Presentation on theme: "Chapter 15 - Cycles Gone Wild Objectives Be able to explain how bacteria can aid in metal recovery from ore Be able to explain the difference between direct."— Presentation transcript:
Chapter 15 - Cycles Gone Wild Objectives Be able to explain how bacteria can aid in metal recovery from ore Be able to explain the difference between direct and indirect leaching of metals Understand the three different approaches to bioleaching of metals Be able to explain how bacteria participate in iron corrosion Be able to explain how bacteria participate in concrete corrosion Be able to give an example of metal methylation that is detrimental and one that is beneficial Be able to describe the major similarities and differences between a soil system and a compost system Be able to describe how the composting process works
Some Beneficial and Detrimental Aspects of Biogeochemical Cycles metal recovery desulfurization of coal acid mine drainage metal corrosion concrete corrosion nitrous oxide emission (ozone) nitrate contamination methylation of metals composting bioremediation Can you give other examples?
Sulfur oxidation is an examples of how a part of a cycle can be harnessed for societal benefit – turning a detrimental acitivity into a beneficial one Detrimental activity: acid mine drainage Coal and ore are found in geological formations under reduced conditions Mining activities expose these materials to O 2 As a result, autooxidation and microbial oxidation occurs 2FeS 2 + 7O 2 + 2H 2 O 2FeSO 4 + 2H 2 SO 4 4FeSO 4 + 2H 2 SO 4 + O 2 2Fe 2 (SO 4 ) 3 + 2H 2 O Fe 2 (SO 4 ) 3 + 6H 2 O 2 Fe(OH) 3 + 3H 2 SO 4
Direct Leaching of Metals MS + 2O 2 MSO 4 (where M is a metal) examples ZnS NiS CoS 2U 4+ + O 2 + 4H + 2UO 2 2+ + 4H + hexa–soluble tetra–insoluble Indirect Leaching of Metals 2FeS + Fe 2 (SO 4 ) 3 + 2H + 2FeSO 4 + H 2 SO 4 2FeSO 4 + 1/2O 2 + H 2 SO 4 Fe 2 (SO 4 ) 3 + H 2 O spontaneous bacterial (a chemoautotrophic process that oxidizes Fe 2+ ) Beneficial activity – metal recovery
Acidothiobacillus ferrooxidans chemoautotrophic, uses O 2 as electron acceptor What types of organisms are useful in metal recovery? Optimal conditions? temp:30 - 50 0 C pH: 2.3 - 2.5 O2:O2:required Fe:2-4 g Fe/L leach liquor 2FeSO 4 + 1/2O 2 + H 2 SO 4 Fe 2 (SO 4 ) 3 + H 2 O CuFeS 2 + Fe 2 (SO 4 ) 3 CuSO 4 + 5FeSO 4 + 2S 0 CuS 2 + 2Fe 2 (SO 4 ) 3 2CuSO 4 + 4FeSO 4 + S 0 CuS + Fe 2 (SO 4 ) 3 Cu 5 FeS 4 + 6Fe 2 (SO 4 ) 3 Some examples of copper-containing minerals: chalcopyrite bornite covellite chalcocite CuSO 4 + 2FeSO 4 + 2S 0 5CuSO 4 + 13FeSO 4 + 4S 0
Approaches to Bioleaching 1. heap leaching 2. reactor leaching 3. in situ leaching 30% Cu and U currently mined using bioleaching In the field, recovery of copper from low-grade ores is between 50- 70% Bioleaching is 1/3 to 1/2 the cost of smelting 1.Heap leaching Requires building an impermeable pad. The ore is then broken up and heaped onto the pad. Water is pumped onto the top of the heap, the leachate is collected, processed, and recycled back onto the heap.
2.Continuous bioreactor The ore is placed into the reactor and water pumped through on a continuously recirculating basis as shown below. Acidothiobacillus
3.In situ leaching This is only practical under favorable geological conditions. Wells are drilled, the outer wells are used to apply leach liquor, and the center well is the recovery shaft. Leach liquor shafts Recovery shaft In all cases, the leached metal can be recovered by electrolysis But the majority of metal recovery operations use a solvent or lixivient extraction The lixivient is a kerosene-like material that contains a metal-chelating agent The metal partitions into the lixivient layer and out of the water phase The metal is then recovered from the lixivient
Metal corrosion It is estimated that 1.6 to 5.0 billion $/yr in damage is due to corrosion of iron pipes. Although this is not solely a microbial process, it is exacerbated by microbial activity. Both iron oxidizing bacteria (aerobic) and sulfate-reducing bacteria (SRBs, anaerobic) participate in these reactions. Corrosion control 1.Coat surfaces with bacteriocides phenolics quaternary ammonia compounds metals (copper) surfactants 2.Remove surface biofilms chemical chlorine surfactants mechanical scraping (pigging)
Metal surface Anaerobic cathodic reaction 2H + + 2e - 2H H 2 Aerobic cathodic reaction O 2 + 2H 2 O + 4e - 4OH - Anodic reaction Fe 0 Fe 2+ + 2e - Sulfate-reducing bacteria 4H 2 + SO 4 2- 4H 2 O + S 2- Fe 2+ + S 2- FeS Iron-oxidizing bacteria Fe 2+ + ½O 2 + 5H 2 O 2Fe(OH) 3 + 4H + Iron corrosion
Concrete corrosion Concrete corrosion at rates of 4.3 to 4.7 mm/yr, causes severe damage and has been well-documented in sewer pipes. The actual corrosion process occurs when sulfuric acid reacts with calcium hydroxide binder in the concrete. Such binding components in concrete as well as ceramics and stone are acid sensitive. Corrosion is a 2-step process that occurs from the inside of the pipe outwards. There are two environments in a sewer pipe, the liquid and the headspace. The action of sulfate-reducing microbes (SRBs) in the liquid generates H 2 S which is volatile and exchanges into the headspace. In the aerobic environment on the concrete in the headspace, sulfur oxidizers oxidize H 2 S to sulfuric acid. The moist environment in the sewer pipe is ideal for growth of the sulfur oxidizers.
