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Azo dye degradation by recycled waste zero-valent iron powder
Abstract and Figures
Neste trabalho é apresentado um método eficiente para a degradação de corantes, usando uma fonte de pó de ferro zero ambientalmente amigável (resíduo de um processo industrial). A influência de vários fatores experimentais (tais como: pH, massa de ferro, tamanho de partícula, concentração do substrato, atmosfera inerte ou oxidante) sobre a eficiência do ferro zero em reduzir o grupo cromóforo e o teor de carbono orgânico total de um azocorante (Preto Remazol B) foi avaliada. O processo de degradação do corante apresentou uma cinética de primeira ordem com uma constante de 0,153 min -1 . Nas condições otimizadas (pH=3, (Fe) = 5 g L -1 , tamanho de partícula ≤ 250 μm), este processo promoveu uma descoloração de 95% e redução de 70% na concentração de carbono orgânico total de uma solução 100 mg L -1 do corante. Este processo também foi empregado na degradação de um efluente industrial têxtil apresentando bons resultados e demonstrando capacidade para ser uma alternativa para a remediação de sistemas aquáticos poluídos. In this paper an efficient method for azo dye degradation using an environmentally friendly zero-valent iron powder source is presented (iron particles discarded from a manufacturing process). The influence of several experimental parameters (such as pH, iron mass, particle size, substrate concentration, oxidizing and inert atmospheres) on the ability of zero-valent iron to reduce the chromophoric groups and total organic carbon content of the azo dye Remazol Black B was evaluated. Kinetic studies revealed that the azo degradation by Fe 0 , iron particle size ≤ 250 μm), the iron-based process produced net a reduction in color and total organic carbon of about 95% and 70%, respectively. The process was also evaluated for the degradation of textile effluent. The studied process showed good characteristics, which can make it an effective alternative for polluted aquatic system remediation.
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... Water is utilized extensively in the textile Fig. 6 Effect of pH on A p-CBA degradation of MWCNTs catalyzed ozonation B rate of ozone decomposition of MWCNTs catalyzed ozonation C rate of ozone decomposition of MWCNTs catalyzed Ozonation industry during dyeing and finishing operations, accounting for over 80% of total effluent (Venkatesh Prabhu and Karthikeyan 2015; Lin and Chen 1997). Water from the dyeing and finishing process has a high concentration of organics, dyestuff and auxiliary textile components, making it potentially hazardous and poorly biodegradable, posing a threat to aquatic life (Arslan-Alaton and Alaton 2007; Pereira and Freire 2006;Sharma et al. 2007). Azo dyes and vat dyes are widely employed in the textile industry; however after dying, approximately 20-50% of these hydrolyzed colors lose their binding capacity with materials and remain in the dye bath, producing colored effluent (He et al. 2013). ...
In the present work, evaluation of two catalysts, activated carbon (AC) synthesized from agricultural waste and multiwalled carbon nanotubes (MWCNT) synthesized from plastic waste, was done for the ozonation of real textile wastewater using para-chloro benzoic acid (p-CBA) as a probe compound. The effect of pH and catalyst dose were studied in terms of •OH exposure, Rct, rate of p-CBA degradation and ozone degradation. The rate constant for the reaction of organic matter with hydroxyl radicals was determined using competition kinetics. The threshold ozone dose for real textile wastewater was found to be 0.51 gm/gm of TOC. With an increase in specific ozone dose, the rate of p-CBA degradation was found to be increasing and has shown a positive effect on •OH exposure and Rct. The increasing pH had shown a positive effect on the rate of degradation and decomposition of p-CBA and ozone, respectively, in the case of AC-catalyzed ozonation. A similar trend was observed in the case of MWCNTs catalyzed ozonation. A positive effect of pH was observed on •OH exposure and Rct, in AC as well as MWCNTs catalyzed ozonation. The effect of catalyst loading has shown significant enhancement in p-CBA degradation, ozone decomposition, •OH exposure and Rct in both AC as well as MWCNTs catalyzed ozonation. However, MWCNTs have proved better than AC as a catalyst for ozonation in studied experimental parameters.
Graphical abstract
Schematic presentation of Ozonation of Textile wastewater
... Iron may also reduce azo dyes; thermodynamically, this is advantageous when zero valent iron reduces the chromophoric group (À N=Nbond). [47] Ponder et al. [48] studied the characteristics of micro and nanosized ZVI such as specific surface area, reactivity, and corrosion behavior of ZVI. Lin and Weng [49] carried out the effect of using nanosized/microsized ZVI on the reductive degradation of Acid Black 24 dye. ...
