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.
Layered FeII–FeIII hydroxides (green rusts, GRs) are promising reactants for reductive dechlorination of chlorinated solvents due to high reaction rates and the opportunity to inject reactive ...slurries of the compounds into contaminant plumes. However, it is necessary to develop strategies that reduce the formation of toxic byproducts such as chloroform (CF). In this study, carbon tetrachloride (CT) dehalogenation by the chloride form of GR (GRCl) was tested in the presence of glycine (GLY) and other selected amino acids. GLY, alanine (ALA), and serine (SER) all resulted in remarkable suppression of CF formation with only ∼10% of CF recovery while sarcosine (SAR) showed insignificant effects. For two nonamino acid buffers, TRIS had little effect while HEPES resulted in a 40 times lower rate constant compared to experiments in which no buffer was added. The FeII complexing properties of the amino acids and buffers caused variable extents of GRCl dissolution which was linearly correlated with CF suppression and dehalogenation rate. We hypothesize that the CF suppression seen for amino acids is caused by stabilization of carbene intermediates via the carbonyl group. Different effects on CF suppression and CT dehalogenation rate were expected because of the different structural and chemical properties of the amino acids.
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•The reactivity of SSI as a heterogeneous electro-Fenton catalyst is much higher than its homogenous counterparts.•Hydroxyl radicals (OH) and superoxide (O2–) are produced on SSI ...cathode during electrolysis.•The SSI electrode is highly stable at least up to 19 cycles and use for 190 h.•The reactivity of SSI electrode was almost the same in the pH range from 7 to 10.•Azo dyes are degraded on SSI cathode via both direct reduction and radical-involved oxidation.
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.
Phosphorus retention in lowland soils depends on redox conditions. The aim of this study was to evaluate how the Fe(III) reduction degree affects phosphate adsorption and precipitation. Two similarly ...P‐saturated, ferric Fe‐rich lowland soils, a sandy and a peat soil, were incubated under anaerobic conditions. Mössbauer spectroscopy demonstrated that Fe(III) in the sandy soil was present as goethite and phyllosilicates, whereas Fe(III) in the peat soil was mainly present as polynuclear, Fe‐humic complexes. Following anoxic incubation, extensive formation of Fe(II) in the solids occurred. After 100 d, the Fe(II) production reached its maximum and 34% of the citrate–bicarbonate–dithionite extractable Fe (FeCBD) was reduced to Fe(II) in the sandy soil. The peat soil showed a much faster reduction of Fe(III) and the maximum reduction of 89% of FeCBD was reached after 200 d. Neoformation of a metavivianite/vivianite phase under anoxic conditions was identified by X‐ray diffraction in the peat. The sandy soil exhibited small changes in the point of zero net sorption (EPC0) and Pi desorption with increasing Fe(III) reduction, whereas in the peat soil Pi desorption increased from 80 to 3100 μmol kg−1 and EPC0 increased from 1.7 to 83 μM, after 322 d of anoxic incubation. The fast Fe(III) reduction made the peat soils particularly vulnerable to changes in redox conditions. However, the precipitation of vivianite/metavivianite minerals may control soluble Pi concentrations to between 2 and 3 μM in the long term if the soil is not disturbed.
