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Completed Activities - Activity IV

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Activity IV Overview

The objective of this activity is to develop, qualify, and screen techniques that show promise for cost-effective remediation of mine waste. The most promising and innovative techniques will undergo bench- or pilot-scale evaluations and applicability studies to provide an important first step to full-scale field demonstrations. Each experiment is assigned as an approved project with specific goals, budget, schedule, and principal team members.


Activity IV, Project 1:
Berkeley Pit Water Treatment

Project Overview

Bench-scale research on treating water from the Berkeley Pit was performed at Montana Tech of the University of Montana, in Butte, Montana.

The Berkeley Pit is an abandoned open-pit copper mine in Butte that has been filling with acidic water since pump dewatering of adjacent underground mines ceased in 1982. Flow into the Berkeley Pit has varied from approximately 7.5 million gallons per day initially to a current rate of approximately 2.5 million gallons per day.

The water in the Berkeley Pit was chosen for this project due to its accessibility, abundance, and the chemical similarities between it and other acidic mine waters. Studies had been conducted since 1986 on the Berkeley Pit water, and substantial analytical data had been developed, providing a foundation for this project.

Technology Description

This project addressed treatability of the acid mine water that is accumulating in the Berkeley Pit. Appropriate treatment techniques were identified and developed. The overall goal was to evaluate technologies that produce clean water, allow for safe waste disposal, and recover selected metals for resale. Technologies were evaluated by considering their effectiveness, technical feasibility, and potential for technology transfer to similar sites.

Experimental testing consisted of four phases:

Physical oxidation, neutralization, and metal removal-this phase consisted of using alkaline reagents such as lime, limestone, or soda ash to neutralize the water and cause metals to precipitate as hydroxides. During neutralization, the water is aerated to oxidize Fe+2 to Fe+3, thereby, enhancing sludge settling characteristics and promoting adsorption reactions. Metals removal efficiency and reaction kinetics were studied.

Metals separation and recovery-this phase was a two-stage hydroxide precipitation process. Sulfide and hydroxide precipitation were combined for more complete removal of metals. In other tests, metal sulfides were precipitated first to recover metal value, and scrap iron was used to cement copper before neutralization.

Use of milling waste-this phase consisted of adding tailings slurry (primarily silicates, clay, lime, and limestone) directly to the Berkeley Pit water. This partially neutralized the water and removed some of the heavy metals. This in situ neutralization could potentially reduce reagent consumption and sludge formation for subsequent processing.

Diversion and treatment of various inflow water sources-this phase consisted of investigating numerous water sources to determine the feasibility of diverting inflow water for treatment. Of the water that flows into the Berkeley Pit, one-third is surface water from the Horseshoe Bend area, and two-thirds is underground water that has penetrated through the mines and surrounding rocks.

Results

All work for this project was completed, and the final report was published. (PDF, 208 Kb, 39 pp)


Activity IV, Project 2:
Sludge Stabilization

Project Overview

The Sludge Stabilization Project for mine waste was a bench-scale research project conducted at Montana Tech of the University of Montana.

The purpose of this research project was to study the properties and stability of sludges generated by remediation of acid mine waters. Results of the study were used to determine the best methods for sludge handling and disposal. One source of acid mine water being studied was from the Crystal Mine located approximately 7 miles north of Basin, Montana. The other source was the water from the Berkeley Pit in Butte, Montana. Besides being acidic, these waters contain toxic concentrations of iron, manganese, copper, zinc, arsenic, and sulfate, which is typical of many hard rock mining operations throughout the western United States.

Past research on remediating acid mine water has focused primarily on water treatment techniques, and little emphasis has been placed on the stability of the sludge that is generated. To address this issue, faculty at Montana Tech of the University of Montana, with expertise in chemistry, geochemistry, metallurgy, and environmental engineering, formed a research team to study the properties and stability of this sludge.

