Chemical Waste Containers for Chemical Waste Disposal

Chemical Waste Containers

This is the companion document to the disposal advice provided within RiskAssess for each of the 3,000 chemicals and solutions in the chemical database.

Legal requirements and recommendations

If your school is subject to special regulations for disposal of liquid wastes (to sewer) or solid wastes (to garbage), you should follow these regulations rather than the general recommendations from RiskAssess. All wastes that cannot be legally disposed of to the sewer or garbage should be retained for collection by a waste collection service. Schools using sewer systems that discharge to inland rivers or to local septic systems should consider any special requirements.

The fundamental principle of chemical waste management is that wastes should be segregated, not mixed together. Segregated wastes can be dealt with easily and, in many cases, metals can be recycled. Mixed wastes cannot be recycled and must be sent to a ‘controlled landfill’, from which the leachate must be monitored and treated for decades. This is bad for the environment and expensive for the school.

The waste containers listed in this document are the ones most commonly used in a school. Not all the waste containers will be required by every school. Containers should come in and out of service as required. Sometimes a dedicated waste container not listed below should be set up, to deal with an unusual waste. This is indicated in the disposal advice for the chemical within RiskAssess.

Each container should have two labels:

a label to indicate the nature of the waste and safety information (e.g., large custom label printed within RiskAssess)
a large blank label, upon which the date, quantity and nature of each waste added to the container can be written by hand.

Ensure that waste containers of flammable liquids are stored in a flammable liquids cabinet!

Small quantities of some wastes may be safety poured down the drain, to the daily limit recommended in the disposal advice for the chemical in RiskAssess. Small quantities may sometimes be poured directly into a waste container partly filled with a receiving liquid that is chosen to precipitate or destroy the chemical. Larger quantities of liquid wastes should be collected in a dedicated container, to be dealt with later.

Some wastes (e.g., ecotoxic metals, non-halogenated organic waste) should never be poured down the drain and should be retained for collection by a waste service. In the case of ecotoxic metal wastes, precipitation of the metal will reduce the volume of solution/precipitate being stored and allow less frequent waste collections, saving money.

Other wastes can be either destroyed (e.g., acid waste + alkaline waste) or can be poured down the drain at the maximum daily rate recommended in RiskAssess, to spread the load on the sewage system and prevent microbial or sludge-contamination issues.

Depending on the school’s environmental policy, the time available and the skill of school staff, the quantity of waste requiring collection by a waste service can be greatly reduced.

Students should be aware of the need to pour wastes into the appropriate waste containers. Minimisation and safe disposal of wastes could be incorporated into learning activities.

Chemical waste processing

There are three stages in processing the chemical wastes in a laboratory:

collection
treatment, in some cases
disposal

Collection of chemical wastes

Sufficient waste containers (1 L to 25 L) are required to allow rapid and efficient collection of chemical wastes. Empty brown Winchester bottles of 2.25 L or 2.5 L capacity (e.g., empty acid bottles, rinsed and with label removed) are often used for waste collection, with a dedicated funnel inserted in the top. Glass or plastic containers of 1 L capacity can be used for collection of small volumes of uncommon wastes, while plastic cubes of 25 L capacity are convenient for large volumes of acid or alkaline waste. Organic wastes (non-halogenated, chlorinated, etc) should usually be collected in glass containers, since the wastes are likely to damage plastic.

Organic waste containers should be placed in a fume cupboard or in a well-ventilated location far from ignition sources, since the wastes are usually volatile and flammable. After a class has finished, the containers should be stored in a flammable liquids cabinet.

Treatment of chemical wastes

After wastes have been collected, some schools may choose to treat the wastes to reduce the quantity and save money. Other schools may simply retain all wastes for collection by a waste service. The decision to treat wastes should take into account the chemical wastes involved, the laboratory facilities and the skill and available time of staff.

Disposal of chemical wastes

Some wastes may be poured down the drain or placed in the garbage, up to the maximum daily quantity recommended below and in RiskAssess.

Wastes that exceed the maximum daily quantity should be retained in a waste container, then

if the ecotoxicity of the waste is low, poured down the drain, over a number of days, at the maximum recommended daily rate
treated in some way to make a more concentrated waste (for retention, then collection) or convert to a harmless waste (for disposal to the garbage), or
removed by a waste collection service.

Advice for specific chemical wastes

A description of the various waste containers, their use, and possible ways of treating some of the wastes is given below.

