Metallic iron for environmental remediation: learning from electrocoagulation

Integrating electrochemical pretreatment with microalgae treatment for nitrogen and phosphorus removal and resource recovery from swine wastewater

This study integrates electrochemical pretreatment with microalgae (Scenedesmus obliquus) treatment to enhance nitrogen and phosphorus removal and resource recovery from swine wastewater. By optimizing electrochemical and microalgae treatment conditions, the dilution factor and the hydraulic retention time for microalgae treatment were reduced to 5 times and 7 days, respectively. Under the optimized operational conditions, removal efficiencies of total nitrogen and ammonia could reach over 89 %and 96 %,respectively, and the removal efficiency of total phosphorus was over 99 %. The study also found that aluminum was more suitable than iron for anode as it produced fewer residues. Additionally, the electrochemical pretreatment reduced Cu2+ and Zn2+ concentrations, mitigating negative impacts on microalgal growth. The microalgae biomass harvested from developed processes was rich in saturated fatty acids, which was desirable for biodiesel production. This approach addresses the challenges of nutrient removal for swine wastewater treatment with high quality biomass recovery.

Effects of common dissolved anions on the efficiency of Fe0-based remediation systems

Quiescent batch experiments were conducted to evaluate the influences of Cl-, F-, HCO3-, HPO42-, and SO42- on the reactivity of metallic iron (Fe0) for water remediation using the methylene blue (MB) method. Strong discoloration of MB indicates high availability of solid iron corrosion products (FeCPs). Tap water was used as an operational reference. Experiments were carried out in graduated test tubes (22 mL) for up to 45 d, using 0.1 g of Fe0 and 0.5 g of sand. Operational parameters investigated were (i) equilibration time (0-45 d), (ii) 4 different types of Fe0, (iii) anion concentration (10 values), and (iv) use of MB and Orange II (O-II). The degree of dye discoloration, the pH, and the iron concentration were monitored in each system. Relative to the reference system, HCO3- enhanced the extent of MB discoloration, while Cl-, F-, HPO42-, and SO42- inhibited it. A different behavior was observed for O-II discoloration: in particular, HCO3- inhibited O-II discoloration. The increased MB discoloration in the HCO3- system was justified by considering the availability of FeCPs as contaminant scavengers, pH increase, and contact time. The addition of any other anion initially delays the availability of FeCPs. Conflicting results in the literature can be attributed to the use of inappropriate experimental conditions. The results indicate that the application of Fe0-based systems for water remediation is a highly site-specific issue which has to include the anion chemistry of the water.

Investigating the Fe0/H2O systems using the methylene blue method: Validity, applications, and future directions

An innovative approach to characterize the reactivity of metallic iron (Fe⁰) for aqueous contaminant removal has been in use for a decade: The methylene blue method (MB method). The approach considers the differential adsorptive affinity of methylene blue (MB) for sand and iron oxides. The MB method characterizes MB discoloration by sand as it is progressively coated by in-situ generated iron corrosion products (FeCPs) to deduce the extent of iron corrosion. The MB method is a semi-quantitative tool that has successfully clarified some contradicting reports on the Fe⁰/H₂O system. Moreover, it has the potential to serve as a powerful tool for routine tests in the Fe⁰ remediation industry, including quality assurance and quality control (QA/QC). However, MB is widely used as a 'molecular probe' to characterize the Fe⁰/H₂O system, for instance for wastewater treatment. Thus, there is scope to avoid confusion created by the multiple uses of MB in Fe⁰/H₂O systems. The present communication aims at filling this gap by presenting the science of the MB method, and its application and limitations. It is concluded that the MB method is very suitable for Fe⁰ material screening and optimization of operational designs. However, the MB method only provides semi-quantitative information, but gives no data on the solid-phase characterization of solid Fe⁰ and its reaction products. In other words, further comprehensive investigations with microscopic and spectroscopic surface and solid-state analyses are needed to complement results from the MB method.

