Zero-Valent Iron-Assisted Autotrophic Denitrification

Porous reactive?barriers containing metallic iron and hydrogenotrophic denitrifying microorganisms may potentially be suitable for in-situ remediation of nitrate-contaminated groundwater resources. The main objective of the research described here was to determine the type and concentration of metallic iron to be used in such reactive?barriers so that ammonia formation through metallic iron-assisted abiotic nitrate reduction was minimized, while a reasonable rate of biological denitrification, sustained by hydrogen produced through metallic iron corrosion, was maintained. Initial experiments included the demonstration of autotrophic denitrification supported by externally supplied hydrogen, either from a gas cylinder or generated through anaerobic corrosion of metallic iron. Next, the effect of iron type on abiotic nitrate reduction was studied, and among those types of iron tested, steel wool, with its relatively low surface-area-to-weight ratio, was identified as the material that exhibited the least propensity to abiotically reduce nitrate. Further, long-term experiments were carried out in batch reactors to determine the effect of steel?wool surface area on the extent of denitrification and ammonia production. Finally, experiments carried out in up-flow column reactors containing sand and varying quantities of steel wool demonstrated biological denitrification occurring in such systems. Based on the results of the final set of experiments, it appeared that to minimize ammonia production, the steel-wool concentration up-flow columns must be even below the lowest value—0.5 g steel wool added to 125?cm3 of sand—used during this study. To counter any detrimental effect of lowered steel wool concentration on the extent of hydrogenotrophic denitrification, increase of the retention time in the columns to values higher than 13 days (the maximum value investigated in this study) may be necessary.     

Factors Influencing Arsenite Removal by Zero-Valent Iron

The effects of pH, alkalinity, and mass transfer efficiency on the removal of arsenite [As(III)] by zero-valent iron (ZVI) were evaluated in this study. The optimum pH range for removal of As(III) was found to be between 7 and 8. As(III) removal varied with salinity, pH, alkalinity conditions, and As(III) concentration. Degradation of As(III) removal performance was observed only under conditions of high alkalinity and arsenic concentrations [alkalinity >10?g CaCO3/L and 2.9?mg/L As(III)]. A strong correlation between As(III) removal and increasing Reynolds number in batch testing suggests that mass transfer efficiency plays an important role in the removal of As(III) by ZVI. A diffusion-limited adsorption model was used to describe the removal of As(III) as the result of adsorption to precipitated iron oxides generated from ZVI corrosion. After an initial period of As(III) rapid adsorption to surface rusts formed during manufacturing and exposure to air, As(III) removal rate is most likely controlled by the rate of iron corrosion and the diffusion of As(III) to adsorption sites in ZVI/iron oxides.     

Simultaneous Removal of Cd and Cr(VI) Using Fe-Loaded Zeolite

Current research focuses on the simultaneous removal of Cd and Cr(VI) in water by a newly developed material having both abilities of sorption and electrochemical reduction. The material was derived from the zeolite modified by Fe(II) chloride followed by sodium borohydride reduction. The Fe-loaded zeolite simultaneously removed Cd and Cr(VI) to below the detection limit at a fairly rapid rate within 1?h for Cd and within 20?h for Cr(VI), under the pH ranging from slightly acid to around neutral. At high concentration of coexisting Cr(VI), the removal efficiency of Fe-loaded zeolite for Cd slightly decreased due to surface fouling by Cr(III) hydroxide precipitations. On the contrary, the coexisting Cd was found to increase the removal rate of Cr(VI) by Fe-loaded zeolite. From the test results, the Fe-loaded zeolite was found to be a possible alternative in simultaneous removal of Cd and Cr(VI) in the aqueous phase.     

In Situ Source Treatment of Cr(VI) Using a Fe(II)-Based Reductant Blend: Long-Term Monitoring and Evaluation

The long-term effectiveness of an FeSO4+Na2S2O4 reductant solution blend for in situ saturated zone treatment of dissolved and solid phase Cr(VI) in a high pH chromite ore processing solid waste fill material was investigated. Two field pilot injection studies were conducted that showed sustained treatment of Cr(VI) over evaluation periods of more than 1,000 days. No well or aquifer formation clogging was observed during injection although treatment was limited to the pore volume displacement radius of the injected reductant. Analysis of posttreatment core samples suggested >85% treatment effectiveness of solid phase Cr(VI), whereas lab tests suggested treatment of the solid phase Cr(VI) can be complete provided the chromite ore processing solid waste sediments are sufficiently dosed with the reductant. The sustained treatment of dissolved phase Cr(VI) migrating through the treatment zones for more than 1,000 days following injection provided strong evidence of a residual treatment capacity having been imparted to the formation solids. Scanning electron microscopy–energy dispersive x-ray spectroscopy analyses of posttreatment core samples indicated that much of the Cr(VI) may be removed through the formation of a Cr-bearing precipitate, possibly a complex carbonate, characterized by an Fe:Cr molar ratio of roughly 1:1.     