Technology Description

The three types of sludge studied were: base initiated sludge, inorganic sulfide-initiated sludge, and sulfate-reducing bacteria-initiated sludge. Appropriate solid-liquid separation techniques were used to isolate the solid phases for chemical characterization and stability tests. Chemical characterization studies included quantifying the various element-solid associations, i.e., adsorbed, surface-precipitated, and coprecipitated contaminants. These studies then identified and quantified the divalent and trivalent forms of iron and the trivalent, pentavalent, and methylated forms of arsenic. Once analytical techniques were verified for each of the sludges, they were applied to as-generated sludge and aged sludge.

Based on the chemical properties of these sludges, various storage environments were proposed and evaluated. The sludge stability research included standard regulatory tests and specifically designed tests, e.g., biostability tests, based on the selected specific disposal options, including storage in the natural environment. The results of these tests were translated into stability-enhancement studies, including the effect of aging the sludge in a temporary storage environment and treating the sludge with chemical additives before final storage.

The results of this sludge characterization and stability study identified characterization techniques and stability procedures that have application to sludges generated through other water-treatment procedures.

Results

All work for this project was completed, and the final report was published.


Activity IV, Project 3:
Photoassisted Electron Transfer Reactions Research

Project Overview

Research efforts under the Mine Waste Technology Program for the remediation of mine wastewaters have focused primarily on removing toxic heavy metal cations from solution. However, little attention has been given to toxic anions that can be associated with the heavy metal cations.

Results

All work for this project was completed. The final report was revised and published. (PDF, 212 Kb, 33 pp)


Activity IV, Project 3A:
Photoassisted Electron Transfer
Reactions for Metal-Complexed Cyanide

Project Overview

Previous research efforts under the Mine Waste Technology Program for the remediation of mine wastewaters predominantly focused on removing toxic heavy metal cations from solution. This was accomplished with chemical processes that generated heavy-metal sludges that were then removed from the water stream by solid-liquid separation processes. However, many of the anions associated with the heavy metal cations in the wastewater are also toxic but remain in solution even after the sludge is generated and separated. In this project, the remediation of metal-complexed cyanide is being investigated using several photolytic methods with the intent to identify and enhance naturally occurring remediation processes.

Overwhelming evidence shows that natural processes occur to heal environmental scars caused by mining activities. These processes include electron-transfer reactions that lower the concentrations of the anionic mobile toxic constituents in surface and ground waters through interactions with electromagnetic radiation (predominantly ultraviolet radiation but some visible light) from the sun. However, such direct natural photolytic processes suffer at night, on cloudy days, and in winter months. During these periods, artificial radiation sources are needed for sustainment. Furthermore, because the photolytic processes usually proceed slowly, catalysts are used to absorb the radiation and transfer the energy to the reactants to remediate the water within more acceptable time frames. Such photocatalysts are either solid semiconductors (heterogeneousphotocatalysts) or dissolved radicals in solution (homogeneous photosensitizers).

Technology Description

Background: When electromagnetic radiation is absorbed, electrons in the absorbing species pass from a singlet ground state (S0) to an excited electronic state (S1). As long as the electron remains in the excited state, the absorbing species are more susceptible to their chemical environment and are, therefore, more apt to participate in electron-transfer reactions. The absorbing species undergo photoreduction when it donates the excited electron. Conversely, photooxidation occurs when the absorbing species accept an electron. In either case, the photoreduction and photooxidation reactions can lead to the destruction of the mobile toxic constituent. For metal-complexed cyanide, only photooxidation can be used and in a reaction similar to cyanide photooxidation (see Activity IV, Project 3) where carbon dioxide and nitrogen gases are reaction products:

M(CN)xx-y + xO2 = My + (½)xN2(g) + xe

Direct Photolysis-In this process, the mobile toxic constituent being remediated must absorb the electromagnetic radiation. Although this phenomenon is rare, it does occur with some metal-complexed cyanides but is dependent on the solution conditions. Research was conducted to identify these conditions.

Homogeneous Photolysis-In this process, aqueous photosensitizers absorb the electromagnetic radiation and then transfer the photon energy to the mobile toxic constituents being remediated. Because the process occurs in bulk solution, its kinetics are dependent on the solution conditions and the concentrations of the photosensitizers and the mobile toxic constituents. When the aqueous photosensitizer is not consumed during the process, it is referred to as homogeneous photocatalysis. In this regard, research is being conducted to identify the conditions needed for using either homogeneous photosensitizers or homogeneous photocatalysts for metal-complexed cyanide remediation.

