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Final Report: Enhanced In Situ Extraction of Heavy Metals Using Surfactant Foams
EPA Contract Number: 68D01021Title: Enhanced In Situ Extraction of Heavy Metals Using Surfactant Foams
Investigators: Hatzinger, Paul B.
Small Business: Envirogen Inc.
EPA Contact:
Phase: I
Project Period: April 1, 2001 through September 1, 2001
Project Amount: $69,975
RFA: Small Business Innovation Research (SBIR) - Phase I (2000)
Research Category: SBIR - Remediation , Hazardous Waste/Remediation
Description:
The objective of this Phase I project was to assess the use of surfactant colloidal gas aphron (CGA) foams for the remediation of heavy metals in contaminated aquifers. One of the most important factors limiting pump-and-treat methods for metal as well as organic pollutant remediation is poor transport of extraction solutions from injection sites to zones of contamination because of movement through preferential flow paths. Surfactant foams may be an ideal medium to improve the transport of metal-extracting agents in the subsurface, because foams move through aquifer sediments in a "plug flow" manner (i.e., entering small micropores as well as preferential flow paths). Microcosm studies were conducted to evaluate the most effective surfactants and surfactant formulations (with chelating agents and pH adjustment) for removing adsorbed lead (Pb), cadmium (Cd), and mercury (Hg) from silica sand. The surfactants were prepared at 0.1 percent concentration. Summary/Accomplishments (Outputs/Outcomes):Among the surfactants tested, Gemtek (plant surfactant mixture) and Ammonyx KP (oleyl dimethyl benzyl ammonium chloride) were the most effective agents for removal of all three metals. Gemtek solubilized 40 percent of the Pb, 61 percent of the Cd, and 38 percent of the Hg adsorbed to silica sand in separate studies. The starting concentrations of the metals on the sand were 41.2 mg/kg for Pb, 8.6 mg/kg for Hg, and 13.0 mg/kg for Cd. Other surfactants, including JBR-425 (mixed rhamnolipids) and 1-dodecylpyridinium chloride also were highly effective for mobilizing one or more of the metals tested. The addition of ethylenediamine tetra-acetic acid (EDTA) to Gemtek and JBR-425 further improved the extraction efficiency of these surfactants, increasing removal efficiency for Pb and Cd to greater than 92 percent. The acidification of specific surfactants using a citrate-citric acid buffer also enhanced metal mobilization by these solutions. For example, the removal of adsorbed Pb and Cd from silica sand by Bioterge AS-40 increased from 26 and 27 percent for surfactant alone to 99 and 88 percent, respectively, after pH adjustment to 5.0 with citrate buffer.
The most effective surfactant formulations from the microcosm tests were prepared as CGA foams using a spinning disc foam generator. The mobilization of lead from silica sand and from an aquifer sediment by liquid surfactants, surfactant foams, and pulsed addition of the liquid and foamed surfactants then was evaluated. Ammonyx KP amended with EDTA (0.1 M) effectively flushed Pb from silica sand (60-70 percent removal), but in the homogenous sand matrix, CGA foams were not more effective than liquid surfactant alone. In aquifer sediment, however, a sequential addition of CGA foam and liquid surfactant proved to be the most effective means for metal mobilization. For example, a pulsed addition of Ammonyx KP with EDTA yielded 68 percent removal of lead compared to 62 percent removal for the liquid formulation. The mobilization of lead from aquifer sediment using Bioterge AS-40 formulations also was greatly enhanced by surfactant foam treatments compared to liquid. Bioterge AS-40 removed 5.5 percent of adsorbed lead from aquifer sediment when added as a liquid surfactant, but this removal increased approximately fourfold to 22 percent using CGA foam.
The acidification of Bioterge AS-40 with citrate-citric acid buffer (0.05 M citrate, pH 5.0) improved lead removal to 45 percent in flow-through aquifer columns when liquid was applied. The removal of Pb from the sediment increased to 73 percent when the acidified Bioterge AS-40 solution was pulsed through the column sequentially as liquid and CGA foam. It is likely that the surfactant foam blocked the more permeable channels in the sediment matrix, forcing the liquid surfactant into less permeable regions and causing an increased removal of adsorbed metal species from these zones. This treatment regimen (pulsed liquid and foam) may be even more effective in natural, undisturbed aquifers or soils, where extreme heterogeneity exists. Additional studies with natural materials as well as engineered heterogeneities are proposed during Phase II of this project.
Conclusions:The results from this project show that surfactants and surfactant foams can be used to effectively remove metals from contaminated sand and sediment. Surfactant foams prepared with acid and metal chelating agents appear to hold particular promise for metals removal, especially when applied in sequence with liquid formulations. The potential application of a foam technology providing enhanced in situ mobilization of heavy metals includes industrial, military, and federal subsurface sites. A technology that dramatically increases the solubilization of metal species could be applied in conjunction with existing pump-and-treat systems to improve the efficiency and shorten the required operational time of those systems. A flushing technique for metals mobilization also will have application apart from pump-and-treat for the one-time removal of metals from heavily contaminated zones.
Additional studies are required to evaluate the most promising formulations from this research in field samples of soils and sediments with natural heterogeneity. The final goal of the Phase I and Phase II projects is to commercialize a new technology to mobilize heavy metals and radionuclides in subsurfaces using foams alone or in conjunction with chemicals capable of extracting metals. Envirogen, Inc., knows of no previous work assessing the potential use of surfactant foams for in situ mobilization of heavy metals; thus, this project represents a completely new approach to metals remediation.
Supplemental Keywords:surfactant flushing, surfactant foam, heavy metals, remediation, lead. , Toxics, Water, Scientific Discipline, Waste, Remediation, Chemical Engineering, Mercury, Chemistry, Environmental Engineering, Environmental Chemistry, Contaminated Sediments, National Recommended Water Quality, 33/50, heavy metal contamination, heavy metals, surfactant enhanced aquifer remediation, in situ flushing, extraction of metals, lead, chelated agents, contaminated aquifers, contaminated sediment, mercury & mercury compounds, surfactant foams
Progress and Final Reports:
Original Abstract