Enhanced Fe(OH)2-driven reductive Dechlorination via shortened Fe-O bonds and colloidal medium.

Fe2+ is usually adsorbed to the surface of iron-bearing clay, and iron (hydr)oxide in groundwater. However, the reductive activity of Fe(OH)2, a prevalent intermediate during the transformation of Fe2+, remains unclear. In this study, high-purity Fe(OH)2 was synthesized and tested for its activity in the degradation of carbon tetrachloride (CT). XRD data confirm that the synthesized material is a pure Fe(OH)2 crystal, exhibiting sharp peaks of (001) and (100) facets. Zeta potential analysis confirms that the off-white Fe(OH)2 is a colloidal suspension with a positive charge of ∼+35-50 mV. FTIR spectra reveal the formation of a coordination compound Fe2+ with OH-/OD-, derived from NaOH/OD. SEM and HRTEM results demonstrate that the Fe(OH)2 crystal has a regular octahedral structure with a size of ∼30-70 nm and average lattice spacings of 2.58 Å. Mössbauer spectrum verifies that the Fe2+ in Fe(OH)2/Fe(OD)2 is hexacoordinated with six Fe-O bonds. XAFS data demonstrate that the Fe-O bonds become shorter as the OH-:Fe(II) ratios increase. DFT results indicate that the (100) crystal face of Fe(OH)2 more readily transfers electrons to CT. In addition to being adsorbed to iron compounds, structural Fe2+ compounds such as Fe(OH)2 could also accelerate the electron transfer from Fe2+ to CT through shortened Fe-O bonds. The rate constant of CT reduction by Fe(OH)2 is as high as 0.794 min-1 when the OH-:Fe(II) ratio is 2.5 in water. This study aims to enhance our understanding of the structure-reactivity relationship of Fe2+ compounds in groundwater, particularly in relation to electron transfer mechanisms.

Fe(II) coordination transition regulates reductive dechlorination: The overlooked abiotic role of lactate.

The coordination environment of Fe(II) significantly affect the reductive reactivity of Fe(II). Lactate is a common substrate for enhancing microbial dechlorination, but its effect on abiotic Fe(II)-driven reductive dechlorination is largely ignored. In this study, the structure-reactivity relationship of Fe(II) is investigated by regulating the ratio of lactate:Fe(II). This work shows that lactate-Fe(II) complexing enhances the abiotic Fe(II)-driven reductive dechlorination with the optimum lactate:Fe(II) ratio of 10:20. The formed hydrogen bond (Fe-OH∙∙∙∙∙∙O = C-) and Fe-O-C metal-ligand bond result in a reduced Fe(II) coordination number from six to four, which lead to the transition of Fe(II) coordination geometry from octahedron to tetrahedron/square planar. Coordinatively unsaturated Fe(II) results in the highest reductive dechlorination reactivity towards carbon tetrachloride (k1 = 0.26254 min-1). Excessive lactate concentration (> 10 mM) leads to an increased Fe(II) coordination number from four to six with a decreased reductive reactivity. Electrochemical characterization and XPS results show that lactate-Fe(II)-I (C3H5O3-:Fe(II) = 10:20) has the highest electron-donating capacity. This study reveals the abiotic effect of lactate on reductive dechlorination in a subsurface-reducing environment where Fe(II) is usually abundant.

Labile Fe(III) phase mediates the electron transfer from Fe(II,III) (oxyhydr)oxides to carbon tetrachloride.

