A new multipurpose remediation media
The number of U.S. hazardous waste sites requiring treatment for soil and groundwater remediation under current federal and state regulations is estimated to be about 217,000. The sites include those that fall under the National Priorities List (NPL, Superfund), Resource Conservation and Recovery Act (RCRA) corrective action and Department of Defense (DOD) and Department of Energy (DOE) installations.
The soil and groundwater at these sites are contaminated with various toxic metals (about 50 to 70 percent of the sites) and with organic contaminants (40 to 70 percent of the sites). In addition, radioactive contamination is found at 90 percent of the DOE installations. DOE estimates that more than 5,700 groundwater plumes have contaminated over 600 billion gallons of water and 50 million cubic meters of soil throughout the DOE complex. Mixed waste containing multiple hazardous and radioactive contaminants is a problem at a number of installations. The types of contaminants present at the sites include:
- Toxic metals such as lead, chromium, arsenic, cadmium, zinc, barium, nickel, copper, beryllium and mercury;
- Organic chemicals such as benzene, toluene, xylenes, chlorinated hydrocarbons such as trichloroethylene (TCE), perchloroethylene (PCE) and energetic chemicals such as nitroesters; and
- Radioactive contaminants such as uranium, plutonium, thorium, cesium, strontium and tritium.
Present treatment methods
The remediation of contaminated surface and groundwater is a prime concern, and is typically attempted with treatments such as precipitation, ion exchange, membrane separation and activated carbon adsorption. The method used most frequently to treat groundwater is conventional pump-and-treat technology. The groundwater is pumped to the surface and treated using various technologies. At sites having mixed contaminants, two different processes are required to remediate a site, an approach that results in complex and costly processing steps. A typical approach is to remove organics using activated carbon followed by ion exchange to remove metals. However, this method is ineffective in meeting the desired cleanup criteria for sites with various types of contaminants, especially when the aquifers are contaminated with non-aqueous phase liquids (NAPLs). These organic compounds are heavier than water, causing them to sink below the water table to the next boundary, making them extremely difficult to access and r
emediate. Pump-and-treat methods are projected to take 30 to 70 years at a number of DOE and DOD sites that contain NAPLs, thus increasing treatment costs.
The limitations of aboveground treatment methods such as pump-and-treat can be overcome by the use of in situ treatment technologies. An attractive option for the treatment of groundwater contaminated with multiple contaminants is to use permeable reactive barriers (PRB). A PRB contains suitable reactive materials constructed to intercept the path of the contaminated groundwater plume. The treatment materials in the barrier remove the contaminants as the groundwater passes through it.
The technology to construct and place barrier materials is well-developed. However, one of the limitations has been the availability of a suitable treatment material that can be used in the barrier. The material must be effective for removal of multiple types of contaminants from complex streams. In addition, the suitable material must not introduce any unacceptable contaminants into the groundwater.
Materials being considered as treatment media in PRBs include Zero-Valent Iron (ZVI), peat, modified zeolites and others. These media can only treat one or two types of contaminants. The limitations of these media include the potential release of contaminants under changing redox conditions.
The remediation of contaminated soil is also a concern, since the contaminants migrate from the soil to groundwater aquifers and other media. In addition, contaminated soil may pose a risk to adjacent populations through ingestion or inhalation. The contaminated soil at the surface can be easily removed and appropriately treated with various methods either on-site or disposed of at a suitable landfill. However, the treatment of contaminated subsurface soils poses a challenge, and various methods such as soil vapor extraction (SVE), bioremediation, thermal desorption, etc. are usually employed. The use of in situ technologies such as SVE and bioremediation for soil remediation is increasing, as these are usually less expensive compared to above-ground treatments such as stabilization and incineration. However, the treatment methods employed at present for soil remediation can treat only one or two types of contaminants. There is a need for treatment methods that can economically treat soils contaminated wit
h multiple contaminants.
New multipurpose treatment material
A new adsorbent/ion-exchange polymer called HUMASORBTM, developed with support from DOE, offers a single-step process to remediate water that contains mixed waste contaminants. The material can be used both in pump-and-treat systems and for in situ groundwater treatment. The new material is based on humic acid derived from coal having the following desirable properties: high cation-exchange capacity, ability to chelate metals, ability to adsorb organics and ability to reduce toxic forms of contaminants such as chromium (VI) to relatively non-toxic forms.
Humic acid is a complex aromatic macromolecule with various linkages between the aromatic groups. It is a highly functionalized carbon-rich biopolymer, with functional groups such as carboxylic, phenolic, enolic and carbonyl structures of various types. Humic acid has been extensively studied and various molecular models have been proposed to explain its unique properties. A recent article discussed the modeling of humic acid structures based on a proposed new humic acid building block (Sein, L.T. et al., ES&T, Vol. 33 No. 4, 1999). The first hypothetical structure for humic acid proposed in 1972 and a new proposed building block are shown in Figure 1.
Metals are bound to the carbon skeleton of humic substances primarily through carboxylic and phenolic oxygen, but heteroatoms such as nitrogen and sulfur also have a positive effect on metal binding. The mechanisms for adsorption of organic compounds by humic acid include hydrophobic bonding, hydrogen bonding, ion exchange and ligand exchange. In addition, humic acid can influence oxidation-reduction of metal species and also stabilize the reduced cationic form by chelation/ion-exchange. This mechanism is responsible for the reduction of chromium (VI) and hexavalent actinides such as plutonium by humic acid.
Humic acid extracted from coal as a liquid is converted to a solid adsorbent/ion-exchange material by cross-linking and immobilization to produce the new polymer, which is insoluble in water. The production process includes cross-linking by aldehydes and encapsulation in an alginate gel matrix. The new polymer incorporates the attributes of humic acid for contaminant removal, and is effective for simultaneous removal of multiple contaminants from mixed waste streams.
