Degradation of TCE,Cr(VI), sulfate,and nitrate mixtures by granular iron in flow-through columns under different microbial conditions

Flow-through aquifer columns packed with a middle layer of granular iron (Fe0) were used to study the applicability and limitations of bio-enhanced Fe0 barriers for the treatment of contaminant mixtures in groundwater. Concentration profiles along the columns showed extensive degradation of hexavalent chromium Cr(VI), nitrate, sulfate, and trichloroethene (TCE), mainly in the Fe0 layer. One column was bioaugmented with Shevanella algae BRY, an iron-reducing bacterium that could enhance Fe0 reactivity by reductive dissolution of passivating iron oxides. This strain did not enhance Cr(VI), which was rapidly reduced by iron, leaving little room for improvement by microbial participation. Nevertheless, BRY-enhanced nitrate removal (from 15% to 80%), partly because this strain has a wide range of electron acceptors, including nitrate. Sulfate was removed (55%) only in a column that was bioaugmented with a mixed culture containing sulfate-reducing bacteria. Apparently, these bacteria used H2 (produced by Fe0 corrosion) as electron donor to respire sulfate. Most of the TCE was degraded in the zone containing Fe0 (50-70%), and bioaugmentation with BRY slightly increased the removal efficiency to about 80%. Microbial colonization of the Fe0 surface was confirmed by scanning electron microscopy.     

Effects of dissolved oxygen on formation of corrosion products and concomitant oxygen and nitrate reduction in zero-valent iron systems with or without aqueous Fe2+

Batch tests were conducted in zero-valent iron (ZVI or Fe0) systems to investigate oxygen consumption and the effect of dissolved oxygen (DO) on formation of iron corrosion products, nitrate reduction, the reactivity of Fe0, the role Fe2+ (aq) played, and the fate of Fe2+. The study indicates that without augmenting Fe2+ (aq), neither nitrate nor DO could be removed efficiently by Fe0. In the presence of Fe2+ (aq), nitrate and DO could be reduced concomitantly with limited interference with each other. Unlike nitrate reduction, DO removal by Fe0 did not consume Fe2+ (aq). A two-layer structure, with an inner layer of magnetite and an outer layer of lepidocrocite, may be formed in the presence of DO. When DO depleted, the outer lepidocrocite layer was transformed to magnetite. The inner layer of magnetite, even in a substantial thickness, might not impede the Fe0 reactivity as much as the thin interfacial layer between the oxide coating and liquid. Surface-bound Fe2+ may greatly enhance the electron transfer from the Fe0 core to the solid-liquid interface, and thus improve the performance of the Fe0 process.     

Hardness and carbonate effects on the reactivity of zero-valent iron for Cr(VI) removal

Zero-valent iron (Fe0) was used to remove hexavalent chromium, Cr(VI), in groundwater via a coupled reduction-oxidation reaction. Nine columns were set up under various groundwater geochemistry to investigate the effects of hardness and carbonate on Cr(VI) removal. The Cr(VI) removal capacity of Fe0 was found to be about 4 mgCr/g Fe0 in the control column (i.e., column 1). A slight decrease in the Cr(VI) removal capacity was found in the presence of calcium hardness. However, there was a 17% drop in the Cr(VI) removal capacity when magnesium hardness was present at low to moderately hard level. Results also revealed that carbonate changed the morphology of the Fe0 by formation of pale green precipitates on the iron filings. Furthermore, there was a 33% decrease in the Cr(VI) removal capacity of Fe0 when both carbonate and hardness ions were present. In general, the presence of hardness ions and carbonate in groundwater have great impact on the Fe0 by formation of passivated precipitates, such as CaCO3, on the Fe0 surface resulting in a diminished lifespan of the Fe0 by blocking electron transfer.     