Acid waste container

Maximum daily quantity: 5-10 mL concentrated acid per day down the drain

Acid wastes with pH >4.5 may be poured down the drain.

Acid wastes from experiments usually have pH <4.5 and small amounts, up to the maximum daily quantity recommended for the acid, may be diluted, then poured down the drain. Maximum daily quantities for concentrated acids are: hydrochloric acid (10 mL of 12 M), nitric acid (10 mL of 16 M), sulfuric acid (5 mL of 18 M). Larger quantities should be placed in an acid waste container. Large quantities of acid waste may be generated in some experiments, requiring multiple waste containers of 2.25-2.5 L capacity or a 25 L plastic cube.

For further disposal advice, see Legal requirements and recommendations and Chemical waste processing. See Disposal of chemical wastes for a general explanation. See Labels for chemical waste containers for labelling advice and to download labels.

Treatment of hydrochloric acid and nitric acid waste

Dilute waste to a concentration <5 M, if more concentrated.

Half-fill a plastic bucket with (diluted) acid waste and add, slowly with stirring, solid calcium hydroxide until a residue of unreacted solid remains on the bottom. Allow the mixture to settle, then pour the supernatant down the drain. The solid residue (excess calcium hydroxide) may be placed in the garden as a soil conditioner, or placed in the garbage.

Treatment of sulfuric acid waste

Dilute waste to a concentration <3 M, if more concentrated.

Half-fill a plastic bucket with (diluted) acid waste and add, slowly with stirring, solid sodium carbonate (anhydrous or hydrated). Carbon dioxide gas will be vigorously evolved and a layer of froth may form on the top of the liquid. Continue slowly adding sodium carbonate until gas is no longer evolved, then pour the supernatant down the drain.

Treatment of acid waste with alkaline waste

Dilute acid waste to a concentration <3 M, if more concentrated.

Half-fill a plastic bucket with (diluted) acid waste and add, slowly with stirring, alkaline waste. If the alkaline waste is a solution of sodium hydroxide, no gas but considerable heat may be evolved. If the alkaline waste contains carbonate and hydrogen carbonate salts, carbon dioxide gas will be vigorously evolved and a layer of froth may form on the top of the liquid. Continue slowly adding alkaline waste until pH >4.5 (test with indicator paper), then pour the liquid down the drain.

Alkaline waste container

Maximum daily quantity: 100 g sodium hydroxide per day down the drain

Alkaline wastes with pH <10.5 may be poured down the drain.

Alkaline wastes from experiments usually have pH >10.5 and small amounts, up to the maximum daily quantity recommended for the alkali may be dissolved in water, poured down the drain. Larger quantities should be placed in an alkaline waste container. Large quantities of alkaline waste may be generated in some experiments, requiring multiple waste containers of 2.25-2.5 L capacity or a 25 L plastic cube.

Treatment of alkaline waste with hydrochloric acid

Dilute waste to a concentration <5 M, if more concentrated.

Half-fill a plastic bucket with (diluted) alkaline waste and add, slowly with stirring, 5 M hydrochloric acid. If the alkaline waste is a solution of sodium hydroxide, no gas but considerable heat may be evolved. If the alkaline waste contains carbonate or hydrogen carbonate salts, carbon dioxide gas will be vigorously evolved and a layer of froth may form on the top of the liquid. Continue slowly adding 5 M hydrochloric acid until pH <10.5 (test with indicator paper), then pour the liquid down the drain.

Treatment of alkaline waste with acid waste

Dilute alkaline waste to a concentration <5 M, if more concentrated.

Carry out the neutralisation and disposal by addition of acid waste (diluted to <5 M) as described above.

Antimony waste container

Maximum daily quantity: 1 g antimony salt per day down the drain

Antimony compounds are toxic to aquatic life with long lasting effects and concentrate in sewage sludge. Only tiny quantities (< 1 g/day) should be poured down the drain in a laboratory. Greater quantities should be placed in a waste container. Antimony compounds are used rarely in school laboratories and only in small quantities. Treatment of antimony waste is difficult. It is recommended that wastes be accumulated for many years, then collected by a waste service.

Bromide waste container

Maximum daily quantity: 10 g bromide salt per day (inland sewage system) or 100 g bromide salt per day (coastal sewage system) down the drain

Bromide ions are abundant in sea water, but may be toxic in river water and lake water, which provide less dilution of sewage effluent. Quantities of bromide in excess of the maximum recommended should be placed in a waste container and either poured down the drain at the maximum recommended daily rate, or retained for collection by a waste service.

