Chapter 15 - Cycles Gone Wild

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Chapter 15 - Cycles Gone Wild

• Objectives
• Be able to explain how bacteria can aid in metal
  recovery from ore
• Be able to explain the difference between direct
  and indirect leaching of metals
• Understand the three different approaches to
  bioleaching of metals
• Be able to explain how bacteria participate in
  iron corrosion
• Be able to explain how bacteria participate in
  concrete corrosion
• Be able to give an example of metal methylation
  that is detrimental and one that is beneficial
• Be able to describe the major similarities and
  differences between a soil system and a compost
  system
• Be able to describe how the composting process
  works

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• Some Beneficial and Detrimental Aspects of
  Biogeochemical Cycles
• metal recovery
• desulfurization of coal
• acid mine drainage
• metal corrosion
• concrete corrosion
• nitrous oxide emission (ozone)
• nitrate contamination
• methylation of metals
• composting
• bioremediation

Can you give other examples?
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Sulfur oxidation is an examples of how a part of
a cycle can be harnessed for societal benefit
turning a detrimental acitivity into a beneficial
one
Detrimental activity acid mine drainage Coal
and ore are found in geological formations under
reduced conditions Mining activities expose
these materials to O2
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Beneficial activity metal recovery
Direct Leaching of Metals
MS 2O2
MSO4 (where M is a metal)
examples
ZnS
NiS
CoS
2UO22
4H
2U4 O2
4H
hexasoluble
tetrainsoluble
Indirect Leaching of Metals
2FeS
Fe2(SO4)3 2H 2FeSO4
H2SO4
spontaneous
2FeSO4
1/2O2 H2SO4

Fe2(SO4)3 H2O
bacterial (a chemoautotrophic process that
oxidizes Fe2)
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What types of organisms are useful in metal
recovery?
2FeSO4
1/2O2 H2SO4

Fe2(SO4)3 H2O
Acidothiobacillus ferrooxidans chemoautotrophic,
uses O2 as electron acceptor

Optimal conditions?
temp
30 - 500 C
pH
2.3 - 2.5
O2
required
Fe
2-4 g Fe/L leach liquor
Some examples of copper-containing minerals
CuFeS2 Fe2(SO4)3
CuSO4 5FeSO4 2S0
chalcopyrite
CuS2 2Fe2(SO4)3
2CuSO4 4FeSO4 S0
chalcocite
CuSO4 2FeSO4 2S0
CuS Fe2(SO4)3
covellite
5CuSO4 13FeSO4 4S0
Cu5FeS4 6Fe2(SO4)3
bornite
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Approaches to Bioleaching
1. heap leaching
2. reactor leaching
3. in situ leaching
30 Cu and U currently mined using
bioleaching In the field, recovery of copper
from low-grade ores is between 50-70
Bioleaching is 1/3 to 1/2 the cost of smelting

• Heap leaching
• Requires building an impermeable pad. The ore is
  then broken up and heaped onto the pad. Water is
  pumped onto the top of the heap, the leachate is
  collected, processed, and recycled back onto the
  heap.

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• Continuous bioreactor
• The ore is placed into the reactor and water
  pumped through on a continuously recirculating
  basis as shown below.

Acidothiobacillus
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1. In situ leaching This is only practical under
   favorable geological conditions. Wells are
   drilled, the outer wells are used to apply leach
   liquor, and the center well is the recovery
   shaft.

Leach liquor shafts
Recovery shaft
In all cases, the leached metal can be recovered
by electrolysis But the majority of metal
recovery operations use a solvent or lixivient
extraction The lixivient is a kerosene-like
material that contains a metal-chelating agent
The metal partitions into the lixivient layer
and out of the water phase The metal is then
recovered from the lixivient
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• Metal corrosion
• It is estimated that 1.6 to 5.0 billion /yr in
  damage is due to corrosion of iron pipes.
  Although this is not solely a microbial process,
  it is exacerbated by microbial activity. Both
  iron oxidizing bacteria (aerobic) and
  sulfate-reducing bacteria (SRBs, anaerobic)
  participate in these reactions.
• Corrosion control
• Coat surfaces with bacteriocides
• phenolics
• quaternary ammonia compounds
• metals (copper)
• surfactants
• Remove surface biofilms
• chemical
• chlorine
• surfactants
• mechanical
• scraping (pigging)

