Cosolvent-enhanced chemical oxidation of perchloroethylene by potassium permanganate

A laboratory study was conducted to examine cosolvent-enhanced in-situ chemical oxidation (ISCO) of perchloroethylene (PCE) using potassium permanganate (KMnO4). The conceptual basis for this new technique is to enhance permanganate oxidation of dense non-aqueous phase liquids (DNAPLs) with the addition of a cosolvent, thereby increasing DNAPL solubility while avoiding mobilization. Among 17 cosolvent candidates screened, tertiary butyl alcohol (TBA) and acetone were the most stable in the presence of KMnO4, both of which increased PCE aqueous solubility significantly, and therefore are suitable to be used as cosolvent in this study. Batch experiments indicated that the second-order rate constant for PCE oxidation by potassium permanganate was 0.043+/-0.002 M(-1) s(-1) in the purely aqueous (no cosolvent) solution. In the presence of 20% cosolvent (volume fraction=fc=0.2), the rate constant decreased to 0.036+/-0.003 M(-1) s(-1) with TBA and to 0.031+/-0.002 M(-1) s(-1) with acetone. However, in the presence of free-phase PCE, chloride ion concentration from PCE oxidation in acetone/water solutions (fc=0.2) was about twice that in aqueous solutions, indicating that the increase in PCE solubility more than compensated for the decrease in reaction rate constant, such that the oxidation efficiency of PCE was increased with cosolvent. A complete chlorine mass balance was observed in the aqueous system, whereas approximately 70% was obtained in TBA/water or acetone/water (fc=0.2). In soil columns containing residual DNAPL and subjected to isocratic flushing with step-wise increases in f(c) cosolvent, TBA at fc=0.2 resulted in PCE mobilization, whereas acetone at fc相似文献   

Laboratory-scale in situ chemical oxidation of a perchloroethylene pool using permanganate

In situ chemical oxidation (ISCO) is an emerging technology for the destruction of some chlorinated solvents present in subsurface environments. A laboratory investigation using a physical model was designed to assess the effectiveness of using permanganate as an oxidant to reduce the mass of a perchloroethylene (PCE) pool. The physical model was filled with silica sand overlying a silica flour base, simulating a two-dimensional saturated sand zone overlying a capillary barrier. PCE was introduced into the model so that it rested on top of the silica flour base, forming a dense nonaqueous phase liquid pool. The experimental methodology involved flushing the model with a permanganate solution for 146 days. During this period, measurements of chloride were used to assess the extent of pool oxidation. Before and after the oxidant flush, the quasi-steady state dissolution from the PCE pool was evaluated. Additionally, tracer studies were completed to assess changes in the flow field due to the oxidation process. At the termination of the experiment nine soil cores extracted from the model were used to detect the presence of MnO2 deposits and to quantify the mass of PCE remaining in the system. Excavation of the remaining material in the model revealed that the MnO2 distribution throughout the model was consistent with that observed in the cores. The oxidant flush was concluded before all of the pure phase PCE had been completely oxidized; however, approximately 45% of the PCE mass was removed, resulting in a fourfold decrease in the quasi-steady state aqueous phase mass loading of PCE from the pool. Measurements of chloride during the oxidant flush and of PCE in the soil cores suggested that the oxidation reaction occurred primarily at the upgradient edge of the PCE pool. MnO2 deposits within the model aquifer decreased the velocity of water directly above the pool, and the overall mass transfer from the remaining PCE pool. The results of this experimental study indicate that ISCO using permanganate is capable of removing substantial mass from a DNAPL pool; however, the performance of ISCO as a pool removal technology will be limited by the formation and precipitation of hydrous MnO2 that occurs during the oxidation process.     

