Compact, Containerizable Systems Suited for Remote, Small Flows and Can Be “Networked” for Cost-Effective Treatment of Multiple Outfalls. Envirogen has recently completed several long-term operational studies with FBR systems in coal mining operations in Appalachia and Western Canada, demonstrating the effectiveness of the technology in the field.

Small-footprint fluidized bed reactor (FBR):

• Can treat flows that start at less than 100 gallons per minute (gpm)
• Reduce dissolved selenium to less than 5 micrograms per liter selenium at discharge — even in the presence of high levels of nitrate.
• Treatment can be automated and delivered in standard-sized ISO containers to difficult-to-reach locations that require minimal site preparation and engineering, and can be monitored remotely.
• relatively low initial capital expenditure
• Possible to relocate the systems to meet changing treatment goals.

Envirogen entered the coal mining selenium treatment business in 2011 with its FBR technology that has been deemed “best-in-class” for performance and cost effectiveness by an independent study developed for the North American Metals Council.

The company now offers containerized and built-in-place FBR systems that can be sized to treat a wide range of flows and influent water compositions, ranging from under 100 gpm to well over 3,000 gpm.

According to David Enegess, Vice President, East Region for Envirogen, the move to develop small FBR systems was motivated by the realities of treating selenium-containing wastewaters in coal mining operations that have multiple outfalls — often in remote locations — as opposed to a single option that would require the construction of a large, centralized facility. “Coal mining is a very dynamic activity, often conducted in rough, remote terrain. Sometimes it is impractical to pipe all the selenium-containing mining waters to a single, centralized facility from both cost and logistics standpoints. The smaller-sized FBR can meet the treatment and discharge requirements of low-flow outfalls. It can be rapidly implemented with a minimal amount of civil engineering, automated and remotely monitored. These systems become an asset that may be relocated if treatment locations vary on one site, or across multiple sites, through time,” he said. “For mines that have multiple outfalls, we envision these being treated by individual small FBRs, with other activities — such as solids treatment and disposal, chemical supply and spare parts serviced from a lower cost centralized location,” he added.

The Fluidized Bed Reactor – “Best-in-Class” Technology

Envirogen’s FBR is an active, fixed-film bioreactor that fosters the growth of microorganisms on a hydraulically fluidized bed of specified media. FBRs have been shown to have some significant advantages over other biological systems in coal mining environments in treating selenium to less than 5 micrograms per liter. First, they operate in a steady-state “plug flow” manner that avoids channeling, upsets in the controlled growth of organisms and gas binding. This ensures that the microorganisms in the system are optimally utilized. Also, they do not require periodic backwash, as do packed bed reactors, for example. These features allow for significantly higher treatment efficiency, resulting in much lower hydraulic residence times (1/5 to 1/10 as long as packed bed). This efficiency results in smaller overall systems and a smaller system footprint — both of which contribute to the dramatically lower capital and installation costs of the FBR compared to other biological treatment systems. Its flexibility in the choice of electron donor chemicals can translate into capital and operating cost savings with reduced solids generation. It also responds well to changes in feed flow and composition, consistently achieving discharge limit conditions.

With a team that has more than 20 years of experience operating the FBR treating similar inorganic compounds like nitrate and perchlorate, Envirogen has the expertise and the ability to offer performance, cost and asset life guarantees in conjunction with these small FBR installations. According to Mr. Enegess, the focus with any solution development will be low lifecycle cost. “As we look around the coal mining industry today, we see the need for high-performing treatment solutions that are flexible enough to meet each coal mine’s approach to handling selenium abatement. But we also understand that cost is a major issue,” he said. “With our FBR technology, we start with the most robust, cost-effective technology on the market. In partnering with the coal mining industry, we can help design and implement technology and long-term operating solutions that will deliver the lowest lifecycle cost over the course of the treatment program,” he added. Mr. Enegess went on to point out that the company is involved in using FBR technology to treat selenium-laden waters in other industries and applications — including other mining industries, power generation, refinery wastewaters and in agricultural and groundwater remediation applications.

For further information, contact:
Julie Mamaux
Voice: 877.312.8950
Fax: 281.358.2443
http://www.envirogen.com

SOURCE Envirogen Technologies, Inc.

Web Site: http://www.envirogen.com

Definition: Black shales are defined as fine grained (silt to clay size) dark colored mud-rock containing >5 % organic carbon ( often preserved as Kerogen).