Effects of Surface-Bound Fe2+ on Nitrate Reduction and Transformation of Iron Oxide(s) in Zero-Valent Iron Systems at Near-Neutral pH

Nitrate reduction in an iron/nitrate/water system with or without an organic buffer was investigated using multiple batch reactors under strict anoxic conditions. Nitrate reduction was very limited (<10%) at near-neutral pH in the absence of the organic buffer. However, nitrate reduction was greatly enhanced if the system: (1) had a low initial pH ( ～ 2–3); (2) was primed with adequate aqueous Fe2+; or (3) was in the presence of the organic buffer. In Cases (1) and (3), nitrate reduction usually was involved in three stages. The first stage was quick, and H+ ions directly participated in the corrosion of iron grains. The second stage was very slow due to the formation of amorphous oxides on the surface of iron grains, while the third stage was characterized by a rapid nitrate reduction concurrent with the disappearance of aqueous Fe2+. Results indicate that reduction of nitrate by Fe0 will form magnetite; Fe2+ (aq.) can accelerate reduction of nitrate and will be substoichiometrically consumed. Once nitrate is exhausted in the system, no more Fe2+ will be consumed. In the presence of nitrate, Fe2+ (aq) will be adsorbed onto the surface of iron grains or iron oxides; the surface-complexed Fe(II) (extracted by acetate with pH = 4.1) might be oxidized and become structural Fe(III), resulting in a steadily increasing ratio of Fe(III)/Fe(II) in the oxides formed. The transformation of nonstoichiometric amorphous iron oxides into crystalline magnetite, a nonpassive oxide, triggers the rapid nitrate removal thereafter.     

Modeling the Removal of Arsenic by Iron Oxide Coated Sand

An arsenic filtration experiment using iron oxide coated sand was modeled using the USGS geochemical program PHREEQC. Despite some uncertainty regarding the initial conditions of the groundwater and the simplicity of the model, it replicated the experimental results within 10%. The original experiment filtered 165 bed volumes to concentrations less than 0.01 mg/L As and approximately 210 bed volumes to 0.05 mg/L As. The model filtered 168 bed volumes to 0.01 mg/L As and 228 bed volumes to 0.05 mg/L.     

Cr(VI) Removal from Water with Amorphous Graphite Concentrate Contaminated by Iron

This work attempted to explore the feasibility of using iron-contaminated graphite concentrate as an effective adsorbent for Cr(VI) removal from polluted water. Adsorption isotherm and kinetics were conducted to investigate the Cr(VI) removal capacity by the iron-contaminated amorphous graphite concentrates. In addition, SEM-EDS, XPS were carried out to further examine the solid samples. The results showed that amorphous graphite concentrate had a 1.52 mg/g adsorption capacity of Cr(VI), with the adsorption being fitted well with the pseudo-second-order kinetics model. In addition, chemical adsorption of Cr(V) on iron-contaminated graphite concentrate due to the formation of ≡Fe-O₄HCr and Fe₂-(CrO₄)₃ complexes was proposed. This study revealed that iron-contaminated amorphous graphite concentrate would be a cheap and good adsorbent for the removal of Cr(VI) from contaminated water.     

Cation Effects on Chromium Removal in Permeable Reactive Walls

Permeable reactive walls have proven to be successful in laboratory and pilot-scale field applications. However, the long-term efficacy of reactive permeable walls has not been established due to the novelty of the technology. Also, the impact of common groundwater ions such as calcium and magnesium (i.e., hardness) on permeable reactive walls is unknown. In theory, the ions may react competitively with chromium in solution and/or other materials on the surface of the zero-valent iron. The ions may also form precipitates that could clog the reactive zone over time, resulting in decreased contaminant removal and a shorter wall lifetime. The purpose of this research was to determine the effects of common groundwater ions on permeable reactive walls. A range of calcium and magnesium concentrations was tested in laboratory columns to determine the effect of these ions on removal of a constant chromium concentration (100 mg/L). Results from the laboratory tests indicated that calcium and magnesium had a significant impact on chromium removal. The most dramatic effects were witnessed at hardness levels up to 140 mg/L as CaCO3 where zero-valent iron capacity was reduced by 45%.     

