Aqueous Environment

Last updated Jul 21, 2023 Edit Source

# ← Chemistry Home

Topic Covers: - Water treatment to maintain safety for humans and the environment - Concentration of solutions, and conversion between different units of concentration - Precipitation reactions as a method to separate dissolved substances - Acidity of solutions, including pH calculations, and neutralisation reactions to maintain water quality - Calculations of quantities of substance related to above concepts (not including limiting reagents)

Sources:

• Chapter 16.9 - 16.12 Lucarelli
• Chapter 15.1 “Essential Water” Pearson Chemistry

• Water is essential to humanity, however, while there is lots of water on Earth, only 2.5% is considered drinkable and only 0.5% of that water is actually accessible
  • Only 0.0125% of water on Earth is accessible and drinkable
• Water quality is essential due to its uses as drinking water, in agriculture and more
  • This means we need to find ways to maintain the quality of water we access
    • We need to purify our water so it is considered potable which can be done using various methods
• For this topic it is good to have an understanding of concentration

Potable Water

Potable water is water that is safe to consume. Potable water needs to be colourless, odourless and contain no pathogens or toxic substances. It is also important to minimise the total dissolved solids (TDS), which includes non-toxic ions and organic matter, in drinking water for good palatability.

• Water is a very effective solvent as seen from the ion-dipole attraction between many compounds and water
  • This can be a problem as water can be easily contaminated
  • Both chemical and biological substances can contaminate water

Chemical contaminants are the elements or compounds that can cause harm if consumed

The main types of chemical contaminants are: Heavy metals Pollutants and fertiliser Organic pollutants

The main sources of chemical contamination are: Run-off from farms and cities Run-off from industrial and mining wastes Lead used in solder in copper water pipes

• Heavy metals typically have severe negative health effects
  • e.g. cancer, brain, organ and nervous system damage and death
• Heavy metal contaminants are removed using precipitation reactions
  • A solution containing an anion that will react with the heavy metal cation to produce a precipitate is added
  • The precipitate is then filtered out of the water

• Water treatment involves transforming sourced water into potable water
  • It involves the removal of contaminants from the water
• Depending on the source water was acquired from, it must be treated differently to purify it
  • Here we’ll look at how different sources of water are treated
• The main sources for drinkable water are:
  • Reservoirs filled by run-off from rivers and streams
  • Water directly from rivers and lakes
  • Groundwater
  • Recycled water
  • Desalinated seawater

• Treatment can vary from city to city but the aim of purification is to remove suspended solids, pathogens, colour and odour from the water

• General water treatment includes four steps:

  • Flocculation
  • Settling or Sedimentation
  • Filtration
  • Chlorination

• The water may also undergo fluoridation (covered below)

• Flocculation

  • Process where small suspended particles in the water are made to join together to form larger, heavier particles
    • These heavier particles sink to the bottom of the water sample
  • Flocculation is achieved by adding alum $(Al_2(SO_4)_3)$ and, if necessary, lime $(Ca(OH)_2)$
    • $Al(OH)_3$ is a gelatinous precipitate known as floc that is produced from the addition of alum
    • This floc traps finer particles, and can remove colour/microorganisms from water, as they are absorbed

• Settling or Sedimentation

  • Suspended solids are removed using gravity
  • The floc from before settles at the bottom as a sludge and is removed

• Filtration

  • The water is filtered to remove any remaining suspended matter
  • Filtration occurs though a filter bed of sand over gravel

• Chlorination

  • The water is treated with chlorine gas to kill pathogens

• Distillation and reverse-osmosis are two approaches typically used to treat seawater
  • In distillation, pure water is evaporated from seawater
• Reverse-osmosis is the process of movement of solvent through a semipermeable membrane from the solution to the pure solvent 450
  • Pure water is pushed by high pressure through the barrier leaving the solute on the other side
  • The substance that goes through the barrier is called the permeate
    • The fresh water is the permeate

• Aeration
  • Groundwater often contains unwanted dissolved gases
    • Aeration removes these gases
  • Oxygen from the air is also incorporated into the water which helps remove dissolved organic compounds
• Clarification
  • Clarification removes suspended fine particles
  • Alum $(Al_2(SO_4)_2)$ and lime $(CaO)$ are added to the groundwater
    • Alum causes the particles to clump together and settle out
      • Alum is a coagulant or flocculating agent
• Sand Filtration
  • Water is passed through filter beds
    • Filter beds are made of granulated anthracite and coarse sand
    • Anthracite is very effective at adsorbing organic materials
• Disinfection
  • Pathogens may still be present in the groundwater and need to be removed
  • Chlorination or chloramination is used to disinfect the water (see above)
• Fluoridation
  • Fluoride ions in water boost tooth health, therefore a certain level of fluoride ions are maintained in the water
  • Fluoridation is the process of adjusting the level fluoride ions to the optimal (0.6-1.0 mg L$^{-1}$)
• pH
  • Water must be in a certain range for pH level (6.5 - 8.5)
  • The pH of the water is corrected by lowering or raising the pH
    • Lime (CaO) increases pH
    • Carbon dioxide lowers pH

• Water monitoring tests several properties:
  • e.g. pH, total dissolved solids, turbidity, bacterial and viral pathogens, heavy metals, etc.
• There are guidelines for the level/concentration that things are allowed to be at for the water to be considered safe
  • The guidelines in Australia are set by the National Health and Medical Research Council (NHMRC)
• Groundwater typically contains heavy metals
  • e.g. iron, aluminium and arsenic

• There are several protocols for water sampling:
  • The container can be rinsed with the sample before the final sample is taken
  • The container should be cleaned before taking another sample
  • The water used for cleaning the container can be tested to ensure there is no contamination between samples
• If pathogens are being tested for, the container should be sterile

Sampling Location

Its important to take samples from multiple locations for large bodies of water. The samples should be taken at the same depth and at a similar time to each other.

• When taking a sample, the depth the sample is taken from in the water body is decided by the temperature of the sample
• If the water body has consistent temperature, the sample is taken from the middle of the water body (half depth)
• If the water body has varying temperature, a sample is taken from the middle of each temperature region

A Van Dorn sampler can be used if the water being sampled is more than 2 metres deep or samples at different depths are required. Van Dorn samplers descend parallel to the bottom.

400