Mercury-Contaminated Soil Remediation by Iodide and Electroreclamation

Mercury was removed from a field-contaminated soil by a combination of redox and complexation processes with iodide/iodine and electrokinetic mobilization. Iodide added to the cathode compartment was transported into the soil and oxidized to iodine near the anode. Mercury was mobilized and transported to the anode as a mercury–iodide complex. After 5 days, some 50% of the total mercury content had migrated to the anode compartment, and another 25% was recovered from the soil water in the vicinity of the anode. No volatile mercury was formed. Electromigration is the dominant transport process for the (charged) mercury–iodide complex, since electro-osmosis would have moved the mercury toward the cathode. The combination of iodide as complexing agent and an electric field for physical mobilization could be developed to a new method for in situ remediation of mercury contaminated soil.     

Iodide-Enhanced Electrokinetic Remediation of Mercury-Contaminated Soils

This study investigates using an iodide-enhanced solution at the cathode during electrokinetic treatment to optimize the removal of mercury from soils. The experimental program consisted of testing two types of clayey soils, kaolin, and glacial till, that were initially spiked with 500 mg/kg of Hg(II). Experiments were conducted on each soil type at two voltage gradients (1.0 or 1.5 VDC/cm) to evaluate the effect of the voltage gradient when employing a 0.1 M KI solution. Additional experiments were performed on each soil type to assess the effect of using a higher iodide concentration (0.5 M KI) when using a 1.5 VDC/cm voltage gradient. The tests conducted on the kaolin soil showed that when the 0.1 M KI concentration was employed with the 1.0 VDC/cm voltage gradient, approximately 97% of the mercury was removed, leaving a residual concentration of 16 mg/kg in the soil after treatment. The tests conducted on glacial till indicated that it was beneficial to use the higher (0.5 M KI) iodide concentration and the higher (1.5 VDC/cm) voltage gradient to enhance mercury removal, because, under these conditions, a maximum of 77% of the mercury was removed from the glacial till, leaving a residual concentration of 116 mg/kg in soil after electrokinetic treatment. Compared to kaolin, the lower mercury removal from the glacial till soil is attributed to the more complicated soil composition, such as the presence of carbonates and organic matter, which caused Hg(II) to adsorb to the soil and/or exist as an immobile chemical species.     

Chemistry of Modified Fenton’s Reagent (Catalyzed H2O2 Propagations–CHP) for In Situ Soil and Groundwater Remediation

The use of modified Fenton’s reagent, or catalyzed H2O2 propagations (CHP), has become increasingly popular for the in situ and ex situ treatment of surface soils and the in situ remediation of the subsurface. The process is based on the catalyzed decomposition of hydrogen peroxide by soluble iron, iron chelates, or iron minerals to generate the strong oxidant hydroxyl radical as well as other reactive oxygen species. Some of these species function as reductants and nucleophiles and may be responsible for the enhanced treatment of sorbed and nonaqueous phase liquid (NAPL) contaminants that is sometimes observed in the field. This paper serves as a review of the process chemistry of CHP; the goal is to provide researchers and practitioners with fundamental concepts that will aid in applying the CHP process to soil and groundwater contamination. Although the importance of well placement and the method of reagent injection must be considered in CHP remediation, understanding and promoting the most effective process chemistry is essential to successful soil and groundwater remediation.     

Development of a Field Design for In Situ Gaseous Treatment of Sediment Based on Laboratory Column Test Data

A testing methodology is presented that supports the development of a field design for in situ gaseous treatment of sediments with diluted hydrogen sulfide. This approach involves the collection of column breakthrough test results at various flow rates, allowing a relationship to be developed between pore velocity of the carrier gas and velocity of the hydrogen sulfide reaction front that permits sizing to the field scale. A regression fit of a set of laboratory column breakthrough test data collected in this study is utilized to illustrate the development of a field design based on a two-dimensional radial flow analytical model. Information regarding treatment time and hydrogen sulfide consumption characteristics associated with in situ gaseous treatment can then be obtained from this model and used as a basis for estimation of treatment schedule and costs. The regression relationship can also be utilized in numerical models in more complex geometries to support the field design of in situ gaseous treatment operations.     

