Bioremediation Technologies In Mining

Transcription

1 Lecture 39 Bioremediation Technologies In Mining Keywords: Active Remediation, Passive Remediation, Sulfate Reducing Bacteria Passive AMD treatment options [ ] Aerobic wetlands Anaerobic wetlands Sulfate-reducing bioreactors Permeable reactive barriers Anoxic limestone drains Shallow, surface flow with vegetation Subsurface flow isolated from outside atmos. Anoxic chamber containing SRB and organics-mine water Permeable barrier - flow of ground water - reactive material Limestone channels under anoxic environment Metal oxidation, precipitation and sorption on biomass Sulfate reduction, alkalinity, sulfide precipitates, sorption by vegetation. Sulfate reduction, alkalinity and metal precipitation Sulfate reduction and metal precipitation-sorptionalkalinity Alkalinity, iron precipitation. Biological sulfate reduction Sulfate reducing bacteria (SRB) capable of anaerobic respiration using sulfate as a terminal electron acceptor. There are four taxonomic groups. Gram-negative, mesophilic SRB (Desulfovibrio, Desulfomicrobium, Desulfobacter etc.) Gram-positive, spore forming SRB (Desulfotomaculum) Thermophilic SRB Thermodesulfobacterium. Archaeal thermophilic SRB 80 0 C. 1

2 Remediation strategies: Chemical/Biological? Metal Solubility product of sulfide Cadmium 6.7 x Copper 5.8 x Iron 3.4 x 10-5 Lead 5.5 x Nickel 6.9 x 10-8 Silver 7.2 x Zinc 2.3 x 10-7 A biological loop: In which sulfate is converted to hydrogen sulfide, using Sulfate Reducing Bacteria. A chemical loop: In which dissolved metals are selectively precipitated by the hydrogen sulfide produced in the biological loop. Biological metal removal and metal recovery from acidic metal contaminated waters Paques Inc. developed bioprocesses to remove sulfur compounds from water. - Sulfate reducing bacteria Convert SO - 4 to H 2 S - dissolved metals precipitate as sulfides Sulfide oxidising bacteria convert excess H 2 S to elemental sulfur. Ground water treatments system installed for the Budelco zinc refinery in the Netherlands in A pilot plant in 1995 at Kennecott s Bingham canyon copper mine. Anaerobic sulfate reducing reactor using hydrogen SO H 2 + H + = HS - + 4H 2 O 2

3 For small sulfate loads, ethanol or organic waste can be used - - 3SO 4 + 2C 2 H 5 OH = 3HS - + 3H 2 O + 3HCO CO 2 Excess sulfide is oxidized by aerobic microorganisms HS - + ½ O 2 = S 0 + OH - Sulfur produced is hydrophobic and separated by sedimentation A hybrid reactor configuration for the above remediation process is illustrated in fig : Metal containing Influents Nutrients ANAEROBIC REACTOR (SRB) Precipitated metal sulfides AEROBIC REACTOR (S-OXIDIZER) Separation of elemental sulfur Air Metal sulfides + sulfur for recovery and treatment Fig. 39.1: A hybrid reactor configuration for active bioremediation of acid mine drainage 3

4 Using the above bioremediation processes, very good metal and anion removal from mining effluents could be achieved. Typical bioremediation results Inflow Outflow Inflow 300 m 3 / h Zinc 100 mg/l < 0.3 Cadmium 1 mg/l < 0.01 Sulfate 1000 mg/l < 200 Biological remediation technologies The following bioremediation processes are commercialized in mining industries. Sulfate - Reducing Bacteria Naturally occurring, indigenous bacteria are used to neutralize acid and precipitate metals in contaminated groundwater. Proven applications include bioreactors, permeable reactive treatment walls, and placement behind hydrostatic bulkheads. Cyanide Destruction Indigenous bacteria have the capability to destroy cyanide and precipitate heavy metals in cyanide heaps and process solutions. This field-proven technology is more cost effective than conventional water rinse and chemical processes. Chromium Reduction Bioreduction of hexavalent chromium to the less toxic trivalent form has been proven both in contaminated soils, groundwater, and industrial wastewater. 4

5 Biological Denitrification Complete biological destruction of nitrate and nitrite in contaminated waters is very costeffective in ex - situ bioreactors. Insitu denitrification is planned for future investigations. Hydrocarbon Remediation Newer remediation alternatives including natural attenuation, surfactant addition, and bioaugmentation. 5