Labile Fe(III) phase (includes Fe(III)aq, Fe(III)ads, or Fe(III)s species) is an important intermediate during the interaction between Fe(II) and Fe(III) (oxyhydr)oxides, but how does labile Fe(III) influence the electron transfer from Fe(II) to oxidant environmental pollutant during this Fe(II)-Fe(III) interaction is unclear. In this work, the dynamic change of Fe(II,III) (oxyhydr)oxides at the same time scale is simulated by synthesizing Fe(III)-Fe(II)-I (Fe(III)+NaOH+Fe(II)+NaOH) with different Fe(II)/Fe(III) ratios. CCl4 is used as a convenient probe to test the reduction kinetics of mixed valence Fe(II,III)(oxyhydr)oxides with different Fe(II):Fe(III) ratios. The Mössbauer spectra results reveal the Fe(III)labile in the solid phase is in octahedral coordination. The electron-donating capability of Fe(II) was improved with increasing Fe(III) content, but suppressed when [Fe(III)] ≥ 30 mM. The reductive dechlorination of CT by Fe(III)-Fe(II)-I decreased gradually with the increase of Fe(III) content, because more amount Fe(III)labile in solid phase is accumulated. This shows that the electron transfer from Fe(II) to Fe(III)labile rather than to CT is enhanced with increasing Fe(III) content. FTIR data shows that the hydroxylation of Fe(II) with Fe(OH)3 occurs preferentially in the non-hydrogen bonded hydroxyl group, causing the decrease of its reductive reactivity. The presence of [Fe(III)-O-Fe(II)]+ in Fe(III)-Fe(II)-I can stabilize the dichlorocarbene anion (:CCl2-), favouring the conversion of CT to CH4 (13.1%). The aging experiment shows that Fe(III)labile surface may maintain the reductive reactivity of Fe(II) during aging when [Fe(III)] = 5-20 mM. This study deepens our understanding of the mass transfer pathway of iron oxyhydroxides induced by Fe(II) and its impact on the reductive dechlorination of CT.

Charging and discharging of humic acid geobattery induced by green rust and oxygenation: Impact on the fate and degradation of tribromophenol in redox-alternating groundwater environments.

Humic acid (HA) and ferrous minerals (e.g. green rust, GR) are abundant in groundwater. HA acts as a geobattery that take up and release electrons in redox-alternating groundwater environments. However, the impact of this process on the fate and transformation of groundwater pollutants is not fully understood. In this work, we found that the adsorption of HA on GR inhibited the adsorption of tribromophenol (TBP) under anoxic conditions. Meanwhile, GR could donate electrons to HA, causing the electron donating capacity of HA rapidly increase from 12.7% to 27.4% in 5 min. The electron transfer process from GR to HA significantly increased the yield of hydroxyl radicals (•OH) and the degradation efficiency of TBP during GR-involved dioxygen activation process. Compared to the limited electronic selectivity (ES) of GR for •OH production (ES = 0.83%), GR-reduced HA improves the ES by an order of magnitude (ES = 8.4%). HA-involved dioxygen activation process expands the •OH generation interface from solid phase to aqueous phase, which is conducive to the degradation of TBP. This study not only deepens our understanding on the role of HA in •OH production during GR oxygenation, but also provides a promising approach for groundwater remediation under redox-fluctuating conditions.

Responses of microalgae under different physiological phases to struvite as a buffering nutrient source for biomass and lipid production.

Microalgae cultivation for biodiesel production is promising, but the high demand for nutrients, such as nitrogen and phosphorus, remains a limiting factor. This study investigated effects of struvite, a low-cost nutrient source, on microalgae production under different physiological phases. Changes in element concentrations were determined to characterize the controllable nutrient release properties of struvite. Results showed that nutrient elements could be effectively supplemented by struvite. However, responses of microalgae under different growth stages to struvite varied obviously, achieving the highest biomass (0.53 g/L) and the lowest (0.32 g/L). Moreover, the microalgal lipid production was obviously increased by adding struvite during the growth phase, providing the first evidence that struvite could serve as an alternative buffering nutrient source to culture microalgae. The integration of microalgae cultivation with struvite as a buffering nutrient source provides a novel strategy for high ammonia nitrogen wastewater treatment with microalgae for biodiesel production.

Reductive dehalogenation in groundwater by Si-Fe(II) co-precipitates enhanced by internal electric field and low vacancy concentrations.

