FIXATION OF METALS AND RADIONUCLIDES AND ACID NEUTRALIZATION BY GEOBIND^TM ACTIVATED RED MUD (ARM)

In this month’s Peroxygen Talk, we move outside the realm of peroxygen chemistries to discuss metals remediation via a technology called activated red mud. Our guest author is Jim Rouse.  He is a geohydrologist with more than forty-five years of relevant pollution control experience, mostly involving in situ remediation of metal and radionuclide contamination of soil and groundwater.  He is a Principal Geohydrologist with MWH and has been involved with the application of activated red mud at mining, dredging and wood treating sites.

INTRODUCTION

A review of relevant literature on remediation of contaminated soil and ground water clearly shows that most of the attention has been and continues to be on remediation of organic contamination, especially contamination with chlorinated volatile and semi-volatile compounds. Techniques include chemical oxidation and reduction, biodegradation, etc, as well as the long-standing pump and treat (P&T) approaches. By contrast, remediation of metal and radionuclide contamination has received little attention, with conventional approaches such as P&T, stabilization / solidification, or excavation commonly used. But an examination of the frequency of the occurrence of metals and radionuclides (inorganics) as the contaminant of concern indicates that there is a real need for more innovative, cost-effective remedial approaches for these contaminants.

For example, a study of the classes of contaminants at 674 CERCLA sites found that metals or radionuclides were present at approximately 75% of the sites. The study looked at sites having one, two or three classes of contaminants, separated between inorganics, volatile or semi-volatile organics. Of the 169 sites with only one class of contaminants, 77 (or 45%) were contaminated with inorganics. Of the 174 sites with two classes of contaminants, 88, or 50%, had inorganic contamination. The remaining 331 sites had all three classes of contaminants, meaning that 496 of the 674 sites included metals or radionuclides. Further, metals and radionuclides including arsenic, radium and uranium are the primary concerns for treatment of drinking water supplies. These cases of contamination of soil and ground water can now be cost-effectively and permanently remediatiated by the application of GeoBind^TM activated red mud (ARM). The availability of a commercial source of high-quality ARM offers a viable alternative for the cost-effective, geologically stable remediation of contaminated soil and ground water, and the removal of metals and radionuclides from the source of supply of drinking water.

GEOBIND^TM ARM MANUFACTURE AND CHARACTERISTICS

The manufacture of GeoBind^TM ARM is truly a ‘green’ process, with the raw material for the manufacturing derived from the residual “red mud” of the alumina refining process. Alumina refining involves the crushing, grinding and caustic leaching of the lateritic bauxite ore to solubilize the aluminum. Approximately two tons of red mud are generated for every ton of produced alumina. In its residual form, the red mud is a finely ground, highly alkaline powder, with high concentrations of sodium hydroxide from the leaching process. The red mud is slurried into large red mud ponds, commonly located near estuaries. Seepage from the ponds is alkaline and causes damage in the receiving waters near the ponds.

The manufacturing process, or “activation”, consists of partial neutralization of the alkalinity by reaction of the sodium hydroxide with salts of calcium and magnesium, to form solid-phase calcium and magnesium oxide and hydroxide, which only tend to become a source of available alkalinity under acidic conditions. Ferrous ions are also added, thus producing additional ferric hydroxide beyond that native to the red mud. The finely ground particles (commonly less than 10 micron in size) actually have been shown to consist of clusters of nano-scale ferric hydrite, which have an immense surface area per unit mass of powder.

The beneficial aspects of activated red mud, and the permanence of the removal of metals, has been studied and reported since the late 1990s [(Clark, 2000)(Genc-Fuhrman, et al, 2004)(McConchie, et al, 1998)(McConchie, et al, 1999)]. The nano-scale particles tend to sorb onto and ‘armor’ sulfide minerals, thus reducing the rate of sulfide oxidation and acid production from sulfide-bearing material such as mine waste and dredged waste (Rouse, 2009). Treatment of acidic waste results in two forms of the neutralization process. First, there is a rapid neutralization involving the hydroxides, carbonates and hydroxycarbonates in the ARM. Then there are slower protonation reactions that take place over hours to days, involving iron and aluminum oxyhydroxides in a slow reaction with hydrogen ion. Metal and radionuclide removal also involves two processes. The initial acid neutralization results in the formation of typical hydrous oxides of metals, similar to common lime neutralization. The second, more permanent reaction involves the metals actually becoming incorporated into the crystal structure of the nano-scale particles in a geochemically stable form. Aging of the metal-bearing ARM shows that the metals become more tightly bound to the ARM over time. Lead, copper, zinc, cadmium and nickel are tightly bound, while cobalt and manganese is more subject to leaching. Arsenic, radium and uranium is also removed and bound to the solids. Testing with the TCLP shows that the tightly bound metals tends to remain bound, assuming the TCLP is done on ‘aged’ material, that has undergone the conversion from the amorphous solid precipitates to the crystalline form.