Brominated organic waste container

Maximum daily quantity: zero

All brominated organic wastes should be placed in a waste container and none allowed to go down the drain or into the garbage. Since many brominated organic compounds are flammable, the waste container should be stored in a flammable liquids cabinet.

Brominated organic wastes should not be placed in a non-halogenated organic waste container. The reason is that non-halogenated wastes, after reprocessing, may be used as fuels, and brominated organic compounds form toxic substances during combustion, including brominated dioxins.

Small quantities of brominated organic waste (occasional <50 g) may be added to a chlorinated organic waste container, since chlorinated compounds and brominated compounds may be treated together by a waste service. If this is done, the label on the waste container should be changed to 'Halogenated organic wastes' and the quantities and identity of each waste recorded. Large quantities of brominated organic wastes should be kept in a separate brominated organic waste container, since other reprocessing options by the waste service may be available.

Brominated organic wastes should be retained for collection by a waste service, either separately (large quantities) or mixed with chlorinated organic wastes (small quantities).

Cadmium waste container

Maximum daily quantity: zero

Cadmium compounds are highly toxic to aquatic organisms They bioconcentrate up the food chain and are a critical contaminant in sewage sludge. No cadmium compounds should be released to the environment. Cadmium compounds are rarely used in schools, due to their extreme human toxicity.

All wastes containing cadmium should be retained for collection by a waste service.

Cerium waste container

Maximum daily quantity: 10 g cerium salt per day down the drain

Limited data indicate that cerium salts are toxic to aquatic organisms and concentrate in sewage sludge. Following the precautionary principle, only small amounts should be released into the environment each day. Cerium salts are not commonly used in school laboratories.

Cerium wastes in excess of the maximum amount recommended for daily release should be retained and either collected by a waste service or, if only small amounts are being retained, poured down the drain at the maximum daily quantity over a number of days.

Chlorinated organic waste container

Maximum daily quantity: zero

All chlorinated organic wastes should be placed in a waste container and none allowed to go down the drain or into the garbage. Since many chlorinated organic compounds are flammable, the waste container should be stored in a flammable liquids cabinet.

Chlorinated organic wastes should not be placed in a non-halogenated organic waste container. The reason is that non-halogenated wastes, after reprocessing, may be used as fuels, and chlorinated organic compounds form toxic substances during combustion, including chlorinated dioxins.

Small quantities of brominated, fluorinated and iodinated organic wastes (occasional <50 g) may be added to a chlorinated organic waste container, since halogenated wastes may be treated together by a waste service. If this is done, the label on the waste container should be changed to 'Halogenated organic wastes' and the quantities and identity of each waste recorded. Large quantities of chlorinated organic wastes should be kept in a separate chlorinated organic waste container, since other reprocessing options by the waste service may be available.

Chlorinated organic wastes should be retained for collection by a waste service, either separately (large quantities) or mixed with small quantities of other halogenated organic wastes, as 'Halogenated organic wastes'.

Chromate waste container

Maximum daily quantity: 10 g chromate salt per day down the drain

Chromium(IV) compounds (chromate and dichromate) are highly toxic to aquatic organisms, but are reduced by organic matter in sewage to chromium(III), which is relatively harmless in the environment. Small quantities may be safely poured down the drain at the maximum daily rate. Quantities in excess of the allowable daily quantity should be placed in a waste container and either retained for collection by a waste service, or treated as described below and the precipitate placed in the garbage. The treatment of chromate wastes is not recommended as a class experiment.

Treatment of chromate waste by reduction and precipitation

Part I: Reduction to chromium(III)

Half-fill a large beaker with chromate waste. Check that the solution is acid (use pH paper) and, if it is not, add dilute sulfuric acid (3 M) until it is. Add saturated iron(II) sulfate solution (~290 g/L) slowly, with stirring, to reduce chromium(IV) ions to chromium(III). The solution will become paler in colour as reduction proceeds and should be colourless when reaction is complete. Check for the absence of chromium(VI) by placing 2 mL of the waste liquid in a test tube and adding a drop of 2 M hydrogen peroxide solution - a blue colour indicates the presence of chromium(VI). If in doubt, add excess iron(II) sulfate solution to ensure completion of reaction.