Enhanced reductive dechlorination in columns treated with edible oil emulsion

The effect of edible oil emulsion treatment on enhanced reductive dechlorination was evaluated in a 14 month laboratory column study. Experimental treatments included: (1) emulsified soybean oil and dilute HCl to inhibit biological activity; (2) emulsified oil only; (3) emulsified oil and anaerobic digester sludge; and (4) continuously feeding soluble substrate. A single application of emulsified oil was effective in generating strongly reducing, anaerobic conditions for over 14 months. PCE was rapidly reduced to cis-DCE in all three live columns. Bioaugmentation with a halorespiring enrichment culture resulted in complete dechlorination of PCE to ethene in the soluble substrate column (yeast extract and lactate). However, an additional treatment with a pulse of yeast extract and bioaugmentation culture was required to stimulate complete dechlorination in the emulsion treated columns. Once the dechlorinating population was established, the emulsion only column degraded PCE from 90-120 microM to below detection with concurrent ethene production in a 33 day contact time. The lower biodegradation rates in the emulsion treated columns compared to the soluble substrate column suggest that emulsified oil barriers may require a somewhat longer contact time for effective treatment. In the HCl inhibited column, partitioning of PCE to the retained oil substantially delayed PCE breakthrough. However, reduction of PCE to more soluble degradation products (cis-DCE, VC and ethene) greatly reduced the impact of oil-water partitioning in live columns. There was only a small decline in the hydraulic conductivity (K) of column #1 (low pH+emulsion, K(final)/K(initial)=0.57) and column #2 (live+emulsion, K(final)/K(initial)=0.73) indicating emulsion injection did not result in appreciable clogging of the clayey sand. However, K loss was greater in column #3 (sludge+emulsion, K(final)/K(initial)=0.12) and column #4 (soluble substrate, K(final)/K(initial)=0.03) indicating clogging due to biomass and/or gas production can be significant.     

Characteristics and applications of controlled-release KMnO4 for groundwater remediation

In situ chemical oxidation (ISCO) using potassium permanganate (KMnO4) has been widely used as a practical approach for remediation of groundwater contaminated by chlorinated solvents like trichloroethylene. The most common applications are active flushing schemes, which target the destruction of some contaminant source by injecting concentrated permanganate (MnO4(-)) solution into the subsurface over a short period of time. Despite many promising results, KMnO4 flushing is often frustrated by inefficiency associated with pore plugging by MnO2 and bypassing. Opportunities exist for the development of new ISCO systems based on KMnO4. The new scheme described in this paper uses controlled-release KMnO4 (CRP) as an active component in the well-based reactive barrier system. This scheme operates to control spreading of a dissolved contaminant plume. Prototype CRP was manufactured by dispersing fine KMnO4 granules in liquid crystal polymer resin matrix. Scanning electron microscope data verified the formation of micro-scale (ID=20-200 microm) secondary capillary permeability through which MnO4(-) is released by a reaction-diffusion process. Column and numerical simulation data indicated that the CRP could deliver MnO4(-) in a controlled manner for several years without replenishment. A proof-of-concept flow-tank experiment and model simulations suggested that the CRP scheme could potentially be developed as a practical approach for in situ remediation of contaminated aquifers. This scheme may be suitable for remediation of sites where accessibility is limited or some low-concentration contaminant plume is extensive. Development of delivery systems that can facilitate lateral spreading and mixing of MnO4(-) with the contaminant plume is warranted.     

