ZVI Surface Research Articles

Capability and Mechanism of Cr(VI) Removal by Phthalic Acid Modified Sulfidated Microscale Zero Valent Iron

Mechanochemically sulfidated microscale zero valent iron (S-mZVIbm) exhibits promising Cr(VI) removal performance but is prone to be passivated by Cr(VI), how to mitigate the passivation is still a challenge. In this study, we successfully synthesized phthalic acid (PA, carboxyl-rich organic acid) modified S-mZVIbm particles (PA-S-mZVIbm (4:1)) through ball milling. The pre-corrosion of the ZVI surface by PA effectively increased the specific surface area of ZVI. Additionally, the carboxyl groups complexed with Cr(VI), thereby enhancing its Cr(VI) removal capacity and alleviating the passivation. The Cr(VI) removal by PA-S-mZVIbm (4:1) was mainly a chemisorption process on the surface and its Cr(VI) removal capacity (55.47mg/g) was 1.26 and 9.94 – 14.83 times that of PA-mZVIbm and S-mZVIbm, respectively. The electron efficiency of Cr(VI) removal by both PA-S-mZVIbm (4:1) and S-mZVIbm was ~100%, however, the Fe(0) utilization efficiency of PA-S-mZVIbm (4:1) was at least 15 times higher than that of S-mZVIbm, explaining the superior performance of PA-S-mZVIbm (4:1). This study confirmed that PA modification could effectively mitigate the passivation and improve the Fe(0) utilization efficiency of S-mZVIbm.

Boosting the Ni(II)-EDTA decomplexation and Ni(II) immobilization in a heterogeneous Fenton-like system with L-cysteine functionalized zero-valent iron

The remarkable stability and detrimental environmental impacts of nickel(II)-ethylenediaminetetraacetic acid (Ni(II)-EDTA) complexes originated from electroplating effluents have sparked a growing curiosity about their efficient remediation. In this study, L-cysteine functionalized zero-valent iron (C-ZVIbm) was synthesized via a ball-milling method and employed as a heterogeneous Fenton-like catalyst to decomplex Ni(II)-EDTA and immobilize Ni(II) from wastewater. The results demonstrated that the pseudo-second-order kinetic rate constant (2.096 L·mol−1·min−1) for EDTA degradation in the C-ZVIbm/H2O2 system was 123.3 times that in the ZVIbm/H2O2 system (0.017 L·mol−1·min−1). The incorporation of L-cysteine onto the surface of ZVI significantly enhanced the H2O2 activation, reactive oxygen species (OH, O2−, and 1O2) generation, and the sequential Ni(II)-EDTA decomplexation and EDTA degradation. The results of capture experiments revealed that 1O2 played a crucial role in the degradation of EDTA in the C-ZVIbm/H2O2 system. The optimization of the experimental variables indicated that the degradation of EDTA and the reduction/adsorption of Ni(II) were antagonistic in the C-ZVIbm/H2O2 system, resulting in an efficient reduction/immobilization of Ni(II) and an inhibition of EDTA degradation at a higher dosage of C-ZVIbm. Our findings provide new insights into the promising heterogeneous Fenton-like process that enables the decomplexation of Ni(II)-EDTA, the degradation of EDTA, and the synchronous immobilization of Ni(II).

Reductive Dechlorination of Chlorinated Ethenes at the Sulfidated Zero-Valent Iron Surface: A Mechanistic DFT Study.

