1. 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

2. Top 20 Hazardous Substances List
ATSDR/EPA 2003
Arsenic
Lead
Mercury
Vinyl Chloride
Polychlorinated Biphenyls (PCBs)
Benzene
Cadmium
Polycyclic Aromatic Hydrocarbons
Benzo(a)pyrene
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.

3. Metals in the Environment
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
71,000 5
0.06
13,700
100 30
38,000
0.03
5,000 10
* Required metals

4. Common metal contaminants found in Superfund sites
Metal Occurrence (%)
Lead (Pb)
Arsenic (As)
Zinc (Zn)
Nickel (Ni)
Mercury (Hg)
Barium (Ba)
Cadmium (Cd)

5. 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.

6. 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.

7. Metals in the Environment
Total metal vs. bioavailable metal
Factors that affect metal bioavailability
1. metal sorption by soil (organic matter, clay minerals, metal oxides)
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

8. Bioavailable vs. total cadmium in soil – example
Total Cd added
mg/Kg
Bioavailable Cd
mg/L
Brazito
Gila
394
483
646
866
1,777
1,059 3 5 10 20
100
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.

9. 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.

10. Effect of increasing Pb on toxicity

11. Metal Toxicity

12. Metal Resistance Mechanisms

14. General remediation approaches for metals
These are based on mechanisms of metal resistance and include:
Removal of metals from wastestreams using biomass as a
sorbent
In situ precipitation of metals by creating anaerobic conditions
Removal of metals from soil using metal-complexing agents,
e.g. biosurfactants
Volatilization of metals, e.g. Selenium
Phytostabilization of metals, e.g. mine tailings

15. 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:
Ion exchange – good Zn removal, no sulfate removal, costly
Liquid membrane extraction – good Zn removal, no sulfate removal, costly
Bioremediation using SRBs
The Solution
After 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).

16. Three effluents are generated:
Solid sludges that are returned to the smelter to recover the precipitated Zn.
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.
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 criteria
Zinc < 0.3 mg/L Original 135 mg/L
Cadmium < mg/L
Sulfate < 200 mg/L Original 1300 mg/L

17. What would you add as an electron donor in the UASB??
ZnSO4 solution
discharge
CH4
flare
Biofilter
scrubber
H2S + CH4
vapor
vapor
Tilted
plate
settler
H2S
liquid
SFF
Groundwater
SO42-, Zn2+
UASB S0
liquid O2
discharge
sandfilter
ZnS solids
ZnS +
biomass
sludge
S0 + biomass
Solids to
zinc smelter
What would you add as an electron donor in the UASB??