Thomas R. Wildeman

Adsorption compared with sulfide precipitation as metal removal processes from acid mine drainage in a constructed wetland

Abstract Metal removal processes from acid mine drainage were studied in an experimental constructed wetland in the Idaho Springs-Central City mining district of Colorado. The wetland was designed to passively remove heavy metals from the mine drainage flowing from the Big Five Tunnel. Concurrent studies were performed in the field on the waters flowing from the wetland and in the laboratory on the wetland substrate. Both studies suggest that there is competition for organic adsorption sites among Fe, Cu, Zn and Mn. Iron and Cu appear to be more strongly adsorbed than Zn and Mn. The adsorption of metals varies with the fluctuation of pH in the outflow water. Also indicated by field and laboratory studies is the microbial reduction of sulfate with a corresponding increase in the sulfide concentration of the water. As sulfide is generated. Cu and Zn are completely removed. The field results suggest that upon start up of a constructed wetland, the adsorption of dissolved metals onto organic sites in the substrate material will be an important process. Over time, sulfide precipitation becomes the dominant process for metal removal from acid mine drainage.

The group separation of the rare-earth elements and yttrium from geologic materials by cation-exchange chromatography

Abstract Demand is increasing for the determination of the rare-earth elements (REE) and yttrium in geologic materials. Due to their low natural abundance in many materials and the interferences that occur in many methods of determination, a separation procedure utilizing gradient strong-acid cation-exchange chromatography is often used to preconcentrate and isolate these elements from the host-rock matrix. Two separate gradient strong-acid cation-exchange procedures were characterized and the major elements as well as those elements thought to provide the greatest interference for the determination of the REE in geologic materials were tested for separation from the REE. Simultaneous inductively coupled argon plasma—atomic emission spectroscopy (ICAP—AES) measurements were used to construct the chromatograms for the elution studies, allowing the elution patterns of all the elements of interest to be determined in a single fraction of eluent. As a rock matrix, U.S. Geological Survey standard reference BCR— 1 basalt was digested using both an acid decomposition procedure and a lithium metaborate fusion. Hydrochloric and nitric acids were tested as eluents and chromatograms were plotted using the ICAP—AES data; and we observed substantial differences in the elution patterns of the REE and as well as in the solution patterns of Ba, Ca, Fe and Sr. The nitric acid elution required substantially less eluent to elute the REE and Y as a group when compared to the hydrochloric acid elution, and provided a clearer separation of the REE from interfering and matrix elements.

The distribution of Mn2+ in some carbonates by electron paramagnetic resonance

Abstract The results from the electron paramagnetic resonance (E.P.R.) spectra of maganese in fourteen natural and synthetic carbonate minerals show that manganese exists as Mn 2+ in highly ionic lattice sites which are isolated in the carbonate mineral and slightly distorted from octahedral symmetry. There are two lattice sites for Mn 2+ in dolomite and they are assigned as the Mg 2+ and Ca 2+ substitutional sites for Mn 2+ . The E.P.R. spectra show that the Mg-site is more populated than the Ca-site. From the results of the spectra of Mn 2+ in sedimentary and hydrothermally altered dolomites from the Leadville Formation of Colorado, it is found that the concentration ratio of Mn 2+ in the Mg-site to Mn 2+ in the Ca-site increased with alteration.

Balance of S in a constructed wetland built to treat acid mine drainage, Idaho Springs, Colorado, U.S.A.

The wetland constructed at the Big Five Tunnel in Idaho Springs, Colorado was designed to remove, passively, heavy metals from acid mine drainage. In optimizing the design of such a wetland, an improved understanding of the chemical processes operating there was required, particularly SO42− reduction and sulfide precipitation. For this purpose, field and laboratory data were collected to study the balance of S in the system. Field data collected included water analyses of the mine drainage and wetland effluents and measurements of H2S gas emissions from the wetland. The concentration of sulfide in the wetland effluent ranged from 10−4 to 10−3 mol/l. The average rates of H2S emission from the surface of the substrate were 150 nmol/cm2/d in the summer and 0.17 and 0.35 nmol/cm2/d in the winter. This maximum estimated loss of sulfide was not significant in reducing the amount of sulfide available for precipitation with metals. Sequential extraction experiments for S on wetland substrates showed that acid volatile sulfides (AVS) increased with time in the wetland substrate. A serum bottle experiment was conducted to study the S balance in the Big Five wetland by quantitatively measuring the amount of S in different phases as microbial SO42− reduction progressed. The increase in AVS reasonably balanced the decrease in SO42− concentration in the experiment, suggesting that the decrease in SO42− concentration represented the amount of SO42− reduced and that nearly all of the sulfide produced was precipitated as AVS. Sulfide precipitation was determined to be the primary metal removal process in the wetland system and amorphous FeS is the primary iron sulfide formed in the substrate.

