The disposal of some chemicals requires their destruction in many cases to make their disposal safer.
Cyanides: Cyanides are prone to oxidation. Reaction of cyanide-containing waste with sodium hypochlorite will form relatively nontoxic cyanate compounds. These can be flushed down a sink.
Chromates: Chromates are oxidizers. Therefore, reaction with ferrous sulfate (available in impure form as moss killer) will reduce the chromium to much less toxic chromium(III). Addition of sodium bicarbonate (baking soda) will further isolate the chromium as the completely harmless and insoluble chromium(III) hydroxide.
Copper, cobalt, nickel: You may want to consider precipitating copper metal from large amounts of pure copper waste as copper commands a high price. However, if the copper solution is contaminated, large amounts may be disposed of by reaction with sodium bicarbonate to produce the insoluble copper(II) carbonate, which can be disposed of in solid waste. Do a similar process for cobalt, nickel, titanium, and molybdenum.
Silver: Large amounts of silver are certainly undesirable to dispose of, but if they must be disposed, reduce the silver solution with sodium hydroxide and glucose or zinc before disposal. Forget about disposal and recover the silver metal.
Permanganates: Permanganates are quite unstable and will spontaneously decompose. But if brown stains are undesirable, add iron(II) sulfate solution, along with some hydrochloric acid. This may produce chlorine, but in the process the permanganate will be reduced to light pink manganese(II) solution. This can then be neutralized with sodium bicarbonate and the resulting precipitate disposed of in solid waste.
Mercury, cadmium, lead, thallium, arsenic: It is best to take these to a solid waste disposal area or use quantities of such small size (10 mg per experiment for mercury, thallium, arsenic) that no significant pollution could occur.
Antimony: Antimony naturally exists as stibnite, antimony trisulfide. Reaction of an antimony solution in dilute hydrochloric acid with iron(II) sulfide will precipitate antimony in this relatively harmless and reddish form.
Zinc: Zinc is not extremely toxic, therefore for reasonable amounts no special disposal means are necessary.
Magnesium, sodium, calcium, strontium, etc: These can be flushed down a sink in great quantities as they are completely harmless.
Barium: Add Epsom salts to precipitate barium as the insoluble barium sulfate. Wash away.
Beryllium: Beryllium naturally exists as the silicate. Precipitation as the silicate is likely the wisest method of disposal of small quantities of this valuable material.
The rare earth metals, as well as aluminium, gallium, indium, the alkali metals, silicates, phosphates, may be flushed in reasonable quantities down a sink without causing any guilt as to toxic effects on environmental habitats.