Effect of voltage intensity on the nutrient removal performance and microbial community in the iron electrolysis-integrated aerobic granular sludge system

The effects of voltage intensity on the nutrient removal performance and microbial community in the iron electrolysis-integrated aerobic granular sludge (AGS) system were investigated over a period of 15 weeks. Results revealed that the application outcomes of iron electrolysis for AGS systems relied on voltage intensity. When a constant voltage of 1.5 V was applied, the sludge granulation was most obviously accelerated with a specific growth rate of the sludge diameter of 0.078 day^-1, and the removal efficiencies of total nitrogen (TN) and total phosphorus (TP) increased by 14.1% and 20.2%, respectively, compared to the control reactor (without the iron electrolysis-integration). Moreover, the AGS developed at different voltages included different microbial communities, whose shifts were driven by the Fe content and the average diameter of AGS. Both heterotrophic nitrifiers and mixotrophic denitrifiers were significantly enriched in the AGS developed at 1.5 V, which effectively enhanced TN removal. Together with the response of the functional genes involved in Fe, N, and P metabolism, the electrolytic iron-driven nutrient degradation pathway was further elaborated. Overall, this study clarified the optimum voltage condition when iron electrolysis was integrated into the AGS system, and revealed the enhancement mechanism of this coupling technology on nutrient removal during the treatment of low-strength municipal wastewater.

Weak magnetic field enables high selectivity of zerovalent iron toward metalloid oxyanions under aerobic conditions

For water treatment/remediation by zerovalent iron (ZVI), of particular concern is its selectivity toward contaminants over natural non-targets (e.g., O₂ and H₂O/H⁺). Hence, the effects of weak magnetic field (WMF) on the selectivity of ZVI toward metalloid oxyanions (i.e., As(III), As(V), Sb(III), Sb(V), Se(IV) and Se(VI)) were in-depth investigated under aerobic conditions. This study unraveled that, despite the electron utilization (EU) of ZVI with and without WMF were almost identical at reaction equilibrium, the application of a WMF could enhance the specific removal capacity (SRC) of ZVI toward metalloid oxyanions from 1.8-19.0 mg/g Fe to 12.6-85.3 mg/g Fe. Particularly, the electron efficiency (EE) of ZVI with WMF for reduction of Se(IV)/Se(VI) were 3.7- to 14.1-fold greater than that without WMF. Since the WMF-induced magnetic gradient force (F_ΔB) can derive the movement of both Fe²⁺ and metalloid oxyanions, the subsequent incorporation of metalloid oxyanions with in-situ generated iron oxides can also been mediated synchronously and thus leading to an enhanced SRC of ZVI (also EE for Se(IV) and Se(VI) reduction by ZVI). In general, our findings prove that WMF should be a promising method to promote the selectivity of ZVI for water decontamination under aerobic conditions.

Water Treatment Using Metallic Iron: A Tutorial Review

Researchers and engineers using metallic iron (Fe0) for water treatment need a tutorial review on the operating mode of the Fe0/H2O system. There are few review articles attempting to present systematic information to guide proper material selection and application conditions. However, they are full of conflicting reports. This review seeks to: (i) Summarize the state-of-the-art knowledge on the remediation Fe0/H2O system, (ii) discuss relevant contaminant removal mechanisms, and (iii) provide solutions for practical engineering application of Fe0-based systems for water treatment. Specifically, the following aspects are summarized and discussed in detail: (i) Fe0 intrinsic reactivity and material selection, (ii) main abiotic contaminant removal mechanisms, and (iii) relevance of biological and bio-chemical processes in the Fe0/H2O system. In addition, challenges for the design of the next generation Fe0/H2O systems are discussed. This paper serves as a handout to enable better practical engineering applications for environmental remediation using Fe0.