Heterogeneous Photocatalysis-In this process, solid semiconductors are used to absorb the electromagnetic radiation and then transfer the photon energy to the mobile toxic constituent being remediated. However, electron transfer reactions can only occur if the mobile toxic constituent is adsorbed at the surface of the semiconductor. Thus, reaction kinetics are dependent on the mobile toxic constituent concentration as well as the rate of adsorption of the constituent, the available surface area of the semiconductor, and the rate of desorption of the reaction products. Consequently, reaction kinetics can be three orders of magnitude slower than reactions with homogeneous photolysis.

Nevertheless, reaction efficiencies are usually higher with heterogeneous photocatalysis due to the higher efficiency of photon capture and the increased life of the electron in the excited state. This is ultimately attributed to the properties of the semiconductor. With semiconductors, electrons are promoted from the valence band and into the conductance band across a band gap. The photon energy must be greater than or equal to the band gap energy. Excited electrons in the conductance band can then be donated to the mobile toxic constituent to induce its reduction. Likewise, the electron vacancy or hole in the valence band can accept electrons from the mobile toxic constituent and, thereby, induce its oxidation. The process is similar to the process described earlier; however, it is evident that solution conditions must also be well-defined to control reactant adsorption and product desorption. In this regard, studies are being conducted to optimize these conditions for metal-complexed cyanide oxidation reactions. Currently, only anatase (TiO2) is being investigated because it has the highest known efficiency of semiconductors.

Results

This project was a continuation of the nitrate and cyanide project (Activity IV, Project 3) but with the inclusion of photolytic research on metalcomplexed cyanides. The final report was published. (PDF, 2.7 M, 89 pp)


Activity IV, Project 3B:
Photoassisted Electron Transfer
Reactions for Berkeley Pit Water

Project Overview

See Activity IV, Project 3A for Project Overview.

Technology Description

Background-When electromagnetic radiation is absorbed, electrons in the absorbing species pass from a singlet ground state (S0) to an excited electronic state (S1). As long as the electron remains in the excited state, the absorbing species are more susceptible to their chemical environment and are, therefore, more apt to participate in electron-transfer reactions. The absorbing species undergo photoreduction when it donates the excited electron. Conversely, photooxidation occurs when the absorbing species accept an electron. In either case, the photoreduction and photooxidation reactions can lead to the precipitation of mobile toxic constituents. For example, ferrous cations can be precipitated as ferri-hydroxide after being photooxidized to ferric cations:

h Fe2+ = Fe3+ + e Fe3+ + 3OH = Fe(OH)3(s) .

This reaction mechanism may account for the natural precipitation events observed in Berkeley Pit water. Once the iron is precipitated and separated, photolysis and/or conventional hydrometallurgical processes can then be used to recover the valuable mobile toxic constituents. On the other hand, a photoreduction reaction is exemplified by sulfate conversion to elemental sulfur:

SO42-(aq) + 8H+ + 6e- S(s) + 4H2O

Clearly, acid mine waters can be remediated through photolysis. However, it is important to note that several competing processes may occur and must be prevented and/or minimized to maximize the efficiency of photoassisted electron transfer reactions.

Nevertheless, reaction efficiencies are usually higher with heterogeneous photocatalysis due to the higher efficiency of photon capture and the increased life of the electron in the excited state. This is ultimately attributed to the properties of the semiconductor. With semiconductors, electrons are promoted from the valence band and into the conductance band across a band gap. The photon energy must be greater than or equal to the band gap energy. Excited electrons in the conductance band can then be donated to the mobile toxic constituent to induce its reduction. Likewise, the electron vacancy or hole in the valence band can accept electrons from the mobile toxic constituent and, thereby, induce its oxidation. The process is similar to the process described earlier; however, it is evident that solution conditions must be well defined to control reactant adsorption and product desorption. In this regard, studies are being conducted to optimize these conditions for metalcomplexed cyanide oxidation reactions. For now, both hematite (Fe2O3) and anatase (TiO2) are being investigated. Hematite is important because it can actually be formed by recycling the precipitated ferrihydrite:

2Fe(OH)3(s) = Fe2O3(s) + 3H2O

Whereas, the anatase is important because it has the highest known efficiency of semiconductors.

