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

Direct Leaching of Metals

MS + 2O2 MSO4 (where M is a metal)

examples

ZnS

NiS

CoS

2U4+ + O2 + 4H+ 2UO22+ + 4H+

hexa–solubletetra–insoluble

Indirect Leaching of Metals

2FeS + Fe2(SO4)3 + 2H+ 2FeSO4 + H2SO4

2FeSO4 + 1/2O2 + H2SO4 Fe2(SO4)3 + H2O

spontaneous

bacterial (a chemoautotrophic process that oxidizes Fe2+)

Beneficial activity – metal recovery

Acidothiobacillus ferrooxidans chemoautotrophic, uses O2 as electron acceptor

What types of organisms are useful in metal recovery?

Optimal conditions?

temp: 30 - 500 CpH: 2.3 - 2.5O2: requiredFe: 2-4 g Fe/L leach liquor

2FeSO4 + 1/2O2 + H2SO4 Fe2(SO4)3 + H2O

CuFeS2 + Fe2(SO4)3 CuSO4 + 5FeSO4 + 2S0

CuS2 + 2Fe2(SO4)3 2CuSO4 + 4FeSO4 + S0

CuS + Fe2(SO4)3

Cu5FeS4 + 6Fe2(SO4)3

Some examples of copper-containing minerals:

chalcopyrite

bornite

covellite

chalcocite

CuSO4 + 2FeSO4 + 2S0

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 50-70%

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.

O reCuFeS

Cu SCuSCu FeS

2

2

5 4

Copper recovery by precipitat ion electrolysiscatalyzed by Fe fi lings

T hiobacillus(oxidat ion of Fe )2+

Fe (S O )2 4 3

FeS O Fe (S O )4 2 4 3

FeS O + CuS O

4

4

pump

Cont inuous Bioreactor S ystem

Fe0

Acidothiobacillus

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 corrosionIt 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 control1. Coat surfaces with bacteriocides

phenolicsquaternary ammonia compoundsmetals (copper)surfactants

2. Remove surface biofilmschemical chlorine

surfactants mechanical scraping (pigging)

Metal surface

Anaerobic cathodic reaction2H+ + 2e- 2H H2

Aerobic cathodic reactionO2 + 2H2O + 4e- 4OH-

Anodic reactionFe0 Fe2+ + 2e-

Sulfate-reducing bacteria4H2 + SO4

2- 4H2O + S2-

Fe2+ + S2- FeS

Iron-oxidizing bacteriaFe2+ + ½O2 + 5H2O 2Fe(OH)3 + 4H+

Iron corrosion

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 CH3Hg+ + H2OCoB12+

CH3CoB12 + CH3Hg+ + H2O (CH3)2Hg+ + H2OCoB12+

methylcobalamine

methylcobalamine

methylmercury

dimethylmercury

Hg 2+Hg2+

Hg 2+

Hg2+ Hg 2+

Hg 2+

Hg2+

Hg2+

Hg 2+

Hg 2+

(CH ) Hg3 2+

CH H g3+CH H g3

+CH H g3

+

Bacterial methylationof mercury

Eating contam inatedfish

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.

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.

S eS eOselenate

6+

2-4

S eS eOselenite

4+

2-3

(CH ) S e

dimethylseleniumvolat ile

S eS elenium (solid)

3 2

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. Static piles lead to uneven product quality and take several months or more.2. 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.

3. 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.

Temperature in composting

0 1 2 3 4 5 6 71020304050607080

mesophilesthermophiles

activity stops

Timete

mpe

ratu

re -

C

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 heap2. mixing3. ventilation

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

The compost ecosystem:

high substrate density

mesophilic thermophilic temp

usually aerobic

diverse microbial populations

community changes rapidly

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))

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

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:

Optimal is 25:1 to 40:1