Azo dyes are known to cause environmental pollution and freshwater contamination, posing carcinogenic risks to human health. Consequently, researchers have directed their attention towards studying effective methods for removing these dyes from textile effluent. Amorphous alloys have emerged as a promising candidate for enhancing degradation efficiency on azo dyes, with various metallic alloys undergoing extensive investigation. Notably, the novel catalytic metallic materials exhibit significantly higher degradation performance compared to traditional options. Among the promising candidates are nanometallic alloys based on Fe, Mg, Co, Al, and Mn, which offer improved potential for cleaning diverse types of azo dyes from wastewater. The purpose of this review is to provide an update of the research regarding the use of amorphous alloys in azo dye degradation. The different production methods of amorphous alloys were discussed. The amorphous alloys used in dye degradation studies were subcategorized into three main types: Fe‐based, Mg‐based and some miscellaneous‐based amorphous alloys. Additionally, the study delves into the impact of crucial parameters, such as solution pH and initial dye concentration, providing valuable insights for the efficient treatment of wastewater.
... A new method for the transformation of Azo dyes into easier bio-decomposition compounds with Nanoscale Zero-Valent Iron NZVI has been developed in this study. NZVI is an effective reducing agent for azo dyes [Nam and Tratnyek, 2000;Cao et al., 1999; and it is less cost than chemical methods, effective and environmentally friendly Masciangioli and Zhang, 2003;Chang et al., 2006;Hou et al., 2007;Shu et al., 2007;Fan et al., 2009], NZVI reduction is widely used in treating and remedying and dechlorinate waste waters contaminated with chlorinated compounds [Cheng et al., 2006], nitro aromatic compounds , nitrates [Huang et al, 1998] and heavy metals [Fiedor et al, 1998], and even for the deoxidization of more complex anthropogenic chemicals including pesticides [Fiedor et al, 1998] and dyes [Eykholt and Davenport, 1998;Bigg and Judd, 2001;Weber, 1996;Pereira and Freire, 2006]. The azo bond is cleaved when azo dyes are degraded by NZVI treatment process and an aromatic amines and amino-naphthol compounds are formed along with hydrazo (-NHNH-) as an intermediate . ...
In this study Nanoscale Zero-Valent Iron Fe 0 (NZVI) and Nano Zero Valent-Iron supported on pillared clay(NZVI/PILC) have been prepared and characterizations by physical method such as Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The degradation of acidic aqueous solutions of the Acid red 315 (AR 315) azo dye has been studied by NZVI, pillared clay (PILC) and NZVI-B. The effect of different process parameters, such as solution pH, amount of dosage (NZVI, PILC and NZVI/PILC), time reaction effect and other experimental variable, such as (Azo dye concentration and inorganic salts effect) has been investigated to determined optimization method for removal. The concentration of azo dye measured before and after treatment by using UV-Vis Spectrophotometry method. The experimental results showed that AR 315 azo dye solution (100 mg/L, 1.6×10-4 M) was completely removed by NZVI at optimum conditions (amount of NZVI = 1.0 g, 120 min and pH = 3). While the removal efficiency with NZVI/PILC and PILC were 80% and 0% respectively.
... The results in Fig. 10 indicate that first-order and second-order kinetic models are appropriate to describe the degradation phenomenon. These results suggest that the chromophore degradation step followed the same mechanism in these different media, probably involving -NHgroup reduction [48]. The second-order model has correlation coefficients close to unity and involves a fast kinetics right from the first minutes. ...
A ferromagnetic activated carbon, CA-HP@FeII was used as a heterogeneous catalyst in the Fenton
oxidation process for the removal of Bezaktiv Brilliant Blue dye from contaminated water. It was prepared by optimizing the activation of rubber seed hull with phosphoric acid, modified with iron(II)
sulfate. The ferromagnetic activated carbon was characterized by Fourier-transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy/energy-dispersive X-ray spectroscopy,
Brunauer–Emmett–Teller and pore-size distribution. The characterized activated carbon was used to
study the degradation of the Bezaktiv Brilliant Blue dye by varying several parameters, namely the
pH of the solution, the concentration of H2O2 solution, the mass of the catalyst, and the concentration of the pollutant. The results of the characterization showed that ferromagnetite was deposited
on the activated carbon during the modification process and the obtained ferromagnetic activated
carbon have a cubic spinel structure and a specific area of 110.53 m2·g–1. The elimination of the dye
was favorable at the acid pH of 3. The removal of the pollutant was favorable when the concentration of H
2O2, the mass of the catalyst and the concentration of the pollutant were increased. The final
elimination percentage was greater than 75%, whatever the condition and the parameter varied. The
second-order kinetic model better described the degradation of Bezaktiv Brilliant Blue pollutant on
CA-HP@FeII. An analysis of the ferromagnetic material after degradation showed that it had undergone very little changes, and hence could be reused several times. The change in the percentage of
carbon from 46.54% to 54.86% in the ferromagnetic activated after degradation can be attributed
to the mineralization of the carbon atoms of Bezaktiv Brilliant Blue.
