Presentation on theme: "Chapter 21 - Microorganisms and Metals Objectives Know the common toxic metals and the main sources that they come from Know factors affecting metal bioavailability."— Presentation transcript:
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 List ATSDR/EPA 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 Arochlor 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 MetalRange for Soils (mg/Kg)Ave. for Soils (mg/Kg) Aluminum Arsenic Cadmium Calcium Chromium Copper Iron Mercury Magnesium Lead 10,000 – 30,000 1 – – 0.7 7,000 – 500,000 1 – 1,000 2 – 100 7,000 – 550, – – 6, , , , , * 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 SoilTotal Cd added mg/Kg Bioavailable Cd mg/L Brazito Gila ,777 1, 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.
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.
Effect of increasing Pb on toxicity
Metal Resistance Mechanisms
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, costly 2.Liquid membrane extraction – good Zn removal, no sulfate removal, costly 3.Bioremediation using SRBs The Solution After pilot-scale testing, a commercial plant with an 1800 m 3 bioreactor was constructed to treat 6000 m 3 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 H 2 S or S 0. This is passed into an aerobic fixed film bioreactor. Here sulfate oxidizers convert H 2 S to S 0. 3.Gas that contains 40% H 2 S, 60% CH 4, and a small amount of CO 2. The H 2 S is removed by passing through a zinc sulfate solution, and the CH 4 is burned. Aqueous effluent design criteria Zinc < 0.3 mg/L Original 135 mg/L Cadmium < 0.01 mg/L Sulfate < 200 mg/LOriginal 1300 mg/L
UASB Groundwater SO 4 2-, Zn 2+ H 2 S + CH 4 vapor ZnSO 4 solution scrubber H 2 S liquid ZnS + biomass sludge Solids to zinc smelter O2O2 S 0 liquid sandfilter discharge Biofilter Tilted plate settler S 0 + biomass ZnS solids CH 4 flare vapor SFF What would you add as an electron donor in the UASB??