2003 Progress Report: Metals Removal by Constructed WetlandsEPA Grant Number: R828770C005
Subproject: this is subproject number 005 , established and managed by the Center Director under grant R828770
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
Center: HSRC (2001) - Midwest Hazardous Substance Research Center
Center Director: Banks, M. Katherine
Title: Metals Removal by Constructed Wetlands
Investigators: Fitch, Mark W. , Burken, Joel
Institution: Missouri University of Science and Technology
EPA Project Officer: Lasat, Mitch
Project Period: October 1, 2001 through September 30, 2004
Project Period Covered by this Report: October 1, 2002 through September 30, 2003
Project Amount: Refer to main center abstract for funding details.
RFA: Hazardous Substance Research Centers - HSRC (2001) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management
This research project focuses on the use of constructed wetlands for the remediation of mine drainage. The primary focus is on lead mine drainage because it is a significant problem in Missouri, and because many industrial effluents are essentially neutral, as is lead mine drainage. The objectives of this research project are to: (1) determine the chemistry of metals removal in constructed wetlands; (2) determine the failure mode of constructed wetlands at low hydraulic residence times; (3) determine the bioavailability of sequestered metals in constructed wetlands; and (4) determine the effect of operational disturbances on constructed wetlands.
This progress summary builds on the previously reported results obtained in Year 1 of the project, and addresses each of the four objectives separately. One significant development during Year 2 of the project was analytical: the graphite furnace atomic absorption (GFAA) used for lead analyses became a "boat anchor" in October 2002. Fortunately, the University of Missouri, Rolla (UMR) was able to purchase a GFAA, partially funded by this research, for the environmental engineering laboratories. This GFAA was set up in August, and the students on this project are steadily analyzing the significant sample backlog that developed during the period in which we lacked analytical capabilities.
Metals Removal Chemistry
The chemistry of metals removal was largely determined during Year 1 of the project. Scanning Electron Microscopy (SEM), performed during late Year 1 and the beginning of Year 2 of the project, confirmed the presence of lead sulfide (PbS). The chemistry of metals removal in wetlands was dominated by three mechanisms: (1) adsorption, primarily to organic substances in the wetlands; (2) coprecipitation with iron oxyhydroxides; and (3) precipitation as metal sulfides. As shown in Figure 1, the majority of removal is because of coprecipitative adsorption on growing iron particles. In the organic substrate, adsorption seems to dominate, but over time, the adsorbed material may transform to metal sulfides. The effluent from the wetlands contains very small concentrations of the dissolved metal, demonstrating the effectiveness of this treatment technology. SEM showed small amounts of PbS on the substrate, shown in Figure 2; to our knowledge, this is the first definitive report (direct detection) of metal sulfide in metals-treating wetlands. These results are to be compared with samples from the field. Finding existing sediment at Doe Run similar to the wetlands has proved to be a modest challenge, but the West Fork facility has an upflow constructed wetland, which we have been allowed to sample for water concentrations of Pb and have discussed obtaining solids samples. A sampling trip in late October to Teck Cominco's wetlands in Trail, British Columbia, Canada, will collect sediment samples for extractive and SEM analysis.
One concern for the effluent is that some of the metals apparently leave the wetland in a form bound to dissolved organic matter. We determined this effluent issue during Year 2 of the project by separating the effluent using ultrafiltration (MWCO = 3,000) membranes.
Failure Mode at Low Hydraulic Retention Times
The failure mode of wetlands is under current investigation. We constructed two vertical flow wetlands to characterize critically high flow rates (the horizontal flow wetlands fail because of surface flow). These wetlands are run at hydraulic retention times (HRT) of 103 minutes for the downflow column, which is limited by the head available in the column, and 15.3 minutes for the upflow column. These HRTs correspond to surface loading rates of 2 and 12.5 L/m2/minute (30 and 190 mL/minute), respectively, and volumetric loading rates of 3 and 20 L/m3/minute. The maximum volumetric loading rate possible in the horizontal laboratory-scale wetlands is more than 100-fold lower, approximately 0.006 L/m3/minute, because of permeability limitations causing ponding.
