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.

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.

Acacia senegal gum attenuates systemic toxicity in CCl4-intoxicated rats via regulation of the ROS/NF-κB signaling pathway.

Acacia senegal (AS) gum (Gum Arabic) is a natural emulsifier exudate from the branches and trunk of Acacia trees and it is recognized by the Food and Drug Administration (FDA) agency as a secure dietary fiber. The present research evaluated the systemic oxidative and necroinflammatory stress induced by CCl4 administration and the alleviating effect of AS gum aqueous extract (ASE, 7.5 g/Kg b.w.). The results demonstrated the presence of certain phenolic compounds in ASE, as well as its in vitro potent scavenging ability against ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), NO, and lipid peroxide radicals. Also, the outcomes revealed an improvement in the CCl4-induced liver, lung, brain, and spleen toxicity by reducing the levels of ROS, lipid peroxidation, NO, and the gene expression of NF-κB and its relevant ROS-mediated inflammatory genes. In contrast, the total antioxidant capacity (TAC), as well as the enzymatic and non-enzymatic antioxidants, were significantly upregulated in these organs after the treatment with ASE. These results were confirmed by improving the morphological features of each organ. Therefore, ASE can ameliorate the systemic toxicity caused by CCl4 via regulation of the ROS/NF-κB signaling pathway in the rat organs, which is owed to its phytochemical composition.

From Selection to Instruction and Back: Competing Conformational Selection and Induced Fit Pathways in Abiotic Hosts.

Two limiting cases of molecular recognition, induced fit (IF) and conformational selection (CS), play a central role in allosteric regulation of natural systems. The IF paradigm states that a substrate "instructs" the host to change its shape after complexation, while CS asserts that a guest "selects" the optimal fit from an ensemble of preexisting host conformations. With no studies that quantitatively address the interplay of two limiting pathways in abiotic systems, we herein and for the first time describe the way by which twisted capsule M-1, encompassing two conformers M-1(+) and M-1(-), trap CX4 (X=Cl, Br) to give CX4 ⊂M-1(+) and CX4 ⊂M-1(-), with all four states being in thermal equilibrium. With the assistance of 2D EXSY, we found that CBr4 would, at its lower concentrations, bind M-1 via a M-1(+)→M-1(-)→CBr4 ⊂M-1(-) pathway corresponding to conformational selection. For M-1 complexing CCl4 though, data from 2D EXSY measurements and 1D NMR line-shape analysis suggested that lower CCl4 concentrations would favor CS while the IF pathway prevailed at higher proportions of the guest. Since CS and IF are not mutually exclusive, we reason that our work sets the stage for characterizing the dynamics of a wide range of already existing hosts to broaden our fundamental understanding of their action. The objective is to master the way in which encapsulation takes place for designing novel and allosteric sequestering agents, catalysts and chemosensors akin to those found in nature.

Effects of losartan and l-serine in a mouse liver fibrosis model.

Hepatic fibrosis is a common liver disease caused by excessive collagen deposition in the liver. Since liver transplantation is the only current treatment for cirrhosis with worsened fibrosis, a new strategy to develop anti-fibrosis drugs with no adverse effects is necessary. In recent studies, amino acids have been applied as a type of therapy in various fields. l-serine plays a major role in antioxidant production via the maintenance of nicotinamide adenine dinucleotide phosphate hydride production in the mitochondria. l-serine may reduce fibrotic lesions in a mouse model of chronic liver injury. This study used 27 six-week-old C57BL/6 mice and injected them three times a week for eight weeks with carbon tetrachloride (CCl4) (1.5 mg/kg, 10% v/v CCl4 in olive oil) to create a hepatic fibrosis mouse model. The mice, which weighed approximately 20-30 g, were randomly classified into four groups: 1) the olive oil group, which received intraperitoneal injection of olive oil (1.5 mg/kg, 3 times per week for 8 weeks); 2) the CCl4-only group; 3) the CCl4 + losartan (10 mg/kg, PO, 5 days on, weekend off for 8 weeks) group; and 4) the CCl4 + l-serine (100 g/L, free access for 8 weeks) group. Hematoxylin and eosin staining and Masson's trichrome staining showed reduced inflammatory cell deposition and collagen deposition in the liver tissue in the l-serine supplemented group. l-serine was found to reduce the spread of hepatic fibrosis and has potential use in clinical settings. Based on these histopathological observations, l-serine is a potential anti-fibrosis drug.

Hepatoprotective effects of Psidium guajava on mitochondrial enzymes and inflammatory markers in carbon tetrachloride-induced hepatotoxicity.

