Microscale Zero-valent Iron Research Articles

Impact of zero valent iron on blackwater anaerobic digestion

The source diverted blackwater treatment is receiving growing attention as an alternative to conventional energy intensive wastewater management and treatment systems. Blackwater, containing concentrated organic materials, can be anaerobically digested to recovery bioenergy. However, the methane recovery from blackwater is often inhibited by the presence of high free ammonia (FA) in blackwater. In order to improve the methane production in blackwater, nano-scale zero valent iron (nZVI, 35 nm or 50 nm) or micro-scale zero valent iron (mZVI, 200 μm) at different dosages (i.e., 0.5, 1, and 10 g/L) were applied respectively in the anaerobic digestion (AD) reactor for blackwater treatment. The results demonstrated that low doses (0.5–1 g/L) of nZVI slightly improved methane (CH4) production, possibly due to a reduced oxidation–reduction potential (ORP) and improved hydrolysis-acidification in the nZVI supplemented systems. However, a lower biochemical methane potential (BMP) of blackwater was observed with high doses (10 g/L) of nZVI which induced a pH increase (>8.5) in AD reactor leading to a higher FA inhibition of CH4 production. In contrast, the effect of mZVI on blackwater AD system was not significant. The study demonstrated the successful application of nZVI for improving AD of blackwater, however, which requires dosage control.

Enhanced sulfamethoxazole degradation by peroxymonosulfate activation with sulfide-modified microscale zero-valent iron (S-mFe0): Performance, mechanisms, and the role of sulfur species

Sulfide-modified microscale zero-valent iron (S-mFe0) was applied to activate peroxymonosulfate (PMS) to degrade sulfamethoxazole (SMX), a typical sulfonamide bacteriostatic antibiotic. In this work, the effects of S/Fe molar ratio, S-mFe0 dosage, PMS dosage, different initial pH value, dissolved oxygen, SMX concentration and inorganic ions on SMX removal by S-mFe0/PMS system were investigated, respectively. Besides, the role of sulfur species (including the FeS, SO32−, S2−) was studied. In contrast to mFe0/PMS system, the removal efficiency of SMX obtained by S-mFe0 /PMS system was increased by 29.4%. Radical quenching and Electron Paramagnetic Resonance spectroscope (EPR) tests identified that both OH and SO4− were committed to degrading SMX, and SO4− was proven to be the dominant one. The electrochemical analysis of S-mFe0 and bare mFe0, implying a better electron transfer ability of S-mFe0 due to the formation of FeS. Furthermore, the activation of S2− for PMS could be ruled out by EPR tests results. Conversely, SO32− could effectively activate PMS to generate reactive oxygen species (ROS). The catalytic mechanisms of S-mFe0/PMS system were clarified by SEM-EDS, XRD, XPS, radical quenching and EPR tests. Based on the detected intermediates via LC-TOF-MS/MS, the degradation pathways of SMX by S-mFe0/PMS system were proposed. Overall, the work suggests that S-mFe0/PMS system has a good potential for the elimination of micropollutants in the aquatic environment.

Aerobic removal of iodinated contrast medium by nano-sized zero-valent iron: A combination of oxidation and reduction.

The removal performance and mechanisms of diatrizoate (DTA), a typical iodinated contrast medium, from water by nano-sized zero-valent iron (nZVI) under aerobic conditions were investigated in this study. Reactive oxygen species (ROS) and transformation products were detected with electron spin resonance and liquid chromatography electrospray ionization tandem mass spectrometry, respectively. Furthermore, the effects of several operational parameters on DTA removal were illustrated. The results showed that nZVI had a much higher DTA removal ability compared to microscale zero-valent iron (mZVI) in the presence of oxygen. Moreover, the detection of ROS and I- as well as the analysis of intermediate products suggested a combination of oxidation and reduction pathways for DTA removal by nZVI under aerobic conditions. Additionally, a high dosage of nZVI and acidic conditions led to the enhancement of DTA removal, while nZVI aging, as well as chloride and nitrate ions in the solution, had negative effects on the degradation of DTA by nZVI in the presence of oxygen.