Sulfidated nano- and microscale zero-valent iron (S-(n)ZVI) has shown enhanced selectivity and reactive lifetime in the degradation of chlorinated ethenes (CEs) compared to pristine (n)ZVI. However, varying effects of sulfidation on the dechlorination rates of structurally similar CEs have been reported, with the underlying mechanisms remaining poorly understood. In this study, we investigated the β-dichloroelimination reactions of tetrachloroethene (PCE), trichloroethene (TCE), cis-1,2-dichloroethene (cis-DCE), and trans-1,2-dichloroethene (trans-DCE) at the S and Fe sites of several S-(n)ZVI surface models by using density functional theory. Dechlorination reactions were both kinetically and thermodynamically more favorable at Fe sites compared to S sites, indicating that maintaining the accessibility of reactive Fe sites is crucial for achieving high S-(n)ZVI reactivity with contaminants. At Fe sites adjacent to S atoms, the reactivity for CE dechlorination followed the order trans-DCE ≈ TCE > cis-DCE > PCE. PCE degradation was hindered at these sites due to the steric effects of S atoms. At the S sites, the energy barriers correlated with the CEs' energy of the lowest unoccupied molecular orbital in the order PCE < TCE < DCE isomers. Our findings reveal that the experimentally observed selectivity of S-(n)ZVI materials for individual CEs can be explained by an interplay of the varying reactivities of Fe and S sites in CE dechlorination reactions.

Decoding the divalent cation effect on sulfidation of zero-valent iron: Phase evolution and FeSx assembly

The decontamination ability of sulfidated zero-valent iron (S-ZVI) can be enhanced by the effective assembly of iron sulfides (FeSx) on neglected heterogeneous surfaces by liquid-phase precipitation. However, S-ZVI preparation with the usual pickling is detrimental to orderly interfacial assembly and leads to an imbalance between electron transfer optimization and electron storage. In this work, S-ZVI was prepared in solutions containing trace divalent cation, and it removed Cr(VI) up to 323.25 times higher than ZVI. This result is achieved by surface sites protonation of divalent cations regulating the phase evolution on the ZVI surface and inducing FeSx chemical assembly. Regulation of divalent cation and S(-II) content further promotes FeSx targeted assembly and reduces electron storage consumption as much as possible. The barrier for FeSx assembly is found to lie at the ZVI interface rather than in the deposition between FeSx. Chemical assembly at heterogeneous interfaces is a prerequisite for the ordered assembly of FeSx. In addition, S-ZVI prepared in simulated groundwater showed extensive preparation pH and universality for remediation scenarios. These findings provide new insights into the development of in-situ sulfidation mechanisms with particular implications for S-ZVI applied to soil and groundwater remediation by the regulation of heterogeneous interfacial assembly.

Synergistic effect and mechanism of zero-valent iron activated persulfate on Cu(II)–EDTA decomplexation enhanced with weak magnetic field

Herein, a weak magnetic field (WMF) was applied to improve the catalytic degradation of the copper-ethylene diamine tetraacetic acid complex (Cu(II)–EDTA) and recovery of Cu using zero-valent iron/peroxomonsulfate (ZVI/PS). The removal rates of Cu(II)–EDTA in the WMF/ZVI/PS system were 7.92 and 1.33 times those in the ZVI and ZVI/PS systems, respectively. The recovery rates of Cu in the WMF/ZVI/PS system were 4.78 and 1.25 times more than those in ZVI and ZVI/PS systems, respectively. Scavenger quenching experiments and electron spin resonance (ESR) results showed that more active radicals (SO4•− and •OH) were produced and both SO4•− and •OH played the important role in Cu(II)–EDTA degradation in the WMF/ZVI/PS system. The ZVI and PS dosages and initial solution pH considerably affected the removal efficiency of Cu(II)–EDTA. X-ray energy-dispersive spectroscopy (EDS) results showed that WMFs could accelerate the enrichment of paramagnetic Cu2+ on the ZVI surface. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results showed that Cu2+ ions were first adsorbed on the corroded ZVI and then reduced to Cu0 with time. A possible Cu(II)–EDTA decomposition and Cu reduction mechanism was also proposed. This study provides a promising and ecofriendly technique for treating industrial wastewater containing organic complexed metals.

