Microbial Fe, Mn, As cycles Flashcards
What is the oxidised form of iron?
Fe(III), Fe 3+, ferric
What is the reduced form of iron?
Fe(II), Fe 2+, ferrous
What conditions are required for ferrous iron oxidation?
Aerobic conditions
What conditions are required for ferric iron reduction?
Anaerobic conditions
What do isotopic tests on BIFs show?
Half of Fe atoms originated in shallow oceans after being processed by microbe 2.5 Gya
When is Fe(II) stable?
Low pH aerobic conditions
What type of organism can use Fe(II) as an electron donor for aerobic respiration?
Acidophilic chemolithotroph
Describe the process of using Fe(II) as an e- donor for aerobic respiration
FeS2 reacts with oxygen to give Fe(II) (initiator reaction, either bacteria or chemical-driven), O2 is an e- acceptor, producing Fe(III)
How can using Fe(II) as an e- donor for aerobic respiration lead to acid mine drainage?
If pyrite is present, Fe(III) can react to give H2SO4
How is autotrophic bioleaching carried out?
By acidophilic S- or Fe- oxidising bacteria
e.g. oxidation of pyrite giving sulfuric acid (acid mine drainage)
What is bioleaching used for?
To extract low grade ores
Describe water affected by AMD
High acidity and elevated Fe(III) and other toxic metals, e.g. Cu, Zn, Ni, As
Toxic to almost all forms of aquatic life
How is AMD treated?
Sulfate-reducing bacteria
Describe how SRBs treat AMD
Reduces sulfate to sulfide, raises pH, forms H2S, precipitates toxic metals as sulfides
What was tested at Wheal Jane Mine to treat AMD and what was used in the end?
Tried active measures and switched to passive measures
Describe the active measures that were tested at the Wheal Jane Mine
Three step system that generates lots of sledge
Removes 99% of metals when pH is adjusted to 8.5
Describe the passive measures used at the Wheal Jane Mine
Aerobic cells, anaerobic cells, shallow rock filters
Name a common form of aerobic cell to treat AMD and describe how it works
Artificial reed beds (commonly used commercially)
Drives the system oxic to precipitate Fe as Fe3 hydroxides that absorb metals
Microbial oxidation around the roots
Describe anaerobic cells to treat AMD
Uses compost bioreactors, e- donors include straw and sawdust
pH is raised by metabolic
Only successful when a conditioning phase allows reactors to stabilise
Describe shallow rock filters to treat AMD
Algal growth and precipitation of metals via alkalinity, driven by consumption of CO2 during oxygenic photosynthesis
Placed downstream of the anaerobic cells
What are the two methods for anaerobic Fe(II) oxidation?
Anoxygenic phototrophic bacteria
Nitrate-reducing bacteria
Describe nitrate-reducing bacteria
Uses Fe(II) as e- donor for the reduction of nitrate to nitrogen
Describe an oxygenic phototrophic bacteria
Transfers e- to anything with a redox potential higher than +100mV (Ferric/ferrous iron redox potential in bicarbonate environment is +100mV
e- flow into photosynthetic system (Fe(III)) from Fe(II)
What type of organisms can aerobically oxidise Mn(II)?
Diverse range of bacteria and some fungi
What are the three mechanisms of aerobic Mn(II) oxidation?
Oxidise dissolved Mn(II) enzymatically (organism gains energy)
Oxidise Mn(II) sorbed to Mn(IV) oxide (some gain energy, some don’t)
Attack Mn(II) non-enzymatically (high pH environment)
What is Fe(III) reduction catalysed by?
Dissimilatory iron reducing organisms
Where does dissimilatory Fe(III) reduction occur?
Outside the cell because Fe(III) is highly insoluble
This is unusual, most e- acceptors are reduced inside the cell
What conditions allow for dissimilatory Fe(III) reduction?
Anaerobic conditions
The ion is the e- acceptor rather than oxygen
What is oxidised in dissimilatory Fe(III) reduction?
A carbon substrate
e.g. acetate to CO2
What happens to the Fe(II) produced in dissimilatory Fe(III) reduction?
Some goes into aqueous environment and some is incorporated into new mineral phases
Describe e- donors for Fe(III) reduction
Hydrogen, large range of organics (e.g. benzene)
Describe e- acceptors for Fe(III) reduction
Range from amorphous (bioavailable) to near crystalline
Alternatives include toxic metals and radionuclides
How can Fe(III) reduction be stimulated? What is this used for?
By adding soluble Fe(III), chelating agent, or e- shuttle
Used for in situ remediation
What are the three mechanisms for Fe(III) reduction?
Direct contact, chelating agent, electron shuttle
What can an e- shuttle do? Where does it come from?
Catalyses direct contact
Is either secreted or exogenous
Describe the role of Geobacter in Fe(III) reduction
Uses e- transfer,
Peripili conducts e- out to iron oxide, increases contact
Give three impacts of Fe(III)-reducing bacteria
Release of Fe(II), formation of new Fe(II)-bearing minerals, release/capture of trace elements, degradation of organics
Give a Fe(II)-bearing mineral and what it can be used for
Magnetite (converted by Geobacter)
Has applications in medicine and water treatment (for biotransformation of waste minerals)
How can Fe(III)-reducing bacteria be used to generate energy?
It can use anodes in fuel cells as en e- acceptor
Also has applications in waste organic treatment and metal recovery (at the cathode)
Give four examples of functional bionanominerals
Nanocrystals, nanomagnets, quantum dots, nanosilver biocide
Give two examples of e- donors for Mn(IV) reduction
Hydrogen and large range of organics
Give four examples of uses for functional bionanominerals
Recording devices, healthcare, photovoltaics, catalysts
Give two examples of e- acceptors for Mn(IV) reduction
Mn(IV) and Mn(III)
What does the reduction of As(V) result in?
The mobilisation of As(III), this cannot be reduced further
What does selenium (selenate/selenite) reduction result in?
The formation of insoluble elemental selenium (immobile), this can be reduced to selenide
What can the reduction of As and Se be carried out by?
Dissimilatory iron reducing bacteria (DIRB)
Give three potential mechanisms for As mobilisation
Oxidation of As-rich pyrite
Reduction of As(V)-rich Fe(III) oxyhydroxides
Mobilisation of As(V) by fertiliser phosphate or biogenic carbonate
Give three examples of how As poisoning can be prevented
Identify at risk aquifers, reoxidation using air/nitrate, stimulate sulfate reduction
How does drinking water from wells become contaminated with As?
Bringing up water for drinking or irrigate pushes down carbon-rich water, stimulating microbes to reduce As(III)
How can arsenic be bioremediated?
Bioprecipitation with Fe(III)-(oxyhydr)oxides and sulfides
Give two examples of how As can be bioremediated
Harnessing natural biogeochemical cycles to remove microbially-mobilised As
Biogenic Fe(III)-(oxyhydr)oxide and sulfides for removal