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Layered FeII–FeIII hydroxides (green rusts, GRs) are efficient reducing agents against oxidizing contaminants such as chromate, nitrate, selenite, and nitroaromatic compounds and ...chlorinated solvents. In this study, we adopted a buffered precipitation approach where glycine (GLY) was used in the synthesis of sulfate-interlayered GR (GRSO4) by aerial oxidation of FeII or co-precipitation by adding FeIII salt to an aqueous solution of FeII at constant pH. In both the oxidation and the co-precipitation methods pure crystalline GRSO4 was precipitated in the presence of 70mM GLY (pH 8.0), whereas in the absence of GLY, synthesis failed under similar conditions. Gycine functions as both a pH buffer and a ligand; FeII-GLY complexes serve as a source of base (FeII-GLY+H2O→FeII+H-GLY+OH−) during GR formation, supplying about 45% of the total base required for the synthesis. The GLY buffer decreases pH fluctuations during base addition and hence allows for fast GRSO4 precipitation, minimizing byproduct formation. The use of other pH buffers 4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid and 2-amino-2-(hydroxymethyl)-1,3-propanediol was also tested but failed. Mössbauer spectroscopy, X-ray diffraction, Fourier transform infrared, transmission electron microscopy, and FeII measurements confirmed the purity, stoichiometry, and pyroaurite-type structure of the obtained GRSO4. The formula of GRSO4 was found to be FeII4.08FeIII1.98(OH)11.6(SO4)1.00, and the tabular GR crystals had a lateral size of 100–500nm and a thickness of about 40nm. Upscaling of the synthesis by either 25 times in volume or 20 times in FeII concentration resulted in pure GRSO4 products. Compared with the conventional unbuffered GRSO4 synthesis method, the present method can provide pure products with a controllable, fast, and low-cost process.
Bone char catalyzed dechlorination of trichloroethylene (TCE) by green rust (iron(II)-iron(III) hydroxide, GR) has introduced a promising new reaction platform for degradation of chlorinated ...solvents. This study aimed to reveal whether a broader class of biochars are catalytically active for the dechlorination reaction and to identify which biochar properties are the most important for the catalytic activity. Biochars produced by pyrolysis of animal, plant, and sewage waste substrates at 950 °C were prepared for catalytic dechlorination of TCE by GR tested in batch experiments with 0.15 g L−1 biochar, 3.2 g L−1 GR, and ~ 20 µM TCE. The results showed that the biochar substrate significantly affects its catalytic activity, with the highest TCE reduction rate observed for bone and shrimp-based biochars (k ≥ 0.18 h−1), whereas no reactivity was seen for graphite and activated carbon references. Multivariate regression indicated that the biochar catalytic activity is controlled by multiple biochar properties - biochar surface area, TCE sorption, abundance of C–O groups, and pore size are the properties that impact the catalytic activity most. Derivation of biochar reactivity relationship for a broad spectrum of biochars provides a new approach for identifying proper biochar catalysts for pollutant degradation.
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•Biochar reactivity for catalyzing dechlorination of trichloroethylene (TCE) varies.•The extent of TCE conversion to acetylene is biochar dependent.•Graphite and activated carbon have no catalytic activity.•Graphitic structure and O functional groups are critical.•High surface area and high TCE adsorption to biochar stimulate biochar reactivity.
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•Graphene oxide can strongly affect the fate of chlorinated solvents under anoxic iron-rich conditions.•Trace amounts of graphene oxide was found to strongly facilitate the reduction ...of carbon tetrachloride by green rust.•Chloroform was the sole and “dead end” dechlorination product.•The amphiphilicity of GO is responsible for the enhanced dechlorination.•GO can provide a suitable reaction platform for carbon tetrachloride by green rust.
Graphene-based nanomaterials can mediate environmentally relevant abiotic redox reactions of chlorinated aliphatic hydrocarbons. In this study as low amounts as ∼0.007 % of graphene oxide (GO) was found to catalyze the reduction of carbon tetrachloride by layered Fe(II)-Fe(III) hydroxide (Green Rust, GR) in aqueous solutions with chloroform being the reduction product. On the basis of sorption studies of carbon tetrachloride onto the GO surface it is suggested that it is the amphiphilicity of GO, which initiates the reaction by providing a suitable reaction platform for the reagents. This study indicates that traces of graphene oxide can affect reaction pathways as well as kinetics for dechlorination processes in anoxic sediments by facilitating a partial dechlorination.