The new media has been used for treatment and demonstration at a number of DOE and DOD facilities. A mobile HUMASORB-based process unit (Figure 2) has been used on-site to demonstrate and treat different waste streams. The application of the new media for remediation of streams contaminated with multiple contaminants has the potential to lower the life-cycle costs by 50 to 70 percent compared to conventional treatment technologies.
Johnston Atoll, Pacific. A treatment system based on the new polymer was used recently at the Johnston Atoll DOD facility for treatment of spent decontamination solution (SDS) to remove lead, mercury and arsenic. SDS was analyzed for residual chemical agent and was found to be below the drinking water standards (DWS) for all agent types. However, much of the waste was found to contain levels of the metals arsenic, lead and mercury exceeding those mandated under RCRA. The original plan for treatment of the SDS was incineration at the Johnston Atoll Chemical Agent Disposal System (JACADS). However, approximately 24,000 gallons of SDS could not be treated at JACADS because of the current RCRA permit limitations on the processing of such material.
The program manager for chemical demilitarization (PMCD) was asked by the U.S. Environmental Protection Agency (EPA) to consider treatment of the liquid waste by some alternative technology to reduce the metals concentration to below the regulatory hazardous levels. Technology application studies were conducted using a process based on the new polymer on SDS samples received from Johnston Island. The results from the batch and column tests conducted showed that the levels of arsenic, lead and mercury were below the RCRA treatment levels. Based on initial technology application tests, the process was subsequently selected by the PMCD as a viable alternative for removal of the metal contaminants from the SDS waste and approved for use by EPA.
Project managers used a process mobile unit that employed the new media to treat 24,000 gallons of SDS. The treated SDS was analyzed, and concentrations of the toxic metals were determined to be below the required regulatory mandated levels. An approved independent laboratory completed the analytical activities for regulatory compliance. The successful treatment showed that the process can be implemented easily, and is potentially effective for treatment of metals in waste brines.
Resource recovery, Berkeley Pit, Mont. HUMASORB technology was also demonstrated for resource recovery and remediation of water from Berkeley Pit, the largest abandoned open mine pit in the world, as part of the DOEÕs Resource Recovery Project (RRP). The RRP focuses on the evaluation of technology systems for reclaiming usable water and the identification of marketable resources from surface and groundwater contaminated with metals. The demonstration in Butte, Mont., was designed to establish applicability of the process to remove toxic heavy metals such as cadmium from Berkeley Pit water samples, while producing a chelated micronutrient-enriched fertilizer product for agriculture applications.
In the two-stage process, the Berkeley Pit water first was treated with a liquid HUMASORB product to remove iron and other metals that are useful as agricultural micronutrients. The remaining metals and other toxic metals were reduced by passing through HUMASORB columns in the second stage. Comparison of metals analysis of Berkeley Pit water before and after treatment indicates that metals concentration in the treated effluent was near or below detection limits.
Tests conducted by Montana State University with the fertilizer material produced in the process indicated that the micronutrients derived from the Berkeley Pit waters are utilized by plants, with no difference between the uptake of nutrients from the Berkeley Pit water versus that obtained from a commercial source. In addition, increased yields of 35 percent for alfalfa and 20 percent for wheat were identified following application of the micronutrients. The results from the demonstration with Berkeley Pit waters show that the new process is an economical approach to utilize acid mine water as a resource for creating effective micronutrient fertilizer for various applications such as agriculture and reclamation.
Groundwater treatment tests at INEEL. The new polymer was evaluated at Idaho National Engineering & Environmental Laboratory (INEEL) as a potential candidate for radionuclide removal technology for radioactive cesium and strontium. Researchers there conducted the tests in two phases. In the first phase, the researchers performed bench-scale column tests using simulated waste streams containing the contaminants present in the INEEL groundwater. The results from the column tests were used to design columns and establish treatment parameters for further tests on-site with groundwater containing radioactive contaminants.
The column tests using the new polymer were conducted at INEEL with Idaho Chemical Processing Plant (ICPP) groundwater spiked with relatively high concentrations of radioactive cesium and strontium compared to the tests conducted with other ion-exchange and adsorbent materials. The input concentration (as measured by picocurie per liter (pCi/L)) of cesium in the HUMASORB tests was nearly three orders of magnitude higher, and that of strontium was four orders of magnitude higher, than that used in tests with other materials.
The column tests conducted at INEEL show that the new media removed both radioactive cesium and strontium. That makes it the only material, other than IONSIV-IE911 (CST), evaluated by INEEL to date, effective in removing both radioactive cesium and strontium. The results from the tests conducted in this project were compared with the results with CST. The amount of radioactive cesium adsorbed per gram of adsorbent was nearly two orders of magnitude higher with the new media as compared to CST.
Simulated barrier tests. The new polymer is being evaluated for contaminant removal under simulated barrier conditions for treatment of groundwater. The conditions simulate barrier installation depths of approximately 10 feet and 100 feet. In these tests, a simulated waste stream containing a mixture of metals, organics and radionuclide surrogates is being passed through the barriers. The tests are on-going for more than 18 months; results indicate that the new media is effective in removing contaminants (metals, organics and radionuclide surrogates) under simulated barrier conditions in a single treatment step.
The DOE has recently selected HUMASORB for evaluation as a potential permeable reactive barrier material for an extensive groundwater plume contaminated with chlorinated compounds and radioactive technetium (present as TcO4-). The ability of the new media to remove both organic and inorganic contaminants has significant potential for cleanup of both contaminated surface and groundwater.
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This article originally appeared in the December, 1999 issue of Environmental Protection magazine, Vol. 10, Number 12, pp. 42-45, 50.
This article originally appeared in the 12/01/1999 issue of Environmental Protection.