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??