Rare earths in the Leadville Limestone and its marble derivates

Abstract Samples of unaltered and metamorphosed Leadville Limestone (Mississippian, Colorado) were analyzed by neutron activation for ten rare-earth elements (REE). The total abundance of the REE in the least-altered limestone is 4–12 ppm, and their distribution patterns are believed to be dominated by the carbonate minerals. The abundances of the REE in the marbles and their sedimentary precursors are comparable, but the distribution patterns are not. Eu is enriched over the other REE in the marbles, and stratigraphically upward in the formation (samples located progressively further from the heat source), the light REE become less enriched relative to the heavy REE. The Eu anomaly is attributed to its ability, unique among the REE, to change from the 3+ to 2+ oxidation state. Whether this results in preferential mobilization of the other REE or whether this reflects the composition of the pore fluid during metamorphism is unknown. Stratigraphically selective depletion of the heavy REE may be attributed to more competition for the REE between fluid and carbonate minerals in the lower strata relative to the upper strata. This competition could have been caused by changes in the temperature of the pore fluid or to the greater resistance to solution of the dolomite in the lower parts of the formation than the calcite in the upper parts.

A NEW MILLENNIUM OF PASSIVE TREATMENT OF ACID ROCK DRAINAGE: ADVANCES IN DESIGN AND CONSTRUCTION SINCE 1988

The history of passive treatment of acid rock drainage dates back over 20 years. It is only recently that engineers and scientists have been able to discern how Mother Nature has been immobilizing metals in natural wetlands and to mimic her handiwork. Since 1988 (when engineers and scientists gathered at two major technical conferences in Pittsburgh and Chattanooga), the geochemistry of metal precipitation in oxidizing and reducing environments has become better understood and the capacity of passive treatment systems for mine drainage has reached levels of 1,200 gpm. Systems operating in tropical and alpine environments indicate that this technology has broad application. While there have been advances, a “cook book” approach to design has yet to be realized. However, a staged design protocol of laboratory, bench-, and pilot-scale testing has yielded full-scale designs that have been functioning as intended. Future advancements needed include a focus on sulfate removal and the recovery of resources that might make this already economical water treatment method even more so. Additional

THE CHALLENGES OF DESIGNING, PERMITTING AND BUILDING A 1,200 GPM PASSIVE BIOREACTOR FOR METAL MINE DRAINAGE WEST FORK MINE, MISSOURI

An active underground lead mine produces water having a pH of 8.0 with 0.4 to 0.6 mg/L of Pb and 0.18 mg/L of Zn. A full-scale 1,200 gpm capacity bioreactor system was designed and permitted based on a phased program of laboratory, bench and pilot scale bioreactor testing; it was constructed in mid-1996. The gravity flow system, covering a total surface area of about five acres (2 ha), is composed of a settling basin followed by two anaerobic bioreactors arranged in parallel which discharge into a rock filter polishing cell that is followed by a final aeration polishing pond. The primary lead removal mechanism is sulfate reduction/sulfide precipitation. The discharge has met stringent in-stream water quality requirements since its commissioning. The system was designed to last about 12 years, but estimates suggest a much longer life based on anticipated carbon consumption in the anaerobic cells.