Coal mining commonly disturbs the overburden completely for surface mining purposes. Even for a deep coal mine, some disturbance of the overburden material is required during the construction of slope, shaft etc. The coal permit application in the USA requires proper geochemical characterization of the materials to be disturbed by the coal mining activities. Commonly acid base accounting (ABA), Sulfur forms and selenium are analyzed for geochemistry purposes. High acid forming zones and total selenium > 1 mg/Kg in the rock is identified and marked as potential problem zones. Such materials are specially handled during the mining.

Black shales are typically enriched in organic matter (up to 6% in some formations). Sulfur content in the black shale could be as high as 16%. (Reference http://www.ias.ac.in/jessci/feb06/vin-3)

In most cases, black shales are known culprits for selenium. Black shales could have metal enrichment factor as high as 50 times for Ag, 10 for Mo.  Black shales are commonly enriched in:

• Arsenic (As)
• Copper (Co)
• Chromium (Cr)
• Molybdenum (Mo)
• Nickel (Ni)
• Uranium (U)
• Selenium (Se)

These metals however do not form minerals of their own and cannot be identified by naked eye or even under the microscope using regular light microscopy. They are usually microscopically distributed among the organic matter in the black shale. Sometimes they substitute for other metals and present in trace amount in other mineral phases.

Black sea is a modern day example when black shale is forming. Black sea is shut off from oceanic currents and reducing condition exists below 150 meter death.

In short, black shale is a problem for the coal industry as they can leach high concentration of various metals (Including RCRA metals) in the surface and groundwater if the condition becomes acid producing. It is often recommended to perform leach tests especially on the black shale samples to evaluate the rate and fate of various elements present in the black shale.

Geochemical condition must be properly evaluated before mining begins. Please contact Ankan Basu, P.G (Geochemist/Hydrogeologist) [email: admin@coalgeology.com] with 6 years of experience in the coal industry for any geochemical investigations.

References:

• Black Shale, http://faculty.umf.maine.edu/eastler/public.www/Black%20Shales.pdf
• Black Shale – Its Deposition and Digenesis: http://www.clays.org/journal/archive/volume%2027/27-5-313.pdf

What is the national primary drinking water standard for chromium?

USEPA standard for total chromium is 0.1 mg/L (note, no MCL is currently developed for different species of chromium)

What is Hexavalent Chromium?

Hexavalent chromium, or chromium (VI), is one the three most common forms of chromium. Hexavalent form of chromium is carcinogenic with the other forms are not.

What is the crustal abundance of chromium?

• Crustal abundance – 100 mg/Kg
• In granites – 20 mg/Kg
• In ultramafics – 2000 mg/Kg
• Black Shale – 20 to 3000 mg/Kg
• Soil (worldwide): ~200 mg/Kg
• US soil: 1.0 to 2000 mg/Kg (average 54mg/Kg)
• Rainwater: 0.2 to 1.9 microgram/L
• Groundwater: generally less than 0.1 microgram/L
• Sea water: average 0.3 microgram/L

What is the average concentration of chromium in Coals?

Average concentration of chromium concentration is US coals is 15 mg/Kg.

What are some of the common use of chromium?

The greatest use of chromium is in metal alloys (cast iron, stainless steel; protective coatings) and pigments for paints, cement, paper, rubber, composition floor covering and other materials.

Learn about chromium from Q&A:

• Name a mineral with Cr (VI): Crocoite – PbCrO4. Generally Cr(VI) minerals are rare in nature. Most of the deep earth minerals with chromium have +3 oxidation states.
• What are the oxidation states of chromium?  0, +3 and +6 oxidation states found in nature.
• What form of chromium is dominant in oxygen rich environment? In pH condition greater than 2 and under oxidizing conditions, Cr(III) is thermodynamically unstable and converts to Cr(VI).

Example of chromium compounds:

• Cr (0): Native chromium, rare in nature.
• Cr (III): chromium oxide (Cr2O3), chromium hydroxide [Cr(OH)3], Cr(OH)++, Cr(OH)2+
• Cr(VI): Chromate (CrO4–) and dichromate (Cr2O7-2)

Did you know?

• Evaporites and chromium: In the Atacama Desert of South America, many Cr(VI) minerals have been found.  Na, K, Ca and Ba are found to form minerals with both chromate (CrO4–) and dichromate (Cr2O7–).
• Chromium and Paradise Valley, Arizona: 100s to 1000s microgram/L of chromium (Cr+6) is naturally present in the groundwater at the Paradise Valley of Arizona.
• Chromium has 26 known isotopes; four of them are stable and naturally occurring.
• Earth’s mantle has chromium concentration between .41-.55 percent.
• Chromite ore is NOT actively mined in the United States, Canada or Mexico. US stopped mining for chromium in 1961.
• Primary chromite deposits are ONLY associated with ultra-mafic rocks either as stratiform or podiform deposits.
• Precambrian Stillwater complex in Montana (sill) is a basic layered intrusion, known for high chromium reserve. Bushveld Complex of South Africa is another example of chromium complex.