... Cases of rising innovations incorporate, but are not limited to: 1. Advanced oxidation processes (AOPs) ; 2. Zero-valent iron degradation processes. (Pereira & Freire, 2006); 3. Better Physico-chemical treatment methods (including precipitation, coagulation, adsorption, flocculation, flotation, electrochemical destruction, mineralization, and decolorization process) (Gogate and Pandit, 2004); 4. Fungal Degradation. (Bumpus, 2004 ...
COVID-19 has disrupted all aspects of human life. To mitigate the impact of the pandemic, several efforts have been taken, including by Indonesian scholars abroad. This book entitled Indonesia Post-Pandemic Outlook: Environment and Technology Role for Indonesia Development explores environment and technology issues and topics related to the COVID-19 pandemic and discusses post-pandemic recovery efforts in Indonesia. Comprising of 19 chapters, this book is divided into four sections. The first section, disaster and greening management development, discusses insights for a better solution in disaster prevention and development of greening management. Second, waste and pollution management development, explores options in development of waste and pollution management such as potentials uses of membrane technology, remediation of textile dyes, biochar industry, and also discussion on persistent organic pollution and microplastics. Third, food defense and security development, explores the potentials of food security and management in utilizing the potential of coastal sand marginal land resources, IoT and smart packaging, and smart food supply chain. The last part, human resource and public service development, discuses developments on management of education system, public transportation, e-government, and health information system. We hope that this book can be a valuable reference for stakeholders, policymakers, as well as society to recover from the pandemic crisis and find better solutions to benefit future generations.
... For example, it was explored that nearly 50% of dyes are lost during textile dyeing and Responsible Editor: Guilherme L. Dotto Avik Kumar Dhar and Humayra Akhter Himu are the authors with equal contributions. 10% to 15% of effluent is wasted (Pereira and Freire 2006). Because of their resistance to degradation, dye molecules prevail in significant quantities in wastewater. ...
In recent decades, increased industrial, agricultural, and domestic activities have resulted in the release of various pollutants into the aquatic systems, which require a reliable and environmentally friendly method to remove them. Adsorption is one of the most cost-effective and sustainable wastewater treatment techniques. A plethora of low-cost bio-based adsorbents have been developed worldwide so far to supplant activated carbon and its high processing costs. Bentonite clays (BCs), whether in natural or modified form, have gained enormous potential in wastewater treatment and have been used successfully as a novel and cost-effective bio-sorbent for removing organic and inorganic pollutants from the liquid suspension. It has become a sustainable solution for wastewater treatment due to its variety of surface and structural properties, superior chemical stability, high capacity for cation exchange, elevated surface area due to its layered structure, non-toxicity, abundance, low cost, and high adsorption capacity compared to other clays. This review encompasses comprehensive literature about various modification techniques and adsorption mechanisms of BCs concerning dyes and heavy metal removal from wastewater. A critical overview of different parameters for optimizing adsorption capacity and regeneration via the desorption technique has also been presented here. Finally, a conclusion has been drawn with some future research recommendations based on technological challenges encountered in industrializing these materials.
Graphical Abstract
... Compared to another Fe 2+ source that may bring secondary contamination such as FeSO 4 , zero-valent iron (ZVI, or Fe 0 ) is a suitable reagent for Fenton reactions (B. Chen et al., 2011;Fu et al., 2014;Guan et al., 2015;Pereira & Freire, 2006). In ZVI-based Fenton system, the yield of Fe 2+ is critical for the reaction process (Feitz et al., 2005), and it can be further generated by the corrosive oxidation of ZVI to J o u r n a l P r e -p r o o f 4 Fe 2+ accompanied by two electrons transfers to O 2 . ...