Chloride Interactions with Iron Surfaces: Implications for Perchlorate and Nitrate Remediation Using Permeable Reactive Barriers

Perchlorate (ClO4?) can be reduced by iron surfaces, suggesting that permeable reactive barriers may represent a useful groundwater remediation strategy. However, chloride produced by the reaction inhibits further perchlorate removal. Adsorption of chloride on iron filings was investigated as a potential mechanism of chloride interference. The effect of chloride on the removal of nitrate, another oxyanion reactive at iron surfaces, was also investigated to draw more general conclusions about anion competition when target compounds adsorb electrostatically. A triple layer adsorption model was used to describe chloride sorption isotherms on the iron filings using magnetite as the model surface and defining a single type of surface hydroxyl sorption site. The model considered electrostatic attraction, specific sorption, and the effect of adsorbed Fe2+ on chloride sorption. Experimental and modeling results indicate that chloride competition is probably not of concern for nitrate reduction in permeable reactive barriers. However, perchlorate reduction is significantly inhibited by chloride in both buffered and unbuffered solutions, possibly because the reactive sorbed Fe2+ sites may be preferentially occupied by chloride.     

Genesis and Effects of Particles Produced during In Situ Chemical Oxidation Using Permanganate

A research effort was undertaken to investigate the genesis of particles produced during in situ chemical oxidation (ISCO) of trichloroethene (TCE) with permanganate (MnO4?) and to explore the effects of those particles on system permeability and metal mobility. The experimental approach included characterization of soil and groundwater samples from an ISCO field site, batch experiments with a replicated 25 factorial design, and flow-through column experiments. Analyses of intact soil cores from an ISCO field site revealed that MnO2 solids were present in the subsurface near an injection well for NaMnO4 but at low levels (2.3–2.5 mg/g dry wt media) calculated to fill <1% v/v of the aquifer porosity. Batch tests revealed that the mass of filterable solids (>0.45 μm) produced during chemical oxidation with MnO4? was increased at higher TCE concentrations (54 versus 7 mg/L) and in the presence of ambient silt/clay-sized particles in the groundwater (750 versus 7.5 mg/L). Under otherwise comparable conditions, increasing the MnO4? dose markedly increases the oxidant consumption and also increases the solids production. The oxidant form (NaMnO4 versus KMnO4) or reaction time (15 versus 300 min) had little effect on oxidant consumption or filterable solids production. During MnO4? oxidation of higher levels of TCE in a groundwater with ambient silt/clay particles present, there can be substantial increases in filterable solids generated, which are <1 μm in size and consist of MnO2, commingled with other mineral matter. Conceivably, low volumetric fillings of these solids could cause permeability loss. Flow-through column experiments revealed that permeability loss was possible during ISCO but only under conditions with very high MnO2 solids production. On the positive side, the MnO2 solids produced can increase the sorption potential for metals such as cadmium and can represent a mode of immobilization. This research demonstrated that ISCO with permanganate has the potential to yield system permeability loss under some conditions as well as to affect metal mobility. The magnitude of these effects is related to the subsurface conditions, target organic chemical mass, and permanganate dose and delivery method. The production of solids during ISCO needs to be carefully considered during process design and operation to avoid solids-related performance problems while exploiting potential benefits.     

Effects of Groundwater Velocity and Permanganate Concentration on DNAPL Mass Depletion Rates during In Situ Oxidation

In situ chemical oxidation (ISCO) using permanganate has been increasingly applied to deplete mass from dense nonaqueous-phase liquid (DNAPL) source zones. However, uncertainty in the performance of ISCO on DNAPL contaminants is partially attributable to a limited understanding of interactions between the oxidant, subsurface hydrology, and DNAPL mass transfer, resulting in failure to optimize ISCO applications. To investigate these interactions, a factorial design experiment was conducted using one-dimensional flow through tube reactors to determine how groundwater velocity, permanganate concentration, and DNAPL type affected DNAPL mass depletion rates. DNAPL mass depletion rates were found to increase with increasing groundwater velocity, or increasing oxidant concentration. An interaction occurred between the two factors, where high oxidant concentrations had little impact on mass depletion rates at high velocities. High oxidant concentration systems experienced gas generation. Mass depletion rates were fastest at high velocities, but required additional oxidant mass and pore volume addition to achieve complete mass depletion. Lower-velocity systems were more efficient with respect to oxidant mass and pore volume requirements, but mass depletion rates were reduced.     