Assessment of Fenton systems based on metabisulphite as a low-cost alternative to hydrogen peroxide

Here we show that a Fenton-like system based on zero-valent iron and metabisulphite (ZVI-MBS) is able to effectively degrade both ibuprofen (a non-steroidal anti-inflammatory drug) and its toxic product 4-isobutylacetophenone in 30 min at pH 3. Based on scavenging experiments, the main reactive species involved in the degradation process would be •OH and SO4•−, with SO4•− likely acting as •OH source. Compared to well-known ZVI-H2O2, ZVI-MBS loses performance considerably at pH > 3. Furthermore, differently from ZVI-H2O2 where the ZVI surface plays major role, dissolved Fe species are important in ZVI-MBS. In presence of MBS, it would thus be more convenient to add Fe2+ directly in the form of ferrous salts than to use ZVI as source of dissolved iron. Fe2+-MBS, similarly to ZVI-MBS, experiences considerable drop in performance above pH 3. MBS is much cheaper, more stable, and safer to operate than H2O2. However, the need to treat water at pH 3 has the important drawback that conductivity/salinity is increased by water acidification for treatment and neutralisation after treatment. That would be appropriate in most cases for water discharge, but could limit reuse of treated water in some fields such as irrigation.

Enhanced arsenic(III) sequestration via sulfidated zero-valent iron in aerobic conditions: Adsorption and oxidation coupling processes

Sulfidated zero-valent iron (S-ZVI) has shown significant potential for the removal of arsenic(III). However, little attention has been paid to the mechanism of As(III) sequestration enhancement and how the phase transformation for S-ZVI strengthens this process in aerobic conditions. In this work, sulfidated ZVI was created by ball-milling (S-ZVIbm) and liquid-mixing (S-ZVIlm) of ZVI with elemental sulfur(S0) to investigate the performance and mechanisms of As(III) sequestration in air-saturated water. Sulfidation was found to significantly enhance the As(III) removal rate constant, which was 2.8 ∼ 6.7 times (S-ZVIbm) and 3.1 ∼ 17.1 times (S-ZVIlm) higher than that without sulfidation. FeS was identified as the predominant sulfur species in the S-ZVI samples using S K-edge XANES spectra. The enhanced electron transfer and ZVI corrosion after sulfidation were verified via electrochemical tests. XANES and Mössbauer spectra suggested that lepidocrocite(γ-FeOOH) was the predominant corrosion product generated on the ZVI surface with the presence of oxygen, and DFT calculations further confirmed the improved performance of γ-FeOOH for As(III) sequestration. Besides, As(III) oxidation occurred dominantly on the heterogeneous surface rather than in solution, and the As(III) sequestration pathway of adsorption followed by oxidation was proposed. This study provides new insight into the enhanced As(III) sequestration by S-ZVI in aerobic conditions.

Efficient Fe(III)/Fe(II) cycling mediated by L-cysteine functionalized zero-valent iron for enhancing Cr(VI) removal

Herein, L-cysteine (Cys) was modified on zero-valent iron (C-ZVIbm) by using a mechanical ball-milling method to improve the surface functionality and the Cr(VI) removal efficiency. Characterization results indicated that Cys was modified on the surface of ZVI by the specific adsorption of Cys on the oxide shell to form a -COO-Fe complex. The Cr(VI) removal efficiency of C-ZVIbm (99.6%) was much higher than that of ZVIbm (7.3%) in 30 min. The attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) analysis inferred that Cr(VI) was more likely to be adsorbed on the surface of C-ZVIbm to form bidentate binuclear inner-sphere complexes. The adsorption process was well-matched to the Freundlich isotherm and the pseudo-second-order kinetic model. Electrochemical analysis and electron paramagnetic resonance (ESR) spectroscopy revealed that Cys on the C-ZVIbm lowered the redox potential of Fe(III)/Fe(II), and favored the surface Fe(III)/Fe(II) cycling mediated by the electrons from Fe0 core. These electron transfer processes were beneficial to the surface reduction of Cr(VI) to Cr(III). Our findings provide new understandings into the surface modification of ZVI with a low-molecular weight amino acid to promote in-situ Fe(III)/Fe(II) cycling, and have great potential for the construction of efficient systems for Cr(VI) removal.