Grades of chromium grade:

• Chemical grade: 28.6% average chromium
• Metallurgical grade: 28.6% average chromium
• Refractory grade: average 23.9% chromium

What are the chemical conditions that favor Cr(VI) stay dissolved in ground water?

• Oxygen rich groundwater
• Neutral to alkaline pH
• Moderate to high concentration of other anions such as sulfate

General geochemistry of chromium:

• Speciation of chromium in natural water depends on concentration of chromium and pH.
• CrO4-2 dominates above pH=6.
• HCrO4- dominates between 0 to 6 pH with low Cr(VI) activity.
• Cr2O7-2 dominates between 0 to 6 pH with high Cr(VI) activity.
• H2CrO4 dominates when pH<0
• Cr(III) speciation: As pH increases, the dominant species changes through Cr+3, CrOH++, Cr(OH)+, Cr(OH)3aq, Cr(OH)4-.
• Cr(VI) Speciation: At higher oxidizing condition, the changes are HCrO4-, CrO4-2, CrO4-3 (See Eh-pH diagram)
• Cr is known to form complexes with organic ligands.
• Cr+6 form soluble compounds with alkali and alkali earth metals.
• In presence of high Mg, chromium forms magnetiochromite and precipitates (see Eh-pH diagram)

Chromiun Eh-pH diagram with Cr activity= 1e-4

Chromium Eh-pH diagram with Cr activity=1e-10

Chromium pe-pH diagram with Cr activity=1e-10

 

Adsorption of Chromium:

• Chromium adsorbs to mineral surfaces better as pH decreases. (Lead in the other hand adsorbs better as pH increases)
• Adsorption of Cr(VI) on soils and sediments depends of the composition of the soil/sediment.

Natural Attenuation of Chromium:

• Cr+6 is carcinogenic. Cr+3 is not. The reduction of hexavalent chromium to trivalent chromium is important in natural attenuation.
• Low pH and high dissolved organic carbon promotes the reduction of chromium from +6 to +3 state.
• Fe+2 could reduce Cr+3 to Cr+3 over wide range of pH

Colloidal transport of chromium:

• Just like lead, chromium could also transport as part of the colloid matter in ground and surface water.

Site characterization for chromium remediation:

• Oxidation – reduction of chromium is characterized by dis equilibrium. Eh-pH diagram may not properly characterize the natural condition.
• Identify dissolved species of chromium in the aquifer. (Cr+6 is most mobile).

This article is based on http://www.epa.gov/nrmrl/pubs/600R07140/600R07140.pdf. Along with other sources and speciation diagrams generated using Geochemist’s Workbench.

References

• EPA’s recommendations for enhanced monitoring for Hexavalent Chromium (Chromium-6) in Drinking Water: http://water.epa.gov/drink/info/chromium/guidance.cfm
• Chemistry, Geochemistry and Geology of Chromium: http://www.engr.uconn.edu/~baholmen/docs/ENVE290W/National%20Chromium%20Files%20From%20Luke/Cr(VI)%20Handbook/L1608_C02.pdf
•  Soil and Water Sampling for Hexavalent Chromium in Northwest Missouri: http://www.epa.gov/region07/pdf/national_beef_leathers-prime_tanning-MODNR_farm_fields_data_summary.pdf
• Sampling method for chromium: http://www.osha.gov/dts/sltc/methods/validated/t-w4001-fv-02-0104-m/t-w4001-fv-02-0104-m.html
• Ion chromatography (IC) ICP-MS for chromium speciation in natural waters - http://www.chem.agilent.com/Library/applications/5989-2481EN.pdf

·         Chromium-6 in Drinking Water Sources:  Sampling Results: http://www.cdph.ca.gov/certlic/drinkingwater/pages/chromium6sampling.aspx

·         Drinking Water Contaminants- Chromium: http://www.freedrinkingwater.com/water-contamination/chromium-contaminants-removal-water.htm

·         Low-Pressure Cr Speciation in Drinking Water using the SC-DX chromFAST System with ICPMS Detection http://www.icpms.com/products/chromfast-cr-speciation.php

Analytical labs for chromium analysis in USA (only few listed)

• http://www.caslab.com/Chromium_Hexavalent.php5
• http://www.californialab.com/
• http://water.epa.gov/scitech/methods/cwa/index.cfm -Methods

Consulting firm for chromium:

Marshall Miller and Associates; http://www.mma1.com, contact Ankan Basu, P.G at admin@coalgeology.com

What is the Maximum Contaminant Limit (MCL) for lead?