Part II: Precipitation of chromium(III) hydroxide

Add, initially dropwise, then slowly with stirring, saturated sodium carbonate solution (~200 g/L). Depending on the quantity of acid in the solution, carbon dioxide gas may be vigorously evolved and a layer of froth may form on the top of the liquid. Continue adding saturated sodium carbonate solution until precipitation of hydrated chromium(III) hydroxide is complete. This can be tested by allowing the precipitate to settle and adding a few more drops of sodium carbonate solution. Heat the beaker and its contents nearly to boiling, with continuous stirring. Allow the mixture to cool and then to settle. Pour the clear supernatant down the drain. Pour the slurry of hydrated chromium(III) hydroxide into a shallow container to evaporate to dryness, then place it in the garbage.

Chromium(III) waste container

Maximum daily quantity: 10 g chromium(III) salt per day down the drain

Chromium(III) compounds have relatively low toxicity in the aquatic environment compared with chromium(VI) compounds (chromate and dichromate), due to the formation of hydroxy- and oxy- species with limited solubility in water. Chromium is regarded as a toxic contaminant of sewage sludge and its discharge to sewage should be minimised. Quantities in excess of the allowable daily quantity should be placed in a waste container and either retained for collection by a waste service, or treated to reduce the volume and allow disposal in the garbage. Alternatively, small quantities of chromium waste may be poured down the drain at the maximum daily rate.

Treatment of chromium(III) waste by precipitation

Half-fill a large beaker with chromium(III) waste and add, initially dropwise, then slowly with stirring, saturated sodium carbonate solution (~200 g/L). If acid is present in the solution, carbon dioxide gas will be vigorously evolved and a layer of froth may form on the top of the liquid. Continue adding saturated sodium carbonate solution until precipitation of hydrated chromium(III) hydroxide is complete. This can be tested by allowing the precipitate to settle and adding a few more drops of sodium carbonate solution. Heat the beaker and its contents nearly to boiling, with continuous stirring. Allow the mixture to cool and then to settle. Pour the clear supernatant down the drain. Pour the slurry of hydrated chromium(III) hydroxide into a shallow container to evaporate to dryness, then place it in the garbage.

Cobalt waste container

Maximum daily quantity: 1 g cobalt salt per day down the drain

As well as being highly toxic to humans, cobalt ions are extremely toxic to aquatic organisms and contaminate sewage sludge.

Tiny quantities of cobalt solutions used for spot tests on tiles or spot plates may be washed down the drain. Otherwise, all wastes containing cobalt should be poured into a cobalt waste container and retained for collection by a waste service, either before or after treatment to reduce the volume of material. Treatment of cobalt wastes should only be attempted if staff have adequate skills in chemistry and suitable facilities are available; otherwise, all cobalt wastes should be retained for collection by a waste service, without treatment. The treatment of cobalt wastes is not recommended as a class experiment.

Treatment of cobalt waste by precipitation

Half-fill a plastic bucket with cobalt waste and add, initially dropwise, then slowly with stirring, saturated sodium carbonate solution (~200 g/L). If acid is present in the solution, carbon dioxide gas will be vigorously evolved and a layer of froth may form on the top of the liquid. Continue adding saturated sodium carbonate solution until a volume has been added that is equal to the original volume of the waste. This creates a solution with high ionic strength and high carbonate ion concentration, both of which favour rapid settling of the hydrated cobalt carbonate. Allow the hydrated cobalt carbonate to settle and pour the clear supernatant down the drain. The slurry of hydrated cobalt carbonate should be retained for collection by a waste service.

Copper waste container

Maximum daily quantity: 1 g copper salt per day down the drain

Although copper ions have low human toxicity, they are extremely toxic to aquatic organisms and contaminate sewage.

Tiny quantities of copper solutions used for spot tests on tiles or spot plates may be washed down the drain. Otherwise, all copper wastes should be poured into a copper waste container. Copper wastes may be treated to reduce their volume by precipitation, redox reaction with an active metal, or electrolysis. All three processes might be used as class experiments.

Treatment of copper waste by precipitation

Half-fill a plastic bucket with copper waste and add, initially dropwise, then slowly with stirring, saturated sodium carbonate solution (~200 g/L). If acid is present in the solution, carbon dioxide gas will be vigorously evolved and a layer of froth may form on the top of the liquid. Continue adding saturated sodium carbonate solution until a volume has been added that is equal to the original volume of the waste. This creates a solution with high ionic strength and high carbonate ion concentration, both of which favour rapid settling of the basic copper(II) carbonate. Allow the basic copper(II) carbonate to settle and pour the clear supernatant down the drain. The slurry of basic copper(II) carbonate should be retained for collection for a waste service. Alternatively, the precipitate may be filtered off, added to water in a beaker, and 3 M sulfuric acid added dropwise at first, then slowly with stirring, to produce a copper sulfate solution that can be used for class experiments (even though it contains some sodium sulfate and excess acid). The solution may be evaporated to obtain copper sulfate pentahydrate by crystallisation.