Development of a New Zero-Valent Iron Zeolite Material to Reduce Nitrate without Ammonium Release

Previous studies have revealed that the application of zero-valent iron (ZVI) in reducing nitrate is limited by ammonium production and the requirement for adequate pH control. The current study focused on developing a new material potentially applicable in permeable reactive barriers, which can reduce nitrate without ammonium release under unbuffered pH. The new material, referred to as ZanF, is derived from zeolite modified by Fe(II), followed by borohydride reduction. The pseudo-first-order rate constant (kobs) of ZanF in the early period of nitrate reduction was 10 times higher than that of the ZVI used in this study. However, the kobs of ZanF decreased in the reaction period that followed. Even though both ZVI and ZanF produced ammonium as a product of nitrate reduction, ZanF removed it to below detection limits via adsorption, whereas ZVI did not remove it to any significant extent. ZanF maintained its high reactivity even under an initial pH of 6.2 without buffer. The higher ZanF/solution ratio increased the removal rate of ZanF as well as the removal efficiency.     

Arsenic Removal by a Colloidal Iron Oxide Coated Sand

A novel treatment process for arsenic removal from contaminated groundwater has been developed for use as a reactive barrier or a small drinking water treatment unit. In this study, modified porous media was made by the deposition of colloidal iron oxide onto sand grains at intermediate pH and ionic strength. Kd values from column experiments were 0.016–0.37?L/kg for As(III) and 0.023–0.85?L/kg for As(V), being lower than those of batch experiments (0.50 and 1.30?L/kg for As(III) and As(V), respectively) due to lower availability of surface adsorption sites in the packed column. Media-independent Kd values reflect the enhancement of arsenic adsorption with an increase of colloidal iron oxide coated sand fraction, apparently due to adsorption equilibration during arsenic transport under the same flow column conditions. The heterogeneous composition of two groundwater samples also reduced arsenic adsorption. Therefore, arsenic elution near the initial breakthrough was regulated by available adsorption surface in a porous coated sand media as well as the effects of competing oxyanions. The exhaustion of adsorption capacity near the critical contamination level is sensitive to geochemical and remedial properties of the contaminants.     

Removal of Nickel(II) from Dilute Aqueous Solution by Sludge-Ash

This work examines the adsorption of Ni(II) onto sludge–ash, a waste produced from a fluidized bed incinerator combusted primarily with biosolids. Results of kinetic experiments showed that the adsorption was rapid. The kinetic adsorption data can be well described by an empirical modified Freundlich equation. The rate of adsorption decreased with either increasing surface loading and ionic strength or decreasing solution pH. The results of equilibrium studies showed that the solution pH was the key factor affecting the adsorption. The modified Langmuir model fit revealed that the hydrogen ion acts as a competitive inhibitor for the adsorption of Ni onto ash. The maximum adsorption capacity for Ni is 5.41 μmol/g. Experimental results indicate that the adsorption is favorable at higher temperature. Thermodynamic adsorption parameters for ΔG°, ΔH°, and ΔS° are ?7.41 kcal/mol, 7.25 kcal/mol, and 48.9 cal/mol?K, respectively.     

Effect of Metallic Iron Concentration on End-Product Distribution during Metallic Iron-Assisted Autotrophic Denitrification

