Ligand-enhanced reactive oxidant generation by nanoparticulate zero-valent iron and oxygen

The reaction of zero-valent iron or ferrous iron with oxygen produces reactive oxidants capable of oxidizing organic compounds. However, the oxidant yield in the absence of ligands is too low for practical applications. The addition of oxalate, nitrilotriacetic acid (NTA), or ethylenediaminetetraacetic acid (EDTA) to oxygen-containing solutions of nanoparticulate zero-valent iron (nZVI) significantly increases oxidant yield, with yields approaching their theoretical maxima near neutral pH. These ligands improve oxidant production by limiting iron precipitation and by accelerating the rates of key reactions, including ferrous iron oxidation by oxygen and hydrogen peroxide. Product yields indicate that the oxic nZVI system produces hydroxyl radical (OH*) over the entire pH range in the presence of oxalate and NTA. In the presence of EDTA, probe compound oxidation is attributed to OH under acidic conditions and a mixture of OH* and ferryl ion (Fe[IV]) at circumneutral pH.     

Biodegradation of metal-complexing aminopolycarboxylic acids

Aminopolycarboxylic acids (APCAs) are used in a broad range of household products and industrial applications to control the solubility and precipitation of metal ions. The two most widely used APCAs are ethylenediaminetetraacetate (EDTA) and nitrilotriacetate (NTA). Because their application is mostly water-based APCAs are disposed off in wastewater from where they reach the environment if not sufficiently well eliminated. A number of NTA and EDTA-degrading bacterial strains have been isolated. Many of them are strictly aerobic members of the alpha-2 subgroup of Proteobacteria. The two newly established NTA-degrading genera Chelatobacter and Chelatococcus appear to be well represented in most aerobic environments. The biochemistry of NTA and EDTA catabolism has been studied in considerable detail and recently, also information on the degradation of ethylenediaminedisuccinate [S,S]-EDDS, a structural isomer of EDTA of natural origin, was reported. For both, NTA and EDTA, a specific monooxygenase/NADH2:FMN oxidoreductase enzyme system has been purified and characterised and their regulation under environmentally relevant growth conditions has been studied. The genes for NTA MO and EDTA MO have been cloned and sequenced and they share many similarities. In addition, an oxygen-independent NTA dehydrogenase has been isolated from a denitrifying NTA-degrading bacterium and the breakdown of [S,S]-EDDS was found to be initiated by a lyase. Information on the transport of APCAs is presently limited to EDTA, where there is strong evidence for an energy-dependent carrier that can transport free and Ca2+-complexed EDTA. Metal speciation of APCAs strongly influences both transport and subsequent metabolism. However, from the data available for the intracellular metabolism of APCAs no generalising pattern with respect to influence of metal speciation on degradation can be deduced yet.     

Structural study of the incorporation of heavy metals into solid phase formed during the oxidation of EDTA by permanganate at high pH

Properties of solid phases formed during the oxidation of EDTA by permanganate in a high-pH, high-ionic strength solution, and the retention of Cu2+, Ni2+, and Zn2+ by them were examined. Morphologically, the solids were agglomerates of particles with sizes <100 nm. X-ray absorption spectroscopy (XAS) analysis indicated that these particles were birnessite. Its precipitation was accompanied by the removal of Zn2+ and Cu2+ released as a result of the breakdown of their complexes with EDTA. However, Ni2+ was not removed from the supernatant. Cu2+ was strongly bound by birnessite and exhibited little mobility in the pH range from 3 to 14. Zn2+ was more mobile, especially at pH > 12. XAS showed that Cu2+ binding sites were located within MnO6 octahedra-comprised sheets that constitute birnessite while Zn2+ was positioned between them.     