Characterizing the reactivity of metallic iron for water defluoridation in batch studies

The suitability of metallic iron (Fe(0)) for water defluoridation is yet to be understood. Fluoride removal ([F^-]₀ = 20.0 mg L^-1) and Orange II discoloration ([Orange II]₀ = 10.0 mg L^-1) by Fe(0)/H₂O batch systems are compared herein. A steel wool (SW) and a granular iron (GI) are used as Fe(0) specimens. Each essay tube contains 0.5 g sand and 0.1 g of the used Fe(0). Investigated systems were: (i) SW/sand at pH 5.0, (ii) GI/sand at pH 5.0 and (iii) SW/sand at pH 8.0. Prior to contaminant addition, Fe(0) was allowed to pre-corrode within the systems for up to 46 days. The systems were then equilibrated for 30 days with a mixture of the two model contaminants. Result confirmed (i) the higher efficiency of SW over GI in removing both contaminants, (ii) the higher efficiency of Fe(0) for Orange II discoloration and (iii) the positive impact of initial low pH values on the efficiency of Fe(0)/H₂O systems. The major output of this research is that conventional Fe(0)/H₂O systems are not suitable for quantitative water defluoridation. It is suggested that ways to avoid defluoridation using Fe⁰ must be explored. One affordable opportunity is blending fluoride-polluted water with carefully harvested rainwater.

The Impact of Selected Pretreatment Procedures on Iron Dissolution from Metallic Iron Specimens Used in Water Treatment

Studies were undertaken to determine the reasons why published information regarding the efficiency of metallic iron (Fe0) for water treatment is conflicting and even confusing. The reactivity of eight Fe0 materials was characterized by Fe dissolution in a dilute solution of ethylenediaminetetraacetate (Na2–EDTA; 2 mM). Both batch (4 days) and column (100 days) experiments were used. A total of 30 different systems were characterized for the extent of Fe release in EDTA. The effects of Fe0 type (granular iron, iron nails and steel wool) and pretreatment procedure (socking in acetone, EDTA, H2O, HCl and NaCl for 17 h) were assessed. The results roughly show an increased iron dissolution with increasing reactive sites (decreasing particle size: wool > filings > nails), but there were large differences between materials from the same group. The main output of this work is that available results are hardly comparable as they were achieved under very different experimental conditions. A conceptual framework is presented for future research directed towards a more processed understanding.

Role of pre-corrosion of Fe0 on its efficiency in remediation systems: An electrochemical study

The duration of in-situ generation of iron corrosion products (FeCPs) prior the remediation process (so called "aging" of metallic iron (Fe⁰)), was found as the key parameter affecting the efficiency of Fe⁰ for water remediation. Batch experiments were performed in buffered solutions (pH 4.0, 4.7 and 5.5) and under oxic conditions (presence of dissolved oxygen) using Zn²⁺ as probe contaminant. The time-dependent (0-16 d) concentration changes of aqueous Fe²⁺ and Zn²⁺ were monitored using differential pulse polarography (DPP). During the time of pre-corrosion varying from 0 to 6 d, an "induction period" of the corrosion occurs in the first one - 2 h when no Fe²⁺ ion is released in the solution. After this period, Fe²⁺ was identified in solution and its concentration progressively increases up to 6 h, then starts to decrease and after 6 d nearly disappears. Experiments with Zn²⁺ reveal that the most efficient Fe⁰ remediation occurs after 6 h of pre-corrosion. This coherence thus proves that the presence, the amount and the age of FeCPs ("degree" of corrosion) significantly impact the removal efficiency of Zn²⁺ in Fe⁰/H₂O systems. The present study severely refute the wording 'reactivity loss' and states that progress in designing sustainable Fe⁰/H₂O systems will not be achieved before the role of "active" FeCPs is clarified.

Enhanced Nitrobenzene reduction by zero valent iron pretreated with H2O2/HCl

In this study a novel iron-based reducing agent of highly effective reduction toward nitrobenzene (NB) was obtained by pretreating zero valent iron (ZVI) with H₂O₂/HCl. During the H₂O₂/HCl pretreatment, ZVI undergoes an intensive corrosion process with formation of various reducing corrosion products (e.g., Fe²⁺, ferrous oxides/hydroxides, Fe₃O₄), yielding a synergetic system (prtZVI) including liquid, suspensions and solid phase. The pretreatment process remarkably enhances the reductive performance of ZVI, where a rapid reduction of NB (200 mg L^-1) in the prtZVI suspension was accomplished in a broad pH range (3-9) and at low dosage. Nitrosobenzene and phenylhydroxylamine are identified as the intermediates for NB reduction with the end-product of aniline. Compared with the virgin ZVI as well as another nanosized ZVI, the prtZVI system exhibits much higher electron efficiency for NB reduction as well as higher utilization ratio of Fe⁰. A rapid reduction of various nitroaromatics in an actual pharmaceutical wastewater further demonstrated the feasibility of the prtZVI system in real wastewater treatment.