• EPA has set the maximum contaminant level for lead at 0.015 mg/L

Lead concentration in common geologic strata:

Lead is similar to K in in ionic size. Ionic substitution is common between lead and potassium in silicates (K-feldspars and biotite in particular). Lead content generally increases from ultramafic to granitic  rocks. Average lead concentration in granites is about 23 mg/Kg

Did you know these FACTS about LEAD?

• Lead was ranked second on the CERCLA Priority List of Hazardous Substances in 1999 and 2001 (First being Arsenic).
• Lead is the most commonly RECYCLED metal – 50% of lead production is secondary in nature.
• 70% of lead produced is used in lead-acid storage batteries.
• Lead was widely used in the construction of water pipes in 20th century.
• Lead picments were common in paints prior to 1978!
• Leaded gasoline (tetraethyl lead) was available between 1923 to 1980s .
• Most of the lead produced in the USA comes from Missouri.

Sources of lead contamination:

• fall out of atmospheric dust
• Industrial/municipal discharge
• fertilizers
• lead based paints
• mining waste

Lead geochemistry is controlled by

• adsorption at the solid/water interface.
• Precipitation
• complexation with organic components

RULE of LEAD: Lead has strong affinity towards soils (adsorbed to soils) and rarely released to surface or ground water.

• Exception to the rule – low pH system or high dissolved organic carbon (DOC).

Remedial technologies for lead contamination:

• Containment (caps and vertical barriers)
• Solidification/ stabilization
• Separation/ concentration

Solubility of Lead:

Lead is highly soluble in water over a wide range of pH conditions. In pure water lead forms stable cation Pb+2 below about pH 7. Lead can also form aqueous complexes with OH-, Cl-, SO4– etc. As the pH increases of the solution, lead forms more stable species such as PbOH+, Pb(OH)2, and Pb(OH)3-.

Lead geochemistry and solubility is highly controlled by solution pH and Eh of the environment.

• Lead is MOBILE in LOW pH and HIGH Eh conditions.
• Lead is usually NOT a metal of concern in at mining related sites. Acid mine drainage produces sulfate apart from low pH condition which allows lead to precipitate as angelsite (PbSO4).
• Lead carbonate (Cerrussite) is highly soluble below pH 6; but highly insoluble above pH 8.
• Thermodymanic data predicts lead hydroxide and lead oxide to be stable. However, kinetically they are difficult to precipitate in room temperature.
• Lead phosphate minerals are highly insoluble and remedial technologies uses this strategy.

Figure 1: Lead Solubility Diagram (HCO3=SO4=.001 activity)

Eh-pH diagram for Lead:

Eh-pH diagrams are not specific to every field conditions that you would encounter. The Eh-pH diagram for lead generated below uses the following activities -

• Pb activity = 1e-5
• HCO3- activity= .001
• SO4– activity = .001

Figure 2: Lead Eh-pH Diagram

It is evident from the Eh-pH diagram that lead is mobile at low pH condition (pH<2). With increasing pH, the lead sulfate Anglesite becomes first to precipitate if enough sulfate is available below pH 6. Above pH 6, the carbonates cerussite and hydrocerussite stable. In a reducing condition, galena could also be stable over wide range of pH.

Fig 3: Lead Eh-pH Diagram-2

Figure 3 is generated with lead activity = 1.E-6. Notice, how lower concentration of lead leads to higher field of soluble phase.

Any Eh-pH diagram could be easily converted in to a pe-pH diagram. The figure below converts the Eh-pH diagram from figure 3 to a pe-pH diagram:

Lead pe-pH Diagram-3

Adsorption of LEAD:

• Hydrous Ferric Oxide (HFO) plays major role in lead mobility as lead adsorbs to HFO stronger than any other divalent metal ions.
• Adsorption increases as pH in increased between 3 (0% adsorbed) to 6 (100% adsorbed).
• Lead could also adsorbs to iron sulfide in reducing environment.

Lead Valence states

• 0, + 2 and +4

Lead Transport:

• Lead transport in colloidal form could be significant in both surface and ground water. Colloidal particles could be either organic or inorganic.
• If you are high dissolved lead in your sample, try using 0.1 micron filters for “dissolved” samples.

References: 

http://www.epa.gov/nrmrl/pubs/600R07140/600R07140.pdf.

A facultative anaerobic organism is an organism, usually a bacterium that makes ATP by aerobic respiration if oxygen is present but is also capable of switching to fermentation. In contrast, obligate anaerobes die in the presence of oxygen.