Lutetium speciation and toxicity in a microbial bioassay: testing the free-ion model for lanthanides

The validity of the free-ion model (FIM) for the element lutetium (Lu), a member of the lanthanides, was assessed in experiments with the bacterium Vibrio fischeri. The FIM is mainly based on divalent metals and synthetic ligands and has not yet been validated for the trivalent lanthanides. The bioluminescence response of V. fischeri was studied at different Lu concentrations in the presence and absence of natural and synthetic organic ligands [citrate, malate, oxalate, acetate, ethylenediaminetetraacetate (EDTA), and nitrilotriacetate (NTA)]. All ligands were tested separately to ensure that their concentrations would not cause adverse effects themselves. Free Lu3+ concentrations were calculated with a speciation program, after extension of its database with the relevant Lu equilibria. The results confirmed the FIM for Lu: that is, in contrast to total dissolved Lu concentrations, free Lu3+ concentrations had an apparent relationship with the response of V. fischeri. However, a contribution of minor inorganic Lu complexes cannot be ruled out. In the presence of malate and oxalate, the EC50 for Lu3+ decreased faster in time than for the other ligands, indicating lower elimination rates. With an EC50 of 1.57 microM, Lu3+ is more toxic than La3+, Cd2+, or Zn2+ and approximately equally as toxic as Cu2+. Although the pH increased slightly during the experiments, it was shown that the influence of pH on Lu speciation was limited.     

Heterogeneous photocatalytic reduction of chromium(VI) over TiO2 particles in the presence of oxalate: involvement of Cr(V) species

Cr(VI) photocatalytic reduction experiments over TiO2 particles under near UV irradiation in the presence of excess oxalate were performed at acid pH (2 and 3) and under air and N2 bubbling. Initial photonic efficiencies for Cr(VI) reduction are nearly the same under aerobic and anaerobic conditions, but show a significant increase at the lowest pH. At pH 2, the addition of oxalate facilitates Cr(VI) reduction, hindering the electron-shuttle mechanism taking place in pure water. The oxalate synergistic effect at pH 2 is lower than that previously found for EDTA and negligible at pH 3. Chromium(V) oxalate concentration profiles were obtained by EPR spectroscopy in the presence of excess oxalate at pH 1.5. Coordinated Cr(V) complexes [Cr(V)(O)(Ox)2]-, [Cr(V)(OH2)(Ox)2]-, and [Cr(V)(O)(OH)2(Ox)]- were identified, on the basis of the comparison of their corresponding g values with recent literature data. The kinetic analysis of the temporal evolution of the paramagnetic Cr(V) species indicates also an effective photocatalytic degradation of chromium(V) oxalate complexes. This new evidence reinforces previous findings regarding sequential one-electron-transfer processes in Cr(VI) photocatalytic reduction, suggesting that this route may represent a general behavior for the Cr(VI) reduction over UV-irradiated TiO2 particles.     

Effect of Fenton reagent dose on coexisting chemical and microbial oxidation in soil

The ability of modified Fenton reactions to promote simultaneous chemical and biological oxidation in an artificially contaminated soil was studied in batch laboratory slurry reactors. Tetrachloroethene (PCE) and oxalate (OA) were used to distinguish chemical oxidation from aerobic heterotrophic metabolism. PCE was mineralized by Fenton reactions, but OA was not oxidized. Indigenous soil microorganisms did not degrade added PCE aerobically but readily assimilated OA. Fenton reactions were promoted at the natural soil pH (7.6) by adding H2O2 and Fe(III), with nitrilotriacetic acid (NTA) as a chelator, at a constant molar ratio of H2O2/Fe(III)/NTA of 50:1:1. The *OH-mediated mineralization of PCE was demonstrated by adding 2-propanol (an *OH scavenger), which inhibited PCE oxidation. In subsequent dosing studies, PCE oxidation served as an indicator of Fenton reactions, while OA assimilation, dissolved oxygen (DO) concentration, and heterotrophic plate counts were indicators of aerobic microbial activity. Increasing Fenton doses to 20 times that required to achieve 95% PCE oxidation only delayed OA assimilation by 500 min and reduced plate counts by 1.5 log units g(-1) soil. Results show that aerobic metabolism can coexist with Fenton oxidation in soils.     

