Red Mud Wrestling
A soil-like industrial by-product may be successful for enhancing or restoring Louisiana coastal marshes
- By Robert Gambrell, Irving Mendelssohn, Norman Murray
- Jul 01, 2002
Louisiana contains approximately 40 percent of the coastal wetlands in the United States. These wetlands consist of swamps and both economically and environmentally important freshwater and saltwater marshes. The wetlands support nearly a third of the fish and shellfish yields in the lower 48 states and approximately 40 percent of its fur harvest, while there are more than 200,000 acres of private oyster leases. Also, the wetlands provide a wintering habitat for more than half of the ducks and geese in the Central and Mississippi Flyways1,2.
It is difficult to provide a statistical analysis of other wetland values, but some of these include the removal of nutrients and suspended sediments from river systems and more recently, some coastal wetlands have been demonstrated to be effective treatment systems for municipal and other wastewaters.3
Since about 1935, the state has lost more than 1,500 square miles of wetlands and is currently losing about 25 to 30 square miles annually, a rate equivalent to just under the area of two football fields every hour.2 This represents approximately 80 percent of the nation's coastal land loss rate.
These losses are manifested by the conversion of marshes and swamps to open water. There are several processes contributing to this loss. One is natural, slow subsidence of wetlands due to ongoing compaction of deep sediments deposited in the region by the Mississippi River for thousands of years. Thus, there has always been coastal land loss in the region, but as the river periodically changed its course and outfall point into the Gulf of Mexico in prehistoric times, new wetland formation near the mouth of the river by sediment deposition exceeded the rate of subsidence and land loss associated with abandoned deltas. In modern times with the river confined behind a levee for flood control purposes, its sediment load is primarily deposited in the deep Gulf of Mexico instead of in coastal wetland areas by annual overbank flooding that used to occur. Another process contributing to wetland loss includes canal construction for oil and gas production and other navigation purposes that speed saltwater encroachment into freshwater marshes contributing to loss of plants that hold the wetland soils.
Steps to Mitigate Coastal Land Loss
There are a number of proposed measures and activities underway to slow and eventually stop the deterioration and loss of coastal wetlands in Louisiana. Some of these include 1) dedicated dredging or beneficial use of dredged material, 2) altering freshwater hydrology to reduce saltwater intrusion into freshwater wetlands, 3) protecting shorelines in selected areas, 4) managing water flow in navigation channels, 5) restoring barrier islands to enhance their ability to protect inland wetlands from the wave action of hurricanes, and 6) by diverting or releasing Mississippi River water at numerous points up to 150 river miles upstream from its mouth to allow freshwater inflow and especially its sediment load to be deposited in interior wetlands where sediment accretion slows or reverses wetland loss and contributes to new wetland formation.
One potential method for not only restoring some coastal wetlands, but also providing a productive use for an industrial by-product that currently is handled mostly as a waste material, is to use red mud as a soil substrate to be applied to deteriorating wetlands.
Potential Use of Red Mud
Red mud is the by-product remaining after alumina is extracted from bauxite ore. Red mud is soil-like in some respects. It is primarily colloidal (clay-like) mineral matter and it contains an abundance of several plant nutrients. These properties make it worthy of consideration to transport and place in deteriorating wetlands as a soil substrate where marshes are deteriorating because of substrate subsidence or erosion.
However, as it is released untreated from an alumina refinery, it does not support positive plant growth. Two of the contributing factors for poor plant growth is its high pH (heat and alkali are used in the extraction of alumina), and it has very little nitrogen, an important macronutrient required for plant growth. Disposal ponds (diked enclosures designed to retain the material) of red mud have been observed more than 100 acres in size, both drained and undrained, where essentially no plants colonize the sites for years in a part of the United States where natural vegetative colonization occurs quickly on any normal soil artificially cleared of vegetation.
Also, some bauxite ores naturally contain elevated levels of several trace and toxic metals (lead, iron, silica, zinc, copper, etc.) that tend to be concentrated in the red mud. Before any productive use can be considered, this environmental risk must be assessed, determining if red mud will release problem levels of these elements to soluble and plant available forms.
