17. The chemistry of oxidation of pyrites, the production of ferrous ions and
subsequently ferric ions, is very complex, and this complexity has
considerably inhibited the design of effective treatment options. Although a
host of chemical processes contribute to AMD, pyrite oxidation is by far the
greatest contributor. General equations for this process are:
2FeS2 + 7O2+ 2H2O → 2Fe2+ + 4SO4
2- + 4H+
The oxidation of the sulfide to sulfate solubilizes the ferrous iron (iron(II)),
which is subsequently oxidized to ferric iron (iron(III)):
4Fe2+ + O2 + 4H+ → 4Fe3+ + 2H2O
Either of these reactions can occur spontaneously or can be catalyzed by
microorganisms that derive energy from the oxidation reaction. The ferric
irons produced can also oxidize additional pyrite:
FeS2 + 14Fe3+ + 8H2O → 15Fe2++ 2SO4
2- + 16H+
The net effect of these reactions is to release H+, which lowers the pH and
maintains the solubility of the ferric ion.
18. When pyrite is enclosed within
rock, only minimal amounts of
pyrite are oxidized through natural
weathering, thereby generating
only small amounts of acid. When
fully exposed to air and water, the
chemical reactions forming AMD
occur at a faster rate.
When acid levels are high, the pH is
low. In low pH conditions, the metals are dissolved and leave
no physical indication of their presence.
34. Anaerobic Digestion
Anaerobic digestion (AD) is a biological process that breaks down organic
materials (feedstocks) in the absence of oxygen (anaerobic conditions) into
methane (CH4) and carbon dioxide (CO2).
It is a process that occurs naturally in bogs, lake sediments, oceans, and
digestive tracts.
Cows contain one of the most well known fermentation vats, the rumen,
which is part of the stomach (in other animals as well). Fermentation takes
place during digestion
Anaerobic degradation
35. Anaerobic degradation phases
There are four basic
phases of anaerobic
digestion, which is a
synergistic process
using anaerobic
microorganisms:
1) hydrolysis,
2) acidogenesis,
3) acetogenesis, and
4) methanogenesis.
36. Hydrolysis is a reaction with water. Acid and base can be used to accelerate the
reaction. However, this occurs in enzymes as well. Through hydrolysis reaction,
cellulose, starch, and simple sugars can be broken down by water and enzymes.
In anaerobic digestion, the enzymes are exoenzymes (cellulosome, protease,
etc.) from a number of bacteria, protozoa, and fungi (see Reaction1).
(1) biomass + H2O → monomers + H2
During acidogenesis, soluble monomers are converted into small organic
compounds, such as short chain (volatile) acids (propionic, formic, lactic,
butyric, succinic acids – see Reaction 2), ketones (glycerol, acetone), and
alcohols (ethanol, methanol – see Reaction 3).
(2) C6H12O6 + 2H2 → 2CH3CH2COOH + 2H2O
(3) C6H12O6 → 2CH3CH2OH + 2CO2
37. The acidogenesis intermediates are attacked by acetogenic bacteria; the
products from acetogenesis include acetic acid, CO2, and H2. The reactions
4-7 shows the reactions that occur during acetogenesis:
(4) CH3CH2COO- + 3H2O → CH3COO- + H+ + HCO3
- + 3H2
(5) C6H12O6 + 2H2O → 2CH3COOH + 2CO2 + 4H2
(6) CH3CH2OH + 2H2O → CH3COO- + 2H2 + H+
(7) 2HCO3
- + 4H2 + H+ → CH3COO- + 4H2O
Several bacteria contribute to acetogenesis, including:
Syntrophobacter wolinii, propionate decomposer
Syntrophomonos wolfei, butyrate decomposer
Clostridium spp., peptococcus anaerobes,
lactobacillus, and actinomyces are acid formers.
38. Methanogenesis
The last phase of anaerobic digestion is the methanogenesis phase. Several
reactions take place using the intermediate products from the other phases,
with the main product being methane. Reactions 8-13 show the common
reactions that take place during methanogenesis:
(8) 2CH3CH2OH + CO2 → 2CH3COOH + CH4
(9) CH3COOH → CH4 + CO2
(10) CH3OH → CH4 + H2O
(11) CO2 + 4H2 → CH4 + 2H2O
(12) CH3COO- + SO4
2- + H+ → 2HCO3 + H2S
(13) CH3COO- + NO- + H2O + H+ → 2HCO3 + NH4
+
Several bacterial contribute to methanogenesis, including:
Methanobacterium, methanobacillus, methanococcus, and methanosarcina,
etc.