Extraction of heavy metals from soils using biodegradable chelating agents

Metal pollution of soils is widespread across the globe, and the clean up of these soils is a difficulttask. One possible remediation technique is ex-situ soil washing using chelating agents. Ethylenediaminetetraacetic acid (EDTA) is a very effective chelating agent for this purpose but has the disadvantage that it is quite persistent in the environment due to its low biodegradability. The aim of our work was to investigate the biodegradable chelating agents [S,S]-ethylenediaminedisuccinic acid (EDDS), iminodisuccinic acid (IDSA), methylglycine diacetic acid (MGDA), and nitrilotriacetic acid (NTA) as potential alternatives and compare them with EDTA for effectiveness. Kinetic experiments showed for all metals and soils that 24 h was the optimum extraction time. Longer times only gave minor additional benefits for heavy metal extraction but an unwanted increase in iron mobilization. For Cu at pH 7, the order of the extraction efficiency for equimolar ratios of chelating agent to metal was EDDS > NTA> IDSA > MGDA > EDTA and for Zn it was NTA > EDDS > EDTA >MGDA > IDSA. The comparatively low efficiency of EDTA resulted from competition between the heavy metals and co-extracted Ca. For Pb the order of extraction was EDTA > NTA >EDDS due to the much stronger complexation of Pb by EDTA compared to EDDS. At higher concentration of complexing agent, less difference between the agents was found and less pH dependence. There was an increase in heavy metal extraction with decreasing pH, but this was offset by an increase in Ca and Fe extraction. In sequential extractions EDDS extracted metals almost exclusively from the exchangeable, mobile, and Mn-oxide fractions. We conclude that the extraction with EDDS at pH 7 showed the best compromise between extraction efficiency for Cu, Zn, and Pb and loss of Ca and Fe from the soil.     

Oxidation of phenolic endocrine disrupting chemicals by potassium permanganate in synthetic and real waters

In this study, five selected environmentally relevant phenolic endocrine disrupting chemicals (EDCs), estrone, 17β-estradiol, estriol, 17α-ethinylestradiol, and 4-n-nonylphenol, were shown to exhibit similarly appreciable reactivity toward potassium permanganate [Mn(VII)] with a second-order rate constant at near neutral pH comparable to those of ferrate(VI) and chlorine but much lower than that of ozone. In comparison with these oxidants, however, Mn(VII) was much more effective for the oxidative removal of these EDCs in real waters, mainly due to the relatively high stability of Mn(VII) therein. Mn(VII) concentrations at low micromolar range were determined by an ABTS [2,2-azino-bis(3-ethylbenzothiazoline)-6-sulfonic acid diammonium] spectrophotometric method based on the stoichiometric reaction of Mn(VII) with ABTS [Mn(VII) + 5ABTS → Mn(II) + 5ABTS(?+)] forming a stable green radical cation (ABTS(?+)). Identification of oxidation products suggested the initial attack of Mn(VII) at the hydroxyl group in the aromatic ring of EDCs, leading to a series of quinone-like and ring-opening products. The background matrices of real waters as well as selected model ligands including phosphate, pyrophosphate, NTA, and humic acid were found to accelerate the oxidation dynamics of these EDCs by Mn(VII). This was explained by the effect of in situ formed dissolved Mn(III), which could readily oxidize these EDCs but would disproportionate spontaneously without stabilizing agents.     

Parabola-Like Shaped pH-Rate Profile for Phenols Oxidation by Aqueous Permanganate

Oxidation of phenols by permanganate in the pH range of 5.0-9.0 generally exhibits a parabola-like shape with the maximum reaction rate obtained at pH close to phenols' pK(a). However, a monotonic increase or decrease is observed if phenols' pK(a) is beyond the pH range of 5.0-9.0. A proton transfer mechanism is proposed in which the undissociated phenol is directly oxidized by permanganate to generate products while a phenolate-permanganate adduct, intermediate, is formed between dissociated phenol and permanganate ion and this is the rate-limiting step for phenolates oxidation by permanganate. The intermediate combines with H(+) and then decomposes to products. Rate equations derived based on the steady-state approximation can well simulate the experimentally derived pH-rate profiles. Linear free energy relationships (LFERs) were established among the parameters obtained from the modeling, Hammett constants, and oxygen natural charges in phenols and phenolates. LFERs reveal that chlorine substituents have opposite influence on the susceptibility of phenols and phenolates to permanganate oxidation and phenolates are not necessarily more easily oxidized than their neutral counterparts. The chlorine substituents regulate the reaction rate of chlorophenolates with permanganate mainly by influencing the natural charges of the oxygen atoms of dissociated phenols while they influence the oxidation of undissociated chlorophenols by permanganate primarily by forming intramolecular hydrogen bonding with the phenolic group.     