Effects of brining on the corrosion of ZVI and its subsequent As(III/V) and Se(IV/VI) removal from water

Zero-valent iron (ZVI) has been extensively applied in water remediation, and most of the ZVI materials employed in practical applications are iron scraps, which have usually been corroded to certain extent under different conditions. In this study, the effects of brining with six solutions (NaCl, Na₂SO₄, NaHCO₃, Na₂SiO₃, NH₄Cl, and NaH₂PO₄) on the corrosion of ZVI and its performance in the removal of As(III/V)/Se(IV/VI) were systematically investigated. All the studied solutions enhanced the corrosion of ZVI except for Na₂SiO₃, and the degrees of corrosion followed the order of NH₄Cl > NaH₂PO₄ > Na₂SO₄ > NaCl > NaHCO₃ > H₂O > Na₂SiO₃. The corrosion products derived from ZVI were identified by SEM and XRD, and the dominant corrosion products varied with the type of brine solution. The positive correlation between the degree of ZVI corrosion and As(III/V)/Se(IV/VI) removal by the pre-corroded ZVI (pcZVI) was verified. In addition, As and Se removal by pcZVI was realized via a comprehensive process including adsorption and reduction, as further supported by the XPS analysis. We believe this study will shed new light upon the selection of iron materials pre-corroded under different saline conditions for practical water remediation.

Formation of macroscopic surface layers on Fe(0) electrocoagulation electrodes during an extended field trial of arsenic treatment

Extended field trials to remove arsenic (As) via Fe(0) electrocoagulation (EC) have demonstrated consistent As removal from groundwater to concentrations below 10 μg L(-1). However, the coulombic performance of long-term EC field operation is lower than that of laboratory-based systems. Although EC electrodes used over prolonged periods show distinct passivation layers, which have been linked to decreased treatment efficiency, the spatial distribution and mineralogy of such surface layers have not been investigated. In this work, we combine wet chemical measurements with sub-micron-scale chemical maps and selected area electron diffraction (SAED) to determine the chemical composition and mineral phase of surface layers formed during long-term Fe(0) EC treatment. We analyzed Fe(0) EC electrodes used for 3.5 months of daily treatment of As-contaminated groundwater in rural West Bengal, India. We found that the several mm thick layer that formed on cathodes and anodes consisted of primarily magnetite, with minor fractions of goethite. Spatially-resolved SAED patterns also revealed small quantities of CaCO3, Mn oxides, and SiO2, the source of which was the groundwater electrolyte. We propose that the formation of the surface layer contributes to decreased treatment performance by preventing the migration of EC-generated Fe(II) to the bulk electrolyte, where As removal occurs. The trapped Fe(II) subsequently increases the surface layer size at the expense of treatment efficiency. Based on these findings, we discuss several simple and affordable methods to prevent the efficiency loss due to the surface layer, including alternating polarity cycles and cleaning the Fe(0) surface mechanically or via electrolyte scouring.

The limitations of applying zero-valent iron technology in contaminants sequestration and the corresponding countermeasures: the development in zero-valent iron technology in the last two decades (1994-2014)