Case Study of Subsidence in Interstratified Sedimentary Bedrock: Parking Garage Distress Investigation and Remediation

This case study illustrates the investigation and remediation of distress and subsidence on a multilevel, posttension, cast-in-place concrete parking garage that was constructed at the crest and over the side of a gentle sloping hillside. A comprehensive exploration program was conducted to determine the cause of the subsidence. The investigation included core borings through the distressed caissons and the underlying bedrock and the use of a downhole camera to determine the rock conditions. Based on the investigation, it was concluded that the caisson subsidence was caused by the collapse of a large cavity underlying the sandstone. A grouting program was performed to reduce the potential of further settlement. This paper summarizes the findings and conclusions derived from this investigation and the remedial measures undertaken to reduce the potential of further distress to the structure.     

Fully Coupled Analysis of Failure and Remediation of Lower San Fernando Dam

The extent of flow deformation in an embankment dam is determined by the driving forces and the residual strength of the soil, as well as by the kinematic constraints. The soil conditions of berm and buttress, as well as of foundation, are also critical factors affecting seismic performance of an embankment dam. A careful examination of these factors is necessary when proposing remedial measures to a seismically deficient dam. This paper presents a set of fully coupled finite element analyses of the response of the well-known lower San Fernando Dam during the 1971 earthquake. A critical state model incorporating the concept of state-dependent dilatancy was employed to describe soil behavior over the full range of loading conditions encountered. The results show clearly that a flow slide occurred on the upstream side, and indicate that a downstream flow slide would occur, too, if the downstream berm had not been constructed before the event. The analyses show also that the addition of an upstream berm could effectively prevent the upstream flow slide.     

Remediation of TCE-Contaminated Groundwater in a Sandy Aquifer Using Pulsed Air Sparging: Laboratory and Numerical Studies

The objective of this study was to investigate, through laboratory and numerical investigations, the effectiveness of a pulsed air sparging system for remediation of groundwater contaminated with trichloroethylene (TCE) in a sandy aquifer. In laboratory experiments, air was pulsed into TCE source zone on a daily basis in order to remediate TCE-contaminated groundwater. Most dissolved TCE was removed at the end of experiments although its concentrations fluctuated due to the air pulsing. The measured gaseous phase TCE concentration increased whereas the aqueous phase TCE concentration decreased during air sparging pulses. Experimental data were assessed by using a numerical code STOMP (subsurface transport over multiphases) with some modification based on the TCE dissolution kinetics. The unmeasured residual TCE mass was predicted through numerical simulations. Results show that aqueous concentrations for TCE are still much higher than the maximum contaminant level in spite of successful removal of 95% of residual TCE. It may imply that it would be more appropriate to apply air sparging combined with other remediation technologies such as bioremediation for remediation of TCE-contaminated groundwater.     

Simulation of Metals Total Maximum Daily Loads and Remediation in a Mining-Impacted Stream