Metal mobility during in situ chemical oxidation of TCE by KMnO4

The potential for trace-metal contamination of aquifers as a side effect of In Situ Chemical Oxidation (ISCO) of chlorinated solvent contamination by KMnO(4) is investigated with column experiments. The experiments investigate metal mobility during in situ chemical oxidation of TCE by KMnO(4) under conditions where pH, flow rate, KMnO(4), TCE, and trace-metal concentrations were controlled. During ISCO, the injection of MnO(4) creates oxidizing conditions, and acidity released by the reactions causes a tendency toward low pH in aquifers. In order to evaluate the role of pH buffering on metal mobility, duplicate columns were constructed, one packed with pure silica sand, and one with a mixture of silica sand and calcite. Aqueous solutions of TCE and KMnO(4) (with 1 mg/L Cu, Pb, Zn, Mo, Ni, and Cr(VI)) were allowed to mix at the inlet to the columns. After the completion of the experiments, samples of Mn oxide were removed from the columns and analyzed by analytical scanning and transmission electron microscopy. In order to relate the results of the laboratory experiments to field settings, the analyses of Mn-oxide samples from the lab experiments were compared to samples of Mn oxide collected from a field-scale chemical-oxidation experiment that were also analyzed by analytical electron microscopy as well as time-of-flight secondary-ion mass spectroscopy. The pH ranged from 2.40 in the silica sand column to 6.25 in the calcite-containing column. The data indicate that aqueous Mo, Pb, Cu and Ni concentrations are attenuated almost completely within the columns. In contrast, Zn concentrations are not significantly attenuated and Cr(VI) is transported conservatively. The results indicate that within the range 2.40 to 6.25, metal mobility is not affected by pH. Comparison of analyses of Mn-oxide from the lab and field demonstrate that a variety of metals are sequestered from solution by Mn oxide.     

Development of a biobarrier for the remediation of PCE-contaminated aquifer

The industrial solvent tetrachloroethylene (PCE) is among the most ubiquitous chlorinated compounds found in groundwater contamination. The objective of this study was to develop a biobarrier system, which includes a peat layer to enhance the anaerobic reductive dechlorination of PCE in situ. Peat was used to supply primary substrate (electron donor) continuously. A laboratory-scale column experiment was conducted to evaluate the feasibility of this proposed system or PCE removal. This experiment was performed using a series of continuous-flow glass columns including a soil column, a peat column, followed by two consecutive soil columns. Anaerobic acclimated sludges were inoculated in all three soil columns to provide microbial consortia for PCE biodegradation. Simulated PCE-contaminated groundwater with a flow rate of 0.25 l/day was pumped into this system. Effluent samples from each column were analyzed for PCE and its degradation byproducts (trichloroethylene (TCE), cis-dichloroethylene (cis-DCE), vinyl chloride (VC), ethylene (ETH), and ethane). Results show that the decrease in PCE concentrations and production of PCE byproducts were observed over a 65-day operating period. Up to 98% of PCE removal efficiency was obtained in this passive system. Results indicate that the continuously released organics from peat column enhanced PCE biotransformation. Thus, the developed biobarrier treatment scheme has the potential to be developed into a cost-effective in situ PCE-remediation technology, and can be utilized as an interim step to aid in system scale-up.     

In-situ oxidation of trichloroethene by permanganate: effects on porous medium hydraulic properties

