Proper disposal of chemicals
Most chemicals used by the amateur chemist come from every day use and do not require special treatment before being discarded in the usual way. However there are many chemicals, such as heavy metal compounds or organic solvents, that require special disposal techniques. In the interest of keeping lab costs low, though, it may be a good idea to look into ways to recycle and recover chemicals such as solvents, which can be reclaimed by distillation.
Contents
Standard procedures
Acids
Acids should be neutralized with a base, such as carbonate/bicarbonate, hydroxide (calcium hydroxide e.g.) usually as a solution. The soluble salts of halogen acids and oxoacids (except perchloric and chloric acids) can be safely poured down the drain. Special acids and their salts, such as hydrazoic acid and azides must not be poured directly down the drain, they must be treated with nitrous acid to destroy them.
Oxidizing mixtures, such as Aqua regia, piranha solution or mixed acid must also never be poured down the drain, as they're much more dangerous than simple acids, and can wreak havoc on your plumbing. Instead, they must be neutralized first.
Bases
Alkali hydroxides can be left in open air to absorb carbon dioxide as well as corrosive and unpleasant gases, forming carbonates and salts. For quicker results, alkali bases can be neutralized with any acid, though for practical and economical purposes, acetic acid or citric acid are sufficient. Bases may be reused to absorb acidic vapors in a desiccator, which mitigates their dangers. Care must be taken to avoid excess heating with strong bases.
Cyanides
Waste containing cyanide, either from gold refining or from organic extractions of alkaloids from cyanide containing plants, must be neutralized with bleach or hydrogen peroxide, to turn them into less harmful cyanates. Cyanates can be further neutralized to nitrogen and carbon dioxide.
Cyanides must never be poured down the drain.
Heavy metals
Since the biggest hazard comes from the volatile and soluble heavy metal compounds, it's best to convert them into insoluble compounds, which are less toxic.
Heavy metals, such as mercury, can often be neutralized with sulfur, generating mercury(II) sulfide, which is generally insoluble in water and has little reactivity.
Very dangerous heavy metals such as cadmium and arsenic should always be properly disposed of at designated facilities, as their effects on human life and the environment is sometimes catastrophic.
Inorganic anions
Inorganic anions, based on halogens, sulfur, phosphorus, nitrogen, carbon and silicon can be flushed down the drain with lots of water. Examples of these are sulfide, sulfate, chloride, chlorate, nitrate, nitrite, thiocyanate.[1]
Metal ions
A good rule of thumb for transition metals is to convert solutions of their ions to the insoluble carbonate or oxide, often one of the most stable and nontoxic forms of these elements, prior to disposal. Another simple solution is to reduce the ions back to the metal.
Copper salts are dangerous to the environment and should be reduced with iron to elemental copper, that can be reused, and the iron salts produced are less toxic.
Nickel-containing compounds, especially organonickel compounds, are carcinogens, and are also dangerous to many other forms of life. It is a good idea to take these to a proper waste disposal facility.
Cobalt salts may be carcinogenic and should be taken to a proper waste disposal facility.
Aluminium compounds should not be disposed in the sewage. They can, however, be converted to aluminium sulfate, which is safe to dispose of in soil, though it will cause a decrease in pH(it is used as a fertilizer for this purpose). It is a good idea to dispose of aluminium sulfate and a safe basic compound (such as calcium carbonate) at the same time.
Many compounds of chromium, especially chromium(VI), are poisonous and carcinogenic. These should be converted to the +3 oxidation state; hydrogen peroxide as well as sulfites or thiosulfate will do this, which is typically the least harmful, preferably to chromium(III) oxide. Chromium dioxide may be another feasible option, as it is found in cassette tapes and can be recycled with them.
Zinc compounds can be refined to zinc metal using electrowinning. It's not recommended to use zinc compounds as zinc supplemements for soil or livestock, as it may contain traces of cadmium.
Some MSDS sheets recommend that rare earths be converted to their oxalates or carbonates and disposed of in the trash.
Alkali metals are minimally toxic as their salts and may be dumped down the drain.
Organic compounds
Organic solvents should be dumped in labeled waste tanks, such as: halogenated, non-halogenated solvents. Flammable organic solvents that safe in low exposures such as ethanol, methanol, and acetone can often simply be burned outside for disposal, as most often their combustion products are simply carbon dioxide and water. It may be a better idea just to recover them by distillation, though.
A good way to destroy halogentated and aromatic compounds is with Fenton's reagent, which breaks them down to simpler non-toxic compounds. It's best however, to avoid dumping large quantities of waste, as the reaction in exothermic and may splash or volatilize some of the waste, so it's best to perform the neutralization in small steps.
Organic salts, such as acetates and oxalates can be pyrolyzed to carbonates and water vapors.
Recycling
Sometimes it is an option to purify waste products into chemicals that are pure enough to be used again. While this can sometimes be time consuming, it can often be more environmentally and economically friendly than discarding the waste in one manner or another is. Before discarding waste, think about what all it contains, and if there is some way that it could be used.
Down the drain vs. into the soil
The availability of these disposal methods varies depending on where you live. If you live in an apartment block, you only have the drain. If you live in a suburban house, you have both. If you live in some quaint rustic shithole like the author of this edit, you only have the "into the soil" option.
Generally, the soil option is safer, because there is no plumbing you should worry about, but some chemicals shouldn't be released into the soil, either. In general, your designated place for pouring chemicals into the soil should be far from any gardens, horticulture or any other agricultural objects. But chemicals marked as "Oh yes!" in the table are harmless and even useful in agriculture as fertilizers. These you can pour down in your garden.
Lists of various chemical compounds that can be safely released in the ground or down the drain can be found here, here or here.
Other
Heavy metal salts or solutions can be mixed with cement, and trapped in the resulted concrete block. The resulting block can also be covered in another layer of cement, to reduce the diffusion of the heavy metals. Unless the resulting concrete is damaged, broken and dissolved in acid, the heavy metal ions will not be released. However, this merely is a method to prevent the heavy metals from being released in the environment, and is not a permanent way of disposal.