BAF工艺处理微污染含铁锰地下水试验研究

采用BAF工艺处理微污染含铁锰地下水,研究了其净化效能和适宜的运行条件.结果表明,在水力负荷为4-5 m3/(m2·h),气水比为3:1-4:1的试验条件下,BAF工艺能有效去除氨氮、锰、铁、CODMn.和浊度.当原水铁含量小于2 mg/L,滤层中部DO在4 mg/L左右时,对氨氮的去除效果最佳.微量Fe2+即可维系滤...     

Long-term ammonia removal in a coconut fiber-packed biofilter: Analysis of N fractionation and reactor performance under steady-state and transient conditions

A comprehensive study of long-term ammonia removal in a biofilter packed with coconut fiber is presented under both steady-state and transient conditions. Low and high ammonia loads were applied to the reactor by varying the inlet ammonia concentration from 90 to 260 ppm_v and gas contact times ranging from 20 to 36 s. Gas samples and leachate measurements were periodically analyzed and used for characterizing biofilter performance in terms of removal efficiency (RE) and elimination capacity (EC). Also, N fractions in the leachate were quantified to both identify the experimental rates of nitritation and nitratation and to determine the N leachate distribution. Results showed stratification in the biofilter activity and, thus, most of the NH₃ removal was performed in the lower part of the reactor. An average EC of 0.5 kg N-NH₃ m⁻³ d⁻¹ was obtained for the whole reactor with a maximum local average EC of 1.7 kg N-NH₃ m⁻³ d⁻¹. Leachate analyses showed that a ratio of 1:1 of ammonium and nitrate ions in the leachate was obtained throughout steady-state operation at low ammonia loads with similar values for nitritation and nitratation rates. Low nitratation rates during high ammonia load periods occurred because large amounts of ammonium and nitrite accumulated in the packed bed, thus causing inhibition episodes on nitrite-oxidizing bacteria due to free ammonia accumulation. Mass balances showed that 50% of the ammonia fed to the reactor was oxidized to either nitrite or nitrate and the rest was recovered as ammonium indicating that sorption processes play a fundamental role in the treatment of ammonia by biofiltration.     

Decreasing ammonium generation using hydrogenotrophic bacteria in the process of nitrate reduction by nanoscale zero-valent iron

An integrated nitrate treatment using nanoscale zero-valent iron (NZVI) and Alcaligenes eutrophus, which is a kind of hydrogenotrophic denitrifying bacteria, was conducted to remove nitrate and decrease ammonium generation. Within 8 days, nitrate was removed completely in the reactors containing NZVI particles plus bacteria while the proportion of ammonium generated was only 33%. That is a lower reduction rate but a smaller proportion of ammonium relative to that in abiotic reactors. It was also found that ammonium generation experienced a biphasic process, involving an increasing period and a stable period. After domestication of the bacteria, the combined NZVI-cell system could remove all nitrate without ammonium released when the refreshed nitrate was introduced. Nitrate reduction and the final product distribution were also studied in batch reactors amended with different initial NZVI contents and biomass concentrations, respectively. Both the nitrate removal rate and the ammonium yield decreased when the initial content of NZVI reduced and the initial biomass concentration increased. However, about 27% of the nitrate was converted to ammonium when excess bacteria (OD₄₂₂ = 0.026) were used, which was higher than that with appropriate amount of bacteria.     

Fluidized bed reactor for the biological treatment of ion-exchange brine containing perchlorate and nitrate

The removal of perchlorate and nitrate from contaminated drinking water using regenerable ion-exchange processes produces a high salt brine (3-10% NaCl) laden with high concentrations of perchlorate and nitrate. This bench-scale research describes the operation of acetate-fed granular activated carbon (GAC) based fluidized bed reactors (FBR) for perchlorate-only, and combined nitrate and perchlorate removal from synthetic brine (6% NaCl). The GAC was inoculated with a salt-tolerant culture developed by the authors and used previously in batch systems. An FBR was an effective design for perchlorate reduction and exhibited first-order degradation kinetics with respect to perchlorate concentrations. Nitrate was also removed by the organisms in the column and had no negative effects on the removal of perchlorate using the FBR design. However, at higher concentrations of nitrate the FBR was more difficult to operate due to loss of carbon and biomass from the formation of nitrogen bubbles and the high recycle flow rates needed.     