The main objective of this study was to determine the optimum composition of a reactive porous medium containing sand and metallic iron, to be used for Fe(0)-assisted hydrogenotrophic denitrification. This determination is important to ensure that the end-product distribution after such treatment is acceptable, i.e., ammonia formation due to abiotic nitrate reduction by metallic iron in such media is minimized, while a reasonable rate of biological denitrification is maintained. Based on a previous study it was established that steel wool, with its relatively low specific surface area, exhibited the least propensity to abiotically reduce nitrate. It was also established that to achieve acceptable end-product distribution, the steel wool concentration in the reactive porous media has to be lowered even below the lowest value, i.e., 4.0?g steel wool/m3 of sand, used during that study. It was further hypothesized that to counter any detrimental effect of lower steel wool concentration on biological nitrate removal rate, increase of the retention time in porous media to values higher than 13 days, the maximum value investigated in that study, may be necessary. In the present study, experiments were conducted in batch reactors containing denitrifying microorganisms and various concentrations of steel wool and in semibatch reactors containing sand seeded with denitrifying microorganisms and various concentrations of steel wool. Based on the results of the semibatch experiments, it appears that to achieve acceptable end-product distribution, the steel wool concentration in the reactive porous media has to be maintained around 2.0?g steel wool/m3 sand and the corresponding retention time in the reactive media must be around 26 days.     

纳米零价铁对溶液中铅镉铬砷的去除性能研究

通过液相还原法,采用KBH4还原Fe2+成功制备了平均粒径40~80nm、比表面积19.713 4m2/g、有较好表面活性的纳米零价铁(NZVI),NZVI在含铅、砷、铬、镉初始浓度为100mg/L的pH分别为2、7、12的溶液中进行去除试验。结果表明,在pH=2与pH=7的条件下NZVI对铅、砷、铬的去除效果较好,去除速率在前30min较快;在pH=12条件下对镉的去除效果明显,去除速率在前40min较快;不同pH条件下各离子的去除率差异较大,这主要与各离子在不同pH条件下的存在形态有关。NVZI去除溶液中的铅、砷、铬、镉,不仅效率高,而且绿色环保,不会对环境造成二次污染。     

Sorption of Cr (VI) onto Olive Stone in a Packed Bed Column: Prediction of Kinetic Parameters and Breakthrough Curves

This paper investigates the ability of olive stone to remove chromium (VI) ions from aqueous solution in a packed bed up-flow column with an internal diameter of 1.5 cm. The experiments were performed with a bed height of 15 g (13.4 cm) and a flow rate of 2 mL/min. To predict the breakthrough curves and to determine the characteristic parameters of the column useful for process design, four kinetic models; Adams-Bohart, Thomas, Yoon-Nelson, and Dose-Response models were applied to the experimental data. All models were found suitable for describing the whole or a definite part of the dynamic behavior of the column. The simulation of the whole breakthrough curve was effective with the Dose-Response model, but the initial part of the breakthrough was best predicted by the Adams-Bohart model. On the other hand, the results indicated that, at pH values of this work, approximately 50% of Cr (VI) is biosorbed by olive stone and the other 50% is reduced to Cr (III), both processes being of equal importance. Therefore, a two-stage biosorption process was developed. The goal of these final experiments was to confirm that Cr (III) [the Cr (VI) reduction product] was also effectively sorbed by olive stone in a second column.     

Degradation of Lindane by Zero-Valent Iron Nanoparticles

Thus far, zero-valent iron has been studied mostly for the degradation of structurally simple one- and two-carbon halogenated organic contaminants such as chlorinated methanes, ethanes, and ethenes. In this research, laboratory synthesized particles of nanoscale iron were explored to degrade lindane, also known as γ-hexachlorocyclohexane, a formerly widely utilized pesticide and well-documented persistent organic pollutant. In general, lindane disappeared from aqueous solution within 24?h in the presence of nanoiron concentrations ranging from 0.015?to?0.39?g/L. By comparison, approximately 40% of the initial lindane dose remained in solution after 24?h in the presence of 0.53?g/L of larger microscale iron particles. However, the surface area normalized first-order rate constants were all within the same order of magnitude regardless of dose or iron type. A key reaction intermediate, γ-3,4,5,6-tetrachlorocyclohexene from dihaloelimination of lindane was identified and quantified. Trace levels of additional degradation products including benzene and biphenyl were detected but only in the high concentration experiments conducted in 50% ethanol. While up to 80% of the chlorine from the lindane molecules ended as chloride in water, only 38% of the expected chloride concentration was observed for the microscale iron experiment. This work together with previous published studies on the degradation of polychlorinated biphenyl, chlorinated benzenes, and phenols suggest that zero-valent iron nanoparticles can be effective in the treatment of more structurally complex and environmentally persistent organic pollutants such as lindane.     