Fe(II) and silicate can form Si-Fe(II) co-precipitates in anoxic groundwater and sediments, but their phase composition and reactivity towards subsurface pollutants are largely unknown. Three types of Si-Fe(II) co-precipitations with the same chemical composition, namely Si-Fe(II)-I, Si-Fe(II)-II, and Si-Fe(II)-III, have been synthesized by different hydroxylation sequences in this work. It was found that Si-Fe(II)-III reduce carbon tetrachloride (CT) much faster (k1=0.04419 min-1) than Si-Fe(II)-I (0 min-1) and Si-Fe(II)-II (7.860 × 10-4 min-1). XRD results show that the main component of Si-Fe(II)-III is ferrous silicate (FeSiO3), which is quite different from that of Si-Fe(II)-I and Si-Fe(II)-II. The unique arrangement of hydroxyl coordination, the less distorted octahedral structure, the polyhedral morphology and the absence of Si-A center vacancies in Si-Fe(II)-III are responsible for its high reductive dehalogenation reactivity. The highest redox activity of Si-Fe(II)-III was shown by electrochemical characterization. The [FeII-O-Si]+ in Si-Fe(II)-III may stabilize the dichlorocarbene anion (˸CCl2-), which favors the transformation of CT to methane (9.2%). The Si-Fe(II) co-precipitates consist of countless internal electric fields, and the transformation of hydroxyl and CT both consumed electrons. The coexistence of hydroxyl and CT increases the electron density in the electron-rich region due to their electronegativity, enhancing their electron-accepting capabilities. This study deepens our understanding of the phase composition and electronic structure of Si-Fe(II) co-precipitates, which fills the gap in the reductive dehalogenation of halides by Si-Fe(II) co-precipitates.

Mineralization of tribromophenol under anoxic/oxic conditions in the presence of copper(II) doped green rust: Importance of sequential reduction-oxidation process.

The groundwater environment often undergoes the transition from anoxic to oxic due to natural processes or human activities, but the influence of this transition on the fate of groundwater contaminates are not entirely understood. In this work, the degradation of tribromophenol (TBP) in the presence of environmentally relevant iron (oxyhydr)oxides (green rust, GR) and trace metal ions Cu(II) under anoxic/oxic-alternating conditions was investigated. Under anoxic conditions, GR-Cu(II) reduced TBP to 4-BP completely within 7 h while GR only had an adsorption effect on TBP. Under oxic conditions, GR-Cu(II) could generate •OH via dioxygen activation, which resulted in the oxidative transformation of TBP. Sixty-five percentage of TBP mineralization was achieved via a sequential reduction-oxidation process, which was not achieved through single reduction or oxidation process. The produced Cu(I) in GR-Cu(II) enhanced not only the reductive dehalogenation under anoxic conditions, but also the O2 activation under oxic conditions. Thus, the fate of TBP in anoxic/oxic-alternating groundwater environment is greatly influenced by the presence of GR-Cu(II). The sequential reduction-oxidation degradation of TBP by GR-Cu(II) is promising for future remediation of TBP-contaminated groundwater.

Hydroxyl groups bridge the electron transfer from Fe(II) to carbon tetrachloride.

