Hydrogeologic modeling for permeable reactive barriers.

The permeable reactive barrier technology for in situ treatment of chlorinated solvents and other groundwater contaminants is becoming increasingly popular. Field scale implementation of this and other in situ technologies requires careful design based on the site-specific hydrogeology and contaminant plume characteristics. Groundwater flow modeling is an important tool in understanding the hydraulic behavior of the site and optimizing the reactive barrier design. A combination of groundwater flow modeling and particle tracking techniques was used to illustrate the effect of hydraulic conductivity of the aquifer and reactive media on key permeable barrier design parameters, such as the capture zone width, residence time, flow velocity, and discharge. Similar techniques were used to illustrate the modeling approach for design of different configurations of reactive barriers in homogeneous and heterogeneous settings.     

Waste green sands as reactive media for groundwater contaminated with trichloroethylene (TCE)

Waste green sands are byproducts of the gray iron foundry industry that consist of sand, binding agents, organic carbon, and residual iron particles. Because of their potential sorptive and reactive properties, tests were conducted to determine the feasibility of using waste green sands as a low cost reactive medium for groundwater treatment. Batch and column tests were conducted to determine the reactivity, sorptive characteristics, and transport parameters for trichloroethylene (TCE) solutions in contact with green sands. Normalized rate constants for TCE degradation in the presence of iron particles extracted from green sands were found to be comparable to those for Peerless iron, a common medium used to treat groundwater. Rate constants and partition coefficients obtained from the batch tests were found to be comparable to those from the column tests. Analytical modeling shows that reactive barriers containing green sand potentially can be used to treat contaminated groundwater containing TCE at typical concentrations observed in the field.     

Performance evaluation of a permeable reactive barrier for remediation of dissolved chlorinated solvents in groundwater

A pilot-scale permeable reactive barrier (PRB) consisting of granular iron was installed in May 1995 at an industrial facility in New York to evaluate the use of this technology for remediation of chlorinated volatile organic compounds (VOCs) in groundwater. The performance of the barrier was monitored over a 2-year period. Groundwater velocity through the barrier was determined using water level measurements, tracer tests, and in situ velocity measurements. While uncertainty in the measured groundwater velocity hampered interpretation of results, the VOC concentration data from wells in the PRB indicated that VOC degradation rates were similar to those anticipated from laboratory results. Groundwater and core analyses indicated that formation of carbonate precipitates occurred in the upgradient section of the iron zone, however, these precipitates did not appear to adversely affect system performance. There was no indication of microbial fouling of the system over the monitoring period. Based on the observed performance of the pilot, a full-scale iron PRB was installed at the site in December 1997.     

Fe-based metallic glass: An efficient and energy-saving electrode material for electrocatalytic degradation of water contaminants

Excessive consumption of electrical energy has hampered the widespread application of electrochemical technology for degradation of various contaminants. In this paper, a Fe-based metallic glass (MG) was demonstrated as a new type of electrocatalyst to effectively and economically degrade an azo dye. In comparison to other typical electrodes, Fe-based MG electrodes exhibit a minimized degradation time, and the specific energy is 4–6 orders of magnitude lower than that of dimensionally stable anode (DSA), metal-like boron-doped diamond (BDD) and other electrodes. As sacrificial electrode materials, Fe-based MGs have less specific electrode mass consumption than iron electrodes. The use of Fe-based MGs will promote the practical application of electrochemical technology and the use of MGs as functional materials.     

Effect of contaminant hydrophobicity on hydrogen peroxide dosage requirements in the Fenton-like treatment of soils

A homologous series of n-alcohols was used as model contaminants to investigate the effect of hydrophobicity on the hydrogen peroxide concentration necessary in Fenton-like treatment for near-complete (>99%) destruction of compounds sorbed to soil. These probe compounds were selected because they exhibit equal reactivities with hydroxyl radicals, but have varied hydrophobicities. The standard Fenton reaction was first used to confirm equal hydroxyl radical reactivity for the n-alcohols. Central composite rotatable design experiments were then used to determine the conditions in an iron(III)-hydrogen peroxide system that resulted in 99% degradation of each of the probe compounds when sorbed to soil. The hydrogen peroxide concentrations required for 99% destruction of the sorbed compounds increased with probe compound hydrophobicity. Furthermore, hydrogen peroxide concentration requirements were directly proportional to the log octanol-water partition coefficients (logK(OW)) of each probe compound. This quantitative relationship may not be directly applicable to other organic contaminants, but a strong correlation between logK(OW) and hydrogen peroxide requirements for other contaminants will likely be found. These results confirm that hydrogen peroxide requirements for soil treatment increase as a function of contaminant hydrophobicity and provide a basis for the development of an algorithm for hydrogen peroxide requirements when modified Fenton's reagent is used for in situ chemical oxidation (ISCO).     