Respitation: Aerobic CH2O + O2 = CO2+ H2O

Phosphate is commonly used as a fertilizer. Phosphate may be added to the lake through sewage, fertilizer and other sources. The excess nutrients stimulate the algal growth in the lake. The algae grow and die that becomes the food source for the aerobes. In such condition, the aerobe population in the lake multiplies exponentially consuming oxygen at a faster rate than it could diffuse through water.  This leads to anaerobic condition in the lake. In such situation, facultative anaerobes thrive killing entire fish population of the lake.

Respiration: Anaerobic 2CH2O + SO4-2 = 2HCO3 + H2S

Eutrophication can be human-caused or natural. Untreated sewage effluent and agricultural run-off carrying fertilizers are examples of human-caused eutrophication. However, it also occurs naturally in situations where nutrients accumulate (e.g. depositional environments), or where they flow into systems on an ephemeral basis.

Hard_water_and_drop

July 11, 2011 (Coal Geology) Hard water is water referes to the alkaline earth ion concentrations. The primary alkaline earth ions are Ca2+ and Mg2+. Calcium usually dominates over magnesium concentration.

Some important Points:

• Hard water is generally not harmful to one’s health
• pose serious problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boilers, cooling towers, and other equipment that handles water. In boilers, the deposits impair the flow of heat into water, reducing the heating efficiency and allowing the metal boiler components to overheat. In a pressurized system, this overheating can lead to failure of the boiler.
• Chelation of organics within soap by the alkaline ions is the main reason of concern for hard water. When soap is chelated, it fails to properly react with the water.
• Hard water also forms deposits that clog plumbing. These deposits, called “scale”, are composed mainly of calcium carbonate (CaCO₃), magnesium hydroxide (Mg(OH)₂), and calcium sulfate (CaSO₄).[
• Calcium and magnesium carbonates tend to be deposited as off-white solids on the surfaces of pipes and the surfaces of heat exchangers. This precipitation (formation of an insoluble solid) is principally caused by thermal decomposition of bi-carbonate ions but also happens to some extent even in the absence of such ions. The resulting build-up of scale restricts the flow of water in pipes.

Measurement Unit: "mg/L as of CaCO3"

How to convern both Ca+2 and Mg+2 concentration to "mg/L as of CaCO3" unit?

Formula for Hardness = {[Ca+2]x(Mol. Wt. of CaCO3/Mol Wt. of Ca+2)}  + { [Mg+2]x(Mol. Wt. of CaCO3/Mol Wt. of Mg+2)
= [Ca+2]x(100.08/40.08)    +    [Mg+2]x(100.08/24.31]

Use the attached excel spreadsheet to  Calculate hardness of water.

Hardness of Water Calculation Spreadsheet -CoalGeology.Com

[ReviewAZON asin="0521891485" display="inlinepost"]Hardness Scale (mg/L of CaCO3):
0-60 – Soft Water
61-120 Moderately Hard Water
121-180 Hard Water
>180    Very Hard Water

Hard water is commonly associated with limestone/dolomite rocks such as in Florida.

Sources:
Hardness in water is defined as concentration of multivalent cations. Multivalent cations are cations (positively charged metal complexes) with a charge greater than 1+. They mainly have the charge of 2+. These cations include Ca2+ and Mg2+. These ions enter a water supply by leaching from minerals within an aquifer. Common calcium-containing minerals are calcite and gypsum. A common magnesium mineral is dolomite (which also contains calcium). Rainwater and distilled water are soft, because they also contain few ions.

The following equilibrium reaction describes the dissolving/formation of calcium carbonate scales:

CaCO3 + CO2 + H2O ? Ca2+ + 2HCO3-

Resources:

http://en.wikipedia.org/wiki/Hard_water

The Unified Soil Classification System (USCS) is a soil classification system used in engineering and geology to describe the texture and grain size of a soil. The classification system can be applied to most unconsolidated materials, and is represented by a two-letter symbol. Each letter is described below (with the exception of Pt):

If the soil has 5–12% by weight of fines passing a #200 sieve (5% < P_#200 < 12%), both grain size distribution and plasticity have a significant effect on the engineering properties of the soil, and dual notation may be used for the group symbol. For example, GW-GM corresponds to “well graded gravel with silt.”

If the soil has more than 15% by weight retained on a #4 sieve (R_#4 > 15%), there is a significant amount of gravel, and the suffix “with gravel” may be added to the group name, but the group symbol does not change. For example, SP-SM with gravel may refer to “poorly graded SAND with silt and gravel.”