High Efficiency Removal of 1,2-Dichloroethane from Groundwater by Microscale Zero-valent Iron Combined with Biological Carbon Source

In order to explore the degradation mechanism of 1,2-dichloroethane (1,2-DCA), which is one of the refractory saturated chlorinated hydrocarbons, the groundwater and aquifer soil from a chlorinated hydrocarbon contaminated site in Beijing were collected to carry out microcosm experiments under anaerobic conditions using zero-valent iron (ZVI) coupled with biological action. The removal rate of 1,2-DCA under different conditions was investigated by adding a composite agent consisting of micron zero-valent iron (mZVI), a biological carbon source, and a few nutrients. Changes of the groundwater physical and chemical parameters were monitored. The results showed that, when the dosage of the composite agent reached 3%, the concentration of 1,2-dichloroethane could be reduced to below the detection limit within 15 days. Neutral pH and the presence of SO42- were more conducive to the dechlorination of 1,2-DCA. After 30 days, an obvious increase in the ethene concentration was detected (0.17-0.52 mg·L-1) and no significant vinyl chloride, chloroethane, or ethane was observed in the microcosms, illustrating that the dihaloelimination was the main degradation pathway of 1,2-DCA in the system. In addition, the groundwater could maintain a low oxidation-reduction potential (-100 to -300 mV), dissolved oxygen (<0.5 mg·L-1), and a suitable pH value (6.5-7.5) for a long time under the synergy of mZVI and a biological carbon source. This was beneficial to the activity of anaerobic microorganisms and to the dechlorination reaction.

Zerovalent iron-sand filtration can reduce the concentration of multiple antimicrobials in conventionally treated reclaimed water

Irrigation with reclaimed water is increasing in areas that lack access to, and infrastructure for, high-level treatment and distribution. Antimicrobial residues are known to persist in conventionally treated reclaimed water, necessitating the investigation of reuse site-based mitigation options to further reduce these contaminants. We examined the effectiveness of a 50:50 volume/volume, particle matched, micro-scale zerovalent iron (ZVI)-sand filter in reducing concentrations of mixtures of antimicrobials present in pH-unadjusted conventionally treated reclaimed water. Twelve antimicrobials (azithromycin, ciprofloxacin, erythromycin, linezolid, oxacillin, oxolinic acid, penicillin G, pipemidic acid, sulfamethoxazole, triclocarban, tetracycline and vancomycin) were quantified using high performance-liquid chromatography-tandem mass spectrometry in reclaimed water, and ZVI-sand filtered reclaimed water, in a two-month long greenhouse-based experiment. Data were analyzed using a non-parametric rank-based approach. ZVI-sand filtration significantly reduced concentrations of azithromycin, ciprofloxacin, oxolinic acid, penicillin G, sulfamethoxazole, linezolid, pipemidic acid and vancomycin. Azithromycin, the antimicrobial with the highest median concentration (320 ng/L), was reduced to below the limit of detection after ZVI-sand filtration. Inorganic element (antimony, beryllium, cadmium, chromium, iron, lead, selenium and thallium) and water quality (free and total chlorine, nitrates, nitrites, pH and total dissolved solids) analyses showed that ZVI-sand filtered reclaimed water quality (nitrate, salinity, and inorganic elements) met the recommended guidelines for agricultural irrigation with reclaimed water. Based on our initial results, ZVI-sand filtration may be a promising basis for a point-of-use filtration system for reclaimed water irrigation on small-scale farms.