Concrete corrosion Corrosion control: inhibit SRBs by addition of alternate electron acceptors treat the concrete with a high pH solution to maintain neutral surface apply a plastic coating
Methylation of metals There are a number of metals and metalloids that are microbially methylated. In some cases the resulting methylated metal is more toxic and in some cases less toxic than the original metal. Two examples: 1.Mercury – mercury is one of the most common metal pollutants found in the environment. Microbes methylate mercury under both aerobic and anaerobic conditions although methylation by SRBs (anaerobic) is thought to be the primary route. Methylation reactions involve vitamin B 12, methylcobalamine. CH 3 CoB 12 + Hg 2+ + H 2 O CH 3 Hg + + H 2 OCoB 12 + CH 3 CoB 12 + CH 3 Hg + + H 2 O (CH 3 ) 2 Hg + + H 2 OCoB 12 + methylcobalamine methylmercury dimethylmercury
The reason for methylation of mercury is not well understood but it is thought that it may be a detoxification mechanism. Unfortunately, methylmercury and dimethylmercury are highly toxic. Since they are more lipophilic than other forms of mercury, methylmercury partitions into lipids and is subject to biomagnification. As a result of methylmercury contamination, there are advisories on levels of fish consumption in some lakes in the US and Europe.
2. Selenium - For selenium, the methylated form is less toxic than the anions selenate and selenite. As a result, methylation has been proposed as a detoxification mechanism. Although not as common a pollutant as mercury, one well-documented case of selenium poisoning is in the Kesterton wildlife refuge in California. Here, the need for irrigation in agriculture caused the accumulation of salts including selenium salts during evaporation of applied water. These salts were washed into the Kesterton wetlands areas creating high levels of selenium and leading to extensive bird kills. Methylation of the selenium has been proposed as a way to reduce selenium concentration in the marsh.
Composting Although there are many backyard compost systems, there are many potential applications on a much larger scale for composting. Essentially, the compost process turns waste products into an organic soil amendment by taking advantage of the normal microbes found in soil and optimizing they carbon cycling activities. There are three approaches to composting. 1.Static piles lead to uneven product quality and take several months or more. 2.Aerated piles have perforated pipes buried inside them to deliver air during the composting process. This allows control of both oxygen and temp. and speeds up the process to 3 to 4 weeks. 3.Continuous feed systems are large scale (used for municipal waste) and use grinders to produce input material of similar size and consistency. The input material is also moistened and oxygen and temp. are controlled. In such a system, the composting process can be completed in 2 to 4 days.
Temperature in composting 01234567 10 20 30 40 50 60 70 80 mesophiles thermophiles activity stops Time temperature - C The objective is to keep the temperature between 60 and 70 0 C to maintain optimal activity. Temperature is controlled through: 1. size and shape of compost heap 2. mixing 3. ventilation Important parameters in composting: temperature moisture (50-60% optimal) oxygen pH compost density The compost ecosystem: high substrate density mesophilic thermophilic temp usually aerobic diverse microbial populations community changes rapidly
Microbiology of composting: Mixed population 5 - 10% substrate used by bacteria (10 8 - 10 12 bacteria/g peaks at 55 – 60 0 C) 15 - 30% used by actinomycetes (10 7 - 10 9 actinomycetes/g which peak after bacteria) 30 - 40% used by fungi (10 5 - 10 8 fungi/g which peak when T declines (< 50 0 C)) Microbial compostion (C/N ratio) bacteria 5:1 fungi 10:1 Substrate composition (C/N ratio) bacteria 10:1 to 20:1 fungi 150:1 to 200:1 Compost density and makeup are important for a successful process. Material that is too dense will not allow good air flow and oxygenation. Also, dense compost tends to get saturated leading to anaerobic conditions. Anaerobic conditions are avoided because of production of gaseous products including volatile organics, ammonia, and sulfide. The carbon:nitrogen ratio is also important: Optimal is 25:1 to 40:1