Reductive dechlorination of chlorinated organic pollutants (COPs) by Fe(II) occurs in natural environments and engineered systems. Fe(II) ions undergo hydroxylation in aqueous solutions to form Ferrous Hydroxyl Complex (FHC), which plays an essential role in Fe(II)-mediated reductive dechlorination. However, how hydroxyl groups of FHC bridge the electron transfer from Fe(II) to COPs is still not fully understood. This work shows that the rate of reductive dechlorination of carbon tetrachloride (CT) by FHC increased with increasing OH- dosage. XRD data shows the increase of OH- dosage transform FHC from Fe2(OH)3Cl to Fe(OH)2, which leads to increased reductive strength of FHC. More non-hydrogen bonded hydroxyl groups coordinate with Fe(II) in FHC with increasing the OH- dosage, which stabilizes the octahedral structure of Fe(II) as shown by Mössbauer data. Electrochemical analysis reveals that the increase of OH- dosage enhances the reductive activity of FHC, which is also confirmed by the decreased HOMO-LUMO gap. It was found that FHC dechlorinated CT to methane, which was attributed to the stabilization of trichlorocarbene anion(˸CCl3-) by [surface-O-Fe(II)-OH]+. This work deepens our understanding on the bridge effect of hydroxyl groups on the electron transfer from Fe(II) to COPs, and provides a theoretical foundation for the reductive dechlorination of COPs in both natural environments and engineered systems.

Interaction between green rust and tribromophenol under anoxic, oxic and anoxic-to-oxic conditions: Adsorption, desorption and oxidative degradation.

As a reductive Fe(II)-bearing mineral, green rust (GR) is able to reduce halogenated compounds in anoxic subsurface environments. The redox condition of subsurface environment often changes from anoxic to oxic due to natural and anthropogenic disturbances, but the interaction of GR with halogenated compounds in oxic, and anoxic-to-oxic transition conditions has not been studied. This study reveals that GR can sequester TBP for a short time (4 to 10 h) under anoxic conditions. Later, GR undergoes structural transformation to ferrihydrite and magnetite with the desorption of TBP. GR-derived iron (hydr)oxides can generate 33.8 µM of •OH upon 50 h exposure to dioxygen, which leads to 67% of oxidative degradation of TBP. The anoxic-to-oxic transition during the TBP adsorption process initiates the TBP desorption immediately, and also results in the oxidative degradation of TBP via the production of •OH. The oxygenation of GR immediately forms magnetite which activate dioxygen to produce •OH. Also, the GR-derived magnetite acts as a Fe(II) source, and free Fe(II) in solution and Fe(II) adsorbed on magnetite surface both contribute to dioxygen activation. This work provides vital evidence on the role of GR in the fate and transformation of TBP in redox alternating subsurface environments.

Pyridinic nitrogen enables dechlorination of trichloroethylene to acetylene by green rust: Performance, mechanism and applications.

Carbonous materials were found to catalyze the dechlorination of trichloroethylene (TCE) by green rust (GR), but the catalytic mechanism was not fully understood. We have developed a facile ball milling method to synthesize N-doped graphene (NG) with various N species, catalyzing fast dechlorination of TCE to acetylene by GR with the highest acetylene production rate of ~0.1 d-1. The adsorption of TCE onto NG is mainly derived from the graphene region of NG, and high pyridinic N is essential for the enhanced TCE reduction by GR. Oxygen species did not enhance the TCE reduction in GR/NG system. High dechlorination rates are correlated to a high amount of defect in NG and a high electron conductivity of NG. Pyridinic N has the highest adsorption energy for TCE among all the N species, which leads to the highest catalytic performance. High electrochemically active surface area resulted from the high content of pyridinic N facilitate the NG-catalyzed dechlorination. The acetylene production rate in real groundwater is still around one-third of that in ultrapure water. This work not only reveals the catalytic mechanism of NG-catalyzed dechlorination by GR, but also provide a feasible approach for practical remediations of TCE-contaminated groundwater using GR-NG mixture.

Phosphorus and nitrogen recovery from wastewater by ceramsite: Adsorption mechanism, plant cultivation and sustainability analysis.

Recovery of the nitrogen (N) and phosphorus (P) in wastewater would help to minimize eutrophication and their reuse would lead to a more sustainable society. Sewage sludge and fly ash were used to fabricate ceramsite in the laboratory. After modified with alkali or lanthanum it was shown in benchtop experiments to effectively recover N and P from real wastewater treatment plant effluent. The N&P-adsorbed ceramsite was then applied as an eco-friendly, slow-release fertilizer to promote the germination, growth and blooming of Impatiens commelinoides, realizing the recycling of N and P from wastewater. Emergy analysis shows that such recycling is more sustainable than the current two approaches (i.e., landfill and incineration) for sludge disposal. This work thus demonstrates a sustainable solution combining the reuse of solid waste, effective wastewater purification and recovery of N and P nutrients. Applying the technologies demonstrated would help to minimize the environmental impact of wastewater and solid waste.