Acceleration of microiron-based dechlorination in water by contact with fibrous activated carbon

Zero-valent iron (ZVI) is widely applied for reduction of chlorohydrocarbons in water. Since the dechlorination occurs at the iron surface, marked differences in rate constants are commonly found for nanoscale and microscale ZVI. It has already been shown for trichloroethene (TCE) adsorbed to activated carbon (AC) that the dechlorination reaction is shifted to the carbon surface simply by contacting the AC with highly reactive nanoscale ZVI particles. Transfer of reactive species to the adsorbed pollutant was discussed.The present study shows that even low price and very low reactive microscale ZVI can also be utilized for an effective dechlorination process. Compared to the reaction rate at the iron surface itself, an enormous acceleration of the dechlorination rate for chlorinated ethenes was observed, reaching activity levels such as known for nanoscale ZVI. When fibrous AC is brought into direct contact with microscale ZVI the iron-surface-normalised dechlorination rate constants increased by up to four orders of magnitude. This implies that the dechlorination reaction is fully transferred to the AC surface. At the same time, the anaerobic corrosion of the same material was not substantially affected. Thus, the utilization of iron's reduction equivalents towards dechlorination (dechlorination efficiency) can be considerably enhanced. A screening with various AC types showed that the extent of rate acceleration depends strongly on the surface chemistry of the AC. By means of temperature-programmed desorption, it could be shown that concentration and type of oxygen surface groups determine the redox-mediation properties. Quinone/hydroquinone groups were identified as being the main drivers for electron-transfer processes, but to some extent other redox-active groups such as chromene and pyrone can also act as redox mediators. AC overall plays the role of a catalyst rather than a reactant. The present study derives recommendations for practical application of the findings in water-treatment approaches.

Removal of roxarsone from synthetic livestock wastewater by sodium alginate loading with micro-scale zerovalent iron

Removal of roxarsone from synthetic livestock wastewater by sodium alginate loading with micro-scale zerovalent iron

Reduced expression of iron transport and homeostasis genes in Pseudomonas fluorescens during iron uptake from nanoscale iron

Plant growth-promoting rhizobacteria (PGPR) have beneficial effects on the host plant that include acquisition of nutrients such as iron (Fe). In this study, the impact of varying concentrations of nanoscale zero-valent iron (NZVI, 1, 2 and 5 g L−1) on a PGPR, Pseudomonas fluorescens' viability, growth and production of siderophores was investigated. Microscale zero-valent iron (MZVI) particles were also included in this study to determine and compare the effects of particle size on Fe acquisition and use by Pseudomonas fluorescens. Siderophores chelate insoluble forms of Fe that are then rendered bioavailable to P. fluorescens as well as some plants. This research indicates that the Fe species derived from lower concentrations of NZVI additions (1 g L−1) are bioavailable and can be utilized as a nutrient by P. fluorescens without the bacteria producing siderophores, an energy intensive process. Also, NZVI application as a fertilizer has promise for better utilization of Fe by plants. P. fluorescens colony forming units (log10 CFU mL−1) growth in cultures containing 1 g L−1 NZVI were significantly higher exceeded (14 units) than those grown without NZVI additions (13 units). The majority of P. fluorescens cells treated with 2 and 5 g L−1 were nonviable and produced limited colony forming units (4 units). The effects of NZVI on siderophore production by P. fluorescens were assessed using chrome azurol S (CAS) plates and real time quantitative reverse transcription PCR (qRT-PCR). Further, expression of genes involved in Fe utilization [putative pvdS (PFL 4190) and a bacterioferritin-associated ferredoxin gene (PFL 4858)] were targeted using real time quantitative reverse transcription PCR (qRT-PCR). Expression of both genes was below detection limits in cultures exposed to NZVI but measurable in MZVI and control treatments, again indicating potential ease in uptake of Fe by P. fluorescens from NZVI. Transmission electron microscopy (TEM) revealed the presence of NZVI particles on the exterior and interior of cells; energy dispersive X-ray spectroscopy (EDS) verified that the material was derived from NZVI and that Fe concentrations were higher in the interior of cells exposed to 1 g L−1 NZVI compared to the control. The research findings from this study indicate that NZVI has the potential to accumulate in plant growth-promoting rhizobacteria and affect the growth of organisms in environments like the rhizobiome which may affect plant growth positively. This research opens up opportunities for future studies to determine the effects of Fe and other nutrients and micronutrients derived from nanomaterials, particularly in oligotrophic systems.

