RESUMEN

Electrokinetically enhanced in-situ flushing using surfactants has the potential to remove polycyclic aromatic hydrocarbons (PAHs) from low permeability clay soils; however, previous research has shown that the applied electric potential produces complex physical, chemical, and electrochemical changes within clay soils that affect mass transfer and overall efficiency. This article presents the results of a laboratory investigation conducted to determine the contaminant mass removal by using a periodic voltage application. The periodic voltage effects were evaluated by performing four different bench-scale electrokinetic tests with the voltage gradient applied continuously or periodically, under relatively low voltage (1.0 VDC/cm) and high anode buffering (0.1 M NaOH) as well as high voltage (2.0 VDC/cm) and low anode buffering (0.01 M NaOH) conditions. For all the tests, kaolin soil was used as a representative clay soil and it was spiked with phenanthrene, a representative PAH, with a target concentration of 500 mg/kg. A nonionic polyoxyethylene surfactant, Igepal CA 720, was used as the flushing solution in all the tests. The voltage was applied according to a cycle of five days of continuous application followed by two days of "down time," when the voltage was not applied. The results of these experiments show that considerable contaminant removal can be achieved by employing a high, 2.0 VDC/cm, voltage gradient along with a periodic mode of voltage application. The increased removal was attributed to increased phenanthrene solubilization and mass transfer due to the reduced flow of the bulk solution during the down time as well as to the pulsed electroosmotic flow that improved flushing action.

Asunto(s)

Fenantrenos/aislamiento & purificación , Hidrocarburos Policíclicos Aromáticos/aislamiento & purificación , Contaminantes del Suelo/aislamiento & purificación , Silicatos de Aluminio , Arcilla , Electroquímica , Cinética , Permeabilidad , Tensoactivos

RESUMEN

Numerous sites have been polluted with mercury as a result of accidental spills and improper disposal practices, and these mercury-contaminated sites may have adverse effects on human health and the environment. Innovative and cost-effective remediation techniques are urgently needed, and this study was performed to investigate the use of electrokinetics for mercury-contaminated soils. Initially, batch tests were performed on two soils, kaolin and glacial till, spiked with mercury(II) to investigate mercury desorption and complexation under different pH environments (pH range 2-12). The complexing agents included disodium ethylenediaminetetraacetate (Na-EDTA), potassium iodide (KI), and sodium chloride (NaCl), and these solutions were used at a concentration of 0.1 M. In addition, deionized water was used for comparison purposes. Based on the batch tests, Na-EDTA and KI were identified as the complexing agents with the greatest potential. The removal efficiency of these complexing agents was then examined by conducting electrokinetic experiments that employed the same solution concentration (0.1 M) and voltage gradient (1.0 VDC/cm) conditions. These tests indicated that for both soils, KI was a more effective complexing agent than Na-EDTA under electrokinetics. For the kaolin soil, the electrokinetic treatment using KI removed approximately 97% of the initial contaminant present (500mg/kg of Hg(ll)), leaving a residual concentration of 16 mg/kg of Hg in the soil, whereas on the glacial till soil, KI removed only 56% of the initial contaminant present (500mg/kg of Hg(II)), leaving a residual concentration of 220 mg/kg of Hg in the soil. The lower Hg removal from glacial till is attributed to the presence of organic matter, which increased mercury adsorption or the formation of insoluble mercury surface complexes.

Asunto(s)

Mercurio/aislamiento & purificación , Contaminantes del Suelo/aislamiento & purificación , Adsorción , Silicatos de Aluminio , Arcilla , Electricidad , Cinética , Mercurio/química

RESUMEN

Polycyclic aromatic hydrocarbon (PAH)-contaminated soils exist at numerous sites, and these sites may threaten public health and the environment because many PAH compounds are toxic, mutagenic, and/or carcinogenic. PAHs are also hydrophobic and persistent, so conventional remediation methods are often costly or inefficient, especially when the contaminants are present in low permeability and/or organic soils. An innovative technique, electrokinetically enhanced in situ flushing, has the potential to increase soil-solution-contaminant interaction and PAH removal efficiency for low permeability soils; however, the electrolysis reaction at the anode may adversely affect the remediation of low acid buffering capacity soils, such as kaolin. Therefore, the objective of this study was to improve the remediation of low acid buffering soils by controlling the pH at the anode to counteract the electrolysis reaction. Six bench-scale electrokinetic experiments were conducted, where each test employed one of three different flushing solutions, deionized water, a surfactant, or a cosolvent. For each of these solutions, tests were performed with and without a 0.01 M NaOH solution at the anode to control the pH. The test using deionized water with pH control generated a higher electroosmotic flow than the equivalent test performed without pH control, but the electroosmotic flow difference between the surfactant and cosolvent tests with and without pH control was minor compared to that observed with the deionized water tests. Controlling the pH was beneficial for increasing contaminant solubilization and migration from the soil region adjacent to the anode, but the high contaminant concentrations that resulted in the middle or cathode soil regions indicates that subsequent changes in the soil and/or solution chemistry caused contaminant deposition and low overall contaminant removal efficiency.

Asunto(s)

Fenantrenos/aislamiento & purificación , Contaminantes del Suelo/aislamiento & purificación , Adsorción , Electrólisis , Concentración de Iones de Hidrógeno , Cinética , Permeabilidad , Fenantrenos/química , Tensoactivos/química , Movimientos del Agua

RESUMEN

In order to avoid the effects of complex soil composition or contaminant interaction, previous bench-scale electrokinetic experiments were generally performed using simplified conditions. An ideal soil such as kaolinite was often used, and testing was frequently restricted to an individual contaminant or a small group of contaminants. However, actual waste sites consist of soils that are usually quite different from ka-olinite, and many sites are polluted by a large number of mixed contaminants. Therefore, this preliminary study was undertaken to assess electrokinetic performance on a site-specific field soil and a simulated sludge mixture containing mixed wastes in the form of metals, organic compounds, and radionuclides. Bench-scale experiments showed that the field soil had a high buffering capacity that resulted in high pH conditions throughout the soil, whereas the simulated sludge had a low buffering capacity that resulted in low pH conditions except near the cathode. The high pH conditions in the soil allowed the migration of anionic metallic contaminants, such as hexavalent chromium, but inhibited the migration of cationic metallic contaminants, such as cadmium. The low pH conditions in the sludge allowed simultaneous migration of both anionic and cationic contaminants in opposite directions, respectively, but the synergistic effects of co-contaminants retarded contaminant removal. The removal of organic compounds and radionuclides from both the soil and the sludge were achieved. However, additional research is warranted to systematically investigate the synergistic effects and the fate of different contaminants as well as to develop electrode-conditioning systems that enhance contaminant migration.