Treatment of copper waste by redox reaction with an active metal

Half-fill a plastic bucket with copper waste and add, initially dropwise, then slowly with stirring, 3 M sulfuric acid until the solution is acid and all precipitated material has dissolved. Carry out the operation in a fume cupboard if you suspect that any copper sulfide is present, since toxic hydrogen sulfide will be evolved. If insoluble material remains, allow it to settle, and decant the clear supernatant solution. Add steel wool (not stainless-steel wool). Metallic crystals of copper will grow out from and through the steel surfaces and some of the iron will dissolve. Some hydrogen gas may also be evolved, depending on the amount of acid present in the waste mixture, so carry out the reaction remote from ignition sources. When reaction is complete, pour the clear supernatant down the drain, leaving behind the crystals of copper and unreacted steel wool, which may be put in the garbage together. Alternatively, wash the copper crystals from the pieces of unreacted steel wool, collect the copper crystals, and wash them again with water. The copper crystals may be dissolved in dilute nitric acid to make a copper nitrate solution that can be used for school experiments.

Treatment of copper waste by electrolysis

Half-fill a plastic bucket with copper waste and add, initially dropwise, then slowly with stirring, 3 M sulfuric acid until the solution is acid (test with pH paper) and all precipitated material has dissolved. Carry out the operation in a fume cupboard if you suspect that any copper sulfide is present, since toxic hydrogen sulfide will be evolved. If insoluble material remains, allow it to settle, and decant the clear supernatant solution into a large beaker.

Insert a carbon electrode (anode) and a copper electrode (cathode) into the decanted solution. A DC power supply may be used to supply the energy. Copper crystals will form at the copper cathode and fall to the bottom of the bucket. Oxygen gas will be evolved and the carbon will be corroded at the carbon anode. Continue electrolysis until no further copper is precipitated. Remove the electrodes and allow the copper crystals to settle. Pour the clear supernatant down the drain. The copper crystals may be dissolved in dilute nitric acid to make a copper nitrate solution that can be used for school experiments.

Fluoride waste container

Maximum daily quantity: 1 g fluoride salt per day (inland sewage system) or 10 g fluoride salt per day (coastal sewage system) down the drain

Although fluorine is an essential trace element and fluoride ions are added to drinking water at the rate ~1 mg/L, fluoride ions are toxic at high concentrations, the problem being greater with sewage effluent that discharges into inland waters than into the ocean.

Quantities of fluoride waste in excess of the maximum recommended should be placed in a waste container and either poured down the drain at the maximum recommended daily rate over a number of days, treated with calcium ions to precipitate fluoride ions, or retained for collection by a waste service.

Treatment of fluoride waste with calcium chloride

Half-fill a plastic bucket with fluoride waste and add, slowly with stirring, a half-saturated solution of calcium chloride (~350 g/L) until no further precipitation of calcium fluoride occurs. This can be tested by allowing the precipitate to settle and adding another drop of calcium chloride solution. When precipitation is complete, allow the precipitate to settle and pour the supernatant down the drain. The residue of calcium fluoride is geologically stable and may be placed in the garden or in the garbage.

Combined collection and treatment of fluoride waste

Small quantities of fluoride waste may be poured into a container half-filled with half-saturated calcium chloride solution (~350 g/L) and the calcium fluoride allowed to settle to the bottom. The container might serve for a number of years before it is necessary to treat the sediment as described above.

Fluorinated organic waste container

Maximum daily quantity: zero

All fluorinated organic wastes should be placed in a waste container and none allowed to go down the drain or into the garbage. Since many fluorinated organic compounds are flammable, the waste container should be stored in a flammable liquids cabinet.

Fluorinated organic wastes should not be placed in a non-halogenated organic waste container. The reason is that non-halogenated wastes, after reprocessing, may be used as fuels, and fluorinated organic compounds form toxic substances during combustion, including hydrogen fluoride.