Precise decomplexation of Cr(III)-EDTA and in-situ Cr(III) removal with oxalated zero-valent iron

Cr(III)-organic complexes removal from wastewater is challenging to decompose the complexes without producing toxic Cr(VI). Herein we demonstrated that oxalated zero-valent iron (Ox-ZVI) could one-step decomplex 95.4% of Cr(III)-EDTA with 94.2% removal of total Cr and 60.5% removal of TOC without generating Cr(VI) at pH 6.0. Ox-ZVI with FeC2O4 shell preferred to adsorb and activate O2 to produce more reactive species than ZVI with iron oxide shell, while the generated 1O2 could selectively break the Cr-O and Cr-N bond to precisely decomplex Cr(III)-EDTA without producing Cr(VI). The released Cr(III) was in-situ precipitated on the surface of ZVI in Cr(OH)3 form. Interestingly, •OH preferred the effective degradation of the released EDTA ligand to the oxidation of the chelated Cr(III) at pH 6.0. This study offers a green and facile method for the Cr(III)-organic complexes treatment, and also sheds light on the importance of reactive species adjustment for the precise pollutant control.

Enhanced dechlorination of trichloroethene by sulfidated microscale zero-valent iron under low-frequency AC electromagnetic field

In this study an electromagnetic heating strategy is proposed for remediation of trichloroethene (TCE) by ball milled, sulfidated microscale zero valent iron (S-mZVIbm) particles. S-mZVIbm is ferromagnetic, which generates heat under the application of a low-frequency alternating current electromagnetic field (AC EMF). We found that the temperature reached up to ~120 ℃ during 30-min electromagnetic induction heating of 10 g/L S-mZVIbm (with S/Fe molar ratio of 0.1), compared with ~55 ℃ and ~80 ℃ for ZVI and ball milled mZVIbm, respectively. The application of AC EMF accelerated the TCE degradation rate (kTCE = 5.5 × 10-1 h-1) by up to 4-fold without compromising or even enhancing electron efficiency of S-mZVIbm compared to no-heating. Furthermore, this process halved the generation of chlorinated intermediate, cis-DCE. In contrast, water-bath heating only increased the dechlorination rate 2-fold with unchanged cis-DCE generation and lowered electron efficiency. This is attributed to both rising temperature by induction heating and accelerated ZVI corrosion and surface Fe0 exposure caused by AC EMF. In real groundwater, the AC EMF maintained the same promoting effects for TCE dechlorination by S-mZVIbm. This study shows that combination of filed-scale available AC EMF with S-mZVIbm provides a promising approach for remediation of chlorinated hydrocarbons in contaminated groundwater.

Combination of immobilized TiO2 and zero valent iron for efficient arsenic removal in aqueous solutions

The photocatalytic removal of arsenic from aqueous solutions was investigated using titania (TiO2) immobilized on a glass support, both bare and combined with synthesized metallic iron nanoparticles (nZVI) or commercial microscale iron (ZVI). Three procedures, namely dip-coating, rotational coating, and sponge coating, were tested for achieving the immobilization of TiO2. The photocatalytic activity of the semiconductor films under UV-irradiation after cumulative coatings was evaluated for 10 mg L−1 aqueous As(III), which was entirely oxidized to As(V) with all settings within 90 and 180 min. Titania immobilized by dip-coating was found to be the most effective as it showed the faster kinetics. The reuse of immobilized TiO2 was also investigated, detecting no changes in the photocatalytic activity after five consecutive reactions. The addition of commercial ZVI particles to the immobilized TiO2 system did not bring about significant changes in the kinetics for As(III) oxidation at the three pH values investigated, i.e., 5, 7, and 9. By contrast, the addition of nZVI not only led to a faster depletion of As(III) compared to bare titania but also the removal of As(V) from the solution to concentrations below 10 μg L−1, the upper limit recommended by the World Health Organization for human consumption.The role of iron species in the arsenic removal process with both [ZVI + immobilized-TiO2] and [nZVI + immobilized-TiO2] systems was further investigated by performing adsorption and irradiation experiments without titania. It was inferred that within the pH range evaluated, the minor corrosion of the ZVI surface, even under UV irradiation, restricts the production of reactive oxidizing species and the generation of sites for arsenic species adsorption. By contrast, adsorption should be the main process responsible for the overall diminution of As(III) and As(V) species in solution attained upon nZVI addition, promoted by the increase of the external oxides/hydroxides layer on iron nanoparticles. Nevertheless, it might be also considered a certain contribution of UV-generated oxidant species formed in nZVI to the photocatalytic oxidation performance of titania.