Over the past 20 years, zero-valent iron (ZVI) has been extensively applied for the remediation/treatment of groundwater and wastewater contaminated with various organic and inorganic pollutants. Based on the intrinsic properties of ZVI and the reactions that occur in the process of contaminants sequestration by ZVI, this review summarizes the limitations of ZVI technology and the countermeasures developed in the past two decades (1994-2014). The major limitations of ZVI include low reactivity due to its intrinsic passive layer, narrow working pH, reactivity loss with time due to the precipitation of metal hydroxides and metal carbonates, low selectivity for the target contaminant especially under oxic conditions, limited efficacy for treatment of some refractory contaminants and passivity of ZVI arising from certain contaminants. The countermeasures can be divided into seven categories: pretreatment of pristine ZVI to remove passive layer, fabrication of nano-sized ZVI to increase the surface area, synthesis of ZVI-based bimetals taking advantage of the catalytic ability of the noble metal, employing physical methods to enhance the performance of ZVI, coupling ZVI with other adsorptive materials and chemically enhanced ZVI technology, as well as methods to recover the reactivity of aged ZVI. The key to improving the rate of contaminants removal by ZVI and broadening the applicable pH range is to enhance ZVI corrosion and to enhance the mass transfer of the reactants including oxygen and H(+) to the ZVI surface. The characteristics of the ideal technology are proposed and the future research needs for ZVI technology are suggested accordingly.

Removal of cationic dye methylene blue by zero-valent iron: Effects of pH and dissolved oxygen on removal mechanisms

Effects of pH and dissolved oxygen on mechanisms for decolorization and total organic carbon (TOC) removal of cationic dye methylene blue (MB) by zero-valent iron (ZVI) were systematically examined. Decolorization and TOC removal of MB by ZVI are attributed to the four potential mechanisms, i.e. reduction, degradation, precipitation and adsorption. The contributions of four mechanisms were quantified at pH 3.0, 6.0 and 10.0 in the oxic and anoxic systems. The maximum efficiencies of decolorization and TOC removal of MB were found at pH 6.0. The TOC removal efficiencies at pH 3.0 and 10.0 were 11.0 and 17.0%, respectively which were considerably lower as compared with 68.1% at pH 6.0. The adsorption, which was favorable at higher pH but was depressed by the passive layer formed on the ZVI surface at alkaline conditions, characterized the effects of pH on decolorization and TOC removal of MB. The efficiencies of decolorization and TOC removal at pH 6.0 under the anoxic condition were 73.0 and 59.0%, respectively, which were comparable to 79.9 and 55.5% obtained under the oxic condition. In the oxic and anoxic conditions, however, the contributions of removal mechanisms were quite different. Although the adsorption dominated the decolorization and TOC removal under the oxic condition, the contribution of precipitation was largely superior to that of adsorption under the anoxic condition.

Role of Iron Anode Oxidation on Transformation of Chromium by Electrolysis

The potential for chemical reduction of hexavalent chromium Cr(VI) in contaminated water and formation of a stable precipitate by Zero Valent Iron (ZVI) anode electrolysis is evaluated in separated electrodes system. Oxidation of iron electrodes produces ferrous ions causing the development of a reducing environment in the anolyte, chemical reduction of Cr(VI) to Cr(III) and formation of stable iron-chromium precipitates. Cr(VI) transformation rates are dependent on the applied electric current density. Increasing the electric current increases the transformation rates; however, the process is more efficient under lower volumetric current density (for example 1.5 mA L(-1) in this study). The transformation follows a zero order rate that is dependent on the electric current density. Cr(VI) transformation occurs in the anolyte when the electrodes are separated as well as when the electrolytes (anolyte/catholyte) are mixed, as used in electrocoagulation. The study shows that the transformation occurs in the anolyte as a result of ferrous ion formation and the product is a stable Fe(15)Cr(5)(OH)(60) precipitate.

Comment on "Reductive dechlorination of γ-hexachloro-cyclohexane using Fe-Pd bimetallic nanoparticles" by Nagpal et al. [J. Hazard. Mater. 175 (2010) 680-687]

The author used a recent article on lindane (γ-hexachloro-cyclohexane) reductive dechlorination by Fe/Pd bimetallics to point out that many other of published works in several journals do not conform to the state-of-the-art knowledge on the mechanism of aqueous contaminant removal by metallic iron (e.g. in Fe(0)/H(2)O systems). It is the author's view that the contribution of adsorbed Fe(II) to the process of contaminant reduction has been neglected while discussing the entire process of contaminant reduction in the presence of bimetallics.