Simulation of metals transport was performed to help develop metals total maximum daily loads (TMDLs) and evaluate remediation alternatives in a mountain stream in Montana impacted by hundreds of abandoned hardrock metal mines. These types of watersheds are widespread in Montana and many other areas of the western United States. Impacts from abandoned hardrock or metal mines include loadings of sediment, metals, and other pollutants causing impairment of multiple beneficial uses and exceedances of water quality standards. The United States Environmental Protection Agency (EPA) Water Quality Analysis Simulation Program (WASP) was used to model and evaluate TMDLs for several heavy metals in Tenmile Creek, a mountain stream supplying drinking water to the City of Helena, Mont. The model was calibrated for baseflow conditions and validated using data collected by the EPA and the United States Geological Survey, and used to assess existing metals loadings and losses, including interactions between metals in water and bed sediment, uncertainty, water quality standard exceedances, TMDLs, potential source areas, and required reductions in loadings. During baseflow conditions, adits and point sources contribute significant metals loadings to Tenmile Creek. Exceedances of standards are widespread throughout the stream under both baseflow and higher flow conditions. Adsorption and precipitation onto bed sediments play a primary role in losses from the water column in some areas. Modeling results indicate that some uncertainty exists in the metal partition coefficients associated with sediment, significance of precipitation reactions, and in locations of unidentified sources and losses of metals. TMDLs and loading reductions were calculated based on variations in flow, concentrations, loadings, and standards (which vary with hardness) along the mainstem. In most cases, considerable reductions in loadings are required to achieve TMDLs and water quality standards. Reductions in loadings from point sources, mine waste near watercourses, and streambed sediment can help improve water quality, but alteration of the water supply scheme and increasing baseflow will also be needed.     

Aquatic Assessment of the Chernobyl Nuclear Accident and Its Remediation

This modeling study evaluated the aquatic environment affected by the Chernobyl nuclear accident and the effectiveness of remediation efforts. The study results indicate that radionuclide concentrations in the Pripyat and Dnieper rivers were well above the drinking water limits immediately after the Chernobyl accident but have decreased significantly in subsequent years due to flushing, burying, and decaying. Because high concentrations of 90Sr and 137Cs, the major radionuclides affecting human health through the aquatic pathways, are associated with flooding, two earthen dikes were constructed along the Pripyat River. The left-bank dike alone was successful in reducing the 90Sr concentration in the river by half. The 100-m-high, movable New Safe Confinement (NSC), which will cover the current Chernobyl Shelter, will reduce radionuclide contamination further in these rivers and nearby groundwater. If the Chernobyl Shelter should collapse before the NSC is built, the resulting peak radionuclide concentrations in the Dnieper River are expected to still remain below the drinking water limits. The radionuclide influx to groundwater through the NSC should not have any effect on concentrations in the Pripyat River.     

Iron-Mediated Aeration: Evaluation of Energy-Assisted Enhancement for In Situ Subsurface Remediation

Laboratory tests using ultraviolet radiation and sonocation energy were found to kinetically enhance an iron-mediated aeration (IMA) process under development to remove chelated metals and radionuclides and associated organics, from groundwater and soils. A model inorganic contaminant (Cd2+) chelated with ethylenediamine tetraacetic acid (EDTA) was used. The IMA process breaks the complex, releasing the target metal for removal. Overall experimental results indicate that the EDTA degradation mechanism can be accelerated compared to nonenergized IMA, by a factor of 2 using sonocation energy and by a factor of 3–4 using photochemical energy at circumneutral pH. No differences in the by-products were indicated in chemical analyses. For both sonocation and photochemical tests, the major breakdown products detected were glyoxylic acid and formaldehyde. Only minor amounts of larger molecular weight species (iminodiacetic acid, nitrilotriacetic acid, and ethylenediamine triacetic acid) were detected.     

Predicting the Performance of Activated Carbon-, Coke-, and Soil-Amended Thin Layer Sediment Caps