Application of sequential extraction analysis to Pb(II) recovery by zerovalent iron-based particles

Zerovalent iron (ZVI) is an environmental-friendly reactive reagent for recovering heavy metals. However, the detailed recovery mechanism remains unclear due to a lack of quantitative analysis of recovery products. Herein, microscale ZVI, nanoscale ZVI and Ni/Fe nanoparticles were used to recover Pb(II) in aqueous solution and a sequential extraction procedure (SEP) was applied to determine the formed lead species quantitatively. At high initial Pb(II) concentration (500 mg L−1), more than 99.5% of Pb(II) was immobilized by Ni/Fe and n-ZVI, whereas m-ZVI caused inferior recovery efficiency (<25%). XRD and XPS results revealed that Pb(II) was reduced to Pb0 prior to the formation of metal hydroxides as the external shell of ZVI. SEP results showed that the fraction bound to carbonates (PbO), fraction bound to iron oxides and exchangeable fraction were the main lead species conducted by Ni/Fe, n-ZVI and m-ZVI, respectively. Consequently, (co-)precipitation and specific adsorption dominated Pb(II) recovery by Ni/Fe and n-ZVI, whereas m-ZVI conducted Pb(II) recovery mainly via weak adsorption. The reactivity of ZVI toward Pb(II) followed the increasing order of m-ZVI << n-ZVI ≤ Ni/Fe. The detailed mechanisms of Pb(II) recovery conducted by different ZVI were proposed.

The effect of zerovalent iron on the microbial degradation of hexabromocyclododecane

Hexabromocyclododecane (HBCD), a commonly used brominated flame retardant (BFR), has been listed as a persistent organic pollutant (POP). In order to remediate HBCD in the environment, the influence of microscale zerovalent iron (MZVI) on the HBCD degrading microcosm was evaluated. In the acclimated microcosm collected from river sediment, 49% of HBCD was initially removed through adsorption and then 30% of HBCD was biodegraded through non-debromination processes. In contrast to MZVI only, over 60% of HBCD was gradually degraded by MZVI through a debromination reaction. In the microcosm-MZVI combined system, the biodegradation ability of the microcosm was inhibited. The aqueous chemistry was changed by the addition of MZVI, which led to the alteration of microbial composition and biodegradation ability. These better understandings can facilitate an evaluation of the impact of MZVI on HBCD biodegradation when ZVI was used to remediate this BFR.

Dynamic interactions between sulfidated zerovalent iron and dissolved oxygen: Mechanistic insights for enhanced chromate removal

Recent research on contaminant removal by zerovalent iron (ZVI) has evolved from investigating simple model systems to systems that encompass increased dimensions of complexity. Sulfidation and aerobic conditions are two of the most broadly relevant complications. Combining these two, this study investigated the dynamic interactions between sulfidated microscale ZVI and dissolved O2, for removal of Cr(VI), a model contaminant for metals and metalloids. The results show that the coupling of sulfidation and oxygenation significantly improves Cr removal, which is attributed to enhanced Fe(II) production that resulted from accelerated corrosion of Fe(0). The Cr(VI) removal rate increased with increasing O2 saturation from 0% to 100% but showed a bimodal dependence on the S/Fe ratio. At the optimal S/Fe ratio, the ZVI exhibits a highly porous surface morphology, which, according to prior literature on sulfur induced corrosion, promotes corrosion. In addition, a novel time series correlation was developed between aqueous Fe(II) and Cr(VI) based on data collected in the presence and absence of 1,10-phenanthroline, to probe for changes of reductants during the reaction time course. The analysis indicated that Fe(0) was responsible for the initial small amount of Cr(VI) removal, which then transitioned to a phase controlled by surface Fe(II). The slopes of the time series correlations during the latter phase of the reaction vary with experimental conditions but are mostly much higher than the theoretical stoichiometric ratio between Cr(VI) and Fe(II) (i.e., 0.33), indicating that Fe(II) regeneration contributes significantly to Cr removal.