Generation of atomic hydrogen by Ni-Fe hydroxides: Mechanism and activity for hydrodechlorination of trichloroethylene.

Atomic hydrogen (H•) is highly reactive for the hydrodechlorination of trichloroethylene (TCE). In this work, we found that the coprecipitation of Ni2+ and Fe2+ at neutral pH led to an unprecedented catalytic generation of H•. The generated H• effectively dechlorinate TCE to nontoxic ethylene and ethane, and Fe2+ is the only electron donor. The abundant adsorbed H• produced with a Ni/Fe ratio of 0.4 enhances hydrogen evolution reaction causing a low efficiency for hydrodechlorination. In contrast, the active absorbed H• is generated in the crystal lattice of Ni-Fe hydroxides with a Ni/Fe ratio of 3.0 causing highly efficient hydrodechlorination of TCE. This work not only reveals the mechanism of catalytic hydrodechlorination by Ni-Fe hydroxides at neutral pH, but also provides a novel approach to detoxify TCE in contaminated water using facile precipitated Ni-Fe hydroxides.

Electrochemical reductive remediation of trichloroethylene contaminated groundwater using biomimetic iron-nitrogen-doped carbon.

Electrochemical dechlorination is a prospective strategy to remediate trichloroethylene (TCE)-contaminated groundwater. In this work, iron-nitrogen-doped carbon (FeNC) mimicking microbiological dechlorination coenzymes was developed for TCE removal under environmentally related conditions. The biomimetic FeNC-900, FeNC-1000, and FeNC-1100 materials were synthesized via pyrolysis at different temperatures (900, 1000, and 1100 °C). Due to the synergistic effect of Fe-N4 active sites and graphitic N sites, FeNC-1000 had the highest electron transfer efficiency and the largest electrochemical active surface area among the as-synthesized FeNC catalysts. The pseudo-first-order rate constants for TCE reduction using FeNC-1000 catalyst are 0.19, 0.28 and 0.36 h-1 at potentials of -0.8 V, -1.0 V and -1.2 V, respectively. Active hydrogen and direct electrons transfer both contribute to the dechlorination from TCE to C2H4 and C2H6. FeNC maintain a high reactivity after five reuse cycles. Our study provides a novel approach for the dechlorination of chlorinated organic contaminants in groundwater.

Stabilized green rusts for aqueous Cr(VI) removal: Fast kinetics, high iron utilization rate and anti-acidification.

Green rusts (GRs) are redox active towards contaminants but they are not stable for long distance transport during the soil and groundwater remediation. In this study, green rust chloride (GR) was stabilized by selected regents, including silicate (Si), phosphate (P), fulvic acid (FA), carboxymethyl cellulose (CMC) and bone char (BC), then these stabilized GR, collectively named GR-X, would be further applied for Cr(VI) removal from aqueous solution. The stabilization experiment demonstrated that the release of Fe(II) from GR was effectively suppressed by above reagents, enabling at least 50% lower Fe(II) leaching from the stabilized GR-X than that from the pristine GR. The intact hexagonal GR plates and crystallinity were also confirmed by the SEM images and XRD patterns after storage for 7 days, indicating the stable structure of GR-X was remained. In the Cr(VI) removal tests, Cr(VI) was eliminated by GR-X in seconds with a Fe(II) utilization efficiency over 90%. The Cr species examination demonstrated that the GR-X was able to transfer Cr(VI) into stable Cr(III)-Fe(III) precipitates (Fe-Mn oxides fraction). After Cr(VI) removal tests, all reactors were exposed to the air for 1 week to monitor pH fluctuation and evaluated the risk of acidification. The results indicate that, except for GR-Si system, the other post-remediation systems are stable and the pH buffering ability of GR-X could avoid acidification and lower the Cr leaching risk.