Surface chemical reactivity in selected zero-valent iron samples used in groundwater remediation

Permeable iron barriers have become a popular choice as a passive, cost-effective in situ remediation technology for chlorinated solvents. However, loss of reactivity over time, due to a build up of corrosion products or other precipitates on the iron surface, is a great concern. Because first-order rate constants for trichloroethylene (TCE) degradation have differed by iron pre-treatment and sonication history, X-ray photoelectron spectroscopy (XPS) was used to explore the changes in near surface chemistry of several iron samples. Both sonicated and unsonicated filings were analyzed in unwashed and groundwater-soaked conditions. Unsonicated acid-washed iron, with the highest first-order rate constant for TCE degradation, was characterized by greater surface oxygen content and was more ionic relative to the unwashed samples. The unsonicated, unwashed sample, with the lowest rate constant, exhibited a mixture of nonstoichiometric iron oxide and oxyhydroxide species. Sonication of groundwater-soaked iron removed weakly bonded iron hydroxide species and decreased the ionic character of the surface as was observed in the unwashed samples. Thus, this type of study might provide a better understanding of the chemical reactivity of selected iron samples and design better material in remediation technology.     

Design and construction techniques for permeable reactive barriers.

Adequate site characterization, bench-scale column testing, and hydrogeologic modeling formed the basis for the design and construction of permeable reactive barriers for groundwater remediation at various sites, such as Dover Air Force Base, DE and Naval Air Station, Moffett Field, CA. Dissolved chlorinated solvents, such as perchloroethylene (PCE) and trichloroethylene (TCE), have been the focus at many sites because the passive nature of the reactive barrier operation makes such barriers particularly useful for treating groundwater contaminants that can persist in the aquifer for several years. A combination of conventional and innovative site characterization, design, and construction techniques were used at these sites to increase the potential cost effectiveness of field application.     

Reaction efficiencies and rate constants for the goethite-catalyzed Fenton-like reaction of NAPL-form aromatic hydrocarbons and chloroethylenes

The contaminants present as nonaqueous phase liquids (NAPLs) in the subsurface are long-term sources for groundwater pollution. Fenton-like reaction catalyzed by natural iron oxides such as goethite in soils is one of the feasible in situ chemical reactions used to remediate contaminated sites. This research evaluated the Fenton-like reaction of five chlorinated ethylenes and three aromatic hydrocarbons using goethite as the catalyst. The reaction efficiencies and rate constants of these compounds in NAPL and dissolved forms were compared. The content of goethite used in batch experiments was in the range similar to those found in subsurfaces. Low H2O2 concentrations (0.05 and 0.1%) were tested in order to represent the low oxidant concentration in the outer region of treatment zone. The results showed that at the tested goethite and H2O2 ranges, the majority of contaminants were removed in the first 120 s. When aromatics and chloroethylenes were present as NAPLs, their removal efficiencies and reaction constants decreased. The removal efficiencies of 0.02 mmol NAPL contaminants were 26-70% less than those of the dissolved. The measured rate constants were in the order of 10(9) M(-1) s(-1) for dissolved chlorinated ethylenes and aromatic hydrocarbons, but were 25-60% less for their NAPL forms. The initial dosage of H2O2 and NAPL surface areas (18.4-38.2 mm2) did not significantly affect reaction efficiencies and rate constants of chlorinated NAPLs. Instead, they were related to the octanol-water partition coefficient of compounds. For both dissolved and NAPL forms, aromatic hydrocarbons were more reactive than chlorinated ethylenes in Fenton-like reaction. These results indicated that the decrease in reaction efficiencies and rate constants of NAPL-form contaminants would pose more negative impacts on the less reactive compounds such as benzene and cis 1,2-DCE during goethite-catalyzed Fenton-like reaction.     

Silicone emulsion-enhanced recovery of chlorinated solvents: batch and column studies

Batch and column experiments were conducted to investigate the feasibility of flushing with silicone oil emulsion for the removal of chlorinated solvents, including trichloroethylene (TCE), perchloroethylene (PCE) and 1,2-dichlorobenzene (DCB). In the batch experiments, solubilization potentials of emulsion and effects of surfactants as additives were examined. The emulsion prepared with 2% (v/v) silicone oil could solubilize 90.7% of 10,000 ppm TCE, 97.3% of 4000 ppm PCE and 99.7% of 7,800 ppm DCB. Results of one-dimensional column studies indicated that aqueous solubility and sorption of contaminants determined the flushing efficiency. The addition of surfactants below their critical micelle concentration (CMC) did not affect the removal of chlorinated solvents in batch and column experiments. The results of this study show that flushing with oil-based emulsion can be applied to treat the chlorinated solvents.     

