Chapter 12: Waste Disposal
12.1 Introduction to waste management
Practical work produces chemical waste. Some of these wastes may be harmful to people, property or the environment if not properly treated before disposal. Regardless of where the waste will go – a sink, a flower bed, a pit latrine – the following procedures should always be followed.
Note, often there are unused reagents at the end of a practical. These are valuable and should be stored for use on another day. When storing left over reagents, label the container with:
- The name of the compound, e.g. ”sodium hydroxide solution”
- The concentration, e.g. 0.1 M
- The date of preparation, e.g. 15 June 2010
- Important hazard information, e.g. ”CORROSIVE, neutralize spills with weak acid.”
Sometimes, there are used reagents that may be recycled. Recycling of chemicals reduces harm to the environment and saves money. Examples of chemical recycling are:
- Regenerating silver nitrate solution from qualitative analysis waste.
- Purification for reuse of organic solvents from distribution/partition law waste
In order to recycle these compounds, students must put their waste in designated containers. Specific instructions for chemical recycling follow in another section.
Some wastes may be discarded without worry. These solutions may be poured down a sink or into a pit latrine. These include:
- The final mixture in the flask after a titration. This is neutral salt water.
- All of the wastes from food tests in biology. Note that unused reagents are not waste! Finally, some wastes require special treatment. These wastes and their treatments follow.
12.2 Special instructions for certain wastes
12.2.1 Organic wastes
These are any substance that does not mix with water, for example kerosene, isobutanol, ether, chloroform, etc. These substances should be placed in an open container and left to evaporate downwind from people and animals. Setting these wastes on fire is usually unnecessary and may be dangerous.
12.2.2 Strong acids
Sulfuric, hydrochloric, and nitric acid solutions will corrode sinks and pipes if not neutralized before disposal. These wastes should be collected in a special bucket during a practical. After the practical, bicarbonate of soda should be added until further addition no longer causes effervescence. The gas produced is carbon dioxide.
12.2.3 Strong bases
Sodium and potassium hydroxide solutions as well as concentrated ammonia solutions are also corrosive. These wastes should be collected in a different special bucket during a practical. After the practical, the waste should be colored with POP or a local indicator and acid waste should be added until the color changes. If there is more base waste than acid waste available to neutralize it, citric acid may be added until the color finally changes.
12.2.4 Heavy metals
Barium, lead, silver and mercury solution are highly damaging to the environment and may poison human or animal drinking water if disposed without treatment. Waste containing barium and lead, generally from qualitative analysis, should be collected in a special container during a practical. After the practical, dilute sulfuric acid should be added drop-wise until further addition no longer causes precipitation. At this point, soluble lead and barium will have been converted to insoluble lead sulfate and barium sulfate. These salts may then be disposed in a pit latrine. The waste should of course first be neutralized with bicarbonate of soda.
Waste containing silver should be collected in a different special container. Ideally, this waste will be treated to regenerate silver nitrate solution according to the instructions in the next section. If such recycling is infeasible, sodium chloride solution should be added drop-wise until further addition no longer causes precipitation. At this point, soluble silver will have been converted to insoluble silver chloride and may be disposed in a pit latrine.
There is no treatment for mercury solutions that may be safely performed in a secondary school. This fact combined with the extreme danger of using mercury compound in schools supports the recom- mendation that mercury compounds never be used. If mercury waste is ever discovered at the school, it should be placed in a well-sealed bottle labelled: MERCURY WASTE. TOXIC. DO NOT USE. MUST NOT ENTER THE ENVIRONMENT. SUMU KALI. USITUMIE NA USIMWAGE.
12.2.5 Strong oxidizers
Concentrated solutions of potassium permanganate, chromate, dichromate, hypochlorites (bleach), and chlorates should be reduced prior to disposal. Grind ascorbic acid (vitamin C) tablets to powder and add until the permanganate decolorizes, chromate and dichromate turn green or blue, and hypochlorites lose their smell. The resulting solutions may be safely disposed in a sink or pit latrine.
12.2.6 Solid waste
Solids clog pipes and should never be put into sinks. If the solid is soluble, dissolve it in excess waste and treat as solution waste. If the solid is insoluble, dispose into a pit latrine.
12.2.7 Unknown compounds
If you do not know what a compound is, you do not know what kind of treatment it requires prior to disposal. That solution that looks like water could be nitric acid, or mercury chloride solution. Before disposing of unknown compounds, please use the Guide to Identifying Unknown Chemicals in the appendix. Even if you cannot identify the compound with these instructions, you can use them to ensure that it is not dangerous to dispose.