Persulfate activation by two-dimensional MoS2 confining single Fe atoms: Performance, mechanism and DFT calculations.

Developing efficient catalysts for persulfate (PS) activation is important for the potential application of sulfate-radical-based advanced oxidation process. Herein, we demonstrate single iron atoms confined in MoS2 nanosheets with dual catalytic sites and synergistic catalysis as highly reactive and stable catalysts for efficient catalytic oxidation of recalcitrant organic pollutants via activation of PS. The dual reaction sites and the interaction between Fe and Mo greatly enhance the catalytic performance for PS activation. The radical scavenger experiments and electron paramagnetic resonance results confirm and SO4- rather than HO is responsible for aniline degradation. The high catalytic performance of Fe0.36Mo0.64S2 was interpreted by density functional theory (DFT) calculations via strong metal-support interactions and the low formal oxidation state of Fe in FexMo1-xS2. FexMo1-xS2/PS system can effectively remove various persistent organic pollutants and works well in a real water environment. Also, FexMo1-xS2 can efficiently activate peroxymonosulfate, sulfite and H2O2, suggesting its potential practical applications under various circumstances.

Enhanced reactivity and mechanisms of copper nanoparticles modified green rust for p-nitrophenol reduction.

This paper describes the reduction of p-nitrophenol by green rusts (GRs) interlayered with common inorganic anions (Cl-, SO42- and CO32-). Modifying of GRs with zero-valent Cu nanoparticles (Cu0 NPs) can greatly enhance the reductive reactivity of GRs via the formation of a galvanic couple between the GRs and the Cu0 NPs, as confirmed by an increased corrosion current. The direct addition of Cu0 NPs excludes the possible formation of less active mono-valent Cu in the GRs/Cu2+ system. Oxidation of GRs does not occur upon the addition of Cu0 NPs, thus a decline in electron transfer from the oxidized GRs to the Cu0 NPs is avoided. The optimum Cu0 NPs loading on GRCl is 0.5% wt. The GRCl/Cu0 NPs retains high reactivity in the studied pH range from 7 to 10, while the presence of NO3-, PO43-, SO42-, CO32- and humic acid inhibits PNP reduction by the GRCl and GRCl/Cu0 NPs. The GRCl/Cu0 NPs system is less susceptible to the presence of CO32- and humic acid compared to the pure GRCl system due to the migration of the PNP reduction sites from the GRs to the Cu0 NPs. This work sheds light on a new strategy for enhancing GR-based materials for use in groundwater remediation.

Transformation of roxarsone during UV disinfection in the presence of ferric ions.

The transformation of roxarsone (ROX) during UV disinfection with Fe(III) has been investigated. Fe(OH)2+, as the main Fe(III) species at pH = 3, produces HO under UV irradiation leading to the oxidation of ROX. Dissolved oxygen plays a very important role in the continuous conversion of generated Fe2+ to Fe3+, which ensures a Fe(III)-Fe(II) cycle in the system. The presence of Cl-/HCO3-/NO3- has little influence on the ROX transformation, whereas PO43- achieves an obvious inhibitory effect. The transformation of ROX leads to the formation of inorganic arsenic consisting of a much higher amount of As(V) than As(III). LC-MS analysis shows that phenol, o-nitrophenol and arsenic acid were the main transformation products. Both the radical scavenger experiment and electron spin resonance data confirm that the HO is responsible for ROX transformation. The toxic transformation products are found to have potential environmental risks for the natural environment, organisms and human beings.

Copper nanoparticles/graphene modified green rusts for debromination of tetrabromobisphenol A: Enhanced galvanic effect, electron transfer and adsorption.

