Chapter 21 - Microorganisms and Metals
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Transcript of Chapter 21 - Microorganisms and Metals
Chapter 21 - Microorganisms and Metals
Objectives
• Know the common toxic metals and the main sources that they come from
• Know factors affecting metal bioavailability in the environment
• Know why are metals are toxic to microorganism and their mechanisms for resistance
• Be able to discuss some general approaches for metal remediation
Top 20 Hazardous Substances ListATSDR/EPA 2003
1. Arsenic
2. Lead
3. Mercury
4. Vinyl Chloride
5. Polychlorinated Biphenyls (PCBs)
6. Benzene
7. Cadmium
8. Polycyclic Aromatic Hydrocarbons
9. Benzo(a)pyrene
10. Benzo(b)fluoranthene
11. Chloroform
12. DDT
13. Arochlor 1254
14. Arochlor 1260
15. Dibenz[a,h]anthracene
16. Trichloroethylene
17. Chromium (+6)
18. Dieldrin
19. Phosphorus, white
20. Chlordane
Five of the top 20 EPA hazardous substances are metals.
Metals in the Environment
Metal Range for Soils (mg/Kg) Ave. for Soils
(mg/Kg)
Aluminum
Arsenic
Cadmium
Calcium
Chromium
Copper
Iron
Mercury
Magnesium
Lead
10,000 – 30,000
1 – 50
0.01 – 0.7
7,000 – 500,000
1 – 1,000
2 – 100
7,000 – 550,000
0.01 – 0.3
600 – 6,000
2 - 200
71,000
5
0.06
13,700
100
30
38,000
0.03
5,000
10
* Required metals
Common metal contaminants found in Superfund sites
Metal Occurrence (%)
Lead (Pb) 71
Arsenic (As) 60
Zinc (Zn) 57
Nickel (Ni) 50
Mercury (Hg) 47
Barium (Ba) 46
Cadmium (Cd) 30
Metals and Metalloids of Concern Quantities Produced and Uses
• Arsenic- As 43,000 tons/yr (1995) used in: insecticides, herbicides, seed additives, wood preservatives, desiccants, ceramics, glass (0.2-1%) additives
• Cadmium- Cd 14,500 tons/yr (1995) used in: battery-powered cellular telephones, camcorders, personal computers, pigments, stabilizers, coatings and alloys
• Cobalt- Co 18,500 tons/yr (1994) used in: alloys, nuclear industry, pigment in glazes, UV protectant in eye protective equipment, paint additive, catalyst in the petroleum industry.
• Lead- Pb 1,510,000 metric tons/yr in the US (2002) (a large portion is recycled) over half of lead is used by the auto industry in batteries. Other uses include manufacture of cable sheathings, sheet, pipe foil and tubes, solders,alloys, ammunition, and paints.
• Mercury- Hg 10,000 tons/yr (1980) major uses include electrical apparatus, the electrolytic preparation of chlorine and caustic soda, the manufacture of mildew-proof paint and in industrial and control instruments.
• Nickel- Ni 875,00 tons/yr (1995) used in alloys, plating, batteries, magnets, electrical contacts,electrodes, spark plugs, machinery parts, and as a\ catalyst.
Metals in the Environment
• Total metal vs. bioavailable metal
• Factors that affect metal bioavailability
2. pH high pH bioavailability metal phosphates/carbonates
low pH bioavailability free ionic species
3. redox potential high Eh bioavailability free ionic species
low Eh bioavailability metal phosphates/carbonates/sulfides
1. metal sorption by soil (organic matter, clay minerals, metal oxides)
Bioavailable vs. total cadmium in soil – example
Soil Total Cd added
mg/Kg
Bioavailable Cd
mg/L
Brazito
Gila
394
483
646
866
1,777
1,059
3
5
10
20
100
10
Note that a very small fraction of the total metal is actually bioavailable (defined here as soluble in water). The majority of the metal is sorbed or precipitated.