Removal of Immiscible Contaminants from Sandy Soils Monitored by Means of Dielectric Measurements

The main objective of this paper is to explore the potential application of electromagnetic waves to evaluate the effect of contaminant removal in granular soils. Thus, various specimens of saturated silica sand were prepared using paraffin oil and lubricant oil as contaminants. Four flushing fluids were used to remove the contaminants from sand columns: Deionized water, water-detergent, water-detergent-alcohol solution, and water vapor. Dielectric permittivity was measured at different stages of the removal process at the frequency from 20?MHz?to?1.3?GHz. The measured permittivity was compared with that determined for clean and fully contaminated specimens. A theoretical mixture formula was calibrated and implemented to estimate the volume fraction of contaminant present in the pore fluid. It is concluded in this work that dielectric parameters reflect the contamination level of the soil for the nonpolar organic compounds used here. Measurement of permittivity allows us to determine that the inclusion of alcohol and detergent in the displacing fluid improved the removal efficiency. However, water vapor was the most efficient removal agent.     

Factorial Design Approach to Investigate the Effects of Groundwater Cooccurring Solutes on Arsenic Removal by Electrocoagulation

Effects of groundwater cooccurring solutes such as phosphate (PO4), silicate (SiO3), bicarbonate (HCO3), calcium (Ca), and iron (Fe) on arsenic removal were investigated in this study. Investigation by two-level full-factorial designed experiments revealed that PO4 and SiO3 have negative effect on arsenic removal, whereas HCO3 has negligible positive effect. The effects of Ca and Fe present in groundwater have positive effect on arsenic removal by electrocoagulation (EC). Hypothesis testing at 5% significance level suggests that alkalinity (HCO3) is not an important parameter in arsenic removal by EC in naturally occurring pH range of water.     

Removal of Heavy Metals from Automotive Wastewater by Sulfide Precipitation

The United States Environmental Protection Agency has proposed new categorical pretreatment effluent standards for the Metal Products and Machinery Industry, which are more stringent than current discharge limits in the automotive industry. Therefore, this study was conducted to evaluate metal-sulfide precipitation chemistry as an alternative to metal-hydroxide precipitation chemistry for removing Cu, Ni, Pb, and Zn. There were three aspects of this study: (1) theoretical analysis of both metal–hydroxide and metal–sulfide chemistry; (2) experimental evaluation of commercially available sulfur-containing precipitants using deionized water; and (3) experimental evaluation of the precipitants using wastewater samples from three automotive manufacturing plants (transmission, engine, and assembly plants). The primary findings are: (1) In theory, metal–hydroxide chemistry can achieve the proposed standards when no chelating agents are present. This is not true when as small as 1 mg/L of ethylenediaminetetra-acetic acid (EDTA) is present. (2) Metal–sulfide precipitation chemistry could achieve solubility limits lower than those of metal–hydroxide chemistry over a wide range of pH. However, EDTA could still inhibit precipitation of Ni, Pb, and Zn to concentrations above the proposed standards. (3) The experiments with wastewater samples showed all precipitants removed Cu well while Ni and Zn were not well removed. The sample from transmission and engine plants were more difficult to treat than from an assembly plant, suggesting that it might have had more chelating agents. The commercially available precipitants did not perform any better than sodium sulfide. (4) Costs for using the precipitants were estimated to range from <$1/1,000 gal to >$5/1,000 gal depending on the precipitant.     

Measured and Predicted Herbicide Removal by Mulch

The removal of the herbicides diuron, isoxaben, oryzalin, and clopyralid by shredded cedar mulch was studied in laboratory batch and column experiments. The distribution coefficients (Kd) for the herbicides diuron, isoxaben, and oryzalin measured in batch experiments ranged from 129 to 187 L/kg for raw mulch and from 153 to 341 L/kg for ground mulch. Kd could not be accurately determined for clopyralid because of its weak sorption. A reactive transport model that described adsorption-desorption using a linear two-rate (site) reversible submodel provided a good fit for the experimental breakthrough data for isoxaben from column studies; adjusting the exchange rate coefficients using molecular diffusion coefficient ratios allowed breakthrough curves for oryzalin and diuron to be predicted effectively using no adjustable parameters. Model sensitivity analysis indicates that over 80% herbicide removal efficiency can be obtained for detention times greater than 8 min and storm durations of less than 100 min for the ground mulch particles. Widespread use of mulch as landscape material and the short detention times required for reasonable removal efficiency suggests that a mulch treatment system may be an efficient best management practice; these experimental and model results provide a basis for future pilot testing.