Iron binding by tannic acid: Effects of selected ligands

Foods with high tannin content inhibit Fe absorption from meals. Presumably, tannins form complexes with Fe in the intestinal lumen reducing Fe bioavailability. Our objective was to assess the influence of various ligands on Fe binding by tannic acid in vitro. Different mixing sequences were employed to determine whether the ligands could prevent Fe from binding to the tannin or could remove Fe already bound. Solutions of Fe ⁺³ (FeC₃ in 0.1 n HCl), ligand (ethylenediamine-tetraacetic acid (EDTA), ascorbic acid, nitrilotriacetic acid (NTA) or citric acid) and tannic acid (200 μg ml⁻¹) in pH 4.4 acetate buffer were combined to obtain a final ligand:Fe molar ratio of 1:1 (89 μm). Three mixing sequences were followed: sequence I (Fe and ligand combined and added to tannin); sequence II (tannin and ligand combined and added to Fe); and sequence III (Fe and tannin combined and added to ligand). Fe-tannin binding was assessed by measuring absorbance at 560 nm (visible absorbance maximum) at 15 s intervals for 5 min. An Fe-tannin mixture without ligand served as the control. With EDTA, sequence I resulted in no binding. In sequence II and III, there was some binding initially, but it decreased with time. With ascorbic acid, sequence I resulted in no binding. In sequence II and III, initial binding was slightly lower than the control. Binding did not change with time. With NTA, initial binding varied with the sequence, but converged with time to a value slightly lower than the control. Citric acid did not affect binding regardless of addition sequence. These findings suggest that ligands with high affinity for Fe (e.g. EDTA) can prevent Fe from binding tannin and can remove Fe already bound. Ligands with lower affinity (e.g. citric acid) have little effect. The implications are that EDTA, ascorbic acid and NTA may affect Fe bioavailability from meals containing tannins.     

Effect of organic co-contaminants on technetium and rhenium speciation and solubility under reducing conditions

The chemical and biogeochemical reduction of pertechnetate (TcO4-) and perrhenate (ReO4-) have been compared alongside complexation of the reduced species by three anthropogenic ligands relevant to nuclear waste (ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), and isosaccharinic acid (ISA)). An HPLC size-exclusion column coupled to ICP-MS was used to separate the species and quantify Tc and Re. During method development, ReO4- showed recalcitrance to direct chemical reduction by Sn(ll) under conditions that readily reduced TcO4- and resulted in Tc(IV)-organic complexes. In microcosm experiments of a silty loam soil containing Tc04-, ReO4-, and ISA (3.0 mM), EDTA (0.17 mM), or NTA (2.4 mM), anoxia developed to iron-reducing conditions during the 42 day experimental period. The majority of the TcO4- was reduced to particle-reactive Tc(IV) and removed from solution during nitrate reduction, but there was no chromatographic evidence of Tc(IV)-organic complexes in the porewater. Overall, the excess organic complexants added did not cause a measurable difference in the solubility of Tc(IV) over the control experiments in this organic-rich (12% organic carbon) soil. ReO4- did not undergo reduction, as shown by the constant porewater concentration and the chromatographic data, and thus Re does notfunction as an analogue forTc under environmental nitrate- and iron-reducing conditions.     

酸及螯合预处理对光催化漂白的影响

通过原子吸收光谱分析知道,未漂浆经过硫酸和乙二胺四乙酸(EDTA)螯合预处理后,金属离子Mg2+、Mn2+、Fe3+、Zn2+、Cu2+的含量有不同程度的降低,其中酸预处理时Cu2+、Mg2+的去除率最高,螯合预处理时Fe3+、Zn2+的去除率最高,并且当硫酸和鳌合剂的加入量分别为1.5%和0.6%时,金属离子的去除量较高。经过酸和螯合预处理后,纸浆的白度分别提高了3.8%ISO-5.0%ISO和4.30%ISO-5.2%ISO,纸浆的硬度下降率分别为6.5%-9.6%和5.3%-10%左右,并且当酸和鳌合剂的加入量分别为1.0%和0.6%时,纸浆的白度提高最显著,硬度下降最大,黏度下降很小。通过对预处理前后的纸浆进行光催化漂后的对比研究发现,酸预处理使纸浆的白度降低了2.3%ISO-3.5%ISO,硬度和黏度各自的降低率相近;而鳌合预处理使纸浆的白度提高了3%ISO左右,硬度降低率提高6%左右,黏度却保持较好。     