The combined effect of copper nanoparticles (Cu NPs) and reduced graphene oxide (RGO) on the reactivity of green rust (GR) towards reductive debromination of tetrabromobisphenol (TBBPA) has been systematically investigated. The removal efficiency of TBBPA increased from 28.78% to 44.70% and the pseudo first-order rate constant (kobs) increased from 0.002 min-1 to 0.004 min-1 when the content of Cu NPs in GRSO4-Cu NPs increased from 0% to 0.5%. Cu NPs enhanced the reductive reactivity of GR via formation of a galvanic cell and Cu0/Cu+ redox cycle. The adsorption capacity of RGO towards TBBPA was 13.75 mg/g. The pseudo first-order rate constant for TBBPA removal increased from 0.0341 min-1 to 0.0866 min-1 when the RGO content increased from 0 to 2% in GR-Cu NPs-RGO. RGO enhanced the debromination efficiency via enhancing the adsorption of TBBPA and accelerating electron transfer amongst GR, Cu NPs and TBBPA. The increased corrosion current demonstrates the enhanced electron transfer by RGO in GR-Cu NPs galvanic cell. Six-electron transfer process of TBBPA reduction was revealed by rotating disk electrode analysis, which was in line with the final debromination products (Mono-BPA) determined by ion chromatography and liquid chromatography-mass spectrometry.

Single sheet iron oxide: An efficient heterogeneous electro-Fenton catalyst at neutral pH.

In order to overcome the inefficiency of heterogeneous electro-Fenton process for water treatment at neutral pH, single sheet iron oxide (SSI) derived from layered Fe(II)-Fe(III) double hydroxides (green rusts) was fabricated on an indium tin oxide electrode via layer by layer assembly and used in an undivided electrolysis cell. Use of radical scavengers demonstrated the formation of oxygen radicals by electrochemical reduction of oxygen at the SSI electrode, and the key role of hydroxyl radicals (OH) and superoxide anion (O2-) radicals in degradation of the azo dye orange II. Analysis of degradation products by UV-vis, LC-MS and GC-MS further demonstrated that direct reduction toke place in addition to indirect oxidation. The reactivity of SSI as a heterogeneous electro-Fenton catalyst is two order of magnitude higher than its homogenous counterparts. The SSI electrode was highly stable as the dye degradation did not decrease after use for 19 h with no Fe leaching. The high dye removal efficiency was maintained in a wide pH range from 7 to 10 and in different supporting electrolytes, demonstrating the application of this process under various conditions mimicking natural waters.

A Silicate/Glycine Switch To Control the Reactivity of Layered Iron(II)-Iron(III) Hydroxides for Dechlorination of Carbon Tetrachloride.

Layered FeII-FeIII hydroxide chloride (chloride green rust, GRCl) has high reactivity toward reducible pollutants such as chlorinated solvents. However, this reactive solid is prone to dissolution, and hence loss of reactivity, during storage and handling. In this study, adsorption of silicate (Si) to GRCl was tested for its ability to minimize GRCl dissolution and to inhibit reduction of carbon tetrachloride (CT). Silicate adsorbed with high affinity to GRCl yielding a sorption maximum of 0.026 g of Si/g of GRCl. In the absence of Si, the pseudo-first-order rate constant for CT dehalogenation by GRCl was 2.1 h-1, demonstrating very high reactivity of GRCl but with substantial FeII dissolution up to 2.5 mM. When Si was adsorbed to GRCl, CT dehalogenation was blocked and FeII dissolution extent was reduced by a factor of 28. The addition of glycine (Gly) was tested for reactivation of the Si-blocked GRCl for CT dehalogenation. At 30 mM Gly, partial reactivation of the GRCl was observed with pseudo-first-order rate constant for CT reduction of 0.075 h-1. This blockage and reactivation of GRCl reactivity demonstrates that it is possible to design a switch for GRCl to control its stability and reactivity under anoxic conditions.