Comments on "Decontamination of solutions containing EDTA using metallic iron" by Gyliene O., et al. [J. Hazard. Mater. (2008)

This letter presents an improved discussion of the data provided in a recent article on EDTA removal from aqueous solutions using elemental iron (Fe(0)) by O. Gyliene and his co-workers. It is shown that the authors have furnished a brilliant validation of the concept that dissolved contaminants are primary removed in Fe(0)/H(2)O systems by adsorption onto iron corrosion products and co-precipitation with iron corrosion products. It is reiterated that "contaminant removal" and "contaminant reduction" should not be interchanged randomly.     

高速镀锡电解液中油污的形成及其防治

高速镀锡电解槽液表面常会出现"油污"现象,严重影响了镀锡件的产品质量,增加了原料消耗和镀液排放量.采用高效液相色谱(HPLC)和电子轰击质谱(EI-MS)对高速镀锡电解液中出现的油污染物进行了组分分离和结构剖析,提出了镀液中的油污染物的形成机理.结合油污染物模拟试验和混浊度试验,研究了原料和添加剂中杂质含量、镀液浓度等影响因素对油污形成的影响.试验结果表明,油污染物的形成与原料和添加剂的成分有关,主要是由疏水性杂质在镀液中聚集而成;通过原料的萃取提纯、镀液循环方式的改善、控制镀液温度和添加剂EN-10浓度等方法,可以明显减少油污染物的产生量.研究结果对高速镀锡生产线进一步推行清洁生产工艺有着积极的指导意义.     

Comparison of reductive dechlorination of p-chlorophenol using Fe0 and nanosized Fe0

Chlorophenols, as a kind of important contaminants in groundwater, are toxic and difficult to biodegrade. Laboratory tests were conducted to examine zero-valent iron as an enhancing agent in the dechlorination of chlorinated organic compounds. Nanoscale iron particles were synthesized from common precursors KBH(4) and FeSO(4). Batch experiments were performed to investigate the reduction of p-chlorophenol (4-CP) by both common Fe(0) and nanoscale Fe(0). Comparison of 300 mesh/100 mesh/commercial reductive iron powders showed that size of iron particles played an important role in reduction process. Initial concentration and pretreatment of iron particles also influenced the chlorination rate. Nanoscale Fe(0) offered much more advantages for treatment of 4-CP compared with common iron particles, such as stability and durability. And they can be used to treat contaminants in groundwater over a long time. Among different parts of synthesized nanoscale iron particle solution, the very fine particles were the major agent for treatment of pollutants. As for preservation of nanoscale Fe(0), ethanol was recommended.     

Pilot-scale Fenton's oxidation of organic contaminants in groundwater using autochthonous iron

A pilot-scale study was conducted to evaluate Fenton's oxidation with autochthonous iron for treating extracted groundwater contaminated with organic solvents. Based on a previous bench-scale treatability study, a batch reactor pilot-plant system was designed and operated to evaluate the effects of various parameters including pH, iron concentration, hydrogen peroxide dose, and reaction time. Effective system conditions were found to be pH of 3.5, hydrogen peroxide to iron molar ratio of 75/1, and autochthonous iron at an average concentration of 10mg/l. The data collected demonstrate the effectiveness of Fenton's oxidation using autochthonous iron for treating this contaminated water, with reductions to below method detection limits for many contaminants. This pilot-scale study provided kinetic rate constants for predicting contaminant disappearance, information necessary for designing a full-scale Fenton's oxidation system.     

Reactivity of Fe(II)/cement systems in dechlorinating chlorinated ethylenes

Ferrous iron (Fe(II)) in combination with Portland cement is effective in reductively dechlorinating chlorinated organics and can be used to achieve immobilization and degradation of contaminants simultaneously. Reactivities of chlorinated ethylenes (perchloroethylene (PCE), trichloroethylene (TCE), 1,1-dichloroethylene (1,1-DCE), vinyl chloride (VC)) in Fe(II)/cement systems were characterized using batch slurry reactors. Reduction kinetics of the chlorinated ethylenes were sufficiently fast to be utilized for the proposed treatment scheme, and were described by a pseudo-first-order rate law. The order of reactivity of the chlorinated ethylenes was TCE>1,1-DCE>PCE>VC. Reduction of TCE and PCE mainly yielded acetylene, implying that the transformation of the two compounds occurred principally via reductive beta-elimination pathways. Transformation of 1,1-DCE and VC gave rise to primarily ethylene, implying that major degradation pathways were a reductive alpha-elimination for the former and a hydrogenolysis for the latter. The reactivity of the Fe(II)/cement systems in dechlorinating TCE was proportional to Fe(II) dose when the Fe(II)/cement mass ratio varied between 5.6 and 22.3%. The Fe(II)/cement systems with a higher Fe(II) loading were less extensively affected by pH in reductive reactions for TCE than in the previous experiments with PCE or chlorinated methanes. Amendment of Fe(II)/cement systems with Fe(III) addition was found effective in increasing the reactivity in the previous study, but the current findings indicated that the extent to which the reaction rate increased by the amendment might be dependent on the source of the cement and/or the compounds tested.     