Oxidative removal of antibiotic resistant E. coli by sulfidated zero-valent iron: Homogeneous vs heterogeneous activation

As an emerging contaminant in water, antibiotic resistant bacteria are threatening the public health gravely. In this study, sulfidated ZVI was used to activate persulfate, for antibiotic resistant E. coli and antibiotic resistant genes removal. Impressively, 7 log of antibiotic resistant E. coli was inactivated within 30 min, in sulfidated ZVI activated persulfate system (S/Fe = 0.05). Electron paramagnetic resonance and free radical quenching experiments suggested that sulfidation treatment did not change the specie of radicals. SO4•−and HO• were the main reactive oxygen species for the removal of antibiotic resistant E. coli and genes. Investigation on the activation mechanism of persulfate indicated that persulfate decomposition was mainly attributed to heterogeneous activation. More importantly, in-situ characterization (ATR-FTIR) indicated that the main charge transfer complex was formed on the surface of sulfidated ZVI, which would predominantly mediate the generation of SO4•− and HO•. Finally, the proposed system was evaluated in modeling water and secondary effluent. Results revealed that only 2.86 log and 0.84 log of antibiotic resistant E. coli were inactivated in the presence of NOM (10 mg/L) and HCO3– (84 mg/L), respectively. Besides, sulfidated ZVI activated persulfate system could be pH-dependent in actual wastewater treatment.

Interaction of zero-valent iron and carbonaceous materials for reduction of DDT

Dechlorination of dichlorodiphenyltrichloroethane (DDT) as a model compound was performed with zero-valent iron (micro-ZVI and nano-ZVI) as reductant and carbonaceous adsorbents as sink and catalyst in water. DDT is rapidly converted to dichlorodiphenyldichloroethane (DDD) in direct contact with ZVI. However, up to 90% of the DDD is transformed into non-identified, most likely oligomeric products. There is no indication of dechlorination at the aromatic rings. DDT is still rapidly dechlorinated when it is adsorbed on carbonaceous adsorbents, even though ZVI particles have no direct access to the adsorbed DDT. The carbonaceous materials function as adsorbent and catalyst for the dechlorination reaction at once. From electrochemical experiments, we deduced that direct physical contact between ZVI particles and the adsorbent is essential for enabling a chemical reaction. Electron conduction alone does not effect any dechlorination reaction. We hypothesize hydrogen species (H∗) which spill from the ZVI surface to the carbon surface and initiate reductive transformations there. The role of carbonaceous adsorbents is different for different degradation pathways: in contrast to hydrodechlorination (reduction), adsorption protects DDT from dehydrochlorination (hydrolysis).

Single-step removal of Hexavalent chromium and phenol using meso zerovalent iron

Novel meso-zero valent iron (mZVI) was investigated for treating complex wastewater containing toxic heavy metal Cr6+ and organic compound phenol. This study is first of its kind illustrating coupled removal in single-step with H2O2 playing a major role as an oxidant and reductant. The mechanism involved was electron transfer from Fe0/2+ to Cr6+ resulting in Fe2+/3+ which in turn was consumed for phenol oxidation returning as Fe2+ into the system for further Cr6+ reduction. While comparing, single-step simultaneous removal of Cr6+ and phenol showed better performance in terms of pollutant removal, Fe2+/3+ recurrent reaction and precipitation generation, double-tep sequential removal performed better in iron active-corrosion time. It was also observed that the entire redox cycle of Cr6+-Cr3+-Cr6+ was reusable for co-contaminant phenol degradation at all pH with the recurrence of Fe2+-Fe3+-Fe2+. The proposed technique was checked for its viability in a single batch reactor and the complex chemistry of the reactions are unfolded by conducting chemical speciation and mass balance study at every stage of reaction. The unique functioning of mZVI was proven with micro-analysis of ZVI's surface and compared with granular ZVI, cZVI. The results obtained from this study open the door for a safer and cleaner single treatment system in removing both toxic heavy metals and organic compounds from contaminated surface water, groundwater and many such industrial effluents.