Time (hours)
0 10 20 30 40 50 60 70 80 90
CF
U/m
l x
108
0
1
2
3
Control
Mn2+ (50 mg/L)
Cu2+ (50 mg/L)
Cd2+ (50 mg/L)
Zn2+ (50 mg/L)
Pb2+ (50 mg/L)
Despite the fact that low amounts of metals in the environment are bioavailable, as the graphs below demonstrate, it does not take much metal to induce toxicity.
Time (hours)
0 50 100 150
CF
U/m
l x 1
08
0
1
2
Pb2+ (0.5 mg/L)
Control
Pb2+ (0.05 mg/L)
Pb2+ (50 mg/L)
Pb2+ (5 mg/L)
Effect of increasing Pb on toxicity
Cell membrane disruptionHg, Pb, Zn, Ni, Cu, Cd
Inhibition of enzymatic activity(antimetabolite)
Hg, Pb, As, Cd, Cu
Protein denaturationHg, Pb, Cd
Inhibition of cell d ivisionPb, Cd, Hg, Ni
DNA mRNA Protein synthesis
DNA damageHg, Pb, Cd, As
Inhibition of transcriptionHg
Inhibition of translationHg, Pb, Cd
Metal Toxicity
Efflux pumps
Cd2+
Cd 2+
Metallothionein-likeprotein production-(cys-cys) - Cdn
Reduction
Hg Hg
Cu Cu
2+o
6+
3+
Precipitation asmetal saltsCd CdSCd CdPO
2+
2+
4
Volatilization
CH Hg(CH ) Hg
3
3 2
+
EPS sequestration
Cd2+Cd2+Cd 2+Cd2+ Cd 2+
Cd 2+
Cd 2+
Cd 2+Cd 2+
Intracellularsequestration
Pb 2+ Pb 2+
Pb 2+
Pb2+Pb2+
Pb 2+
Pb 2+
Pb 2+
Pb 2+
Pb 2+
Outer membraneor cell wall binding
Cd2+
Cd2+
Cd2+
Cd 2+
Cd2+
Cd 2+
Metal Resistance Mechanisms
FAD FAD
Mer T
Mer P
Periplasm
Inner membrane
Cytoplasm
Hg +
Hg o
Hgo
Reductase
Hg 2+
?
DNA
Lyase
General remediation approaches for metals
These are based on mechanisms of metal resistance and include:
• In situ precipitation of metals by creating anaerobic conditions
• Removal of metals from wastestreams using biomass as a sorbent
• Volatilization of metals, e.g. Selenium
• Removal of metals from soil using metal-complexing agents, e.g. biosurfactants
• Phytostabilization of metals, e.g. mine tailings
Example of a successful bioremediation
Zinc smelter (100 yrs old) with 135 mg/L zinc and 1300 mg/L sulfate in groundwater.
(Budelco in the Netherlands)
Cleanup was mandated, choices included:
1. Ion exchange – good Zn removal, no sulfate removal, costly2. Liquid membrane extraction – good Zn removal, no sulfate removal,
costly3. Bioremediation using SRBs
The SolutionAfter pilot-scale testing, a commercial plant with an 1800 m3 bioreactor was constructed to treat 6000 m3 of groundwater per day ( 55-gallon drum every 3 seconds).
Three effluents are generated:
1. Solid sludges that are returned to the smelter to recover the precipitated Zn.
2. Liquid containing 80% sulfur mostly as H2S or S0. This is
passed into an aerobic fixed film bioreactor. Here sulfate
oxidizers convert H2S to S0.3. Gas that contains 40% H2S, 60% CH4, and a small amount
of CO2. The H2S is removed by passing through a zinc sulfate solution, and the CH4 is burned.
Aqueous effluent design criteriaZinc < 0.3 mg/L Original 135 mg/LCadmium < 0.01 mg/LSulfate < 200 mg/L Original 1300 mg/L
UASBGroundwater
SO42-, Zn2+
H2S + CH4
vapor
ZnSO4 solution
scrubber
H2S
liquid
ZnS + biomass
sludge
Solids to zinc smelter
O2
S0
liquid sandfilterdischarge
discharge
Biofilter
Tilted plate settler
S0 + biomass
ZnS solids
CH4flare
vapor
SFF
What would you add as an electron donor in the UASB??