Part 3: Microbiology of Metals and Metalloids

Question 1

Which groups of organisms are able to aerobically oxidise metal species?

Answer 1

Lithotrophic bacteria and archaea.

Question 2

Why is oxidation of metals possible without organic matter being available?

Answer 2

Metal-oxidising organisms are usually autotrophic and can therefore oxidise metal ores wherever they are exposed to air (oxygen)..

Question 3

What are the two major problems associated with oxidation of metals and metalloids by bacteria/archaea?

Answer 3

1) Acid formation e.g. from pyrite or from sulphur oxidation

2) Mobilisation of toxic metals and metalloids

Question 4

Explain the problem of acid formation from metals/metalloids during oxidation.

Answer 4

Most metals become very soluble in low-pH conditions. When pyrite is oxidised to Iron Hydroxide, 8H+ is produced per O2.
When H2S is oxidised, sulphuric acid is produced.

Question 5

Explain the problem of mobilisation of toxic metals and metalloids during oxidation.

Answer 5

Many metals or metalloids become soluble when in an oxidised state, making them highly mobile - e.g. Uranium, Chromium, Arsenic.

(does not apply to iron or manganese.)

Question 6

Explain why bacteria are required for oxidation of metals and metalloids.

Answer 6

Oxidation of metal sulphides in air is a very slow process. Lithotrophic metal and sulphur oxidising bacteria speed up the process. e.g. Acidithobacillus ferroxidans.

Question 7

Which metal sulphides are preferentially utilised by bacteria?

Answer 7

Metals that react quicker in air (without) bacteria are used since these are easier to degrade and less energy must be exerted by the bacteria.

i.e. FeS > CuS > PbS

Question 8

When is bacterial ore leaching used in industry?

Answer 8

When the metal content of an ore is too low for conventional methods to be effective

e.g. if copper ore is less than 1% Cu e.g. Covellite

Question 9

Describe the industrial process of bacterial ore leaching.

Answer 9

1) Low grade ore is crushed to increase surface area
2) Acid leachate water is percolated through the crushes ore
3) The leachate is then so acid-rich that it easily precipitates and can be harvested.

Question 10

How does oxidised Uranium become a threat to drinking waters?

Answer 10

Uranium ores dumped as waste are oxidised at the surface by air and bacteria such as A. ferooxidans. When oxidised from +4 to +6, Ur becomes soluble and can reach drinking waters.

Note that Ur is not directly oxidised by bacteria, instead bacteria oxidise Fe2+ to Fe3+ which then chemically oxidises Ur.

Question 11

List 5 metals/oids that can be microbiologically oxidised.

Answer 11

1) Uranium
2) Arsenic
3) Chromium
4) Silver
5) Colbalt

Question 12

Which two metals are insoluble when oxidised but soluble when reduced?

Answer 12

Iron and Manganese.

Question 13

Explain the reciprocating cycle in pyrite oxidation.

Answer 13

At low pH, Iron(II) is stable, but can be oxidised to Iron(III) by bacteria. Iron(III) is then able to react with pyrite to release more Iron(II) which can be oxidised and so on.

Question 14

Which organisms conduct bacterial metal reduction and under what conditions is this possible?

Answer 14

Anaerobic heterotrophs can reduce metals in the absence of oxygen.

Question 15

Describe chromate reduction.

Answer 15

Performed by Micrococcus spp. CrO4(2-) –> Cr(OH)3. Product is less soluble and less toxic.

Question 16

Describe Selenium reduction.

Answer 16

Performed by Bacillus spp. SeO4(2-) –> Se(o). Product is insoluble. Required a modified amino acid called selenocysteine.

Question 17

Describe Uranium reduction.

Answer 17

Performed by Desulfovibrio spp. UO2(2+) –> UO2. Product is insoluble.

Question 18

Why is Mercury particularly problematic in the environment?

Answer 18

Most forms of Mercury are highly toxic, and high concentrations are found in places like the Florida Everglades. The only known method of Hg immobilisation is precipitation as HgS. However, because of the low redox potentional (0.8 mV) it is not preferable for bacteria to do so. Hg is more likely to be converted to the dangerous CH3Hg+.

Question 19

Why is Arsenic particularly problematic in the environment?

Answer 19

Easily soluble and toxic and creates groundwater issues in South Asia. As is mobilised when bacterially-produced Iron(III) oxidises arsenopyrite (FeAsS). Most forms are volatile and some are volatile methylated forms. Only reliable method of As removal is ppt as As2S3 (arsenic sulphate). Desulfosporosinus spp. can use arsenic and sulphate (e acceptor) for anaerobic respiration. However, sulphate is often lacking, so method is unreliable.

Question 20

Why are organometal compounds particularly problematic in nature? Give an example.

Answer 20

Organometals and particularly problematic since they act as a pathway for heavy metals into the cell.

e.g. tributyl-tin (used as antifoul) - degrades extremely slowly since the metal-covalent bonds are unnatural.

Question 21

How can steel be protected from corrosion?

Answer 21

Can be connected to a metal with lower redox potential, causing this other metal to lose electrons instead. (i.e. sacrificial cathode).

Question 22

Why is Arsenic particularly problematic in the environment?

Answer 22

Easily soluble and toxic and creates groundwater issues in South Asia. As is mobilised when bacterially-produced Iron(III) oxidises arsenopyrite (FeAsS). Most forms are volatile and some are volatile methylated forms. Only reliable method of As removal is ppt as As2S3 (arsenic sulphate). Desulfosporosinus spp. can use arsenic and sulphate (e acceptor) for anaerobic respiration. However, sulphate is often lacking, so method is unreliable.

Question 23

Why are organometal compounds particularly problematic in nature? Give an example.

Answer 23

Organometals and particularly problematic since they act as a pathway for heavy metals into the cell.

e.g. tributyl-tin (used as antifoul) - degrades extremely slowly since the metal-covalent bonds are unnatural.

Question 24

How can steel be protected from corrosion?

Answer 24

Can be connected to a metal with lower redox potential, causing this other metal to lose electrons instead. (i.e. sacrificial cathode).