Different effects of nano-scale and micro-scale zero-valent iron particles on planktonic microorganisms from natural reservoir water

This study compares the effect of nanoscale and microscale zerovalent iron on natural freshwater microplankton over 21 days.

Degradation of sulfamethoxazole by a heterogeneous Fenton-like system with microscale zero-valent iron: Kinetics, effect factors, and pathways

Abstract Five commercial microscale zero-valent iron (mZVI) samples showed different reactivity in sulfamethoxazole (SMX) removal in the presence of H2O2, which was independent of respective iron surface area but was consistent with the dissolved iron releasing rate. Of five mZVI samples, mZVIYF2 possessed the highest reactivity in H2O2 and the mZVIYF2/H2O2 system could remove 97.9% SMX at room temperature within 10 min. The degradation curve of SMX by mZVI/H2O2 at pHini 3.0 could be divided into three phases: a lag phase, a rapid degradation phase, and a final stationary phase. Effects of pHini, mZVI loading, SMX concentration, and H2O2 concentration on SMX degradation by mZVI/H2O2 were analyzed. Removal rates of SMX by mZVI/H2O2 dropped sharply upon increasing pHini. Increasing dosages of Fe0 ranged from 25 to 75 mg/L and H2O2 at low concentrations (below 0.5 mM) both enhanced SMX degradation, but a higher concentration of Fe0 and H2O2 also quickly decreased oxidative efficiency of SMX. Increasing dosages of SMX within the range of 5–100 µM progressively decreased the oxidation rates of SMX by mZVI/H2O2. Furthermore, three degradation pathways of SMX by mZVI/H2O2 process were proposed based on the combination of theoretical calculations and intermediates identification.

Mechanochemically Sulfidated Microscale Zero Valent Iron: Pathways, Kinetics, Mechanism, and Efficiency of Trichloroethylene Dechlorination.

In water treatment processes that involve contaminant reduction by zerovalent iron (ZVI), reduction of water to dihydrogen is a competing reaction that must be minimized to maximize the efficiency of electron utilization from the ZVI. Sulfidation has recently been shown to decrease H2 formation significantly, such that the overall electron efficiency of (or selectivity for) contaminant reduction can be greatly increased. To date, this work has focused on nanoscale ZVI (nZVI) and solution-phase sulfidation agents (e.g., bisulfide, dithionite or thiosulfate), both of which pose challenges for up-scaling the production of sulfidated ZVI for field applications. To overcome these challenges, we developed a process for sulfidation of microscale ZVI by ball milling ZVI with elemental sulfur. The resulting material (S-mZVIbm) exhibits reduced aggregation, relatively homogeneous distribution of Fe and S throughout the particle (not core-shell structure), enhanced reactivity with trichloroethylene (TCE), less H2 formation, and therefore greatly improved electron efficiency of TCE dechlorination (εe). Under ZVI-limited conditions (initial Fe0/TCE = 1.6 mol/mol), S-mZVIbm gave surface-area normalized reduction rate constants (k'SA) and εe that were ∼2- and 10-fold greater than the unsulfidated ball-milled control (mZVIbm). Under TCE-limited conditions (initial Fe0/TCE = 2000 mol/mol), sulfidation increased kSA and εe ≈ 5- and 50-fold, respectively. The major products from TCE degradation by S-mZVIbm were acetylene, ethene, and ethane, which is consistent with dechlorination by β-elimination, as is typical of ZVI, iron oxides, and/or sulfides. However, electrochemical characterization shows that the sulfidated material has redox properties intermediate between ZVI and Fe3O4, mostly likely significant coverage of the surface with FeS.