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Iron corrosion
Metal surface
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Concrete corrosion
Concrete corrosion at rates of 4.3 to 4.7 mm/yr,
causes severe damage and has been well-documented
in sewer pipes. The actual corrosion process
occurs when sulfuric acid reacts with calcium
hydroxide binder in the concrete. Such binding
components in concrete as well as ceramics and
stone are acid sensitive. Corrosion is a
2-step process that occurs from the inside of the
pipe outwards. There are two environments in a
sewer pipe, the liquid and the headspace. The
action of sulfate-reducing microbes (SRBs) in the
liquid generates H2S which is volatile and
exchanges into the headspace. In the aerobic
environment on the concrete in the headspace,
sulfur oxidizers oxidize H2S to sulfuric acid.
The moist environment in the sewer pipe is ideal
for growth of the sulfur oxidizers.
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Concrete corrosion

• Corrosion control
• inhibit SRBs by addition of alternate electron
  acceptors
• treat the concrete with a high pH solution to
  maintain neutral surface
• apply a plastic coating

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Methylation of metals

• There are a number of metals and metalloids that
  are microbially methylated. In some cases the
  resulting methylated metal is more toxic and in
  some cases less toxic than the original metal.
• Two examples
• Mercury mercury is one of the most common metal
  pollutants found in the environment. Microbes
  methylate mercury under both aerobic and
  anaerobic conditions although methylation by SRBs
  (anaerobic) is thought to be the primary route.
  Methylation reactions involve vitamin B12,
  methylcobalamine.

CH3CoB12 Hg2 H2O CH3Hg
H2OCoB12
methylcobalamine
methylmercury
CH3CoB12 CH3Hg H2O (CH3)2Hg
H2OCoB12
methylcobalamine
dimethylmercury
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The reason for methylation of mercury is not well
understood but it is thought that it may be a
detoxification mechanism. Unfortunately,
methylmercury and dimethylmercury are highly
toxic. Since they are more lipophilic than other
forms of mercury, methylmercury partitions into
lipids and is subject to biomagnification. As a
result of methylmercury contamination, there are
advisories on levels of fish consumption in some
lakes in the US and Europe.
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2. Selenium - For selenium, the methylated form
is less toxic than the anions selenate and
selenite. As a result, methylation has been
proposed as a detoxification mechanism. Although
not as common a pollutant as mercury, one
well-documented case of selenium poisoning is in
the Kesterton wildlife refuge in California.
Here, the need for irrigation in agriculture
caused the accumulation of salts including
selenium salts during evaporation of applied
water. These salts were washed into the
Kesterton wetlands areas creating high levels of
selenium and leading to extensive bird kills.
Methylation of the selenium has been proposed as
a way to reduce selenium concentration in the
marsh.
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Composting
Although there are many backyard compost systems,
there are many potential applications on a much
larger scale for composting. Essentially, the
compost process turns waste products into an
organic soil amendment by taking advantage of the
normal microbes found in soil and optimizing they
carbon cycling activities.

• There are three approaches to composting.
• Static piles lead to uneven product quality and
  take several months or more.
• Aerated piles have perforated pipes buried inside
  them to deliver air during the composting
  process. This allows control of both oxygen and
  temp. and speeds up the process to 3 to 4 weeks.
• Continuous feed systems are large scale (used for
  municipal waste) and use grinders to produce
  input material of similar size and consistency.
  The input material is also moistened and oxygen
  and temp. are controlled. In such a system, the
  composting process can be completed in 2 to 4
  days.

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• Important parameters in composting
• temperature
• moisture (50-60 optimal)
• oxygen
• pH
• compost density

The objective is to keep the temperature between
60 and 700C to maintain optimal activity.
Temperature is controlled through
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Microbiology of composting

Mixed population
5 - 10 substrate used by bacteria (108 - 1012
bacteria/g peaks at 55 600C)
15 - 30 used by actinomycetes (107 - 109
actinomycetes/g which peak after bacteria)
30 - 40 used by fungi (105 - 108 fungi/g which
peak when T declines (lt 500C))
Compost density and makeup are important for a
successful process. Material that is too dense
will not allow good air flow and oxygenation.
Also, dense compost tends to get saturated
leading to anaerobic conditions. Anaerobic
conditions are avoided because of production of
gaseous products including volatile organics,
ammonia, and sulfide. The carbonnitrogen ratio
is also important


bacteria 51
Microbial compostion (C/N ratio)
fungi 101
Substrate composition (C/N ratio)
bacteria 101 to 201
Optimal is 251 to 401
fungi 1501 to 2001