In situ capping manages contaminated sediment on-site without creating additional exposure pathways associated with dredging, e.g., sediment resuspension, and potential human exposure during transport, treatment, or disposal of dredged material. Contaminant mass is not immediately removed in sediment capping, which creates concerns over its long-term effectiveness. Groundwater seepage can also decrease the effectiveness of in situ capping. This study compares the effectiveness of commercially available sorbents that can be used to amend sand caps to improve their ability to prevent contaminant migration from the sediments into the bioactive zone. Amendments evaluated include coke, activated carbon, and organic-rich soil. The properties relevant to advective-dispersive transport through porous media (sorption, porosity, dispersivity, and bulk density) are measured for each material, and then used as inputs to a numerical model to predict the flux of 2,4,5-polychlorinated biphenyl (PCB) through a sand cap amended with a thin (1.25-cm) sorbent layer. Systems with and without groundwater seepage are considered. Isolation times provided by the sorbent layers increased with increasing sorption strength and capacity (activated carbon?coke ≈ soil?sand). The effective porosity, dispersivity, and bulk density of the sorbent layer had little effect on cap performance compared to sorption strength (Kf). In the absence of seepage, all sorbents could isolate PCBs in the underlying sediment for times greater than 100?years and would be effective for most cap applications. With groundwater seepage (Darcy velocity = 1?cm/day), activated carbon was the only sorbent that provided contaminant isolation times greater than 60?years. Long isolation times afforded by sorbent-amended caps allow time for inherently slow natural attenuation processes to further mitigate PCB flux.     

Electron Beam Irradiation for Mercury Oxidation and Mercury Emissions Control

A variety of air pollution control strategies have been investigated to reduce mercury emissions from coal-fired sources. The most developed and deployed technologies are based on adsorption of mercury onto powdered activated carbon followed by carbon collection. Mercury oxidation over selective catalytic reduction catalysts followed by wet scrubbing is another potential technique, and tests suggest that emissions reductions of 20–80% are possible, but test results are variable and ultrahigh removal (95%+) is unusual. The objective of this study was to investigate the effectiveness of electron beam irradiation to oxidize mercury vapor, to improve mercury removal with wet scrubbers or wet electrostatic precipitators (ESPs). Metallic mercury vapor samples in air and other atmospheres were prepared at concentrations of approximately 16?μg/m3. Samples were electron irradiated at energy levels of 2.5–10 kGy, equivalent to 3.1–12.4?kJ/m3 stack gas. Results show that mercury oxidation rate was dependent on both the gaseous atmosphere composition and the irradiation energy level. At medium energy levels, approximately 98% of gaseous mercury vapor was readily oxidized. Electron beam irradiation demonstrated high levels of mercury oxidation at the bench scale, and the technology might help improve mercury removal in wet scrubbers or wet ESPs when employed as a primary or secondary mercury oxidation technique.     

Comparison of Enzyme Activities in Vegetated and Nonvegetated Sediments

This study focuses on the effect of aquatic plants on the changes of enzyme activities in wetland sediments. Wetland plants play essential roles both as a carbon supplier for microbes which synthesize enzymes and as a regulator for enzyme activity by modifying hydrochemistry in the rhizosphere. Although numerous studies have been carried out on soil enzymes, little information is available on the vertical distribution and temporal variation of enzyme activities affected by the presence of plants in wetlands. Our results clearly show that sediments with wetland plants exhibit significantly higher enzyme activities of β-glucosidase, arylsulfatase, phosphatase, and N-acetylglucosaminidase (P<0.05) up to a depth of 15?cm throughout the year, whereas only lower values were observed even at the surface of sediments (0–3?cm) without plants. However, in the field, there were no statistically significant changes of enzyme activities associated with the changes of season and the vertical position along the depth (P<0.05). This indicates that the organic carbon supplemented by root exudates, root debris, and plant residue played an important role in increasing enzyme activities in the sediments with plants. The mechanisms driven by aquatic plants such as oxygen diffusion and transpiration-induced advection did not induce the short-term changes in enzyme activities. Exceptionally, the changes of sulfate availability and the increase of temperature have implications in the changes of arylsulfatase activities depending on the location (vegetated versus nonvegetated sediment) (P = 0.000), season (growing season versus senescence) (P = 0.042), and sediment depth (P = 0.002). Since wetlands treat wastewaters with variable carbon sources, it would be beneficial to maintain increased enzyme activities in the regeneration of inorganic nutrients from organic materials. In addition, the presence of plants would vertically extend the area where the higher enzyme activities are observed and the movement of wastewater takes place and, consequently, could accelerate wetland treatment efficiency.     