BETA-AMYLASE SULPHYDRYL AND DISULPHIDE GROUP REACTIONS. ADDITIONAL ASPECTS ON ENZYME INHIBITION BY ASCORBIC ACID

Cysteine, EDTA and certain other sulphydryl and chelating agents activated barley β-amylase at pH 7 but not at pH 4·35. Apart from oxalate and citrate the same reagents prevented ascorbic acid inhibition of the enzyme. A partial purification of the enzyme was effected on SP-Sephadex C-50 in the presence of mercaptoethanol and Ellman's reagent was reduced when incubated with ascorbic acid and the more active fraction which was isolated. When Ellman's reagent was added to an enzyme system containing additional copper which was incubated with ascorbic acid, a reduction of the synthetic disulphide reagent still occurred. An observable increase in absorbance at 360 nm was also noted in a similar enzyme system. The results suggest that the ascorbate free radical is readily attracted to the exposed disulphide bonds of β-amylase     

Use of diffusive gradients in thin films to measure cadmium speciation in solutions with synthetic and natural ligands: comparison with model predictions

The performance of the technique of diffusive gradients in thin films (DGT) was characterized in well-defined systems containing cadmium with chloride and nitrate ions, simple organic ligands (nitrilotriacetic acid and diglycolic acid), and Suwannee river fulvic acid for the pH range 5-8. Cd was fully labile in all Cd, Cl-, and NO3- solutions tested (I= 0.1 and 0.01 M), even atvery low Cd concentrations (10 nM), consistent with there being no binding of Cd to the diffusive gel. Diffusion coefficients of Cd-nitritotriacetic acid (NTA) and Cd-diglycolic acid (DGA) species were measured and found to be ca. 25-30% lower than the equivalent coefficient for free metal ions. These values were used to calculate concentrations of labile Cd from DGT measurements in solutions of Cd with NTA or DGA. Cd-NTA and Cd-DGA species were found to be fully DGT-labile. DGT devices that used a diffusive gel with a reduced pore size, which retarded the passage of fulvic acid species through the gel, were used to estimate the proportion of Cd complexed by fulvic acid. These results were compared with predictions of the solution speciation from models with default parameter values. ECOSAT, incorporating the NICA-Donnan model, correctly predicted the magnitude of the binding and its pH dependence, while predictions from WHAM V (with humic ion binding model V) and WHAM 6 (with humic ion binding model VI) were less satisfactory at predicting the pH dependence. Reasonable fits to the data could be obtained from WHAM 6 when the effective binding constant log K(MA) was changed from 1.6 to 1.5, the value of deltaLK1 from 2.8 to 1.0 to minimize the dependence on pH, and the value of deltaLK2 from 1.48 to 1.0 to decrease the strength of the strong bidentate and tridentate binding sites.     

Influence of pH on Egg Yolk Lipid Oxidation

The influence of pH on lipid oxidation of liquid egg yolk was studied. Liquid yolk samples were prepared to contain 60% yolk, 40% distilled water, 0.012% sodium azide, and pH was adjusted from the normal pH of 6.0 to 2.0, 3.0, 4.0, and 5.0 with 1N HCl. Samples were agitated at room temperature (ca 23°C) for 20 days and lipid oxidation measured using the distillation thiobarbituric acid test. No increase in thiobarbituric acid reactive substances occurred in the control (pH 6) or pH 5 yolk samples. The degree of oxidation significantly increased (P<0.01) in the more acidic samples in the following order: pH 3 > pH 2 > pH 4. Addition of 1.2% EDTA significantly lowered (P<0.01) the degree of oxidation but did not inhibit oxidation completely.     

Iron Binding by Sodium Alginate

Sodium alginate was dispersed in iron chelate solutions and incubated for 10 min. The alginate was precipitated with acetone and the iron in the supernatant measured. Bound iron was calculated. The effects of pH, iron: alginate ratio, chelator, and competing cation on iron binding by alginate were determined. Binding increased as the pH decreased and as the iron:alginate ratio increased; the chelator used (EDTA, NTA, ascorbic acid, citrate) affected complexing. Binding decreased in the presence of calcium. In vitro iron availability studies indicated that alginate-iron complexes are disrupted during digestion.     