Results

The final report was published. (PDF, 6 M, 79 pp)


Activity IV, Project 4:
Metal Ion Removal from Acid Mine
Waste Waters by Neutral Chelating Polymers

Project Overview

A bench-scale research project was conducted at Montana Tech of the University of Montana to eliminate or minimize some current economic or technical difficulties that exist in treatment technologies for acid mine wastewater. The novel technology was based on neutral chelating polymers that can have their chelating property turned on and off. The chelate switch was based on known electrochemical or photochemical properties of electrically conducting polymers.

Technology Description

Chelates are chemical substances that have more than one binding site on the molecule. These added binding sites attach a molecule to a metal ion more strongly than a single binding site. The result is that chelates can be very effective at removing metal ions from wastewater. Chelates can be ionic or neutral. Ionic chelates exchange a cation (H+ or Na+) for the metal ion removed from the solution. Neutral chelates are electrically neutral and do not add material to the solutions when the metal ions are removed.

The removal of metal ions from aqueous solutions is presently accomplished by a variety of chemical and electrochemical processes. These techniques have distinct advantages in the appropriate situations (pH range, concentration range, matrix composition, etc.); however, they may not be practical under less-than-optimum operating conditions.

The goal of this project was to develop an alternate technology that required no additional chemicals, could produce a marketable product (such as pure metals), and could reduce costs and waste volume. The research project was a collaborative effort between academic and government resources, including the Haskell Indian Nations Universities' Haskell Environmental Research Studies Center. Initially, the project focused on the design of chelating polymer systems for laboratory study and for theoretical study (molecular modeling). The first polymer systems were based on current literature information. Modeling results were compared to experimental and literature results as a means to test the validity of the theoretical data.

The validated modeling procedure was used to design and test a variety of neutral chelating systems for their capability to remove metal ions and associated anions from acid mine wastewater. The neutral chelating polymers determined to be most effective for water cleanup by the preliminary experimentation and the modeling studies were studied more thoroughly. The polymeric systems were evaluated for their removal efficiencies, contaminant capacity, ruggedness, ease of use, and cost effectiveness. Other important parameters identified in the preliminary studies were also used in the systems evaluations.

A more detailed process evaluation procedure was developed from the results of the refined experimentation. The selected polymeric system was then completely studied using a variety of synthetic and actual mine wastewater.

Results

All work for this project was completed, and the final report was published. (PDF, 235 Kb, 53 pp)


Activity IV, Project 5:
Removal of Arsenic as Storable
Stable Precipitates

Project Overview

The objective of this project was to strip arsenic from solutions in such a way as to produce apatite mineral-like precipitated products that are stable for long-term storage in tailing pond environments. Substitution of arsenic into an apatite structure will provide a solid solution mineral compound that is environmentally stable for outdoor pond storage.

Technology Description

Earlier research demonstrated that a precipitation technique is effective in removing arsenic (to low micrograms per liter concentrations) from aqueous solutions [U.S. Environmental Protection Agency (EPA)-supported project]. The precipitation is conducted in a way to form a solid solution compound containing arsenate and phosphate in an apatite mineral-like phase. This solid is stable to EPA's toxicity characteristic leachate procedure, and more importantly, the solubility is one to two orders of magnitude less than calcium arsenate in aqueous solutions over the pH range of 9 to 12 (the range of pH values maintained in tailing ponds).

In the early 1980s, it was demonstrated that lime precipitation of calcium arsenate with subsequent storage in a tailings pond environment is unacceptable because at pH levels above approximately 8.5, calcium arsenate will be converted to calcium carbonate (by carbon dioxide in air) with the release of arsenic into the aqueous phase. Removal of arsenic by precipitation as calcium arsenate has been discontinued by industry and has been replaced by ferric arsenate precipitation (EPA's Best Demonstrated Available Technology for arsenic-bearing solutions). However, even though low concentrations of arsenic in solutions can be achieved by ferric precipitation, it has been demonstrated that the removal from solution is actually an adsorption phenomena. Therefore, long-term stability of such residues in tailings pond environments may not be appropriate, hence, the need for the present study.