Pyrite enhanced the reactivity of zero‐valent iron for reductive removal of dyes

AbstractBACKGROUNDZero‐valent iron (ZVI) is an environmentally friendly and cost‐efficient material for the removal of contaminants. However, its application is challenged by problems such as surface passivation and pH increase in the reaction media. As a potential solution for this problem, pyrite, a waste of the mining industry, was used together with ZVI for reductive removal of azo dyes.RESULTSIn comparison with ZVI alone, the ZVI/pyrite mixture showed dramatically enhanced efficiency and reusability for dye removal, although pyrite itself can hardly remove dyes. The reaction rate constant kobs for the removal of Orange II at the initial pH (pH0) of 7.0 was enhanced 9.4‐ to 28.0‐fold by using the mixture at a pyrite/ZVI mass ratio ranging from 1 to 4, respectively. The mechanism studies indicate that pyrite not only helped produce more iron(II) by facilitating iron corrosion and the iron(II)/iron(III) cycle in the mixed system, but also activated the ZVI surface by in situ sulfidation. The high reactivity of the adsorbed iron(II) in the mixed system significantly contributed to dye removal, especially at the alkalescent pH.CONCLUSIONPyrite accelerated the reductive removal of dyes by ZVI, through inhibiting its surface passivation and suppressing the pH increase in the reaction media. Therefore, this research proposes a facile method for enhancing the reactivity and applicability of ZVI in environmental remediation. © 2020 Society of Chemical Industry

Coexistence of humic acid enhances the reductive removal of diatrizoate via depassivating zero-valent iron under aerobic conditions

The coexistence of humic acid (HA) could accelerate the ZVI-mediated reductive removal of organic contaminants via ZVI surface depassivation under aerobic conditions.

Surfactant-facilitated dechlorination of 2,2′,5,5′-tetrachlorinated biphenyl using zero-valent iron in soil/sediment solution: Integrated effects of plausible factors

Surfactants are used to assist the zero-valent iron-mediated reductive dechlorination (ZVI-RD) of hydrophobic organic contaminants (HOCs). Although the effect of surfactants has been investigated in single-factor systems, the relationships between the surfactant and the matrix properties during RD are not well understood. Thus, an orthogonal experiment and post-experiment characterization of ZVI were conducted in the present study to estimate the integrated effects of plausible factors. The results showed that the introduction of surfactants significantly influenced the reduction of 2,2′,5,5′-tetrachlorinated biphenyl (PCB-52) by altering the contact between ZVI and PCB-52. An anionic surfactant was able to alleviate the adverse impact of high amounts of non-ionic surfactants and humic acid (used as representative soil organic matter) by changing their sorption behaviors, which were also influenced by the initial pH value. However, the reduction of ZVI by humic acid decreased the electron transfer efficiency of ZVI, and also reduced the contact between ZVI and PCB-52 by generating FeCO3. These results suggest that the rate-limiting process for the ZVI-RD of HOCs in the soil/sediment solution is the contact between ZVI and HOCs, which can be improved by the addition of surfactants at concentrations corresponding to the maximum adsorption capacity of HOCs on the ZVI surface.

Effects of Good's Buffers and pH on the Structural Transformation of Zero Valent Iron and the Oxidative Degradation of Contaminants.