Source: http://en.wikipedia.org/wiki/Unified_Soil_Classification_System

Symbol chart

The upper mantle of Earth is composed mainly of olivine and pyroxene.

Pyroxene Names

The chain silicate structure of the pyroxenes offers much flexibility in the incorporation of various cations and the names of the pyroxene minerals are primarily defined by their chemical composition. Pyroxene minerals are named according to the chemical species occupying the X (or M2) site, the Y (or M1) site, and the tetrahederal T site. Cations in Y (M1) site are closely bound to 6 oxygens in octahedral coordination. Cations in the X (M2) site can be coordinated with 6 to 8 oxygen atoms, depending on the cation size. Twenty mineral names are recognised by the International Mineralogical Association’s Commission on New Minerals and Mineral Names and 105 previously used names have been discarded (Morimoto et al., 1989).

A typical pyroxene has mostly silicon in the tetrahedral site and predominately ions with a charge of +2 in both the X and Y sites, giving the approximate formula XYT₂O₆. The names of the common calcium – iron – magnesium pyroxenes are defined in the ‘pyroxene quadrilateral’ shown in Figure 2. The enstatite-ferrosilite series ([Mg,Fe]SiO₃) contain up to 5 mol.% calcium and exists in three polymorphs, orthorhombic orthoenstatite and protoenstatite and monoclinic clinoenstatite (and the ferrosilite equivalents). Increasing the calcium content prevents the formation of the orthorhombic phases and pigeonite ([Mg,Fe,Ca][Mg,Fe]Si₂O₆) only crystallises in the monoclinic system. There is not complete solid solution in calcium content and Mg-Fe-Ca pyroxenes with calcium contents between about 15 and 25 mol.% are not stable with respect to a pair of exolved crystals. This leads to a miscibility gap between pigeonite and augite compositions. There is an arbitrary separation between augite and the diopside-hedenbergite (CaMgSi₂O₆ – CaFeSi₂O₆) solid solution. The divide is taken at >45 mol.% Ca. As the calcium ion cannot occupy the Y site, pyroxenes with more than 50 mol.% calcium are not possible. A related mineral wollastonite has the formula of the hypothetical calcium end member but important structural differences mean that it is not grouped with the pyroxenes.

Magnesium, calcium and iron are by no means the only cations that can occupy the X and Y sites in the pyroxene structure. A second important series of pyroxene minerals are the sodium-rich pyroxenes, corresponding to nomenclature shown in Figure 3. The inclusion of sodium, which has a charge of +1, into the pyroxene implies the need for a mechanism to make up the “missing” positive charge. In jadeite and aegirine this is added by the inclusion of a +3 cation (aluminium and iron(III) respectively) on the Y site. Sodium pyroxenes with more than 20 mol.% calcium, magnesium or iron(II) components are known as omphacite and aegirine-augite, with 80% or more of these components the pyroxene falls in the quadrilateral shown in figure 2.

Table 1 shows the wide range of other cations that can be accommodated in the pyroxene structure, and indicates the sites that they occupy.

In assigning ions to sites the basic rule is to work from left to right in this table first assigning all silicon to the T site then filling the site with remaining aluminium and finally iron(III), extra aluminium or iron can be accommodated in the Y site and bulkier ions on the X site. Not all the resulting mechanisms to achieve charge neutrality follow the sodium example above and there are several alternative schemes:

1. Coupled substitutions of 1+ and 3+ ions on the X and Y sites respectively. For example Na and Al give the jadeite (NaAlSi₂O₆) composition.
2. Coupled substitution of a 1+ ion on the X site and a mixture of equal numbers of 2+ and 4+ ions on the Y site. This leads to e.g. NaFe²⁺_0.5Ti⁴⁺_0.5Si₂O₆.
3. The Tschermak substitution where a 3+ ion occupies the Y site and a T site leading to e.g. CaAlAlSiO₆.

In nature, more than one substitution may be found in the same mineral.

