Transcript Indirect Leaching of Metals

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
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?
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
As a result, autooxidation and microbial oxidation occurs
2FeS2 + 7O2
+ 2H2O
2FeSO4 + 2H2SO4
4FeSO4 + 2H2SO4 + O2
2Fe2(SO4)3 + 2H2O
Fe2(SO4)3 + 6H2O
2 Fe(OH)3 + 3H2SO4
Beneficial activity – metal recovery
Direct Leaching of Metals
MS + 2O2
MSO4
(where M is a metal)
examples
ZnS
NiS
CoS
2U4+ + O2 + 4H+
2UO22+ + 4H+
hexa–soluble
tetra–insoluble
Indirect Leaching of Metals
2FeS + Fe2(SO4)3 + 2H+
spontaneous
2FeSO4 + 1/2O2 + H2SO4
2FeSO4 + H2SO4
Fe2(SO4)3 + H2O
bacterial (a chemoautotrophic process that oxidizes Fe2+)
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:
pH:
O2 :
Fe:
30 - 500 C
2.3 - 2.5
required
2-4 g Fe/L leach liquor
Some examples of copper-containing minerals:
CuFeS2 + Fe2(SO4)3
chalcopyrite
CuSO4 + 5FeSO4 + 2S0
CuS2 + 2Fe2(SO4)3
chalcocite
2CuSO4 + 4FeSO4 + S0
CuS + Fe2(SO4)3
covellite
CuSO4 + 2FeSO4 + 2S0
Cu5FeS4 + 6Fe2(SO4)3
bornite
5CuSO4 + 13FeSO4 + 4S0
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 5070%
Bioleaching is 1/3 to 1/2 the cost of smelting
1.
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.
2.
Continuous bioreactor
The ore is placed into the reactor and water pumped through on a continuously
recirculating basis as shown below.
Fe2(SO4)3
Ore
CuFeS2
Cu2S
CuS
Cu5FeS4
pump
FeSO4
+ CuSO4
Fe0
Copper recovery by
precipitation electrolysis
catalyzed by Fe filings
Continuous Bioreactor System
Thiobacillus
Acidothiobacillus
2+
(oxidation of Fe )
FeSO4
Fe2(SO4)3
3.
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
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
1. Coat surfaces with bacteriocides
phenolics
quaternary ammonia compounds
metals (copper)
surfactants
2.
Remove surface biofilms
chemical
chlorine
surfactants
mechanical
scraping (pigging)
Iron corrosion
Iron-oxidizing bacteria
Fe2+ + ½O2 + 5H2O
2Fe(OH)3 + 4H+
Aerobic cathodic reaction
O2 + 2H2O + 4e4OH-
Metal
surface
Anaerobic cathodic reaction
2H+ + 2e2H
H2
Anodic reaction
Fe0
Fe2+ + 2eSulfate-reducing bacteria
4H2 + SO424H2O + S2Fe2+ + S2FeS
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.
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
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:
1. 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
methylcobalamine
CH3CoB12 + CH3Hg+ + H2O
methylcobalamine
CH3Hg+ + H2OCoB12+
methylmercury
(CH3)2Hg+ + H2OCoB12+
dimethylmercury
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.
Hg
+
(CH3)2Hg
Hg
+
CH3Hg+ CH Hg+
CH3Hg
3
Bacterial methylation
of mercury
2+
2+
Hg
Hg
2+
2+
Hg Hg Hg2+
Hg2+
2+
Hg
Hg
2+
2+
2+
Eating contaminated
fish
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.
Se
SeO42selenate
6+
Se
2SeO3
selenite
(CH3)2Se
dimethylselenium
volatile
4+
Se
Selenium
(solid)
0
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.
1.
2.
3.
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.
The compost ecosystem:
high substrate density
mesophilic
thermophilic temp
usually aerobic
diverse microbial populations
Temperature in composting
community changes rapidly
Important parameters in composting:
• temperature
• moisture (50-60% optimal)
• oxygen
• pH
• compost density
80
70
60
50
40
30
20
10
activity stops
thermophiles
mesophiles
0
1
2
3
4
5
Time
The objective is to keep the temperature between 60 and 700C to maintain
optimal activity. Temperature is controlled through:
1. size and shape of compost heap
2. mixing
3. ventilation
6
7
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 (< 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 carbon:nitrogen ratio is also important:
Microbial compostion (C/N ratio) bacteria 5:1
fungi
10:1
Substrate composition (C/N ratio)
bacteria 10:1 to 20:1
fungi
150:1 to 200:1
Optimal is 25:1 to 40:1