Phenol removal using zero-valent iron powder in the presence of dissolved oxygen: roles of decomposition by the Fenton reaction and adsorption/precipitation

The mechanism for removal of phenol by zero-valent iron (ZVI) was quantitatively evaluated in the presence of dissolved oxygen by varying the pH from 2 to 8.1 (natural). The measurement of OH radical concentration suggests that the removal of phenol by ZVI was occurred due to the decomposition by the Fenton reaction besides the adsorption/precipitation to the iron surface. From the measurements of dissolved organic carbon (DOC) in the filtrate with the 0.45 μm syringe filter and the solution obtained from acidification of suspended precipitates, the roles of decomposition by the Fenton reaction and adsorption/precipitation were separately evaluated. At solution pH 3, 91% of phenol removal was achieved and 24% of TOC (total organic carbon) decreased. The contribution of the Fenton reaction was found to be 77% of overall TOC reduction. When the pH values were 4 and 5, the overall TOC removal was found to be mainly due to the adsorption/precipitation. At pH 2 and 8.1, the reduction of TOC was very small. The pH and dissolved oxygen significantly affected the dissolution of iron and the production of OH radicals and changed the roles of phenol removal by the Fenton reaction and adsorption/precipitation.

Electrocoagulation treatment of simulated floor-wash containing Reactive Black 5 using iron sacrificial anode

Floor-wash from dye finishing plant is a major source of color and wastewater volume for dyes industries. Batch electrocoagulation (EC) of simulated floor-wash containing Reactive Black 5 (RB5) was studied as a possible pretreatment option. More than 90% of initial 25mg/L of RB5 was removed at current densities of 4.5, 6, and 7.5 mA/cm(2) in the presence of Na(2)SO(4) and NaCl as supporting electrolytes; in less than one hour. Identical k(obs) (pseudo first-order reaction rate constant) values were obtained at initial pH of 3.74 for both electrolytes. However, at initial pH of 6.6, k(obs) values decreased in the presence of Na(2)SO(4) and remained same for NaCl as compared to that at pH 3.74. Highest extent of decolorization and k(obs) values were obtained at initial pH 9.0 for both electrolytes. Under identical conditions, specific energy consumption (SEC) was almost half in the presence of NaCl (~29 kWh/kg RB5) than that of Na(2)SO(4). Vinyl sulfone (VS) was detected as one of the products of EC indicating reduction of azo bonds as a preliminary step of decolorization. Mechanism of decolorization with respect to various experimental conditions was delineated. Generation and accumulation of VS was dependent on initial pH and type of electrolyte. Results of this study revealed that EC in the presence of sodium chloride can be efficiently used as a primary treatment for decolorization of floor-wash containing RB5.

Iron and aluminium based adsorption strategies for removing arsenic from water

Arsenic is a commonly occurring toxic metal in natural systems and is the root cause of many diseases and disorders. Occurrence of arsenic contaminated water is reported from several countries all over the world. A great deal of research over recent decades has been motivated by the requirement to lower the concentration of arsenic in drinking water and the need to develop low cost techniques which can be widely applied for arsenic removal from contaminated water. This review briefly presents iron and aluminium based adsorbents for arsenic removal. Studies carried out on oxidation of arsenic(III) to arsenic(V) employing various oxidising agents to facilitate arsenic removal are briefly mentioned. Effects of competing ions, As:Fe ratios, arsenic(V) vs. arsenic(III) removal using ferrihydrite as the adsorbent have been discussed. Recent efforts made for investigating arsenic adsorption on iron hydroxides/oxyhydroxides/oxides such as granular ferric hydroxide, goethite, akaganeite, magnetite and haematite have been reviewed. The adsorption behaviours of activated alumina, gibbsite, bauxite, activated bauxite, layered double hydroxides are discussed. Point-of-use adsorptive remediation methods indicate that Sono Arsenic filter and Kanchan™ Arsenic filter are in operation at various locations of Bangladesh and Nepal. The relative merits and demerits of such filters have been discussed. Evaluation of kits used for at-site arsenic estimation by various researchers also forms a part of this review.