Small quantities of fluorinated organic waste (occasional <50 g) may be added to a chlorinated organic waste container, since chlorinated compounds and fluorinated compounds may be treated together by a waste service. If this is done, the label on the waste container should be changed to 'Halogenated organic wastes' and the quantities and identity of each waste recorded. Large quantities of fluorinated organic wastes should be kept in a separate fluorinated organic waste container, since other reprocessing options by the waste service may be available.

Fluorinated organic wastes should be retained for collection by a waste service, either separately (large quantities) or mixed with chlorinated organic wastes (small quantities).

Iodide waste container

Maximum daily quantity: 1 g iodide salt per day (inland sewage system) or 10 g iodide salt per day (coastal sewage system) down the drain

Iodide ions and iodine are toxic to aquatic organisms. Quantities of iodide in excess of the maximum recommended should be placed in a waste container and either poured down the drain at the maximum recommended daily rate over a number of days, treated to recover iodine or retained for collection by a waste service.

Treatment of iodide waste

Half-fill a 2 L beaker with clear iodide waste (filter the waste to remove suspended particles, if necessary) and add, slowly with stirring, a solution of sodium hypochlorite (1-2 M, liquid bleach, 5-16% available chlorine) until no further precipitation of iodine occurs. This can be tested by allowing the precipitate to settle and adding another drop of sodium hypochlorite solution. When precipitation is complete, allow the precipitate to settle, pour the bulk of the supernatant down the drain, then filter off the solid iodine. The iodine may be reused within the school for further experiments.

Iodinated organic waste container

Maximum daily quantity: zero

All iodinated organic wastes should be placed in a waste container and none allowed to go down the drain or into the garbage. Since many iodinated organic compounds are flammable, the waste container should be stored in a flammable liquids cabinet.

Iodinated organic wastes should not be placed in a non-halogenated organic waste container. The reason is that non-halogenated wastes, after reprocessing, may be used as fuels, and iodinated organic compounds form toxic substances during combustion, including hydrogen iodide.

Small quantities of iodinated organic waste (occasional <50 g) may be added to a chlorinated organic waste container, since chlorinated compounds and iodinated compounds may be treated together by a waste service. If this is done, the label on the waste container should be changed to 'Halogenated organic wastes' and the quantities and identity of each waste recorded. Large quantities of iodinated organic wastes should be kept in a separate iodinated organic waste container, since other reprocessing options by the waste service may be available.

Iodinated organic wastes should be retained for collection by a waste service, either separately (large quantities) or mixed with chlorinated organic wastes (small quantities).

Lead waste container

Maximum daily quantity: zero

Lead compounds are highly toxic to aquatic organisms. They bioconcentrate up the food chain and are a critical contaminant in sewage sludge. No lead compounds should be released to the environment. Spot reactions with lead solutions should be carried out on dedicated tiles or spot plates, then all material rinsed, with a wash bottle, into a lead waste container.

All wastes containing lead should be retained for collection by a waste service, either before or after treatment to reduce the volume of material. Treatment of lead wastes should only be attempted if staff have adequate skills in chemistry and suitable facilities are available; otherwise, all lead wastes should be retained for collection by a waste service, without treatment. The treatment of lead wastes is not recommended as a class experiment.

Treatment of lead waste by precipitation

Half-fill a plastic bucket with lead waste and add, initially dropwise, then slowly with stirring, saturated sodium carbonate solution (~200 g/L). If acid is present in the solution, carbon dioxide gas will be vigorously evolved and a layer of froth may form on the top of the liquid. Continue adding saturated sodium carbonate solution until a volume has been added that is equal to the original volume of the waste. This creates a solution with high ionic strength and high carbonate ion concentration, both of which favour rapid settling of the lead carbonate. Allow the lead carbonate to settle and pour the clear supernatant down the drain. The slurry of lead carbonate should be transferred to a waste container and retained for collection for a waste service. If only small amounts of lead waste are being generated, they may be poured into a waste container half-filled with saturated sodium carbonate solution (~200 g/L). Such a waste container may be used for several years, with the lead carbonate being allowed to settle and the supernatant being decanted periodically down the drain and replaced with further saturated sodium carbonate solution.

Manganese waste container

Maximum daily quantity: 10 g manganese salt per day down the drain

Manganese compounds are toxic to aquatic organisms and contaminate sewage sludge. Manganese wastes in excess of the maximum daily amount recommended for pouring down the drain should be retained and either collected by a waste service, or treated and placed in the garbage.