Stability of Mineral-Like Residues-Montana Tech of the University of Montana researcher's approach to arsenic storage was to form a mineral-like phase that showed equilibrium-phase stability under tailings pond environmental conditions. If equilibrium-phase stability was achieved (for a given environment), then long-term stability would be ensured (at least for as long as the environmental conditions were maintained). This project was supporting an intensive investigation of the formation of arsenic precipitates in two systems, i.e., the calcium-arsenic-phosphate (apatite-like solid solutions of arsenate and phosphate) system, and the ferric-arsenic-phosphate (phosphoscorodite-like solid solutions of arsenate and phosphate) system. Both of these systems showed great promise for industrial application, if long-term stability could be demonstrated.

Results

The precipitation recipe was applied to two industrially contaminated waters, and the long-term stability of the resulting products were tested. Successful demonstrations resulted in a new way to treat arsenic-bearing wastewaters and mine drainage solutions.

All work for this project was completed, and the final report was published. (PDF, 402 Kb, 87 pp)


Activity IV, Project 7:
Berkeley Pit Innovative
Technologies Project

Project Overview

The purpose of the Berkeley Pit Innovative Technologies Project was to provide a test bed for high risk/innovative technologies for the remediation of Berkeley Pit water. The project focused on bench-scale testing of remediation technologies to help assist in defining alternative remediation strategies for the U.S. Environmental Protection Agency's (EPA) future cleanup objectives for Berkeley Pit waters.

Individuals, companies, or academic institutions with existing remediation technologies were invited to demonstrate their process for the project and write a report summarizing their process including the results of their bench-scale test. A copy of the report from each test was forwarded for evaluation by the EPA Region VIII field office, and the EPA National Risk Management Research Laboratory.

Results

Nine demonstrations were completed and reports are available.

Berkeley Pit Innovative Technologies Project
(a) Purity Systems, Inc. (PDF, 160 Kb, 55 pp)
(b) Geo2 Limited (PDF, 95 Kb, 43 pp)
(c) SPC International Corporation (PDF, 54 Kb, 25 pp)
(d) Technical Assistance International, Inc. (PDF, 72 Kb, 35 pp)
(e) HydroPlus Technologies (PDF, 34 Kb, 22 pp)
(f) International Hydronics (PDF, 72 Kb, 22 pp)
(g) Geo2 Limited (PDF, 73 Kb, 38 pp)
(h) Metre General, Inc. (PDF, 41 Kb, 25 pp)
(i) Mine Remediation Services (PDF, 70 Kb, 31 pp)
(j) Hydrometrics, Inc. (PDF, 68 Kb, 26 pp)
(k) Louisiana State University

Activity IV, Project 8:
Pit Lake System— Characterization
and Remediation for the Berkeley Pit

Project Overview

An interdisciplinary team of Montana Tech of the University of Montana researchers undertook a preliminary study of several aspects of the Berkeley Pit to gather specific information about that pit lake system and to gather information that could be generally applied to all pit lakes.

In this work, the chemical and biological characteristics of the water and sediments in the Berkeley Pit were studied to provide water quality data that can be used to predict future water quality, to evaluate the potential for natural remediation by bacteria such as sulfate-reducers, and to determine if partial in situ remediation would be practical prior to the pump and treat technologies prescribed in the U.S. Environmental Protection Agency's Record of Decision.

To provide the water and sediment for the characterizations, the Montana Bureau of Mines and Geology sampled the water at two locations and at depths from the surface to the bottom (0 to 1,200 feet) at set intervals. Sampling was also done in both the spring and the fall to account for climatic effects on surface water quality.

Results

The water chemistry of the Berkeley Pit lake varies with the volume of water entering it from various sources and the changes in the seasons. The amount of metals precipitated from the surface water layer depends on the area of the water surface exposed to the air and the climatic changes associated with the four seasons. The chemistry of the deep water is relatively constant throughout the year.