Pyroxene minerals

• Clinopyroxenes (monoclinic)
  • Aegirine (Sodium Iron Silicate)
  • Augite (Calcium Sodium Magnesium Iron Aluminium Silicate)
  • Clinoenstatite (Magnesium Silicate)
  • Diopside (Calcium Magnesium Silicate, CaMgSi₂O₆)
  • Esseneite (Calcium Iron Aluminium Silicate)
  • Hedenbergite (Calcium Iron Silicate)
  • Jadeite (Sodium Aluminium Silicate)
  • Jervisite (Sodium Calcium Iron Scandium Magnesium Silicate)
  • Johannsenite (Calcium Manganese Silicate)
  • Kanoite (Manganese Magnesium Silicate)
  • Kosmochlor (Sodium Chromium Silicate)
  • Namansilite (Sodium Manganese Silicate)
  • Natalyite (Sodium Vanadium Chromium Silicate)
  • Omphacite (Calcium Sodium Magnesium Iron Aluminium Silicate)
  • Petedunnite (Calcium Zinc Manganese Iron Magnesium Silicate)
  • Pigeonite (Calcium Magnesium Iron Silicate)
  • Spodumene (Lithium Aluminium Silicate)
• Orthopyroxenes (orthorhombic)
  • Hypersthene (Magnesium Iron Silicate)
  • Donpeacorite, (MgMn)MgSi₂O₆
  • Enstatite, Mg₂Si₂O₆
  • Ferrosilite, Fe₂Si₂O₆
  • Nchwaningite (Hydrated Manganese Silicate)

What is Thiobacillus ferrooxidans?

• Thiobacillus ferrooxidans is truly the most common type of bacteria in mine waste piles.
• Thiobacillus ferrooxidans is acid loving (acidophilic) in nature and increases the rate of pyrite oxidation in mine tailings piles and coal deposits. They prefer a pH of 1.5 to 2.5.
• Thiobacillus are strictly aerobic bacteria (lives only in oxygen rich environment). All species of Thiobacillus are respiratory organisms. (Note: Although Thiobacillus is classified as a strictly aerobic organism, anaerobic oxidation has been demonstrated with T. ferrooxidans using elemental sulfur with ferric sulfate. However, elemental sulfur or ferric iron must be present in order for the bacterium to grow.)
• Thiobacillus are also “obligate autotrophic organisms”. They require inorganic molecules as an electron donor and inorganic carbon (such as carbon dioxide) as a source. They obtain nutrients by oxidizing iron and sulfur with O2.
• Thiobacillus are Gram-negative Proteobacteria. Their life cycle is typical of bacteria, with reproduction by cell fission.
• Thiobacillus are colorless, rod-shaped, bacteria with polar flagella. They possess an iron oxidase, which allows them to metabolize metal ions such as ferrous iron:      Fe²⁺ + 1/2 O₂ + 2H⁺ –> Fe³⁺ + H₂O
• The genus Thiobacillus is “thermophilic” meaning that they prefer temperatures of 45-50 degrees Celsius.
• The bacterium is capable of oxidizing iron and inorganic sulfur compounds producing sulfuric acid, which could lead to acid mine drainage. They are also resistant to heavy metal toxicity.

Where do they normally found?

Thiobacillus ferrooxidans is commonly found in pyrite deposits, metabolizing iron and sulfur and producing sulfuric acid.

What is the use of Thiobacillus ferrooxidans in Mining?

Thiobacillus ferrooxidans has been widely used in a mining technique called bioleaching where metals are extracted from their sulfide ores through biological oxidation. The bacteria is used as catalysts in the chemical process

The discovery of T. ferrooxidans also led to the development of “biohydrometallurgy” which deals with all aspects of microbial mediated extraction of metals from minerals or solid wastes and acid mine drainage etc.

Can we use Bioleaching as a remediation technique?

Yes, we can. BacTech’s patented BACOX technology uses naturally occurring bacteria, harmless to both humans and the environment, to oxidize the contained sulphides and separate metal from the difficult-to-process tailings. In the process, toxic elements such as arsenic are stabilized. The tailings created by bioleaching are benign, and zero environmental damage occurs as a result of the process. An added bonus is its ability to recover valuable metals such as gold, silver, cobalt and nickel that remain in the tailings. Bioleaching is an environmental reclamation solution that also creates a profit. (Source: http://www.bactechgreen.com/s/Home.asp)

The diagram above presents the general geochemical concept of sulfate oxidation. The zone above the groundwater table is the aerobic zone, where the Thiobacillus ferrooxidans could thrive eating pyrite. It would be very common to find sulfate minerals above the water table, such as malenterite (FeSO4.7H2O) while sulfide minerals would be stable below the water table (anaerobic environment).

Thiobacillus Ferrooxidans and Acid Mine Formation: Chemical Reactions

Thiobacillus Ferrooxidans could enhance leaching of heavy metals from sulfidic ores under aerobic conditions about 104 fold or more compared with weathering without bacteria.