Magnetic ion exchange resin (MIEX) pretreatment has been widely used to enhance the performance of waterworks, but the high-salinity organic desorbent produced during the resin regeneration is difficult to be safely disposed of. The present study aimed to investigate the removal of DOM and retention of salts from MIEX desorbent via zero-valent iron (ZVI)-assisted Fenton reaction. ZVI/H2O2 mediated Fenton reaction was further validated by various analytical approaches such as 3D-EEM, Molecular weight distribution, Zeta potential, size, dissolution of iron ions, etc. In batch experiments, 73.26% reduction of DOC and 97.66% of UV254 were achieved in 40 min at optimal H2O2 concentration (20 mM), 2 g/L dose of ZVI, and pH of 3, respectively. The removal of organics was ascribed to the direct oxidation of ZVI/H2O2 and flocculation of the iron released during the oxidation. It should be noted that the high concentration of Cl⁻ in system favored the organic removal and its enhanced performance lied in the production and oxidation of HOCl/OCl⁻. The treated liquid can be reused to regenerate the saturated resin with only a small amount of salt supplementation to obtain a similar recovery performance to that of a freshly configured 10% NaCl solution. Moreover, the retrieval and utilization of ZVI particles and other properties of MIEX desorbent after ZVI/H2O2 treatment were all promising and up to the mark: after four reuses, the mineralization of organic matter differs by 8.81% from the first time, which could be compensated by supplying new ZVI. This research will facilitate the recycling of resin desorbate and expand the market for MIEX applications.
... Zero-valent iron, which is short as ZVI, Fe(0), or Fe 0 , has been widely studied in environmental science as a good reducing reagent for the elimination or alleviation of heavy metal [1,2] and organic pollutants [3][4][5] owing to its good reactivity, low cost and pollution. Despite its intrinsic strong reducibility, the defection of solely ZVI, such as the formation of passivation surface or particle agglomeration, has limited its application in the contamination removal process. ...
Although it has been widely accepted that the sulfidation of zero-valent iron (ZVI) represents a typical diffusionless process, which mainly happens on the surface of the ZVI particle and results in a FeSx@Fe core-shell structure, such an assumption has never been experimentally proved due to the difficulties in probing the interior domains of the large particles, especially those in micro size. In this work, we demonstrate that the sulfidation process not only takes place on the surface but also synchronizes throughout the entire body of the micron-sized ZVI, which was confirmed by the close inspection of the cross-section of a finely sliced ZVI particle. Coupled with the X-ray powder diffraction and X-ray absorption near-edge structure analysis, it is revealed that the whole S-ZVI particle consists of a mixed lattice of the dominant metallic iron (87.5%) and minor iron sulfides (12.5%), according to the established averaged oxidation state for iron. This work is the first to unambiguously reveal the compositional information of the micron-sized S-ZVI particles, especially the inner components that are typically difficult to reach. It clarified a long-time misunderstanding that the sulfidation process on ZVI is initiated from the surface and continuously undergoes a diffusion pathway, reaching to the center of the particle.
We report on the synthesis of zero valent iron nano particles (nZVI) via chemical reduction method. The large peak visible in the XRD pattern reveals the presence of an amorphous phase of iron. SEM and TEM images signify the dendritic morphology and core-shell-like structure of manufactured nZVI particles respectively. Methylene blue dye (MB) was used as model contaminant to assess the reductive degradation proficiency of nZVI. With complete elimination of MB, the fresh synthesized nZVI exhibited the best performance (97%), while the regenerated nZVI had an 85.1% MB removal efficiency after five regenerations. The equilibrium data of adsorption were fitted to eight different kinetic and isothermal models. The effects of critical operating factors such as pH, varied amounts of nZVI and dye concentration, adsorption temperature, and adsorption time were also studied. A presumptive reaction mechanism and function of core-shell construction in contaminant sequestration has also been investigated.
