Final Report: A Novel Approach to Prevention of Acid Rock Drainage (ARD)

EPA Contract Number: 68D99054
Title: A Novel Approach to Prevention of Acid Rock Drainage (ARD)
Investigators: Olson, Gregory J.
Small Business: Little Bear Laboratories Inc.
EPA Contact: Richards, April
Phase: I
Project Period: September 1, 1999 through March 1, 2000
Project Amount: $69,968
RFA: Small Business Innovation Research (SBIR) - Phase I (1999) RFA Text |  Recipients Lists
Research Category: Watersheds , SBIR - Water and Wastewater , Small Business Innovation Research (SBIR)

Description:

The project evaluated a novel chemical agent for its potential to 1) control acid rock drainage (ARD) at its source and 2) stop ARD once it starts. The agent is a potent inhibitor of acidophilic (acid-loving) pyrite-oxidizing bacteria that play a major role in formation of ARD from sulfidic mine tailings. The agent at low concentrations is a highly specific inhibitor of acidophilic bacteria, but has low toxicity to other forms of life. Thus, it could be used safely at low concentrations in the environment.

The effectiveness of the agent was evaluated in the laboratory on three samples of sulfidic, acid-producing U.S. mine rock. The tests employed accelerated weathering laboratory humidity cells following methods described in American Society for Testing and Materials (ASTM) procedure D5744-96. The procedure involved a weekly leach with deionized water followed by aeration for 3 days with moist air and 3 days with dry air. The cycle was repeated weekly for 8 to 14 weeks. The agent was spiked at various concentrations into leach solutions applied to some humidity cells. For comparison, other humidity cells were untreated controls, receiving only deionized water. Leach solutions were routinely measured for the following parameters indicative of ARD: pH, redox potential, sulfate, iron, conductivity and total acidity. Certain samples were also analyzed for the concentrations of heavy metals. Additional preliminary tests investigated production of a controlled release form of the agent.

Summary/Accomplishments (Outputs/Outcomes):

Control humidity cells not treated with the agent generated ARD. Humidity cells receiving leach solution amended with the agent at a low concentration of 5 to 20 parts per million (ppm) did not develop biocatalyzed ARD. When applied to control humidity cells that were already generating ARD, the agent at the same concentration (5 to 20 ppm) stopped biooxidation of sulfides. In one case, a single application of the agent at 20 ppm stopped biocatalyzed ARD for several weeks. Whether applied prior to onset of biooxidation or where microorganisms were already generating ARD, the agent reduced iron and sulfate release from sulfidic rock in the humidity cells down to the estimated background chemical oxidation rate. The agent also significantly reduced the acidity and leaching of heavy metals from the rock compared to untreated controls.

The novel agent selectively inhibits acidophilic bacteria. It is biodegradable at neutral pH but was not biodegraded in acidic zones where pyrite oxidizing bacteria act to catalyze the production of ARD. The agent did not adsorb significantly to rock.

A controlled release form of the agent was prepared and tested in preliminary investigations. This form of the agent was readily soluble in dilute sulfuric acid but was only slightly soluble in deionized water. Thus, low concentrations of the agent would be released from contact with rainfall, but higher concentrations would be released if ARD began to form.

Tests were done to determine if the toxicity of the agent could be increased in the presence of surfactants. No advantage was found in inhibiting biooxidation of sulfides by blending the agent together with surfactants.

Conclusions:

The agent was shown to have the potential to be a more effective, less expensive inhibitor of pyrite-oxidizing bacteria than is available from existing technology. It was shown effective in preventing biocatalyzed ARD and in stopping ongoing ARD. Consequently, it could be applied to fresh tailings as well as to old tailings at abandoned sites. Its lack of binding to rock means it can readily percolate in spoils heaps, or can be injected into wells or bore holes, to stop biogenic ARD. An acid-soluble form of the agent was produced that could provide controlled release of the agent in response to acid generation, yet protect it from biodegradation under neutral pH conditions. Such a product could be blended with or plowed into tailings. Still another application of the agent was suggested in terms of improved solution management strategies in mines. For example, the agent is known to be present in process solutions in certain types of mining operations. Rather than being treated, these solutions could be applied to tailings heaps during mine closure to prevent ARD. Furthermore, the agent might also be used in closure of copper and precious metal ore bioleaching mining operations to prevent ARD following bioprocessing.

Supplemental Keywords:

acid mine drainage prevention, acid rock drainage prevention, mine closure, mine wastes, pyrite-oxidizing bacteria, remediation., Scientific Discipline, Waste, Water, Contaminated Sediments, Remediation, Wastewater, Chemistry, Microbiology, Wet Weather Flows, Biochemistry, Engineering, Engineering, Chemistry, & Physics, inhibitory compounds, wastewater treatment, acid mine drainage, mining, acid rock drainage, acid rock drainage (ARD), ARD, control technologies, mining impacted watershed, acid mine runoff, waste water treatment

SBIR Phase II:

A Novel Approach To Prevention of Acid Rock Drainage (ARD)  | Final Report