In the rainy season, rice growing areas in Vietnam often become flooded by up to 1.5 m water. The floodwater brings contaminants from cultivated areas, farms and villages to the rice fields resulting ...in widespread contamination. In 2012 and 2013, the inorganic and organic contaminants in floodwater was investigated in Thanh Hoa and Hue. Water samples were taken at 16 locations in canals, paddy fields and rivers before and during the flood. In total, 940 organic micro-pollutants in the water samples were determined simultaneously by GC-MS method with automatic identification and quantification system (AIQS), while ICP-MS was used for determination of ten trace elements in the samples. The concentrations of 277 organic micro-pollutants and ten elements (As, Cu, Cd, Cr, Co, Pb, Zn, Fe, Mn, Al) ranged from 0.01 to 7.6 μg L
−1
and 0.1 to 3170 μg L
−1
, respectively, in the floodwater. Contaminants originated from industrial sources (e.g. PAH) were detected at low concentrations, ranged from 0.01 to 0.18 μg L
−1
, while concentrations of pollutants originated from domestic sources (e.g. sterols, pharmaceuticals and personal care products and pesticides) were ranged from 0.01 to 2.12 μg L
−1
. Isoprocarb had the highest detection frequency of 90%, followed by isoprothiolane (88%) and fenobucarb (71%). The results indicated that contaminants in floodwater come from untreated wastewater from villages, and the agricultural activities are the major sources of increased pesticides resuspended in the floodwater in this study.
•Total oxalate extractable iron(III)oxide pool reduced within 21 days of incubation.•Aluminium- and non-reduced iron(III)oxides recaptures mobilized phosphorus (P).•Langmuir sorption equilibrium ...between residual sorption capacity and P in solution.•Reciprocal relationship between residual sorption capacity and phosphorus release.
Rewetting of drained agricultural lowland peat soils is followed by the risk of increased phosphorus (P) release to downstream systems, thereby challenging their restoration as nutrient sinks for years to decades. While extensive knowledge is available on P mobilization under anaerobic conditions caused by reductive (Fe(III))-oxide dissolution, the net P release to the aqueous phase (PSol) is only poorly understood due to unknown significance of P re-sorption to aluminum (Al) oxides and non-reduced Fe(III)-oxides. We therefore hypothesize that PSol is a function of the sorption capacity and P saturation of Al-oxides and non-reduced Fe(III)-oxides. A comprehensive set of 47 Danish topsoil and subsoil samples from agricultural lowlands were incubated for up to 148 days in the laboratory under anoxic and water-saturated conditions at room temperature. Oxalate-extractable Fe, Al and P (Feox, Alox and Pox) varied by three orders of magnitude, with Feox ranging between 1.8 and 1590 mmol kg−1; Alox and Pox also showed high variation with maximum contents of 883 and 153 mmol kg−1, respectively. Bicarbonate-dithionite generally extracted 2 times less Fe, Al and P (FeBD, AlBD, PBD) than oxalate. Oxalate extraction data were used to calculate the degree of P saturation (DPS) and P sorption capacity (PSC). The extent of Fe(III)-oxide reduction measured as 0.1 M HCl extractable Fe(II)HCl was well described by first-order kinetics with rate constants ranging between 0.01 and 0.3 d-1.
The estimated maximum Fe(II)HCl produced (Fe(II)max) ranged between 3 and 1490 mmol Fe(II) kg−1, with Feox corresponding to Fe(II)max values very closely. For most soils almost full dissolution of the entire Feox pool was achieved within 21 days of incubation. PSol concentrations, measured in the soil solution extract, ranged between 0.05 and 5.05 mg L-1, increasing with incubation time for most of the soils. While PSol was not correlated with FeBD:PBD and Feox:Pox, DPS, total P (PT) or total Fe (FeT):PT ratio, the investigations revealed a strong reciprocal relationship between the residual sorption capacity (RSC) of the soils, i.e., PSC subtracted the Fe(III)-oxides reduced, and the PSol/Pox ratio. Moreover, the results showed low or lack of P release to the aqueous phase, even for the soil samples where all Feox pools were reduced, if the RSC of the soil was above 100 mmol kg−1. This highlights the importance of redox-stable Al-oxides to capture mobilized P from rewetted lowland peat soils.