OBJECTIVE: The present investigation was aimed to evaluate the hepatoprotective potential of ethanolic extract of Psidium guajava (P. guajva) and its isolated quercetin fraction on carbon tetrachloride (CCl4)-induced hepatotoxicity. MATERIALS AND METHODS: The rats were divided into 6 groups and each group contained 6 rats. CCl4 (1.5 ml/kg b.w.) was used to induce the hepatotoxicity. Ethanolic extract of P. guajava (300 mg/kg b.w.), isolated quercetin fraction (20 mg/kg b.w.) were used as a treatment and silymarin (25 mg/kg b.w.) was used as a standard drug. After the study period, the liver tissues were collected and evaluate the levels of liver functional markers, mitochondrial enzymes, histopathological analysis and the expressions of inflammatory markers. RESULTS: The levels of liver functional markers were increased and protein, albumin and A/G ratio levels were decreased and the decreased levels of mitochondrial enzymes were noted in CCl4-induced rats and the levels were restored near to normal significantly when the administration ethanolic extract of P. guajava, isolated quercetin fraction and silymarin. The normal architecture of liver tissues were altered and the mRNA expressions were up-regulated in CCl4-induced rats and the liver tissues were normalized and the mRNA and protein expressions were down-regulated near to normal significantly when the administration of ethanolic extract of P. guajava, isolated quercetin fraction and silymarin. CONCLUSION: From these results, the isolated quercetin fractions have better activity than that of the ethanolic extract of P. guajava leaves. Hence, the isolated quercetin may be used as the safest drug for hepatotoxicity in future.

Evaluation of carbon tetrachloride fraction of Actinodaphne angustifolia Nees (Lauraceae) leaf extract for antioxidant, cytotoxic, thrombolytic and antidiarrheal properties.

Actinodaphne angustifolia Nees (Family: Lauraceae) is commonly used in folk medicine against urinary disorder and diabetes. The objective of the present study was to evaluate the antioxidant, cytotoxic, thrombolytic, and antidiarrheal activities of carbon tetrachloride (CCl4) fraction of leaves of A. angustifolia (CTFAA) in different experimental models. Antioxidant activity was evaluated by using qualitative and quantitative assays, while antidiarrheal effects assessed with castor oil-induced diarrheal models in mice. The clot lysis and brine shrimp lethality bioassay were used to investigate the thrombolytic and cytotoxic activities, respectively. CTFAA showed antioxidant effects in all qualitative and quantitative procedures. The fraction produced dose-dependent and significant (P<0.05 and P<0.01) activities in castor oil-induced diarrheal models. Moreover, CTFAA significantly (P<0.05) demonstrated a 15.29% clot lysis effect in the thrombolytic test, and the brine shrimp lethality assay LC50 value was 424.16 µg/ml bioassay. In conclusion, the current study showed CTFAA has significant antidiarrheal effects along with modest antioxidant and thrombolytic effects, and these data warrant further experiment to justify and include CTFAA as a supplement to mitigate the onset of diarrheal and cardiovascular disease.

Hepatoprotective Activity of Linalool in Rats Against Liver Injury Induced by Carbon Tetrachloride.

This study aimed to investigate and compare hepatoprotective activity of Coriandrum sativum (Cs) and it is major component linalool (Ln) against experimentally induced hepatotoxicity in rats. Essential oil of Cs was isolated by hydrodistillation method and chemical composition was determined by GS-MS analysis. 42 male Wistar Albino rats were divited into 7 groups each containing 6. The experimental groups were designed as: Normal control group, 1 ml/kg CCl4 administirated group, 25 mg/kg Silymarin and CCl4 administirated group, 100 and 200 mg/kg Cs and CCl4 administirated groups, 100 and 200 mg/kg Ln and CCl4 administered groups. The protective activities were determined according to the results of liver biomarkers (AST, ALT, ALP), antioxidant parameters (GSH, GPx, CAT), lipid peroxidation (MDA) and histopathological examination. Linalool percentage of Cs was 81.6%. The groups treated with linalool (100 and 200 mg/kg) (p < 0.01) and coriander (200 mg/kg) (p < 0.05) had significantly reduced AST (262-375) and ALT (101-290) levels (U/L) compared to the CCl4 (600-622) group. The levels (nmol/g protein) of MDA (11-12) were significantly lower (p < 0.01), the levels of GSH (11-12) and the activities of CAT (23-24) were significantly higher (p < 0.01) in linalool groups (100 and 200 mg/kg) compared to the CCl4 (18-5-10 respectively) group. These results were also supported by histopathological findings and indicate that Cs and Ln shows hepatoprotective activity against liver damage. In this regard, evaluation of activities of major components are needed to compare to medicinal plants in experimental diseases models.

Determination of toxic heavy metals in rice samples using ultrasound assisted emulsification microextraction combined with inductively coupled plasma optical emission spectroscopy.

A straightforward and efficient method was developed by ultrasound assisted emulsification microextraction (USAEME) combined with inductively coupled plasma optical emission spectroscopy (ICP/OES) to trace some toxic heavy metal ions in eight select farmed and four select imported rice samples. In this study, carbon tetrachloride (CCl4) and 1-(2-pyridylazo)2-naphthol (PAN) were used as the extraction solvent and complexing reagent, respectively. The extraction parameters were optimized by means of a central composite design (CCD). Detection and quantification limits were set to be between 0.02 and 0.08 µg L-1 and 0.07-0.30 µg L-1, respectively. The accuracy of our method was further verified against two certified reference materials namely NCS ZC73029 and NIES No 10-b. While no trace of contaminations was found in some rice samples (Tarom, Hashemi, Sadri, Khazar and one imported sample), others such as Kazemi, Jamshidi, Dom Siah, Guilanehand and three of the imported samples were considerably polluted.