In-situ oxidation of dense nonaqueous-phase liquids (DNAPLs) by strong oxidants such as potassium permanganate (KMnO4) has been proposed as a possible DNAPL remediation strategy. In this study, we investigated the effects of in-situ trichloroethene (TCE) oxidation by KMnO4 on porous medium hydraulic properties. In particular, we wanted to determine the overall effects of concurrent solid phase (MnO2) precipitation, gas (CO2) evolution and TCE dissolution resulting from the oxidation reaction on the porous medium's aqueous-phase relative permeability, krw. Three TCE removal experiments were conducted in a 95-cm long, 5.1-cm i.d. glass column, which was homogeneously packed with well-characterized 30/40-mesh silica sand. TCE was emplaced in the sand-pack in residual, entrapped form through a sequence of water/TCE imbibition and drainage steps. The column was then flushed under constant aqueous flux conditions for up to 104 h with either deionized water (reference experiment), deionized water containing 5 mM KMnO4 or deionized water containing 5 mM KMnO4 and 300 mM Na2HPO4. Aqueous-phase relative permeabilities were computed from measured flow rates and measurements of aqueous-phase pressure head, h obtained using pressure transducers connected to tensiometers distributed along the column length. A dual-energy gamma radiation system was used to monitor changes in fluid saturation that occurred during each experiment. In addition, column effluent samples were collected for chemical analyses. Dissolution of TCE during deionized water flushing led to an increase in krw by approximately 22% and a local reduction in h. On the other hand, vigorous CO2 gas production and precipitation of MnO2 was visually observed during flushing with deionized water that contained 5 mM KMnO4. As a consequence, krw declined by approximately 96% and h increased locally by more than 1000 cm H2O during the first 24 h of the experiment, causing sand-pack ruptures and pump failure. Conversely, less CO2 gas production and MnO2 precipitation was visually observed during flushing with deionized water that contained 5 mM KMnO4 and 300 mM Na2HPO4. Consequently, only small increases in h (< 15 cm H2O) were observed in this experiment due to a reduction in krw of approximately 53%. While we must attribute changes in h due to variations in krw to our specific experimental design (constant aqueous flux, one-dimensional flow experiments), these experiments nevertheless confirm that successful application of in situ chemical oxidation of TCE requires consideration of detrimental processes such as MnO2 precipitation and CO2 gas formation. In addition, our results indicate that utilization of a buffered oxidant solution may improve the effectiveness of in-situ oxidation of TCE by KMnO4 in otherwise weakly buffered porous media.     

Kinetics and mechanism of oxidation of tetrachloroethylene with permanganate

The kinetics, reaction pathways and product distribution of oxidation of tetrachloroethylene (PCE) by potassium permanganate (KMnO4) were studied in phosphate-buffered solutions under constant pH, isothermal, completely mixed and zero headspace conditions. Experimental results indicate that the reaction is first-order with respect to both PCE and KMnO4 and has an activation energy of 9.3+/-0.9 kcal/mol. The second-order rate constant at 20 degrees C is 0.035+/-0.004 M(-1) s(-1), and is independent of pH and ionic strength (I) over a range of pH 3-10 and I approximately 0-0.2 M, respectively. The PCE-KMnO4 reaction may proceed through further oxidation and/or hydrolysis reaction pathways, greatly influenced by the acidity of the solution, to yield CO2(g), oxalic acid, formic acid and glycolic acid. Under acidic conditions (e.g., pH 3), the further oxidation pathway will dominate and PCE tends to be directly mineralized into CO2 and chloride. Under neutral (e.g., pH 7) and alkaline conditions (e.g., pH 10), the hydroxylation pathway dominates the reaction and PCE is primarily transformed into oxalic acid prior to complete PCE mineralization. Moreover, all chlorine atoms in PCE are rapidly liberated during the reaction and the rate of chloride production is very close to the rate of PCE degradation.     

Continuous-flow column study of reductive dehalogenation of PCE upon bioaugmentation with the Evanite enrichment culture

A continuous-flow anaerobic column experiment was conducted to evaluate the reductive dechlorination of tetrachloroethene (PCE) in Hanford aquifer material after bioaugmentation with the Evanite (EV) culture. An influent PCE concentration of 0.09 mM was transformed to vinyl chloride (VC) and ethene (ETH) within a hydraulic residence time of 1.3 days. The experimental breakthrough curves were described by the one-dimensional two-site-nonequilibrium transport model. PCE dechlorination was observed after bioaugmentation and after the lactate concentration was increased from 0.35 to 0.67 mM. At the onset of reductive dehalogenation, cis-dichloroethene (c-DCE) concentrations in the column effluent exceeded the influent PCE concentration indicating enhanced PCE desorption and transformation. When the lactate concentration was increased to 1.34 mM, c-DCE reduction to vinyl chloride (VC) and ethene (ETH) occurred. Spatial rates of PCE and VC transformation were determined in batch-incubated microcosms constructed with aquifer samples obtained from the column. PCE transformation rates were highest in the first 5 cm from the column inlet and decreased towards the column effluent. Dehalococcoides cell numbers dropped from approximately 73.5% of the total Bacterial population in the original inocula, to about 0.5% to 4% throughout the column. The results were consistent with estimates of electron donor utilization, with 4% going towards dehalogenation reactions.     