Synthesis of Fly Ash and Bentonite-Supported Zero-Valent Iron and Its Application for Removal of Toxic Cationic Dyes from Aqueous Solutions

Abstract A new adsorbing material for crystal violet (CV) and methylene blue (MB) dye removal, fly ash and bentonite-supported microscale zero-valent iron (FB-mZVI), was synthesized by using iron-ore tailings (IOT) as the iron source. Palm kernel shells (PKSs) were used to reduce the iron oxide of the IOT to Feo. A statistical orthogonal method was used to evaluate the factors determining the adsorbent synthesis. It was found that the important factors were the mass ratio of the fly ash, bentonite, IOT, and PKSs as well as the synthesis time, temperature, and heating rate. Optimum mass ratio of the fly ash, bentonite, PKSs, and IOT was found to be 2:2:1:1, and optimum synthesis temperature, time, and heating rate were found to be 800°C, 10 min, and 10°C/min, respectively. An FB-mZVI adsorbent was used to remove the CV and MB from aqueous solutions. Removal efficiency for CV was much higher than that for MB. UV, XRD, and FTIR analyses revealed that the adsorption mechanisms for removal of CV were different...

Effect of solution pH on aging dynamics and surface structural evolution of mZVI particles: H2 production and spectroscopic/microscopic evidence.

A microscale zero-valent iron (mZVI)-based in situ reactive zone is a promising technology for contaminated groundwater remediation. Estimation of mZVI aging behavior after its injection into the subsurface is essential for efficiency and longevity assessments. In this study, batch tests were conducted to investigate the effect of initial pH on mZVI aging dynamics, as well as the formation and evolution of aging products over 112days. Results indicated that mZVI aging accelerated with decreasing initial pH. Corrosion rates of mZVI particles under pH 6.0 and 7.5 were approximately two orders of magnitude higher than those observed at pH 9.0. The morphological, structural, and compositional evolution of mZVI particles in three systems (pH=6.0, 7.5, and 9.0) were investigated using scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. In acidic and neutral solutions, a thick passivation layer with loosely and unevenly distributed aging precipitates was observed, and Fe3O4 was the final aging precipitate. Nevertheless, in alkaline solutions, minute aging precipitates were detected on the mZVI surface at 112day. Characterization results suggested that mZVI was oxidized via the Fe0-Fe(OH)2-Fe3O4 route. These findings shed new light on mZVI aging mechanisms, particularly its physicochemical characteristics and the structural evolution of mZVI in field-scale groundwater remediation applications.

Preparation of microscale zero-valent iron-fly ash-bentonite composite and evaluation of its adsorption performance of crystal violet and methylene blue dyes.

New microscale zero-valent iron adsorbent on fly ash and bentonite matrix for removal of crystal violet (CV) and methylene blue (MB) was synthesized through direct reduction of iron oxide using coke and palm kernel shell. The adsorbent was prepared as cylindrical shaped pellets to remove the CV and MB from the aqueous solution. Nitrogen adsorption-desorption isotherm and scanning electron microscopy (SEM) studies showed that the adsorbent is highly porous, and the iron particles are finely dispersed on the supporting material surfaces. FTIR and UV studies indicated that the C=C bonds in CV and C=N+(CH3)2 bonds in MB were affected in the adsorption process. MB switched to the reduced MBH2 species while CV was reduced to two small-size molecular compounds, explaining the higher CV adsorption in comparison to that of MB. The reduction of these compounds was coupled to the oxidation of Fe0 to Fe2O3 as revealed by XRD characterization of the adsorbent after adsorption. CV and MB adsorption isotherms fitted well with the Langmuir adsorption model. Different adsorption and reduction kinetic models were examined for the MB and CV removal processes. A better fit of the experimental data with the pseudo-second-order model was observed. CV and MB adsorption increased with temperature in the 30-50°C range. At 50°C, adsorption capacities of CV and MB reached to 89.9 and 42.8mg/g, respectively. This new adsorbent showed a superior adsorption capacity for CV and MB when compared to other adsorbents.

Evaluation of zeolite-supported microscale zero-valent iron as a potential adsorbent for Cd2+ and Pb2+ removal in permeable reactive barriers.