Figure 2. SEM Showing Lead and Iron Sulfides, Molecular Composition Determined via EDS
The low HRTs have resulted in aerobic conditions in these wetlands. Sulfide is not evolved, nor is sulfate significantly removed, and measurements of redox potential and dissolved oxygen in both the bulk and pore water confirm a largely aerobic condition throughout the columns. Therefore, sulfide precipitation is not a significant removal mechanism. The other two removal mechanisms, adsorption and iron oxyhydroxide coprecipitation, are active. Given that iron oxyhydroxide coprecipitation does not remove all lead and zinc, adsorptive breakthrough is anticipated at some point. This breakthrough is becoming apparent, as shown in Figure 3.
Figure 3. Lead Concentrations in Upflow Wetland Column
As an extension of the work on efficacy and failure modes, an acid mine drainage wetland was started in April 2002, in parallel to a new lead mine drainage system and a negative control receiving distilled water. Although these wetlands were designed to grow plants for bioavailability testing, the metal removal performance, shown in Figure 4, is of interest. The acid mine drainage system generally is losing zinc rather than removing this metal (Ceffluent > Cinfluent). The neutral mine drainage system is performing as expected. Presumably, the mix of metals in the synthetic acid mine drainage at pH 3.5 prevents the removal of zinc. The plants are, however, growing well in this wetland.
Figure 4. Results From New Wetlands Receiving Synthetic Acid Mine Drainage (squares) and Synthetic Lead Mine Drainage (triangles). Solid symbols are effluent values, empty symbols are influent values.
The bioavailability of metals in wetlands receiving mine water is being examined at both the laboratory and field scales. Measurement of metals in plant and insect tissues from the laboratory-scale wetlands have shown no toxicologically significant lead or zinc concentrations. Three plant species that are dominant in local wetland environments, cattails (Typha latifolia), bulrush (Scirpus validus), and duck potato (Saggitarria latifolia), were planted in laboratory-scale wetlands. Cattails and bulrush had been planted in our original laboratory-scale wetlands, with significant uptake apparent in plant samples assayed by total digestion. Maximum concentrations encountered were Zn at 45 mg/kg in Typha and 85 mg/kg in Scirpus. All three species have been planted in wetlands now operating (and as described above, one is receiving acid mine drainage). These plants will be harvested for analysis in the final year of the project, with some minor samples taken during operation.
Cominco had measured metals in six plant species at their wetland prior to its reconstruction. During our sampling trip to Canada, we will collect samples of these plants (also insects) and perform independent measurements. Cominco reported that the content of cadmium was in the range of 5-14 mg/kg (controls, various grasses, and three of the six species found in the wetlands). Lead content was 19-74 mg/kg compared to 6 mg/kg in controls; arsenic ranged from 9-157 mg/kg (5-6 mg/kg in controls), with the highest concentrations in Rheum rhaponticum and Epilobium grandifolia; and the metal with the most uptake was, as observed in other studies and in our laboratories, zinc, at 950-2,300 mg/kg compared to 350-370 mg/kg in controls. The toxicological importance of these zinc levels is unknown. A review of literature information on metals concentrations in wetland plants, with a focus on treatment wetlands, is in preparation.
One of the original wetlands, in operation for 5 years, was partitioned along its length into two parts. One of these is being allowed to air dry to the point of no visible water in the pore space, and will be challenged with distilled water. This experiment examines what happens to a disused wetland that dries partially and is saturated again (drought and rained). The effluent will be examined for release of metals, and the substrate will be examined to show whether metals chemistry changes in the substrate as it changes from strongly anaerobic to aerobic in the pore space.
The other half of the wetland was removed by hand and shifted into a new, smaller container. This churned wetland then was restarted with synthetic mine water. The effluent is being analyzed to show whether the mechanical agitation causes a loss of the metals previously removed in the substrate.
An objective of this research project is to disseminate information gathered, as well as existing data that have not been widely utilized or distributed. Dr. Burken is a member of the Interstate Technology and Regulatory Council (ITRC) Constructed wetlands group. He is working on the current technical assistance document, particularly on bioavailability issues and construction guidelines. The guidelines are scheduled for release in 2004. Dr. Burken is conducting final reviews and adding information regarding bioavailability concerns. This document was started well before Dr. Burken’s involvement, but his participation will have great impact on the document overall, and the results of this research project will have considerable bearing on the technical acceptance and understanding of constructed wetlands for the removal of heavy metals.