Treatment of manganese waste by precipitation

Half-fill a plastic bucket with manganese waste and add, initially dropwise, then slowly with stirring, saturated sodium carbonate solution (~200 g/L). If acid is present in the solution, carbon dioxide gas will be vigorously evolved and a layer of froth may form on the top of the liquid. Continue adding saturated sodium carbonate solution until a volume has been added that is equal to the original volume of the waste. This creates a solution with high ionic strength and high carbonate ion concentration, both of which favour rapid settling of the manganese(II) carbonate. Allow the manganese(II) carbonate to settle and pour the clear supernatant down the drain. The slurry of manganese(II) carbonate should be dried in the air and placed in the garbage.

Mercury(II) waste container

Maximum daily quantity: zero

Mercury(II) compounds are highly toxic to aquatic organisms. They bioconcentrate up the food chain and are a critical contaminant in sewage sludge. No mercury(II) compounds should be released to the environment. Mercury(II) compounds are rarely used in schools, due to their extreme human toxicity.

All wastes containing mercury(II) should be retained for collection by a waste service.

Molybdenum waste container

Maximum daily quantity: 1 g molybdenum salt per day down the drain

Molybdenum compounds are not used in large quantities in schools, but are toxic to aquatic organisms and contaminate sewage sludge. Molybdenum wastes usually contain molybdate ions and can be converted to insoluble ammonium phosphomolybdate, which has low toxicity.

Treatment of molybdenum waste by precipitation

Half-fill a beaker with molybdenum waste and add excess sodium dihydrogen phosphate solution (2 M), then slowly with stirring, add concentrated nitric acid until precipitation of yellow ammonium phosphomolybdate is complete. Allow the precipitate to settle and pour the clear supernatant down the drain. Dry the slurry of ammonium phosphomolybdate and place it in the garbage.

Nickel waste container

Maximum daily quantity: 1 g nickel salt per day down the drain

As well as being highly toxic to humans, nickel ions are extremely toxic to aquatic organisms and contaminate sewage sludge.

Tiny quantities of nickel solutions used for spot tests on tiles or spot plates may be washed down the drain. Otherwise, all nickel wastes should be poured into a nickel waste container. All wastes containing nickel should be retained for collection by a waste service, either before or after treatment to reduce the volume of material. Treatment of nickel wastes should only be attempted if staff have adequate skills in chemistry and suitable facilities are available; otherwise, all nickel wastes should be retained for collection by a waste service, without treatment. The treatment of nickel wastes is not recommended as a class experiment.

Treatment of nickel waste by precipitation

Half-fill a plastic bucket with nickel waste and add, initially dropwise, then slowly with stirring, saturated sodium carbonate solution (~200 g/L). If acid is present in the solution, carbon dioxide gas will be vigorously evolved and a layer of froth may form on the top of the liquid. Continue adding saturated sodium carbonate solution until a volume has been added that is equal to the original volume of the waste. This creates a solution with high ionic strength and high carbonate ion concentration, both of which favour rapid settling of the nickel carbonate. Allow the nickel carbonate to settle and pour the clear supernatant down the drain. The slurry of nickel carbonate should be retained for collection by a waste service.

Non-halogenated organic waste container

Maximum daily quantity: zero

Non-halogenated organic wastes principally comprise hydrocarbons, alcohols and ketones. The waste is usually distilled to obtain mixtures with different boiling-point ranges, which are sold as a solvent for cleaning or burnt as a fuel.

It is critical that halogenated compounds (especially chloro-compounds and bromo-compounds) are not present, since they form highly toxic chlorodioxins and bromodioxins and other toxic compounds upon combustion.

Some non-halogenated organic compounds, e.g., acetone, dissolve or cause swelling of many plastics. It is advisable to use a glass container, if the effect of the organic compounds on the plastic of a container is not known.

All wastes containing non-halogenated organic compounds should be retained for collection by a waste service. Do not pour the waste down the drain, since it might form an explosive air/vapour mixture in the sewer pipe.

Silver waste container

Maximum daily quantity: 1 g silver salt per day down the drain

Silver ions are highly toxic to aquatic organisms and contaminate sewage sludge. Since silver is valuable, it should be recovered and recycled, whenever possible.

Tiny quantities of silver solutions used for spot tests on tiles or spot plates may be washed down the drain. Otherwise, all silver wastes should be poured into a silver waste container.

Silver wastes usually comprise precipitated particles of silver halides (especially AgCl) and solutions containing silver ions, and may be treated to reduce their volume.