Large volumes of the red mud material are produced annually, and the management expense and land requirements for storing it as a waste make it worthwhile to explore the possibility of utilizing this material for a productive use. If found suitable for maintenance and/or creation of wetlands, it has been reported that hundreds of acres of wetlands could be created annually.4
Three main criteria must be satisfied before red mud could be used to enhance coastal wetlands: 1) red mud must be treated or amended such that it does support positive wetland plant growth, 2) transporting red mud from the refinery to coastal areas where it is needed must be feasible from an engineering and economic perspective, and 3) the elevated levels of trace and toxic metals must not be released to the point of generating water quality problems and accumulation of these metals in the food chain.
Bauxite consists primarily of the monohydrate and trihydrate forms of alumina in varying percentages. Major impurities include oxides of iron, titanium and silica. However, lesser amounts of phosphorous, nickel, vanadium, zinc, copper, cadmium, lead, chromium, nickel, manganese and several other elements may be present in the original ore as well. All of these elements, plus some unextracted alumina, are found in red mud, concentrated somewhat over original ore levels. Also, depending on the properties of the original ore, process additions sometimes include lime, sodium and calcium compounds. As part of the extraction process, large amounts of the strong base, sodium hydroxide is added. Though this alkalinity of most red mud is often "neutralized" to various degrees by adding acid, the resulting pH of red mud is still high, generally nine to 11.5
All soils and sediments contain some quantities of essentially every trace and toxic metal, and the amount of naturally occurring metals they contain can vary widely depending on the geologic materials from which the soils were derived. While red mud contains substantially greater levels of several metals relative to most uncontaminated soils, more important than total quantities is the mobility and bioavailability of these metals.
Of these elements, those of particular interest from a perspective of potential toxicity because of their elevated concentrations are copper, zinc, nickel, chromium, cadmium and lead. Under some conditions, the elevated levels of iron and manganese could be stressful to wetland vegetation.
For red mud to be considered as a soil-like substrate to restore deteriorating coastal wetlands, two concerns related to the scientific feasibility of this productive use must be satisfactorily addressed. One of these questions is: Since red mud by itself does not support positive plant growth, can it be amended with other readily available substrates to overcome this potential problem? The other question is: Are the elevated levels of certain trace and toxic metals susceptible to being released to water soluble and plant available forms possibly contributing to adverse environmental effects?
Because of these apparent obstacles to productive use of red mud for marsh restoration and subsequent habitat development, the logical approach for a research effort was to start small -- evaluate initial results and proceed with further research only if early studies indicated the potential problems were manageable. Thus the initial effort consisted of one simple laboratory study on release of metals by red mud, and a small greenhouse study evaluating modifications to red mud to help it support plant growth, and if successful, a preliminary look at metal uptake by the plants.
A laboratory microcosm study was done evaluating the release of trace and toxic metals under the range of pH and oxidation-reduction conditions associated with wetland environments.
The laboratory results for all trace and toxic metals were generally as found for cadmium. These results for cadmium, a toxic element present in the red mud at a level
more than 500 times higher than typical soil, indicated the cadmium was held very tightly by the red mud. In fact, the uncontaminated control marsh soil released about 75 percent as much soluble cadmium as did the red mud because the pH of the marsh soil naturally became more acidic upon drainage and oxidation. Thus these preliminary results were encouraging.
The results of the initial greenhouse study were also encouraging. Mature saltwater marsh plants (Spartina alterniflora) were transplanted to: 1) 100 percent red mud (where the plants did not thrive, but did exhibit some growth), 2) a marsh soil control, 3) a mixture of red mud with marsh soil and 4) a mixture of red mud with compost. The significant findings were that where red mud was mixed with marsh soil or compost, plant growth was good and trace and toxic metal uptake by the plants were not excessive, especially considering the elevated levels of the trace and toxic metals present in the red mud relative to the salt marsh control soil.