高锰酸钾降解地下水中PCE的研究

以氯代有机污染物中常见的PCE为目标污染物,以自制高锰酸钾溶液为氧化剂,采用批实验方法,探讨了高锰酸钾降解PCE的反应动力学、影响因素以及反应机理。反应结果表明,高锰酸钾降解PCE的反应符合一级动力学方程,反应活化能E为57.119 kJ/mol,在30℃条件下,反应速率常数为0.0076 min^-1,半衰期为91.20 min。在pH在3～10,离子强度在0～0.1030 mol/L之间变化时,反应速率不受明显影响。     

Intermediate-scale 2D experimental investigation of in situ chemical oxidation using potassium permanganate for remediation of complex DNAPL source zones

In situ chemical oxidation is a technology that has been applied to speed up remediation of a contaminant source zone by inducing increased mass transfer from DNAPL sources into the aqueous phase for subsequent destruction. The DNAPL source zone can consist of one or more individual sources that may be present as an interconnected pool of high saturation, as a region of disconnected ganglia at residual saturation, or as combinations of these two morphologies. Potassium permanganate (KMnO(4)) is a commonly employed oxidant that has been shown to rapidly destroy DNAPL compounds like PCE and TCE following second-order kinetics in an aqueous system. During the oxidation of a target DNAPL compound, or naturally occurring reduced species in the subsurface, manganese oxide (MnO(2)) solids are produced. Research has shown that these manganese oxide solids may result in permeability reductions in the porous media thus reducing the ability for oxidant to be transported to individual DNAPL sources. It can also occur at the DNAPL-water interface, decreasing contact of the oxidant with the DNAPL. Additionally, MnO(2) formation at the DNAPL-water interface, and/or flow-bypassing as a result of permeability reductions around the source, may alter the mass transfer from the DNAPL into the aqueous phase, potentially diminishing the magnitude of any DNAPL mass depletion rate increase induced by oxidation. An experiment was performed in a two-dimensional (2D) sand-filled tank that included several discrete DNAPL source zones. Spatial and temporal monitoring of aqueous PCE, chloride, and permanganate concentrations was used to relate changes in mass depletion of, and mass flux, from DNAPL residual and pool source zones to chemical oxidation performance and MnO(2) formation. During the experiment, permeability changes were monitored throughout the 2D tank and these were related to MnO(2) deposition as measured through post-oxidation soil coring. Under the conditions of this experiment, MnO(2) formation was found to reduce permeability in and around DNAPL source zones resulting in changes to the overall flow pattern, with the effects depending on source zone configuration. A pool with little or no residual around it, in a relatively homogeneous flow field, appeared to benefit from resulting MnO(2) pore-blocking that substantially reduced mass transfer from the pool even though there was relatively little PCE mass removed from the pool. In contrast, a pool with residual around it (in a more typical heterogeneous flow field) appeared to undergo increased mass transfer as MnO(2) reduced permeability, altering the water flow and increasing the mixing at the DNAPL-water interface. Further, the magnitude of increased PCE mass depletion during oxidation appeared to depend on the PCE source configuration (pool versus ganglia) and decreased as MnO(2) was formed and deposited at the DNAPL-water interface. Overall, the oxidation of PCE mass appeared to be rate-limited by the mass transfer from the DNAPL to aqueous phase.     

Is it possible to remediate a BTEX contaminated chalky aquifer by in situ chemical oxidation?