A new composite adsorbent, zeolite-supported microscale zero-valent iron (Z-mZVI) was evaluated as a potential adsorbent for the removal of Cd2+ and Pb2+ from aqueous solution using batch and column experiments. Adsorption isotherms were well fitted by Langmuir model, and the maximum adsorption capacity was 63.14mg/g for Cd2+ and 154.61mg/g for Pb2+, respectively. Both adsorption processes followed the pseudo-second-order model which indicated that the rate-limiting step for different initial concentration was dominated by chemical adsorption process. The coexistence of Cd2+ and Pb2+ caused the reduction of Cd2+ removal efficiency, but not for Pb2+. Z-mZVI has a high removal capacity for Cd2+ and Pb2+ over a wide pH range (3.0-6.8) as well as in the presence of competitive Ca2+ or Mg2+ ions (<2mmol/L). Moreover, Z-mZVI shows a high immobilization capacity for the adsorbed Cd2+ and Pb2+ products, even at the acid solution (pH=3.95). Column experiment confirmed that Z-mZVI could simultaneously remove Cd2+ and Pb2+ from solution efficiently. Thomas model can simulate the equilibrium adsorption capacity of Cd2+ and Pb2+ of the Z-mZVI column well. This study demonstrates that Z-mZVI is an efficient and promising reactive material in permeable reactive barriers for Cd2+ and Pb2+ removal from aqueous solution.

Effect of boron on reactivity and apparent corrosion rate of microscale zerovalent irons

The reactivity and apparent corrosion rate of newly produced boron-alloyed microscale zerovalent iron (mZVI) particles toward a variety of chlorinated aliphatic hydrocarbons (CAHs) were assessed in laboratory-scale batch reactors. For comparison, commercially available nanoscale zerovalent irons (nZVIs) were also included in the study. Based on mass normalized (kM) pseudo first-order degradation rate constants, the reactivity of boron-alloyed mZVIs appeared to be one order of magnitude lower than most reactive nZVI. However, specific surface area normalized reaction rate constants (kSA) were 1–2 orders of magnitude higher than for nZVIs showing that boron-alloyed mZVIs can compete with nZVIs. The high reactivity of boron-alloyed mZVIs was attributed to (1) the prevented iron surface passivation during anaerobic corrosion due to the simultaneous iron and boron dissolution kinetics preventing the formation of insoluble oxidized iron species, (2) the involvement of boron in facilitating electron transfer by inducing β-elimination followed by hydrogenation to ethane and/or (3) involvement of boron in improved particle size reduction during milling process leading to increased active surface areas of the particles. Results of this study suggest that boron-alloyed mZVIs could be an efficient alternative to nZVIs for in situ groundwater remediation of different CAHs in terms of their reactivity and apparent corrosion rate.

Nitrate reduction by micro-scale zero-valent iron particles under oxic condition

The denitrification by zero-valent iron (ZVI) particles under oxic condition at various conditions was investigated in this study. After a 2-h reaction under oxic condition, NO3−-N removal and conversion to NH4+-N was 97.6% and 35.2%, respectively, when the initial NO3−-N concentration was 25 mg/L, the dose of 85 μm ZVI was 2 g/L, and the initial pH was 2.0. The injection of an additional electron doner (50 mg/L Fe2+) did not promote the reaction, but the initial pH greatly affects NO3−-N reduction, regardless of the presence of Fe2+. Phosphate promoted and inhibited NO3−-N sorption and reduction at low and high concentration, respectively, suggesting its role of both electron transfer enhancement and competition. The results of continuous experiments using a horizontal flow ZVI reactor showed that NO3−-N reduction efficiency was in a range of 88-100%, through a 1700 h operation. In addition, field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray (EDX) analysis, and X-ray diffraction (XRD) analysis showed that hematite is the major reaction products of ZVI under oxic condition. The results in this study suggest that ZVI can successfully be used for denitrification under oxic condition, which can be an advantage over nano-scale ZVI.