This project also was the basis for a proposed treatment wetland at the Doe Run Buick Resource Recovery facility. A consulting engineering firm designed the wetland based (in part) on publications from this project, and a sampling plan designed by UMR was included in the submission to Missouri's Department of Natural Resources. Unfortunately, Doe Run has informed us that they decided not to construct the wetland at this time, because of a greater than $1 million bid for construction combined with questions of ability to meet toxicity standards. The Buick facility cracks batteries and has complex and high-strength wastewater, differing substantially from the mine water studied in this research project. This lack of certain compatibility of our results to their unique situation led to substantial doubts of efficacy in light of the capital costs for the research project. Nonetheless, Doe Run has informed us that they remain interested in the use of wetlands for mine water and tailings leachate remediation.
Sampling at the Cominco wetlands in Trail, British Columbia, Canada, was anticipated for spring of 2002, but was delayed, and the wetland cells were reconstructed. The initial sampling round now is scheduled for October 20-22. In addition to sampling their wetlands, we will place media samples into their wetland. These samples consist of "virgin" media placed in porous fiberglass mesh and inserted in flow-through samplers placed into the wetland media. The samples will be collected sequentially over the coming year. Analysis of the media will provide enumeration of sulfate-reducing bacteria, samples for SEM analysis, and samples for total and sequential extraction analysis. Sampling at Stiefel laboratories also is anticipated; however, the exact protocols have not yet been determined.
At the laboratory scale, the breakthrough (hydraulic failure) experiment should be complete during the coming year, completing Objective 2, as should the disturbance test, completing Objective 4.
Objective 3 is to measure bioavailability. Assays will continue to be performed in the coming year with the neutral mine drainage, field samples, and the acid mine drainage systems. The review article on metals concentrations in wetland plants will be submitted for publication.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
|Other subproject views:||All 9 publications||2 publications in selected types||All 2 journal articles|
|Other center views:||All 108 publications||22 publications in selected types||All 14 journal articles|
||Song Y, Fitch M, Burken J, Nass L, Chilukiri S, Gale N, Ross C. Lead and zinc removal by laboratory-scale constructed wetlands. Water Environment Research 2001;73(1):37-44.||
||Song Y, Fitch M, Burken J, Ross C. Adsorption of lead and zinc in the substrates of constructed wetlands. Water Environment Research. 2001;73(1):37-44.||
Supplemental Keywords:lead mine drainage, constructed wetlands, metal geochemistry, acid mine drainage, alternative technology, aqueous waste, aqueous waste stream, aqueous waste streams, effluents, hazardous waste treatment, heavy metal contamination, heavy metals, industrial wastewater, lead, lead compounds, metal removal, metal wastes, metals, mine drainage, mining wastes, wastewater remediation, bioremediation of soils, bioavailability, biochemistry, biodegradation, bioremediation, contaminant, contaminated sediment, contaminated soil, contaminated soils, degradation, denaturing gradient gel electrophoresis, contaminants in soil., RFA, Industry Sectors, Scientific Discipline, Toxics, Waste, Water, Hydrology, Remediation, Wastewater, Mining - NAIC 21, Hazardous Waste, Engineering, Hazardous, 33/50, Engineering, Chemistry, & Physics, Environmental Engineering, hazardous waste treatment, wastewater treatment, industrial wastewater, wastewater remediation, lead & lead compounds, acid mine drainage, lead, alternative technology, aqueous waste, constructed wetlands, metals removal, effluents, mine drainage, metal wastes, heavy metal contamination, heavy metals, metal removal, aqueous waste stream, mining wastes, aqueous waste streams
Progress and Final Reports:Original Abstract
Main Center Abstract and Reports:R828770 HSRC (2001) - Midwest Hazardous Substance Research Center
Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
R828770C001 Technical Outreach Services for Communities
R828770C002 Technical Outreach Services for Native American Communities
R828770C003 Sustainable Remediation
R828770C004 Incorporating Natural Attenuation Into Design and Management Strategies For Contaminated Sites
R828770C005 Metals Removal by Constructed Wetlands
R828770C006 Adaptation of Subsurface Microbial Biofilm Communities in Response to Chemical Stressors
R828770C007 Dewatering, Remediation, and Evaluation of Dredged Sediments
R828770C008 Interaction of Various Plant Species with Microbial PCB-Degraders in Contaminated Soils
R828770C009 Microbial Indicators of Bioremediation Potential and Success