Fe⁰ (ZVI) is one of the most effective reactants for treatment of water contaminated with chlorinated hydrocarbons, pollutants recalcitrant to degradation. Unfortunately, ZVI use is limited by corrosion and loss of reactivity over time. In order to suppress oxidation and passivation phenomena of iron, in this study ZVI was coupled to a suitable semiconductor (ZnS). Under UV light, electrons in the valence band of ZnS are promoted in the conducting band (ECB = − 1.04 eV vs NHE) and are able to reduce Fe²⁺ to Fe⁰ (E0 = − 0.44 eV vs NHE). The ZnS_ZVI composite was simply prepared by coating commercial ZnS with Fe⁰ obtained by reducing Fe (II) salt with sodium borohydride. The achieved ZnS_ZVI catalytic system was characterized by FESEM, WAXD, Raman and N2 adsorption at −196 °C, with the aim to analyze morphology and crystalline structure of both components of the ZnS_ZVI system. Catalytic activity tests showed that ZnS_ZVI is effective in degrading probe molecules, eriochrome black T dye (EBT) and chlorobenzene (PhCl), from aqueous solutions under UV light irradiation. The experimental results show that in both cases the reductive action of Fe⁰ on the N = N bond of EBT and C-Cl of PhCl is synergistically coupled to the oxidative action of ZnS towards by-products. OH radicals, generated by the reaction between the electron holes in the valence band of ZnS and H2O (OH⁻), were indeed confirmed by photoluminescence analysis. Recyclability tests indicate that ZnS_ZVI is highly stable since its catalytic activity, on both probe molecules, remains unchanged even after several cycles of reuse under UV irradiation. ZnS_ZVI catalytic behavior towards EBT has been carefully analyzed and a possible mechanism of EBT degradation has been proposed, which involves the formation of a Fe²⁺-EBT complex whose structure has also been suggested.
The results into investigations of the decomposition of an anthraquinone dye (polan blue E2R) in an aqueous solution induced by ozone, hydrogen peroxide and UV radiation are discussed in the paper. The effect of the ozone dose and concentration, as well as the temperature on decolouration at different initial dye concentrations, were investigated. Low ozone concentrations enabled its better utilisation in the reaction with the dye, although the process rate was slow. An increase of gas flow rate and ozone concentration at the reactor inlet caused a decrease of ozone consumption from 80% to 49%. The hydrogen peroxide concentration from 0.05 to 0.1 mol/dm3 allowed us to obtain a 45% decolouration degree after 48 hours, with the process proceeding most quickly in the first 2 hours. It was found that the yield of anthraquinone dye photolysis in the aqueous solution was affected by both the power of the UV lamp and the character of the light which it emitted. It appeared most advantageous to use mono-energetic radiation in the UV range below 310 nm.
Anthropogenic pollution of groundwater and surface water has become a very serious environmental problem around the world. A wide range of toxic pollutants is recalcitrant to the conventional treatment methods, thus there is much interest in the development of more efficient remediation processes. Degradation of organic pollutants by zero-valent iron is one of the most promising approaches for water treatment, mainly because it is of low cost, easy to obtain and effective. After a general introduction to water pollution and current treatments, this work highlights the advances, applications and future trends of water remediation by zero-valent iron. Special attention is given to degradation of organochloride and nitroaromatic compounds, which are commonly found in textile and paper mill effluents.
Discoloration of aqueous reactive dyes C.I. reactive red 2, C.I. reactive blue 4, and C.I. reactive black 8 is enhanced in the UV/Fe0 system. We examined the effects of pH, dye concentration, and iron dosage on the discoloration of dye solutions in the UV/Fe0 system.
The degradation of Acid Red 88 azo dye in water was investigated in laboratory-scale experiments using 21 oxidation processes. Colour, COD and TOC removals were evaluated for each oxidation system. The processes were studied in three groups: processes deriving from the heterogeneous photocatalysis (using TiO2), oxidation systems based on Fenton-type reactions (homogeneous photocatalysis) and processes based on the application of ozone (ozone catalysis). The results obtained showed that the decolourization rate was quite different for each oxidation process. After 15min reaction time the relative decolourization order established was: H2O2=vis
The degradation of Acid Red 88 azo dye in water was investigated in laboratory-scale experiments using 21 oxidation processes. Colour, COD and TOC removals were evaluated for each oxidation system. The processes were studied in three groups: processes deriving from the heterogeneous photocatalysis (using TiO2), oxidation systems based on Fenton-type reactions (homogeneous photocatalysis) and processes based on the application of ozone (ozone catalysis). The results obtained showed that the decolourization rate was quite different for each oxidation process. After 15 min reaction time the relative decolourization order established was: H2O2 = vis < H2O2/Fe3+/vis < TiO2/H2O2/vis = TiO2/vis < UV < UV/S2O82− < H2O2/Fe3+/C2O42−/vis < H2O2/UV < H2O2/Fe3+/UV < TiO2/S2O82−/UV < TiO2/UV = TiO2/H2O2/UV < O3 < O3/H2O2 < O3/H2O2/UV < O3/UV = O3/vis < O3/H2O2/UV/Fe3+ < O3/TiO2/vis < O3/TiO2. Nevertheless, taking into account the mineralization of the compound (measured as TOC and COD removal) at the end of the experiment, the order was slightly different. For example, the photo-Fenton-ozone process (O3/H2O2/UV/Fe3+) seems to be more appropriate than O3/TiO2. Also, in a kinetic study, the pseudo-first-order kinetic rate was determined for each oxidation system. This overall constant was split up into two components: direct oxidation by irradiation (photolysis) and oxidation by free radicals (mainly HO).
