Heavy Metal Pollution

Hard water contains calcium and magnesium cations which can be removed using a suitable ion exchange resin.
Heavy metal ions such as cadmium, copper, chromium, mercury, nickel, lead and zinc, may also be present in water at unacceptable levels. This is referred to as heavy metal pollution of water.
Run off from roads is a large source of heavy metal contamination in water:

• Lead: leaded petrol (gasoline), tire wear, lubricating oil and grease, bearing wear
• Zinc: tire wear, motor oil, grease, brake emissions, corrosion of galvanized parts
• Iron: car body rust, engine parts
• Copper: bearing wear, engine parts, brake emissions
• Cadmium: tire wear, fuel burning, batteries
• Chromium: air conditioning coolants, engine parts, brake emissions
• Nickel: diesel fuel and petrol (gasoline), lubricating oil, brake emissions

Many of these heavy metals form negatively charged complex ions in water.
For example, chromium can form the chromate ion, CrO₄²⁻, and the dichromate ion, Cr₂O₇^2-.
Existing water treatment processes to remove these negatively charged ions can be inefficient and expensive.

Chemists at the University of California, Santa Cruz, have now developed a new type of material that can soak up negatively-charged pollutants from water. The new material called SLUG-26 contains copper hydroxide ethanedisulfonate.

Copper hydroxide ethanedisulfonate has a layered structure of positively-charged two-dimensional sheets with a high capacity for holding onto negative ions. These positively charged sheets are made up of [Cu4(OH)6]2+ ions as shown to the right. Copper atoms are shown in green and oxygen atoms are shown in red. The ethanedisulfonate exists as a counteranion.

This material could be used to treat polluted water through an ion exchange process similar to water softening. SLUG-26 provides a positively-charged substrate that can exchange a nontoxic negative ion for the negatively-charged pollutants.
The researchers are currently focusing on the use of SLUG-26 to trap the radioactive metal technetium, which is a major concern for long-term disposal of radioactive waste. Technetium is produced in nuclear reactors and has a long half-life of 212,000 years. It forms the negative ion pertechnetate in water and can leach out of solid waste, making groundwater contamination a serious concern.

Reference
Honghan Fei, Scott R. J. Oliver. Copper Hydroxide Ethanedisulfonate: A Cationic Inorganic Layered Material for High-Capacity Anion Exchange. Angewandte Chemie, 2011; DOI: 10.1002/ange.201104200

Further Reading
Periodic Table
Oxidation States
Zeolites

Suggested Study Questions

1. Give the chemical symbol for each of the following elements:
   • calcium
   • magnesium
   • cadmium
   • copper
   • chromium
   • mercury
   • nickel
   • lead
   • zinc
   • technetium
2. Identify which group of the periodic table each of the metals above belongs to.
3. Give the oxidation state for the metal atom in each of the following compounds:
   • CrO₄²⁻
   • Cr₂O₇^2-
   • [Cu₄(OH)₆]²⁺
4. Zeolites can be used to soften hard water. Write an equation to describe the process in which calcium ions in hard water are exchanged for sodium ions.
5. Suggest some possible non-toxic anions that could be exchanged for the toxic pertechnetate anion using SLUG-26.
6. Write an equation to show how the pertechnetate anion could be exchanged for one of the non-toxic anions in question 5.
7. Describe some other possible uses, apart from the removal of pertechnetate anions, for SLUG-26.