Option E: Environmental Chemistry: Water (HL)







Objective:

After this section you should be able to-

  • Solve problems relating to the removal of heavy-metal ions, phosphates and nitrates from water by chemical precipitation.
NOTE: Knowledge of Equilibrium is required.

Introduction

As stated earlier in the "Water Treatment" section, water is the most essential and one of the basic necessities of life.But currently, there is a very little amount of potable water available on earth to support lives. Not only do we humans use it just about every day, but every living thing needs it to live. "Waste Water" is any water that has been adversely affected in quality by anthropogenic influences (human influences). There are numerous examples of anthropogenic contaminants that can be found in water supplies. There are different means through which water is contaminated and have a variety of effects on human health and the quality of water supply. One of the reason of pollution is the presence of Heavy metals in water.

Heavy Metals

Heavy metals are serious water pollutants because the are toxic. The heavy metal ions of mercury, lead and cadmium interfere with the behaviour of other necessary ions in body such as
Ca2+ , Mg2+ and Zn2+

Even very small traces of heavy metals can have very significant harmful effects.

The table below summarizes the sources of each of these pollutants and their possible health and ways to reduce the pollutant-

Sources
Health hazard
Environmental hazard
Ways to reduce the pollutant
Mercury
  • Paints
  • Batteries
  • Agriculture
  • The most dangerous of the metal pollutant; causes serious damage to the nerves and the brain
  • Symptoms of mercury poisoning result from damage to the nervous system; depression, irritability, blindness and insanity
  • Minamata diseases
  • Reproductive system failure in fish
  • Inhibits growth and kills fish
  • Biological magnification in the food chain
  • Granular Activated Carbon
  • Lime-softening
  • Reverse-osmosis
Lead
  • Lead pipes
  • Lead paint and glazes
  • tetraethyl lead in petrol
  • Burning paints in the mouth and digestive system and followed by a constipation or diarrhoea
  • In severe cases there is a failure of the kidneys, liver, and heart which can lead to coma and death.
  • Can cause brain damage particularly in young children.
  • Toxic to plants and domestic animals
  • Biological magnification in the food chain.
  • Refrain from consuming water that has been in contact with your home's plumbing
Cadmium
  • Metal plating
  • rechargeable batteries
  • Pigments
  • By-products of zinc refining.
  • Replaces zinc in enzymes making then ineffective.
  • Itai-Itai disease makes bones brittle and easily broken.
  • Kidney and lung cancer in humans.
  • Toxic to fish
  • Produces birth defects in mice.
  • Coagulation/filtration
  • Distillation
  • Ion exchange
  • Reverse osmosis
CO
  • Incomplete combustion in internal combustion engines
  • Volcanic activity and forest fires, incomplete oxidation of methane formed by anaerobic decomposition of organic matter.
  • Replaces the oxygen carried in haemoglobin in RBCs, reducing oxygen availability and causing suffocation
  • An extremely dangerous gas because it inhibits the transport of oxygen to the blood.
  • Catalytic converters, the use of lean-burn engines.
NOx
  • Formed at high temperatures in internal combustion engines
  • Lightening strikes, denitrification in soils
  • Harmful to people already suffering from lung disease, can damage healthy lung tissue

  • Catalytic converters, the use of lean-burn engines.
SOx
  • Combustion of sulfur-containing coal, smelting of sulfide ores and roasting processes
  • Volcanic eruptions, sour natural gas, sea spray
  • Irritates the respiratory system

  • Wash coal prior to burning, usage of alkaline scrubbers or fluidized beds
VOCs
  • By-products of combustion in internal combustion engines, emitted from paints,cleaning supplies, building materials, furnishings
  • Anaerobic decomposition by bacteria
  • Sick-building' syndrome due to poor air quality
  • Inhibits growth
  • Catalytic converters, the use of green plants
Particulates
  • By-products of industrial processes, and incomplete combustion
  • Volcanic eruptions, forest fires
  • Decreased lung function, aggravation of asthma, development of bronchitis, emphysema and respiratory cancer.

  • Electrostatic precipitation
Since these heavy metal-ions are very dangerous and brings living thing's health at stake. It is extremely important to get rid of these metals to an appropriate extent, as soon as possible. Thus, Heavy metal ions and phosphate ions can be removed from water during the tertiary treatment of waste water precipitation.
The removal of metal ions from solution by chemical precipitation, in which the metal is removed as a sparingly soluble salt. The so-called insoluble salts such as silver chloride dissolve to a very small extent in water, so not all heavy metals will be precipitated out.
Hydrogen sulfide (H2S) can be used to remove heavy metal ions of heavy metals such as mercury:


Hg2+(aq) + H2S (aq) --> HgS (s) + 2H+ (aq)

mercury.gif
Figure 12.1.1 Mercury in watersheds and water-bodies



Similarly, heavy metal ions such as copper, cobalt and iron can be removed as hydroxides from water sewage and soil by chemical precipitaion:


Cu2+ (aq) + 2OH- (aq) --> Cu(OH)2 (s)

To help you understand!
The following general solubility rules help explain the concept of using chemical precipitation for the removal of heavy-metal ions and phosphates from water:

  1. All Carbonates, CO32- and phosphatesPO43-except those of Li+, Na+, K+ and NH4+are insoluble.
  2. All sulfides, S2- except those in Groups I,II and NH4+are insoluble.
  3. All hydroxides and oxides except alkali metals are insoluble (those of Sr2+,Ca2+ and Ba2+ are sparingly soluble).
  4. All Cl-, Br- and I- salts are water soluble except those of Ag+, Hg­22+ and Pb2+
  5. All sulfates are water soluble except those of Ca2+, Ba2+,Pb2+ and Sr2+
  6. Most nitrate (NO3-) salts are water soluble
  7. Most salts of Na+, K+, and NH4+ are water soluble.

Quantitative aspects of Solubility equilibria-

We can quantitatively determine the concentration of various chemical species, such as heavy-metal ions, nitrates and phosphates, when they form precipitates in water. In a reaction in which a precipitate is formed, the precipitate dissolves to some extent allowing a dynamic equilibrium to be established between the precipitate and dissolved ion. Suitabilities can therefore be expressed in terms of equilibrium constants. The equilibrium between solid silver chloride and its saturated solution is given:
AgCl (s) ↔Ag+(aq) + Cl-­(aq)
The equilibrium constant for this heterogeneous eqilibrium is called the solubility product, Ksp.
Ksp is called the solubility product constant (note: the concentration of solid does not appear because it is also a constant) and, because Ksp is temperature dependent, the particular temperature at which it is measured must be quoted. Thus, in a saturated solution of a slightly of a slightly soluble ionic compound, the product of the concentrations of the individual ions raised to the powers indicated by the coefficients is a constant at a given temperature. Precipitation depends on Ksp; a precipitate will form if the product of the ions in the mixture of solutions, when substituted into the Ksp expression, exceeds the numerical value of Ksp at that temperature.
Example:
Zinc(II) ions (Zn2+) can be removed by bubbling hydrogen sulfide through polluted water. The solubility product of zinc sulfide is 1.60 x 10-24 mol2dm-6 at 25
C.
Calculate the concentration of Zn2+ ions in a saturated solution of zinc sulfide.
-->
In a saturated solution: ZnS(s)<--> Zn2+(aq) + S2-(aq)
Ksp
[Zn2+][S2-]
1.60 x 10-24
When no other ions are present: [Zn2+] = [S2-]
[Zn2+]2 = 1.60 x 10-24
[Zn2+] = external image moz-screenshot-6.png1.26 x 10-12 mol dm-3

The example shows that an increase in the concentration of sulfide ions lead to a decrease in the solubility of the zinc ions in solutions. This is a general result known as a common ion effect.
Consider, the solubility of calcium phosphate:
Ca3(PO4)2(s) <--> 3Ca2+(aq) + 2PO43-(aq)
An increase in the either phosphate ions or calcium ions, that is ions common to the compound and the added solution, will - according to the Le Chatelier's Principle- shift the equilibrium to the left and decrease the solubility of the compound.

Summary:
Metal ions are removed from solution by chemical precipitation as a sparingly soluble salt.
Hg2+(aq) + H2S(aq) --> HgS(s) + 2H+(aq)

The insoluble sulfides can also be formed when a soluble sulfide is added.
Pb2+(aq) + S2- --> PbS(s)

Some metals can be removed as insoluble hydroxides on the addition of aqueous sodium hydroxide.
Cr3+(aq) + 3OH-(aq) --> Cr(OH)3(s)

Some metals can be removed as insoluble phosphates.
Al3+(aq) + PO43-(aq) --> AlPO4(s)

A dynamic equilibrium is set up between soluble and the aqueous ions. Consider, for example, mercury sulfide:
HgS(s) <--> Hg2+(aq) + S2-
The equilibrium constant for this reaction, Kc, can be deduced from the equilibrium law as:
{ [Hg2+ (aq)] [S2-(aq)] } / [HgS(s)]

Ksp = [Hg2+ (aq)] [S2-(aq)]

Given the equilibrium formed by a metal M and a non metal X:
MX(s) <--> M+(aq) + X-(aq)
The Keq for this system is given by Ksp = [M+][X-}, and is called the solubility product constant. Ksp depends only on temperature.



Bibliography-
1)Introduction- Cartin Brown, Mike Ford, (2009). Retrieved on April 5, 2011. From- Pearson Baccalaureate Chemistry Developed specifically for the IB Diploma.
2)Importance of water- N.A, (n.d). Retrieved on April 5, 2011. From- http://importanceofwater.org/12/importance-of-water-to-life/
3)Table 1- Cartin Brown, Mike Ford, 2009. Retrieved on April 5, 2011. From- Pearson Baccalaureate Chemistry Developed specifically for the IB Diploma.