An electrokinetic/Fe0 permeable reactive barrier system for the treatment of nitrate-contaminated subsurface soils

Effective nitrate removal by Fe⁰ permeable reactive barriers (Fe⁰ PRB) has been recognized as a challenging task because the iron corrosion product foamed on Fe⁰ hinders effective electron transfer from Fe⁰ to surface-bound nitrate. The objectives of this study were (i) to demonstrate the effectiveness of an electrokinetic/Fe⁰ PRB system for remediating nitrate-contaminated low permeability soils using a bench-scale system and (ii) to deepen the understanding of the behavior and fate of nitrate in the system. Bench-scale laboratory experiments were designed to investigate the influence of the Fe⁰ content in the permeable reactive barrier, the pH in the anode well, and the applied voltage on remediation efficiency. The experimental results showed that the major reaction product of nitrate reduction by Fe⁰ was ammonium and that nitrate reduction efficiency was significantly influenced by the variables investigated in this study. Nitrate reduction efficiency was enhanced by either increasing the Fe⁰ content in the Fe⁰ reactive barrier or decreasing the initial anode pH. However, nitrate reduction efficiency was reduced by increasing the applied voltage from 10 V to 40 V due to the insufficient reaction time during nitrate migration through the Fe⁰ PRB. For all experimental conditions, nearly all nitrate nitrogen was recovered in either anode or cathode wells as nitrate or ammonium within 100 h, demonstrating the effectiveness of the system for remediating nitrate-contaminated subsurface soils.     

Zero-valent iron reduction of nitrate in the presence of ultraviolet light,organic matter and hydrogen peroxide

This paper describes the use of metallic iron (Fe(0)) powder for nitrate removal in a well-mixed batch reactor. Important variables explored include Fe(0) dosage (1-3g/L), UV light intensity (64-128 W), and the presence of propanol (20 mg/L as DOC) and H(2)O(2) (100-200 mg/L). Accumulation of ferrous ions released from the Fe(0) surface can be expressed by an S-curve, which involves lag growth phase, exponential phase, rate-declining phase, and saturation phase. The removal of nitrate increases with increasing Fe(0) dosage; however, the removal makes no difference as the Fe(0) dosage is greater than 2 g/L. UV irradiation retards the dissolution of ferrous ion and the removal of nitrate. The species of propanol, which has a functional group of -OH, plays a role of organic inhibitor for Fe(0) corrosion. The presence of H(2)O(2) appears to inactivate all reactions as the Fe(0) of 10 microm was used; the final H(2)O(2) remains intact throughout the entire reaction period, and there were no removal of nitrate and no dissolution of ferrous ion. Surprisingly, with the use of a larger Fe(0) particle size of 150 microm, the H(2)O(2) was seen to decompose rapidly through Fenton reaction. Nevertheless, the rate of ferrous accumulation or nitrate removal is slow.     

Electrochemical denitrificaton of simulated ground water

Electrochemical denitrification of ground water was studied with an objective to maximize nitrate transformation to nitrogen gas. Aluminum, graphite, iron and titanium were selected as electrode materials. While aluminum, iron and titanium electrodes showed 70-97% nitrate reduction, with graphite electrode the removal was only 8%. Nitrate was transformed to ammonia with iron and aluminum electrodes but with titanium electrodes nitrogen was apparently the major end product. Iron electrodes exhibited the maximum reducing condition (ORP=-463mV) and titanium showed the minimum (ORP=-206mV). Nitrate reduction with titanium electrodes was retarded in the presence of chloride ions possibly due to formation of hypochlorite ions. The first-order nitrate transformation rate constant with respect to time decreased with decrease in current density. However, when the rate constant was expressed with respect to charge passed (Coulomb/l) it was nearly same under different experimental conditions (current density and pH). The study indicates that the process might be suitable for denitrification of drinking water.