Detection of an Aromatic Compound at the Roots of Cyperus Hermaphroditus by Photoacoustic Techniques

Some plants are used to remove organic and inorganic contaminants like chlorinated solvents, petrochemicals, pesticides, and explosives. These contaminants can be captured by the plant roots and then sequestered, degraded, immobilized, or transformed into other less toxic or non-toxic products. Cyperus hermaphroditus is a species endemic to the Santa Alejandrina swamp in Veracruz, Mexico, which is a site highly contaminated with industrial and petroleum wastes. The capability of Cyperus hermaphroditus to remove phenanthrene is reported in this study. Removal occurs by adsorption of this contaminant on the plant roots and can be assessed by analyzing the evolution of the absorption spectrum of the root system. Cyperus hermaphroditus plants of three months were cultivated in a hydroponic culture and exposed to 40, 80, and 120ppm phenanthrene for twelve days. Photoacoustic spectra of the root system indicate that higher amounts of phenanthrene are adsorbed with increasing phenanthrene concentrations, suggesting the use of Cyperus herma-phroditus for phenanthrene removal.     

Polychlorinated naphthalenes and other chlorinated tricyclic aromatic hydrocarbons emitted from combustion of polyvinylchloride

Chloronaphthalenes (CNs) and phenanthrenes or/and anthracenes (CP/CAs) were detected in the emissions of polyvinylchloride (PVC) combustion at 900 degrees C. The presence of metallic iron, copper, or aluminum increased the formation of highly chlorinated CNs (tri- to octachloro-homologues) in the PVC combustion process. Total levels of CNs and CP/CAs were 40-48 and 76-116 mg/kg PVC, respectively, in the emissions from combustion of PVC with metals. Monochloronaphthalenes, dichloronaphtahlenes, monochlorophenanthrenes, and monochloroanthracenes were the predominant homologues. The other CN homologues were minor combustion byproducts. Detection of CNs in the PVC combustion emissions suggests that CN formation from solid waste incineration is a source of CNs in the environment.     

Degradation of chemical warfare agent simulants using gas-liquid pulsed streamer discharges

This study determines the effectiveness of pulsed streamer discharges (PSD), a type of advanced oxidation technology (AOT) to clean water contaminated with chemical agents. For the purpose of this study, experiments were conducted with G and H agent simulants to determine the degradation kinetics and to determine the effects of various electrical and chemical parameters in the degradation of these contaminants. The energy efficiency of contaminant degradation shows that pulsed streamer discharges can be an efficient technology in treating water contaminated with chemical agents. The maximum energy yields of degradation of H and G agent simulants by the pulsed corona discharges are 0.029 and 0.008 molecules/100 eV, respectively, in the series configuration with ferrous sulfate salt in solution.     

Development of High-T c SQUID and Application to Ultra-Sensitive Contaminant Detection System

We have developed magnetic metallic contaminant detectors using multiple high-T _c SQUIDs (superconducting quantum interference devices) for food and industrial products. For manufacturers producing items such as processed food and industrial products, problems with metallic contaminants are critical issues. When contamination occurs, the manufacturer of the product suffers a great loss from recalling the tainted product. The lower detection limit for practical X-ray usage is on the order of 1 mm. A detection system using a SQUID is a powerful tool for sensitive inspections. For practical use, the detection width of the system should be increased by employing multiple sensors. We designed and set up a multi-channel high-T _c SQUID inspection system with a width of at least 130 mm for food and 70 mm for industrial products. As a result, a small ?0.3-mm stainless steel particle for food and a ?27-μm iron particle for industrial products were successfully detected.     

Degradation of carbon tetrachloride by modified Fenton's reagent

The degradation of tetrachloromethane (carbon tetrachloride-CT) by modified Fenton's reagent (catalyzed hydrogen peroxide) was investigated using a range of hydrogen peroxide concentrations and 1 mM iron(III) catalyst. The documented reactive species in modified Fenton's reactions, hydroxyl radical (OH*), is not reactive with CT, yet CT degradation was observed in the Fenton's reactions and was confirmed by chloride generation. Because CT is not reactive with OH*, a reductive mechanism which may involve superoxide radical anion is proposed for CT degradation in modified Fenton's systems. Scavenging of reductants by excess chloroform prevented CT degradation, confirming a reductive mechanism. Similar to CT, three other oxidized aliphatic compounds, hexachloroethane, bromotrichloromethane, and tetranitromethane, were also degraded by modified Fenton's reagent. The results show that modified Fenton's reactions act through a reductive mechanism to degrade compounds that are not reactive with OH*, which broadens the scope of this process for hazardous waste treatment and remediation.