Nisin-producing Lactococcus spp. from mayonnaise-based products and their raw materials

Three bacteriocins were purified from lactic acid bacteria isolated from mayonnaise-based finished products and the raw materials used in the production of the finished products. All three bacteriocin-producing strains (EN3a, EN14a, and EN15b) were identified by sequencing of the 16S rDNA as Lactococcus lactis. The RP-HPLC fractions showed strong antimicrobial activity against Listeria monocytogenes. Mass spectrometry analysis of active fractions showed that all strains produced nisin A (3,353.7 Da). These results were confirmed by amplification of nisin structural gene by PCR, by lack of activity against a nisin-producing strain, and by total loss of activity after reaction of purified bacteriocins with glutathione. The strains Lactococcus lactis EN3a, EN14a, and EN15b are the first nisin-producing strains isolated from mayonnaise-based commercial products and from the raw materials used in their production. The ability of these strains to produce nisin could be useful to improve food quality of these and related products.     

Natural Antioxidants Preserve the Lipid Oxidative Stability of Minimally Processed Avocado Purée

ABSTRACT: Lipid oxidation is one of the major phenomena that limit the shelf-life of avocado products. The effects of adding 100 ppma-tocopherol, 200 ppm ascorbic acid, and 200 ppm ethylenediaminetetraacetic acid (EDTA) on he stability of the lipidic fraction of minimally processed avocado purées were evaluated throughout storage. α tocopherol, followed by ascorbic acid, reduced oil rancidity processes during storage. Peroxide formation was minimal when a-tocopherol was added to avocado pulp, prolonging the induction stage of oils for at least 12 wk. When preserved in vacuum, iodine and specific extinction coefficient at 270 nm (k₂₇₀) values were more stable, with changes of 5.87 g I₂/100 g oil and 0.237, respectively, during 24 wk. On the other hand, EDTA was not shown to be effective in preserving the stability of the lipid fraction of avocado preserved by combined methods.     

Mechanism of initiation of oxidation in mayonnaise enriched with fish oil as studied by electron spin resonance spectroscopy

 Electron spin resonance spectroscopy (spin trapping technique) has been used to identify the most important single factor for initiation of lipid oxidation in mayonnaise enriched with fish oil. Low pH increases the formation of radicals during incubation under mildly accelerated conditions at 37  °C as quantified using 12-doxylstearic acid. Sugar, NaCl and potassium sorbate have no effect on radical formation while EDTA (down to 50 μg/g) has an antioxidative effect. Iron bound to phosvitin in egg yolk, inactive at pH∼6, is considered to be exposed to the solvent (the aqueous phase) at low pH and capable of initiating peroxide cleavage reactions when not protected by EDTA in a mixed complex. Received: 11 February 2000     

Influence of sediment components on the immobilization of Zn during microbial Fe-(hydr)oxide reduction

The fate of Zn and other sorbed heavy metals during microbial reduction of iron oxides is different when comparing synthetic Fe-(hydr)oxides and natural sediments undergoing a similar degree of iron reduction. Batch experiments with the iron-reducing organism Shewanella putrefaciens were conducted to examine the effects of an aqueous complexant (nitrilotriacetic acid or NTA), two solid-phase complexants (kaolinite and montmorillonite), an electron carrier (anthraquinone disulfonic acid or AQDS), and a humic acid on the speciation of Zn during microbial reduction of synthetic goethite. Compared to systems containing only goethite and Zn, microbial Fe(III) reduction in the presence of clay resulted in up to a 50% reduction in Zn immobilization (insoluble in a 2 h 0.5 M HCl extraction) without affecting Fe(II) production. NTA (3 mM) increased Fe(II) production 2-fold and resulted in recovery of nearly 75% of Zn in the aqueous fraction. AQDS (50 microM) resulted in a 12.5% decrease in Fe(II) production and a 44% reduction in Zn immobilization. Humic acid additions resulted in up to a 25% decrease in Fe(II) production and 51% decrease in Zn immobilization. The results suggest that all the components examined here as either complexing agents or electron shuttles reduce the degree of Zn immobilization by limiting the availability of Zn for incorporation into newly formed biogenic minerals. These results have implications for the remediation of heavy metals in a variety of natural sediments.