DEVELOPMENT OF A SIMPLE SCHEME TO DETERMINE THE CHEMICAL TOXICITY OF MINE WASTES 1

A decision tree that uses simple physical and chemical tests has been developed to determine whether a mine waste poses a toxicity threat to the aquatic environment. For the chemical portion of the tree, leachate tests developed by the US Geological Survey (USGS), the Colorado Division of Minerals and Geology (CDMG), and modified 1311 TCLP test of the EPA have been extensively used. The multi-element power of modern inductively coupled plasma, atomic-emission spectroscopy (ICP-AES) is also a necessary component of the scheme. At two sites in Colorado, Virginia Canyon in the Idaho Springs/Central City Superfund Site and in the Upper Animas River Basin, 25 sediment samples and the water flowing over the sediments were collected. General analytical measurements were made in the field, and then, the water and extracts from the three leachate tests were analyzed for 31 elements by ICP-AES. Then, element concentration pattern graphs (ECPG) were produced that compared selected groups of the elements from the three leachates and the water. When the pHs of the water and the leachate were below 5.0, the element concentration patterns of all four solutions were quite similar and aquatic toxicity from metals such as Pb, Cu, Zn, Mn and Al was clearly indicated. When the pHs of the water and the leachate were above 5.0, the element concentration patterns from the four solutions were different and inferred aquatic toxicity depended on the leachate test. Usually when there was a difference, it was found that in the TCLP test, elements from carbonate minerals and oxides dissolved and these elements in the CDMG and USGS tests were not as readily released from solution. In a study done in 2002 in Russell Gulch near Central City, CO, that rated mine waste piles, it was necessary to rate the contamination possibility of the piles on separate physical and chemical scales for the most complete assessment.

CONCEPTUAL METHODS FOR RECOVERING METAL RESOURCES FROM PASSIVE TREATMENT SYSTEMS 1

Recovering mineral resources retained in passive systems for treating acid drainage may be a way for mining companies to achieve sustainability goals. While the development of metallurgical methods for Fe oxide recovery from passive systems is underway, no parallel research effort has apparently been undertaken for the recovery of precious metals, sulfides, or carbonates from sulfate reducing bioreactors (SRBRs), another effective passive treatment technique. The examination of conceptual beneficiation, pyrometallurgical, and hydrometallurgical processes that might be used in resource recovery is a logical first step in this effort. Resource recovery process challenges include dealing with the abundance of organic matter in the SRBR substrate media and the dispersed and probably microscopic character of the metal precipitates. Screening, wet classification, and roasting of the SRBR media appear to be common threads for recovering the four elements considered in the paper: copper, gold, silver, and uranium. As the SRBR technology matures further, research could help close the loop in a process that is now considered to be solely treatment. Additional Keywords: sustainability, recycling, metals, beneficial uses, sulfate reducing bioreactors

A Simple Scheme to Determine Potential Aquatic Metal Toxicity from Mining Wastes

A decision tree (mining waste decision tree) that uses simple physical and chemical tests has been developed to determine whether effluent from mine waste material poses a potential toxicity threat to the aquatic environment. For the chemical portion of the tree, leaching tests developed by the United States Geological Survey, the Colorado Division of Minerals and Geology (Denver, CO), and a modified 1311 toxicity characteristic leaching procedure (TCLP) test of the United States Environmental Protection Agency have been extensively used as a surrogate for readily available metals that can be released into the environment from mining wastes. To assist in the assessment, element concentration pattern graphs (ECPG) are produced that compare concentrations of selected groups of elements from the three leachates and any water associated with the mining waste. The MWDT makes a distinction between leachates or waters with pH less than or greater than 5. Generally, when the pH values are below 5, the ECPG of the solutions are quite similar, and potential aquatic toxicity from cationic metals, such as Pb, Cu, Zn, Cd, and Al, is assumed. Below pH 5, these metals are mostly dissolved, generally are not complexed with organic or inorganic ligands, and hence are more bioavailable. Furthermore, there is virtually no carbonate alkalinity at pH less than 5. All of these factors promote metal toxicity to aquatic organisms. On the other hand, when the pH value of the water or the leachates is above 5, the ECPG from the solutions are variable, and inferred aquatic toxicity depends on factors in addition to the metals released from the leaching tests. Hence, leachates and waters with pH above 5 warrant further examination of their chemical composition. Physical ranking criteria provide additional information, particularly in areas where waste piles exhibit similar chemical rankings. Rankings from physical and chemical criteria generally are not correlated. Examples of how this decision tree has been applied in assessing mine sites are discussed.