An industrial coating site in activity located on a chalky plateau, contaminated by BTEX (mainly xylenes, no benzene), is currently remediated by in situ chemical oxidation (ISCO). We present the bench scale study that was conducted to select the most appropriate oxidant. Ozone and catalyzed hydrogen peroxide (Fenton’s reaction) were discarded since they were incompatible with plant activity. Permanganate, activated percarbonate and activated persulfate were tested. Batch experiments were run with groundwater and groundwater-chalk slurries with these three oxidants. Total BTEX degradation in groundwater was reached with all the oxidants. The molar ratios [oxidant]:[Fe²⁺]:[BTEX] were 100:0:1 with permanganate, 100:100:1 with persulfate and 25:100:1 with percarbonate. Precipitation of either manganese dioxide or iron carbonate (siderite) occurred. The best results with chalk slurries were obtained with permanganate at the molar ratio 110:0:1 and activated persulfate at the molar ratio 110:110:1. To avoid precipitation, persulfate was also used without activation at the molar ratio 140:1. Natural Oxidant Demand measured with both oxidants was lower than 5% of initial oxidant contents. Activated percarbonate was not appropriate because of radical scavenging by carbonated media. Permanganate and persulfate were both effective at oxidant concentrations of ca 1 g kg⁻¹ with permanganate and 1.8 g kg⁻¹ with persulfate and adapted to site conditions. Activation of persulfate was not mandatory. This bench scale study proved that ISCO remediation of a chalky aquifer contaminated by mainly xylenes was possible with permanganate and activated or unactivated persulfate.     

原位化学氧化法在土壤和地下水修复中的研究进展

综述了原位化学氧化法修复污染土壤及地下水的最新进展 ,着重介绍了几种氧化剂 ,如二氧化氯、双氧水及Fenton试剂、高锰酸钾和臭氧在土壤修复中的应用、不足及改进 ,并对化学氧化修复技术的发展前景进行了展望     

Experimental determination of the kinetic rate law for the oxidation of perchloroethylene by potassium permanganate

Flushing soils contaminated with trichloroethylene (TCE) and perchloroethylene (PCE) with a permanganate (MnO₄⁻) solution has been shown to reduce the solvent content of the soil. Experiments were performed to quantify the rate at which KMnO₄ oxidizes aqueous solutions of PCE over a range of concentrations. In a series of homogeneous reactors, aqueous phase PCE concentrations were monitored over time in nine experimental trials with excess oxidant concentrations ranging from 5 to 30 g/l. Analysis of the data was performed to quantify the oxidation reaction order with respect to PCE and KMnO₄ and the reaction rate constant. The reaction between PCE and KMnO₄ was determined to be first-order with respect to both PCE and KMnO₄ with an overall specific reaction rate coefficient of 2.45±0.65 M⁻¹ min⁻¹.     

The reactive transport of trichloroethene is influenced by residence time and microbial numbers

The dechlorination rate in a flow-through porous matrix can be described by the species specific dechlorination rate observed in a liquid batch unless mass transport limitations prevail. This hypothesis was examined by comparing dechlorination rates in liquid batch with that in column experiments at various flow rates (3-9-12 cm day(-1)). Columns were loaded with an inoculated sand and eluted with a medium containing 1mM trichloroethene (TCE) for 247 days. Dechlorination in the column treatments increased with decreasing flow rate, illustrating the effect of the longer residence time. Zeroth order TCE or cis-DCE degradation rates were 4-7 folds larger in columns than in corresponding batch systems which could be explained by the higher measured Geobacter and Dehalococcoides numbers per unit pore volume in the columns. The microbial numbers also explained the variability in dechlorination rate among flow rate treatments marked by a large elution of the dechlorinating species' yield as flow increased. Stop flow events did not reveal mass transport limitations for dechlorination. We conclude that flow rate effects on reactive transport of TCE in this coarse sand are explained by residence time and by microbial transport and that mass transport limitations in this porous matrix are limited.     