Predictions of long-term performance of granular iron permeable reactive barriers: field-scale evaluation

Long-term performance is a key consideration for the granular iron permeable reactive barrier (PRB) technology because the economic benefit relies on sustainable operation for substantial periods of time. However, predictions on the long-term performance have been limited mainly because of the lack of reliable modeling tools. This study evaluated the predictive capability of a recently-developed reactive transport model at two field-scale PRBs, both having relatively high concentrations of dissolved carbonate in the native groundwater. The first site, with 8 years of available monitoring data, was a funnel-and-gate installation, with a low groundwater velocity through the gate (about 0.12 m d(-1)). The loss in iron reactivity caused by secondary mineral precipitation was small, maintaining relatively high removal rates for chlorinated organics. The simulated concentrations for most constituents in the groundwater were within the range of the monitoring data. The second site, with monitoring data available for 5 years, was a continuous wall PRB, designed for a groundwater velocity of 0.9 m d(-1). A comparison of measured and simulated aqueous concentrations suggested that the average groundwater velocity through the PRB could be lower than the design value by a factor of two or more. The distribution and amounts of carbonate minerals measured in core samples supported the decreased groundwater velocity used in the simulation. The generally good agreement between the simulated and measured aqueous and solid-phase data suggest that the model could be an effective tool for predicting long-term performance of granular iron PRBs, particularly in groundwater with high concentrations of carbonate.

Effects of dissolved oxygen on dye removal by zero-valent iron

Effects of dissolved oxygen concentrations on dye removal by zero-valent iron (Fe(0)) were investigated. The Vibrio fischeri light inhibition test was employed to evaluate toxicity of decolorized solution. Three dyes, Acid Orange 7 (AO7, monoazo), Reactive Red 120 (RR120, diazo), and Acid Blue 9 (AB9, triphenylmethane), were selected as model dyes. The dye concentration and Fe(0) dose used were 100 mg L(-1) and 30 g L(-1), respectively. Under anoxic condition, the order for dye decolorization was AO7>RR120>AB9. An increase in the dissolved oxygen concentrations enhanced decolorization and chemical oxygen demand (COD) removal of the three dyes. An increase in gas flow rates also improved dye and COD removals by Fe(0). At dissolved oxygen of 6 mg L(-1), more than 99% of each dye was decolorized within 12 min and high COD removals were obtained (97% for AO7, 87% for RR120, and 93% for AB9). The toxicity of decolorized dye solutions was low (I(5)<40%). An increase in DO concentrations obviously reduced the toxicity. When DO above 2 mg L(-1) was applied, low iron ion concentration (13.6 mg L(-1)) was obtained in the decolorized AO7 solution.

Aqueous removal of diclofenac by plated elemental iron: bimetallic systems

The aqueous removal of diclofenac (DF) by micrometric iron particles (Fe(0)) and amended Fe(0) (Me(0)(Fe(0))) under oxic and anoxic conditions was investigated. Bimetallic systems were obtained by plating the surface of Fe with Co, Cu, Ir, Ni, Pd and Sn. Experimental results confirmed the superiority of (Me(0)(Fe(0))) for DF removal except for IrFe (oxic) and SnFe (anoxic). Under anoxic conditions, Pd was by far the most efficient plating element followed by Ir, Ni, Cu, Co and Sn. However, under oxic conditions, Pd and Cu showed almost the same efficiency in removing DF followed by Ni, Co, Sn and Ir. Oxidative and reductive DF transformation products were identified under oxic and anoxic conditions respectively. In some systems (e.g. CoFe and SnFe oxic/anoxic; PdFe oxic; NiFe anoxic), no transformation products could be detected. This was ascribed to the nature of the plating element and its impact on the process of the formation of metal corrosion products (MCPs). MCPs are known for their high potential to strongly adsorb, bond, sequestrate and enmesh both the original contaminant and its reaction products. Obtained results corroborate the universal validity of the view, that aqueous contaminants are basically removed by adsorption and co-precipitation.