The presence of Good's buffers caused rapid ZVI corrosion and a dramatic release of Fe(II) leading to the Fe(II)-catalyzed transformation of ferrihydrite to lepidocrocite and/or the direct formation of lepidocrocite from the oxidation of Fe(II) in the pH range 4.0-6.2. In comparison, in the absence of Good's buffers, elution of Fe(II) was insignificant with ferrihydrite being the only Fe(III) oxyhydroxide detected following the oxidative transformation of ZVI. The rapid ZVI corrosion in the presence of Good's buffer is possibly due to either (i) disruption of the Fe oxide surface layer as a result of attack by Good's buffers and/or (ii) interaction of Good's buffer with the outer Fe oxide surface and surface-associated Fe(II)/Fe(III) causing the Fe oxide surface layers to be more porous with both these processes facilitating continuous O2 access to the Fe(0) core and allowing the diffusion of Fe atoms outward. Our results further show that the deprotonated forms of Good's buffers and the surface charge of the Fe oxides formed at the ZVI surface strongly affect the sorption of the target compound (i.e., formate) and hence the oxidation of these compounds via surface-associated Fe(II)-mediated heterogeneous Fenton processes.

Unexpected effect of buffer solution on removal of selenite and selenate by zerovalent iron

Many studies used buffers to control pH during the reaction of ZVI, but the effect of buffer solutions received little attention. In this study, the effect of buffers on selenite (Se(IV)) and selenate (Se(VI)) removal by ZVI was systematically investigated. It was found that the addition of buffers can hinder the corrosion of ZVI, and then affect Se removal. Although the removal of Se(IV) by ZVI at initial pH (pH0) 4.0 was accelerated due to the presence of buffer, less Se(IV) was removed in buffered systems than that in unbuffered systems at pH0 ≥ 6.0. Buffers also dramatically inhibited Se(VI) sequestration at pH0 4.0–10.0. These effects were caused by the buffers instead of their influence on pH variation during the reaction, evidenced by the experiments that decoupled pH effect from overall buffer influences. The zeta potentials of the suspension in buffered systems were more negative than those in the unbuffered systems, implying the adsorption of buffer onto the ZVI surface. Accordingly, the effect of buffers observed in this study was mainly caused by its adsorption on ZVI surface, which may interfere the corrosion of ZVI as well as the adsorption of Se(IV)/Se(VI) on the surface of corroded ZVI, the first step of Se(IV)/Se(VI) sequestration by ZVI. These results suggest that it requires careful evaluation for the suitability of buffers in some reactions to study the kinetics and mechanisms.

Core-shell structured mZVI/Ca(OH)2 particle: Morphology, aggregation and corrosion

A calcium hydroxide shell was coated onto the surface of micro-sized zero valent iron (mZVI) particles by hydrothermal approach in oversaturated Ca(OH)2 solution. The heterogeneous nucleation of nano-scale Ca(OH)2 particle on micro-scale spherical ZVI surface was clearly observed by scanning electronic microscope (SEM). The moderate solubility of Ca(OH)2 was demonstrated as the crucial factor in inducing slow nucleation rate and in facilitating the abundant growth of Ca(OH)2 nuclei on mZVI surface. The growth of shell thickness was found to obey the zero order kinetics with the rate constant at about 15nm/h. The Ca(OH)2 shell was demonstrated to be anticorrosive to protect reactive Fe0 from oxidation based on standard corrosion test. In addition, the instant aggregation process of mZVI within 120s was slowed down after Ca(OH)2 shell coating. The saturation magnetization of mZVI, measured by a vibrating sample magnetometer (VSM), was gradually diminished along with the shell formation with a 32% reduction after excluding the Fe0 content change effect. This indicated that Ca(OH)2 shell coating can partially eliminated particle-particle or cluster-cluster magnetic attraction force to enhance the dispersion stability and resultantly facilitate the transportation. The dissolution of Ca(OH)2 shell was greatly dependent on the pH value of the background water environment. The pH gradient change resulted from the Ca(OH)2 shell dissolution along mZVI particle transport was illustrated by a conceptual model.