Treatment of silver waste

A silver waste container usually contains a precipitate at the bottom (silver halides, etc) and a clear supernatant (silver ions, plus other ions). It may also contain ammonia or other amines that form water-soluble complexes with silver ions. When sufficient waste has accumulated (maybe several years), add a drop of 5 M nitric acid to the waste container. If a precipitate forms, continue adding the acid dropwise until no further precipitation occurs (test by allowing the precipitate to settle and add another drop of acid. Allow the precipitate to settle. Pour the clear supernatant into a large beaker and add a few pieces of zinc metal (e.g., granules). Shiny metallic crystals of silver will grow out from the zinc surface and some of the zinc will dissolve. Some hydrogen gas may also be evolved, depending on the amount of acid present in the waste mixture, so carry out the reaction remote from ignition sources. When reaction is complete, pour the clear supernatant down the drain, leaving behind the crystals of silver and unreacted zinc. Remove the pieces of unreacted zinc with forceps and wash the remaining crystals of silver metal by adding water and allowing the mixture to settle and again decanting the supernatant. The silver crystals may be dissolved in 3 M nitric acid to make a silver nitrate solution that can be reused for school experiments. Recovery and recycling of silver from the supernatant solution might be used as a class experiment. The precipitate in the silver waste container should be left there, and more wastes should be added to the container. After many years, the precipitate may be sold (if there is sufficient) or removed by a waste service.

Vanadium waste container

Maximum daily quantity: 10 g vanadium salt per day down the drain

Vanadium compounds are not used in large quantities in schools, but are toxic to aquatic organisms and contaminate sewage sludge. The toxicity of vanadium in aquatic systems is not well understood, so the precautionary principle should be followed.

Vanadium waste should be retained for collection by a waste service or poured down the drain at no more than the maximum rate. Vanadium waste may be reacted with iron(III) to form iron(III) vanadate, which has low solubility in water and may be placed in the garbage.

Zinc waste container

Maximum daily quantity: 1 g zinc salt per day down the drain

Although zinc ions have low human toxicity, they are extremely toxic to aquatic organisms and contaminate sewage sludge.

Tiny quantities of zinc solutions used for spot tests on tiles or spot plates may be washed down the drain. Otherwise, all zinc wastes should be poured into a zinc waste container. Zinc wastes may be treated to reduce their volume by precipitation or electrolysis. Both processes might be used as class experiments.

Treatment of zinc waste by precipitation

Half-fill a plastic bucket with zinc waste and add, initially dropwise, then slowly with stirring, saturated sodium carbonate solution (~200 g/L). If acid is present in the solution, carbon dioxide gas will be vigorously evolved and a layer of froth may form on the top of the liquid, depending on the presence of surface-active substances. Continue adding saturated sodium carbonate solution until a volume has been added that is equal to the original volume of the waste. This creates a solution with high ionic strength and high carbonate ion concentration, both of which favour rapid settling of the zinc hydroxide carbonate. Allow the zinc hydroxide carbonate to settle and pour the clear supernatant down the drain. The slurry of zinc hydroxide carbonate should be retained for collection for a waste service. Alternatively, the precipitate may be filtered off, added to water in a beaker, and 3 M sulfuric acid added dropwise at first, then slowly with stirring, to produce a zinc sulfate solution that can be used for class experiments (even though it contains some sodium sulfate and excess acid). The solution may be evaporated to obtain zinc sulfate heptahydrate by crystallisation, for use in further class experiments.

Treatment of zinc waste by electrolysis

Half-fill a plastic bucket with zinc waste and add, initially dropwise, then slowly with stirring, 3 M sulfuric acid until the solution is acid (test with pH paper) and all precipitated material has dissolved. Carry out the operation in a fume cupboard if it is suspected that any zinc sulfide is present, since toxic hydrogen sulfide will be evolved. If insoluble material remains, allow it to settle, and decant the clear supernatant solution. Insert a carbon electrode (anode) and a zinc electrode (cathode) into the supernatant solution. A DC power supply may be used to supply the energy. Zinc metal crystals will form at the zinc cathode and fall to the bottom of the bucket. Oxygen gas will be evolved and the carbon will be corroded at the carbon anode. Continue electrolysis until no further zinc is precipitated. Remove the electrodes and allow the zinc crystals to settle. Pour the clear supernatant down the drain. The zinc crystals may be washed with water, then used for further class experiments.

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