The use of ion exchange in the treatment of industrial waste

Industrial waste contains a lot of heavy dissolved metal ions which should be removed before waste is introduced to rivers etc.. treated by ion exchange. The liquid is passed over resin(insoluble compound) beds. The cations and anions in waste react with resin and bind to resin and they are removed from waste as shown:

i) resin-OH + M⁺ → R-O^–M⁺ + H⁺
ii) R-OH + A^– → R-A + OH^–
The ions liberated can react together to form water as shown : iii) OH^– + H⁺ → H₂O

The resin beds are continuously replaced or they are recycled by reacting them with acids eg sulphuric acid or alkalis eg sodium hydroxide. The safe waste can then be allowed to enter into the rivers…

Methods of solid waste disposal

• solid waste is produced from domestic or industrial wastes can be disposed by various methods:

i) Land filling and dumping

• involves the dumping of solid waste on open land.
• Waste is usually covered by soil to promote decomposition of waste by bacteria.
• Initially the decomposition of waste occurs by anaerobic means , however as waste is decomposing aerobic conditions occur.
• Landfilling must be correctly managed because organic acids are produced which can carry heavy metal cations and leach out to the surrounding environment where it remain persistant as shown:
• R-COOH + M⁺ → RCOO^–M⁺ + H⁺.
• As waste is decomposing under anaerobic conditions gases eg CH₄(explosive and a greenhouse gas) , H₂S are produced because anaerobic process favours reduction.
• Non-biodegradable materials eg plastics cause pollution.

Leachate

• Black liquid that oozes from refuse tips when water percolates through.
• It contains partly decomposing organic matter and metal ions ,some of which are hazardous.

ii) Incineration

• involves the combustion of solid waste.
• Complete combustion of solid waste occur at a temperature of 800-100 degrees celcius.
• However this process involves liberation of large amounts of carbon dioxide(greenhouse gas).
• To prevent large amounts of carbon dioxide emissions during incineration, waste is burnt under anaerobic conditions at temperature of 850-1050, pyrolsis.
• Temperatures higher than 1050can lead to production of stable ,non-degradable organic compounds called dioxines and dibenzofurans which persist in the environment.
• Dioxines cause liver, kidney diseases and birth defects if they enter food chain.

 

iii) Recycling

• involve smoking of new material from used material eg paper, plastics iron, steel, aluminium etc…

 

Advantages of recycling

• it reduces costs of waste disposal
• reduction int the demand for new landfill sites
• reduction in environmental damage
• there may be substantial savings in energy costs e.g recycling Al
• reduce the amounts of heavy metal ions in the environment.

 

Difficulties of recycling

• produces low grade materials eg recycled paper
• sometimes expensive eg recycling of paper involves de-inking
• recycling plastic waste from mixed domestic refuse is too expensive

 

iv) Dumping at sea and in rivers

• Dumping of waste eg sewage and oils accurs at sea.

 

Advantages

• it provides a source of nutrients for aquatic plants and animals

 

Disadvantages

• oils contains toxic substances eg benzene which is cosmogenic and cause liver and kidney diseases when taken by animals
• it destroys ecological areas eg beaches
• sea birds have their wings flappy, due to oils and can not fly and eventually die
• oils can cause ground water contamination which can affect animals when they enter in the food chain
• eutrophication.

 

The problems associated with heavy metals in the environment

• when mercury is released into the environment during mining activities undergoes a series of reactions which include oxidation methylation ( Hg → Hg²⁺)
• conversion of methane forming bacteria in the environment to form methylmercury, CH₃Hg
• methylmercury is toxic and can enter food chain via drinking water and cause damage to the nerve
• Chromium obtained from mining activities, mined as chromate which is a mixture of chromate, iron and carbon dioxide
  • they are poisonous and can damage liver and kidney disease
  • during extraction heavy metals are also produced eg iron
• lead compounds causes brain damage in children and mental depression in adults

 

The problems associated with the disposal of radioactive waste

• strontium-90 accumulates in the bones to cause cancer and tissue degeneration in number of organs
• radiactive iodine may cause cancer of thyroid glands
• Uranium causes skin cancers and tumour to miners
• Radon-222 causes leukemia, brain tumour and kidney cancers
• cesium-137 brings about nervous, muscular and genetic change
• these may cause ionization of various body fluids , chromosomal aberrations and gene mutations