Oxidation of hydrogen sulfide by Thiobacilli

HS^- + 2O₂ –> S0₄^– + H⁺

Oxidation of elemental Sulfur by Thiobacilli

S° + H₂0 + 1½O₂ à S0₄^– + 2 H⁺

Oxidation of ferrous and ferric iron by T. ferrooxidans

2Fe⁺⁺ + 2H⁺ + ½O₂ —->    2Fe⁺⁺⁺ + H₂0

2Fe⁺⁺⁺ + 6H₂0       —->    2Fe(OH)₃ + 6H⁺

2Fe⁺⁺ + 5H₂0 + ½O₂ —->    2Fe(OH)₃ + 4H⁺

Oxidation of pyrite by T. ferrooxidans

FeS₂ + H₂0 + 3½O₂ à Fe⁺⁺ + 2 SO₄^– + 2 H+

During the oxidation process, H+ ion is generated.  So, the growth of the bacteria reduces the pH of the environment, often below 2.  The low pH of the solution leaches our easily soluble elements such as iron and other trace elements in the solution leading to metal laden acid mine drainage (AMD) situation.

How to stop the growth of Thiobacilli

Adding lime to the overburden or the mine tailing could easily stop the growth of Thiobacilli as the sulfide leaching bacteria are acidophilic and could not grow in neutral environment. Which also stop the generation of sulfuric acid if proper amount is added to the coal mine rejects. In many cases the overburden material could have high natural alkalinity due to the presence of carbonate minerals and many not need any additional application of carbonates to fight acid mine drainage problems.

Recommendations:

A complete geochemical study of the target coal seam and the overburden material must be performed and included in mine permit. State and Federal recommendations should be followed to meet mining requirements. In the US, Acid Base Accounting (ABA) is the most commonly used test to evaluate nature of the overburden. In general elemental analyses of the overburden rock material and the coal seam are also performed. In many other countries, such as in Australia, it is common to perform other tests like “Net Acid Generation Test” (NAG) and leach tests to evaluate metal leaching potential over time.

Who to contact?

Recommended Geological Consulting Firm: Marshall Miller & Associates (http://www.mma1.com)

MM&A is a diverse consulting and engineering firm headquartered in Bluefield, Virginia, U.S.A., offers a wide spectrum of services to clients in North America, South America, Asia, and Europe. Over its 35-year history, MM&A has evolved into a leader in the mineral resource, environmental, and carbon management industries. The company’s growth is based on a commitment to applying and developing advanced engineering and scientific technologies and maintaining our talented staff of geologists; hydrologists; earth scientists; and mining, petroleum, environmental, and civil engineers.

References:

• Thiobacillus
• Acidithiobacillus
• Bacterial Leaching

Article by: Ankan Basu, Geologist/Geochemist, Marshall Miller and Associates.

[ReviewAZON asin="1583215778" display="inlinepost"]“Protecting public health is EPA’s top priority. As we continue to learn more about the potential risks of exposure to chromium-6, we will work closely with states and local officials to ensure the safety of America’s drinking water supply,” said Administrator Jackson. “This action is another step forward in understanding the problem and working towards a solution that is based on the best available science and the law.”

The enhanced monitoring guidance provides recommendations on where the systems should collect samples and how often they should be collected, along with analytical methods for laboratory testing. Systems that perform the enhanced monitoring will be able to better inform their consumers about any presence of chromium-6 in their drinking water, evaluate the degree to which other forms of chromium are transformed into chromium-6, and assess the degree to which existing treatment affects the levels of chromium-6 in drinking water.

EPA currently has a drinking water standard for total chromium, which includes chromium-6, and requires water systems to test for it. Testing is not required to distinguish what percentage of the total chromium is chromium-6 versus other forms such as chromium-3, so EPA’s regulation assumes that the sample is 100 percent chromium-6. This means the current chromium-6 standard has been as protective and precautionary as the science of that time allowed.

EPA’s latest data show that no public water systems are in violation of the standard. However, the science behind chromium-6 is evolving. The agency regularly re-evaluates drinking water standards and, based on new science on chromium-6, has already begun a rigorous and comprehensive review of its health effects. In September 2010, the agency released a draft of the scientific review for public comment. When the human health assessment is finalized in 2011, EPA will carefully review the conclusions and consider all relevant information to determine if a new standard needs to be set. While EPA conducts this important evaluation, the agency believes more information is needed on the presence of chromium-6 in drinking water. For that reason, EPA is providing guidance to all public water systems and encouraging them to consider how they may enhance their monitoring for chromium-6.

More information on the new guidance to drinking water systems: http://water.epa.gov/drink/info/chromium/guidance.cfm

More information on chromium:
http://water.epa.gov/drink/info/chromium/index.cfm

More information on the status of the ongoing risk assessment:
http://cfpub.epa.gov/ncea/iris_drafts/recordisplay.cfm?deid=221433