With the encouraging results of the preliminary laboratory and greenhouse effects, several additional, more comprehensive studies were done. These studies included additional substrate modifiers, looking at the performance of several other plant species grown on red mud mixtures and the uptake of metals by these plants. These studies confirmed that: 1) amending red mud with other materials, sometimes with less than 1:1 mixtures, generally resulted in good plant growth (a paper mill by-product being an exception), 2) several different plant species tested performed well in the amended red mud, many of them freshwater wetland species, and 3) despite the substantially elevated concentrations of several trace and toxic metals in red mud, the concentrations of these metals in wetland plants were similar to or only slightly higher than plants growing in natural wetland soils. Some of these studies extended over a growing season.
The series of sequentially more comprehensive laboratory, greenhouse and controlled field plot experiments revealed that red mud, a by-product of the aluminum refining industry, showed potential for use as a substrate to enhance or restore deteriorating coastal marshes. This would be desirable because an industrial by-product that has been considered a waste material product might be used productively and make a beneficial contribution to slowing a wetland loss problem common to the state of Louisiana.
However, several precautionary points should be mentioned. First, the work reported here should be considered preliminary. The capacity of amended red mud to maintain a near neutral to slightly basic pH for an extended period may be critical to its long-term capacity to tightly hold onto trace and toxic metals. If there is another phase to this work, it should be a multi-year experiment in a natural wetland environment.
The second point to be made is that this work, and possible long-term field work, represents an evaluation of this concept only from a scientific perspective. The economic and engineering feasibility of this productive use of red mud would have to be evaluated, as well.
Finally, it should be mentioned that this is not the first time red mud has been evaluated for productive use purposes. A report in the May 7, 2002, issue of the Sydney Morning Herald indicated problems were encountered where red mud was applied to soils in Australia. In this environment, the fine colloidal nature of the red mud, coupled with its application to dry ground and windy conditions, resulted in the widespread transport of the material as dust. With wetland applications in controlled field plots, there is essentially no wind transport of the material and apparently little loss, if any, is expected.
1. Gambrell, Robert and Manjukiran Prasana. 1994. "A Laboratory Study of Trace and Toxic Metal Chemistry in Red Mud under Simulated Productive Use Application for Wetland Enhancement." Contract Completion Report submitted to Kaiser Aluminum and Chemical Corporation, Baton Rouge, Louisiana.
2. Louisiana Coastal Wetlands Conservation and Restoration Task Force and the Wetlands Conservation and Restoration Authority. 1998. "Coast 2050: Toward a Sustainable Coastal Louisiana and Executive Summary." Louisiana Department of Natural Resources, Baton Rouge, LA. 12 pp.
3. Breaux, Andre. "The Use of Hydrologically Altered Wetlands to Treat Wastwater in Coastal Louisiana." Dissertation, Louisiana State University, Baton Rouge, La. 236 pages.
4. Kaiser Aluminum and Chemical Corporation. 1994. Red Mud Coastal Restoration Demonstration Project (draft). Project developed by Kaiser Aluminum and Chemical Corporation; The Governor's Office of Coastal Activities, State of Louisiana; U.S. EPA; and the Louisiana Department of Natural Resources.
5. Hofstede, H. and G. Ho. 1991. "The Effect of Addition of Bauxite Refining Residue (Redmud) on the Behavior of Heavy Metals in Compost." Trace Metals in the Environment. I. Heavy Metals in the Environment. Elsevier Science Publishing Company, Inc., New York, pp 67-94.
6. Gambrell, Robert and Medhi Aarabi. 2002. "Report on Terrebonne Red Mud Demonstration Project." Contract Completion Report submitted to Kaiser Aluminum and Chemical Corporation, and U.S. EPA.
7. Kong, T. and I.A. Mendelssohn. 1996. "The Impact of Red Mud on Growth of Wetland Vegetation and Substrate Fertility." Wetlands Ecology and Management 4(1):3-14.
This article originally appeared in the 07/01/2002 issue of Environmental Protection.