Advanced Oxidation for Treatment of Aqueous Extracts from EDTA Extraction of Pb and Zn Contaminated Soil

The feasibility of using advanced oxidation processes (AOPs): ozone, ozone/sonification, and ozone/ultraviolet (UV) irradiation in treatment to remove heavy metals and ethylenediamine tetraacetate (EDTA) from aqueous extracts, obtained after soil extraction with EDTA, was examined. Extraction of soil contaminated with 1,243?mg?kg?1 Pb and 1,190?mg?kg?1 Zn with 40?mmol?kg?1 EDTA removed 41.8±0.9 and 7.2±.0.2% of Pb and Zn. Of the AOPs tested, only the use of ozone/UV enabled the decomposition of EDTA–heavy metals complexes in aqueous soil extracts, and recovery of released Pb and Zn by sorption on a commercial sorbent Slovakite. After treatment, the concentration of Pb, Zn, and EDTA in the extracts was fairly low (2.87±1.15?mg?L?1, 7.58±2.12?mg?L?1, and 0.012±0.002?mmol?L?1, respectively), and could presumably be reduced even further with a continuation of treatment. The treated extract was used for subsequent soil rinsing, which removed an additional 12.7±1.6 and 2.7±0.1% of soil Pb and Zn. The results of our study indicate that the use of ozone/UV is a feasible option for treatment of aqueous soil extracts from EDTA extraction. Treated extracts could be safely discharged or reused to lower requirements for process water.     

Comparison of Ion-Selective and Carbon Fiber Mercury Film Microelectrodes for Cadmium Measurement

Cadmium ion-selective microelectrodes and carbon fiber mercury film microelectrodes were fabricated and compared in this research. In order to determine whether either microelectrode could be used in biofilm microenvironmental profile studies, their performances were evaluated under different pH, buffer concentration, and ionic strength conditions. Both microelectrodes showed good linearity during calibration. pH did not have a significant impact on the ion-selective microelectrode, but significantly changed the performance of the carbon fiber mercury film microelectrode. Tris buffer was evaluated for pH control and found to increase readings of both microelectrodes. Ionic strength had a similar impact on both microelectrodes. Both microelectrodes had high selectivity for Cd2+, but potential interferences should be considered when real samples are to be measured.     

Multiple-Porosity Model for the Transport of Reactive Contaminants in Fissured Soils with Nonequilibrium Partitioning

A multiple-porosity model for the transport of reactive contaminants in fissured media with multiple-source, nonequilibrium partitioning is proposed, widening the scope of existing models. The proposed model extends the bicontinuum, dual-porosity concepts by combining five contaminant compartments: (1) mobile water in the fissures; (2) immobile water in the fissures; (3) water diffusing into the soil matrix; (4) sorption in the fissures; and (5) sorption in the soil matrix. Both instantaneous and nonequilibrium sorption are represented in the fissures. Mobile/immobile compartments, fissured soils, and nonequilibrium sorption have been hitherto treated separately or in pairs. Exchange of contaminants occurs between all compartments. Equations for the model are formulated and transformed into the Laplace domain. Solutions for the one-dimensional problem of a leaking storage tank overlying a fissured soil are found. The effects of the inclusion of various contaminant compartments and exchange parameters are analyzed through numerical experiments.     

Approach to Control the Depth of Water in Basin Irrigation and Wetland Flooding

The controlled ponding of water over level terrain in basin irrigation or wetland flooding is described quantitatively as a three-phase process. During the first phase, water is applied at a known rate until ponding emerges at the time of ponding initiation. In the second phase, water continues to be applied at the same rate until a desired ponded depth is attained. In the third phase, water is applied to maintain the desired ponded depth during an arbitrarily long period. The desired ponded depth is maintained by adjusting the water-application rate to equal the infiltration rate plus the evaporation rate. The time of ponding, the ordinary differential equations (ODEs) governing cumulative infiltration during the second and third phases, and the water-application rate during the third phase are derived in this work using an extended Green-and-Ampt formulation of infiltration. Computational examples illustrate the solutions of the derived ODEs and their application in the control of basin irrigation and wetland flooding.