GEOBIND^TM ARM DELIVERY

There are two secrets to successful remediation of soil and ground-water contamination (Blessing and Rouse, 2002):

1. Selection of the best remediation reagent, considering the geochemical conditions of the site, and

2. Selection of the best reagent delivery system, based on site geohydrological conditions, to achieve contact and reaction between the reagent and contamination.

As outlined above, GeoBind^TM ARM appears to be the best reagent for much of the acid, metals and radionuclide contamination of soil and ground water. But an equally important requirement is to assure that the contamination and the reagent achieve intimate contact, thereby allowing for successful reaction. This contact has been accomplished in a number of ways for various sites. This was discussed in some detail in an article (Rouse and Jonas, 2007), as it applies to the dredging industry. Delivery systems can be designed for both source control and residual contamination treatment.

For source control of soil contamination, the dry GeoBind^TM powder can easily be tilled into the surface or mixed into the upper several meters of depth with conventional mixing equipment. This commonly enhances revegetation of disturbed areas, with the proviso that slight overdosing may be required due to soil non-uniformity conditions. Dose rates are low, commonly less than one percent of the soil by weight.

For contaminated, acidic ponds, a slurry of GeoBind^TM ARM in water can be broadcast sprayed over the pond surface with conventional large-capacity sprinkler systems such as are widely used for irrigation systems. In some cases, the sprinklers may need to be mounted on floating barges to achieve sufficient coverage. As the slurry covers the pond surface, the solids settle through the water column, achieving acid neutralization and fixing the dissolved metals into the solids. Once the solids settle to the pond bottom, they are still active at removing dissolved contaminants present in ground-water inflow to the pond or in treating seepage recharging the ground water from the pond contents.

Ground-water pollution plumes can be treated by the grid hydrofracture injection of a slurry of GeoBind^TM ARM or by creation of a bore-hole placed permeable reactive barrier. Such a grid-based source control effort was conducted at an arsenic site in California, and resulted within one week in the decrease to non-detectable levels of the arsenic concentration over the source area (Rouse, Jonas and Thomasser, 2008).

Residual area treatment can be achieved using many of the delivery systems outlined above. Storm-water treatment has been achieved by mixing GeoBind^TM ARM and sand or gravel to form a ‘french drain’, or by placing granulated material into gabions to build permeable reactive dams across flow channels.

Water-supply wells containing arsenic, radium, or uranium can be treated at the well head by passing the produced water through granulated GeoBind^TM ARM housed in a filter holder. Alternatively, the ground water could be treated in situ by injection of a slurry of GeoBind ^TM ARM through a series of bores around the producing well, thereby fixing the contaminants in place in the aquifer.

REFERENCES

Blessing, Todd and Rouse, J.V., 2002, Keys to successful in-situ remediation of hexavalent chromium in soil and ground water, Proceedings, American Wood Preserver’s Association

Clark, M.W.. 2000, Geological cycling of heavy metals in Brisbane River estuary dredge sediments during sub-aerial disposal and consolidation at the Port of Brisbane Authority’s Fisherman Islands Reclamation Paddocks, PhD Thesis, Southern Cross university, Lismore

Genc-Fuhrman, H., Tjell, J.C., and McConchie, 2004, Adsorption of arsenic from water using activated neutralized red mud, Environmental Science and Technology, 38(8), p. 2428-2434

McConchie, D., Clark, M.W., and Hanahan, C., 1998, The use of seawater neutralized red mud from bauxite refinery residues (red mud) to control acid mine drainage and heavy metal leachate, Geological Society of Australia Abstract 49, Townsville, P. 298

McConchie, D., Clark, M.W., Hanahan, C. and Fawkes, R., 1999, The use of seawater-neutralized bauxite refinery residues (red mud) in environmental remediation programs, In I. Goballah, J. Hager, and R. Solozabal (Ed), REWAS ’99 Proceedings of the Global Symposium on Recycling, Waste Treatment and Clean Technology, San Sebastian, Spain, p. 391-400

Rouse, Jim V. 2009, Acid neutralization and metals fixation by activated red mud, Proceedings of the Fifth International Symposium on Remediation of Contaminated Sediments, Battelle, Jacksonville, FL

Rouse, Jim V. and Jim Jonas, 2007, Delivery options for remediation of dredged material using ViroBind^TM technology, World Dredging, Mining and Construction, 43(7), July 2007

Rouse, Jim V., Jim Jonas and Rick Thomasser, 2008, Sequential in-situ remediation of hexavalent chromium and arsenic in ground water, Sixth International Conference on Remediation of Chlorinated and recalcitrant Compounds, Battelle, Monterey, CA