In situ iron activated persulfate oxidative fluid sparging treatment of TCE contamination--a proof of concept study

In situ chemical oxidation (ISCO) is considered a reliable technology to treat groundwater contaminated with high concentrations of organic contaminants. An ISCO oxidant, persulfate anion (S(2)O(8)(2-)) can be activated by ferrous ion (Fe(2+)) to generate sulfate radicals (E(o)=2.6 V), which are capable of destroying trichloroethylene (TCE). The property of polarity inhibits S(2)O(8)(2-) or sulfate radical (SO(4)(-)) from effectively oxidizing separate phase TCE, a dense non-aqueous phase liquid (DNAPL). Thus the oxidation primarily takes place in the aqueous phase where TCE is dissolved. A bench column study was conducted to demonstrate a conceptual remediation method by flushing either S(2)O(8)(2-) or Fe(2+) through a soil column, where the TCE DNAPL was present, and passing the dissolved mixture through either a Fe(2+) or S(2)O(8)(2-) fluid sparging curtain. Also, the effect of a solubility enhancing chemical, hydroxypropyl-beta-cyclodextrin (HPCD), was tested to evaluate its ability to increase the aqueous TCE concentration. Both flushing arrangements may result in similar TCE degradation efficiencies of 35% to 42% estimated by the ratio of TCE degraded/(TCE degraded+TCE remained in effluent) and degradation byproduct chloride generation rates of 4.9 to 7.6 mg Cl(-) per soil column pore volume. The addition of HPCD did greatly increase the aqueous TCE concentration. However, the TCE degradation efficiency decreased because the TCE degradation was a lower percentage of the relatively greater amount of dissolved TCE by HPCD. This conceptual treatment may serve as a reference for potential on-site application.     

Manganese and trace-metal mobility under reducing conditions following in situ oxidation of TCE by KMnO4: a laboratory column experiment

The stability of Mn oxides, and the potential for mobilization of associated trace metals, were assessed by simulating the onset of microbially-mediated reducing conditions in a continuous-flow column experiment. The column had previously been used for an in situ chemical oxidation (ISCO) experiment in which trichloroethylene was reacted with permanganate in the presence of aqueous trace metals, which produced Mn oxyhydroxides (MnO(x)) that sequestered the trace metals and coated the column sand. The column influent solution represented the incursion of ambient groundwater containing dissolved organic carbon (DOC) into an ISCO treatment zone. The influx of DOC-containing groundwater initiated a series of cation-exchange, surface-complexation and reductive-dissolution reactions that controlled the release of aqueous metals from the system. Peak concentrations in the effluent occurred in the order Na, Mo, Cr, Zn, K, Mn, Fe, Pb, Mg, Ni, Cu and Ca. Manganese release from the column was controlled by a combination of cation exchange, reductive dissolution and precipitation of rhodochrosite. The trend in Fe concentrations was similar to that of Mn, and also resulted from a combination of reductive dissolution and cation exchange. Cation exchange and/or surface-complexation were the primary mechanisms controlling Cu, Ni, Mo and Pb release to solution, while Zn and Cr concentrations did not display coherent trends. Although metal release from the treatment zone was evident in the data, concentrations of trace metals remained below 0.05 mg L(-1) with the exception of Mo which reached concentrations on the order of 1 mg L(-1). The establishment of anaerobic conditions in ISCO-treated aquifers may result in a prolonged flux of aqueous Mn(II), but with the exception of MoO(4)(2-), it is unlikely that trace metals sequestered with MnO(x) during ISCO will be released to the groundwater in elevated concentrations.     