Organic carbon, a food source for bacteria is present in the water. The concentration of organic carbon is relatively that of the natural occurring springs in the area of the Berkeley Pit.

No sulfate-reducing bacteria activity was detected in the water or the sediments. However, a number of fungi and yeasts were isolated, and these will be further studied.

The report for this study is complete. (PDF, 359 Kb, 97 pp) This work lead to the more specific research presented in Activity IV, Projects 9, 10, 11, and 16.


Activity IV, Project 9:
Pit Lake System—Deep Water
Sediment/Pore Water Characterization
and Interactions

Project Overview

Research under this project involved collecting various water and solid samples from the Berkeley Pit to characterize them and formulate a conceptual environmental model of this well known pit lake.

The work involved collecting deep water-upper layer sediment samples (600 to 700 feet below surface), collecting subsurface sediment/pore water samples (717 feet below surface), characterization and speciation of these sediment solids and pore water, and modeling the system to understand the controlling sediment forming reactions.

Results

Significant differences did not appear in the elemental content of the upper water column and the deep water (near-sediment) solution. Iron shows a slight (approximately 5%) increase in concentration with depth. The ferrous-to-ferric ratio shows an increase from the surface to approximately 100 feet; the ratio then remains constant from 100 to 717 feet. Sulfate showed a generally increasing concentration as a function of depth. The dissolved oxygen concentration was relatively high near the surface, dropped dramatically from 2 to 18 feet, and then rose to levels exceeding the surface level. It then became relatively constant with increasing depth from approximately 100 feet to near the sediment surface. The dissolved oxygen data appears to suggest that surface water turnover may have occurred down to the 100-foot level; however, additional data is required to confirm this conclusion.

Pore water is the water present within the sediments. This water was separated as a function of depth into a series of samples. The pore water was not clean and had appreciable elemental content. Pore water had lower concentrations of aluminum, zinc, manganese, magnesium, arsenic, potassium, and phosphorus than deep water 3 feet above the sediments. Ferrous iron concentrations in the pore water were as much as four times higher in the upper sediment layers than in the deep water. The reaction of potassium jarosite and/or schwertzmannite with organic carbon to form ferrous species appears to be feasible for the conditions existing in the sediments, and it is the likely reaction controlling the ferrous concentration in the pore water.

Sediment solids showed varying composition trends. Elements that showed decreasing concentration with depth include arsenic, calcium, iron, magnesium, phosphorus, lead and sulfur. The sediments were composed of detrital and precipitated compounds. The major precipitated compounds were jarosite and gypsum. The major detrital compounds were quartz, biotite and muscovite, which are predictable wall rock element at the Berkeley Pit. It was observed that the precipitated materials had a higher concentration at the surface of the core, which suggests that the precipitated solids formed in the water column and settled to the sediment surface. With time, wall rock joined the sediments and diluted them with detrital materials.

All work was completed, and the final report was published. (PDF, 236 Kb, 66 pp)


Activity IV, Project 10:
Pit Lake System—Biological
Survey of Berkeley Pit Water

Project Overview

The purpose of this research was to begin to gain an understanding of the microbial ecology of the Berkeley Pit lake system and to provide necessary data for bioremediation studies of this pit lake and others. The study goals were to determine species diversity and numbers for organisms present in the pit lake system and to begin to understand their potential role in bioremediation.

Results

The research shows that bacterial abundances are high throughout the surface water column; on average, approximately 116,000 bacteria per milliliter were found, which is only nine times less than levels in a freshwater lake. Water samples from a lower depth contain far fewer bacteria (7,000 per milliliter). Sixteen morphotypes of heterotrophic protists were identified. Very few live cells were found in fresh samples of water and sediments, suggesting that active populations are rare and most may exist as cysts, particularly in deeper anaerobic layers. The sulfate-reducing bacteria that were expected to be found in the water and sediments did not exist.

All work was completed, and the final report was published. (PDF, 783 Kb, 54 pp)


All figures and tables can be found in the Mine Waste Technology Annual Report.
(PDF, 5.6 M, 74 pp)

 


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