Redn. of azo dyes by zero-valent iron metal (Fe0) at pH 7.0 in 10 mM HEPES buffer was studied in aq., anaerobic batch systems. Orange II was reduced by cleavage of the azo linkage, as evidenced by prodn. of sulfanilic acid (a substituted aniline). Dye adsorption on Fe particles was <4% of the initial concn., and >90% mass balance was achieved by summing aq. dye and product amine concns. All of 9 azo dyes tested were reduced with first-order kinetics. Decolorization kinetics at the λmax of each dye was rapid: a typical kobs was 0.35 ± 0.01/min for Orange II at 130 rpm on an orbital shaker, corresponding to a surface area normalized rate const. (kSA) of 0.21 ± 0.01 L/m2-min. The rate of Crocein Orange G redn. varied with initial dye concn. in a way that suggested satn. of surface sites on the Fe0, and varied with the square-root of mixing rate (rpm) in a manner indicative of mass transfer-limited kinetics. Correlation anal. using kobs for all azo dyes, ests. of their diffusion coeffs., and calcd
The degradation of a reactive dye by combined sonolysis (520 kHz) and ozonation was studied using C.I. Reactive Black 5 as a model dye. It was found that the joint action of ultrasound and ozone induced a synergistic effect on both the decolorization of the dye and the overall degradation process. Due to the inefficiency of ultrasonic irradiation by itself to render any significant degradation under the conditions employed, the synergy was attributed mainly to mechanical effects of ultrasound to enhance the mass transfer of ozone in solution. The radical chain reactions taking place during thermolysis of water and ozone in collapsing cavities may also have contributed chemically to the synergy by providing additional decomposition pathways for ozone, and an excess of electron-deficient chemicals in solution.
Granular iron has been determined to be a potentially useful reductant for the removal of common organic contaminants from groundwater. This research is aimed at improving our understanding of the processes that control the reactivity and longevity of the iron particles when they are used for groundwater treatment. A suite of nitroaromatic compounds (NACs) including 4-chloronitrobenzene (4ClNB), 4-acetylnitrobenzene (4AcNB), nitrobenzene, 2-methylnitrobenzene (2MeNB), and 2,4,6-trinitrotoluene (TNT) was used to investigate granular iron reactivity in anoxic pH 10, 0.008 M KNO3 solution. Master Builder's brand of granular iron with a surface area of about 1 m2/g was used in all experiments. The NACs were reduced rapidly to anilines that were found to sorb reasonably strongly to the solid particles and to interfere with the reduction of NACs. The granular iron was found to lose reactivity quite rapidly over the first few days of exposure and then more slowly over the next several months. Reactivity loss due to reversibly sorbed products was minimized by flushing the system with background electrolyte between experiments. Competition experiments with binary mixtures of 4ClNB and each one of the other NACs were performed to investigate relative affinities of these compounds for the solid surface. Despite the overall loss in reactivity observed for the granular iron, the relative rate constants in the competition experiments appeared to remain constant in time.
To address some of the fundamental questions regarding the kinetics of reduction of contaminants by zero-valent iron (Fe0), we have taken advantage of the mass transport control afforded by a polished Fe0 rotating disk electrode (RDE) in an electrochemical cell. The kinetics of carbon tetrachloride (CCl4) dechlorination at an Fe0 RDE were studied in pH 8.4 borate buffer at a potential at which an oxide film would not form. In this system, the cathodic current was essentially independent of electrode rotation rate, and the measured first-order heterogeneous rate constant for the chemical reaction (kct = 2.3 × 10-5 cm s-1) was less than the estimated rate constant for mass transfer to the surface. Thus, for the conditions of this study, the rate of reduction of CCl4 by oxide-free Fe0 appears to be dominated by reaction at the metal−water interface rather than by transport to the metal surface. Activation energies for reduction of CCl4 and hexachloroethane by oxide-covered granular Fe0 (measured in batch systems) also indicate that overall rates are limited by reaction kinetics. Since mass transport rates vary little among the chlorinated solvents, it is likely that variation in kct is primarily responsible for the wide range of dechlorination rates that have been reported for batch and column conditions.
