neutralised. If you are going to attempt it, be careful, the reaction is exothermic and if you apply too much too fast you can end up with the mixture getting exceedingly hot and release a cloud of Chlorine gas. The sludge left at the end will be primarily Iron Oxides (rust) and Copper Hydroxides, which are not particularly harmful.
However, my advice would be to check local regulations, and see if there are any waste disposal sites around which can deal with the waste properly.
When using Ferric Chloride as an etchant, you can end up with large amounts of wastage because the Copper Chloride that is left behind ends up weakening the reaction, until you end up with a mixture that is incapable of performing its job, even though there is still large amounts of FCl3 left.
If you want to minimise the amount of wasted etchant, the best technique I've found is to use a wiped sponge approach to etching rather than a submersion approach.
Typically people dunk their boards in the etchant and leave it a while, perhaps with some agitation. This means that your etchant solution become full of the Copper Chloride remnants.
Instead, a better approach is to, wearing rubber gloves, dip a small sponge (e.g. old washing up sponge) into the etchant solution, and then wipe that sponge gently across the surface of the copper. The combined agitation and strong etchant will rapidly etch the board. Once it stops etching, drain the excess fluid from the sponge (CuCl) into a container, and then repeat the process. The end result is you end up with a container of etchant with little copper contaminant, and a container of mostly copper chloride. This way your echant lasts much longer and there is less wastage.