DNAPL remediation with in situ chemical oxidation using potassium permanganate. II. Increasing removal efficiency by dissolving Mn oxide precipitates

In situ chemical oxidation (ISCO) schemes using MnO4- have been effective in destroying chlorinated organic solvents dissolved in ground water. Laboratory experiments and field pilot tests reveal that the precipitation of Mn oxide, one of the reaction products, causes a reduction of permeability, which can lead to flow bypassing and inefficiency of the scheme. Without a solution to this problem of plugging, it is difficult to remove DNAPL from the subsurface completely. In a companion paper, we showed with batch experiments that Mn oxide can be dissolved rapidly with certain organic acids. This study utilizes 2-D flow-tank experiments to examine the possibility of nearly complete DNAPL removal by ISCO with MnO4-, when organic acids are used to remove Mn oxide. The experiments were conducted in a small 2-D glass flow tank containing a lenticular silica-sand medium. Blue-dyed trichloroethylene (TCE) provided residual, the perched and pooled DNAPL. KMnO4 at 200 mg/l was flushed through the DNAPL horizontally. Once plugging reduced permeability and prevented further delivery of the oxidant, citric or oxalic acids were pumped into the flow tank to dissolve the Mn oxide precipitates. Organic ligands removed the Mn oxide precipitates relatively quickly, and permitted another cycle of MnO4- flooding. Cycles of MnO4-/acid flooding continued until all of the visible DNAPL was removed. The experiments were monitored with chemical analysis and visualization. A mass-balance calculation indicated that by the end of the experiments, all the DNAPL was removed. The results show also how heterogeneity adds complexity to initial redistribution of DNAPL, and to the efficiency of the chemical flooding.     

Quantification of potassium permanganate consumption and PCE oxidation in subsurface materials

A series of laboratory scale batch slurry experiments were conducted in order to establish a data set for oxidant demand by sandy and clayey subsurface materials as well as to identify the reaction kinetic rates of permanganate (MnO(4)(-)) consumption and PCE oxidation as a function of the MnO(4)(-) concentration. The laboratory experiments were carried out with 31 sandy and clayey subsurface sediments from 12 Danish sites. The results show that the consumption of MnO(4)(-) by reaction with the sediment, termed the natural oxidant demand (NOD), is the primary reaction with regards to quantification of MnO(4)(-) consumption. Dissolved PCE in concentrations up to 100 mg/l in the sediments investigated is not a significant factor in the total MnO(4)(-) consumption. Consumption of MnO(4)(-) increases with an increasing initial MnO(4)(-) concentration. The sediment type is also important as NOD is (generally) higher in clayey than in sandy sediments for a given MnO(4)(-) concentration. For the different sediment types the typical NOD values are 0.5-2 g MnO(4)(-)/kg dry weight (dw) for glacial meltwater sand, 1-8 g MnO(4)(-)/kg dw for sandy till and 5-20 g MnO(4)(-)/kg dw for clayey till. The long term consumption of MnO(4)(-) and oxidation of PCE can not be described with a single rate constant, as the total MnO(4)(-) reduction is comprised of several different reactions with individual rates. During the initial hours of reaction, first order kinetics can be applied, where the short term first order rate constants for consumption of MnO(4)(-) and oxidation of PCE are 0.05-0.5 h(-1) and 0.5-4.5 h(-1), respectively. The sediment does not act as an instantaneous sink for MnO(4)(-). The consumption of MnO(4)(-) by reaction with the reactive species in the sediment is the result of several parallel reactions, during which the reaction between the contaminant and MnO(4)(-) also takes place. Hence, application of low MnO(4)(-) concentrations can cause partly oxidation of PCE, as the oxidant demand of the sediment does not need to be met fully before PCE is oxidised.     

用于四氯乙烯降解的厌氧污泥的培养与驯化研究

由于四氯乙烯 (PCE)的大量使用和不合理的处置使其成为常见的污染物之一。PCE在好氧条件下不发生生物降解 ,只在厌氧条件下通过还原脱氯发生生物降解。本研究主要是对从不同处理厂获得的厌氧污泥进行培养 ,选出合适的厌氧污泥 ,进行降解PCE的厌氧污泥的驯化 ,为以后进行降解PCE的动力学研究和优势菌种的筛选做准备。同时 ,在实验中检测到了三氯乙烯 (TCE) ,表明PCE是通过还原脱氯发生生物降解的。