Cyanosomes: A pilot study.

Cyanobacteria are the most primitive, oxygen-evolving prokaryotic organism. Several, conspicuous investigations have been done around lipids from cyanobacteria. However, to date and to the best of our knowledge, no study was conducted to formulate and evaluate liposomal vesicles prepared from cyanobacterial lipid. In the present study, cyanosomes were formulated using natural lipids extracted from the cyanobacterium Synechococcus elongatus PCC7942 which were further loaded with thymoquinone. The results showed that the prepared cyanosomes were homogenous with an overall negative charge of particle size ranging between 200 to 210 nm and entrapment efficacy was 70.9 ± 1.86%. The obtained release profiles demonstrated sustained drug release pattern. The study of encapsulated thymoquinone on CCl4 induced liver insult in balb/c mice revealed the better efficacy of encapsulated thymoquinone as compared to thymoquinone alone indicating cyanosome as a promising candidate for drug carrier. However, more studies are required to establish the safety profile, pharmacokinetic pattern and biodistribution parameters of cyanosome for its clinical intricacies in future applications.

Molecular simulations of COFs, IRMOFs and ZIFs for adsorption-based separation of carbon tetrachloride from air.

Covalent organic frameworks (COFs), metal organic frameworks (MOFs) and zeolitic imidazolate frameworks (ZIFs) have been widely studied in gas separation applications due to their large surface areas, high pore volumes, tunable pore sizes and chemical stabilities. In this study, separation performances of 153 COFs, 14 IRMOFs and 8 ZIFs were assessed for efficient removal of carbon tetrachloride (CCl4) from CCl4/Ar, CCl4/N2, CCl4/O2 mixtures at 298 K and infinite dilution. The top performing three materials in each group, namely, borazine-linked polymer (BLP-2H-AA), IRMOF-11 and ZIF-6 were identified. Single-component, binary mixture and quaternary mixture adsorption isotherms of argon (Ar), CCl4, nitrogen (N2) and oxygen (O2) in these materials were computed at 298 K and various total pressures from 10-3 to 1.5 × 104 kPa. Mixture adsorption selectivities and separation potentials were then calculated and the effect of relative humidity on the performance of adsorption-based CCl4 separation was examined. Single-component and quaternary mixture diffusion coefficients of Ar, CCl4, N2 and O2 were finally computed. Our results showed that ZIF-6 exhibits the highest adsorption selectivity and the highest separation potential for CCl4/Ar, CCl4/N2 and CCl4/O2 mixtures, followed by IRMOF-11 and BLP-2H-AA. Results of this computational study will be highly useful to identify the promising materials for removal of CCl4 from air.

Extracorporeal Barbotage Detoxification of the Blood Plasma.

We studied the efficiency of bubbling air as a method of detoxification of the blood plasma and its mixtures with plasma substitutes from CCl4. The efficiency of detoxification depended on the characteristics of the obtained foams: foam forming ability, frequency rate, dispersion, stability of a foam skeleton, syneresis, etc. A hardware-software complex for extracorporeal blood plasma detoxification was developed and the programs for the control and regulation of this process were tested. Physical and mathematical models of toxicant excretion from the body during extracorporal blood detoxification were proposed. Verification of the mathematical model demonstrates high correspondence between the theoretical and experimental data.

CCl4-Enhanced Ultrasonic Irradiation for Ciprofloxacin Degradation and Antibiotic Activity.

In this study, an ultrasound/CCl4 system was used to degrade the fluoroquinolone antibiotic, ciprofloxacin, in aqueous solution. The effect of CCl4 concentration and initial solution pH on ciprofloxacin degradation were investigated. The results showed that ciprofloxacin degraded effectively under an ultrasound/CCl4 system, with degradation efficiency increasing from 0.51% to 50.92%, when the CCl4 concentration ranged from 0.0 to 41.4 mmol/L in 40 min. Radical scavenging experiments certified that both •OH and chlorine-containing radicals contributed to ciprofloxacin degradation. Eight intermediates were detected using ultra high-performance liquid chromatography-mass spectrometry (UHPLC-MS) method, including three chloro-intermediates. Based on these results, the possible degradation pathways of ciprofloxacin are proposed. Agar diffusion tests with E. coli and S. aureus showed that ciprofloxacin's antibacterial activity was completely removed in 40 min. This study indicates that an ultrasound/CCl4 system can degrade ciprofloxacin and remove its antibacterial activity, and thus is a promising process for treating fluoroquinolone antibiotics in wastewater.

KOtBu as a Single Electron Donor? Revisiting the Halogenation of Alkanes with CBr4 and CCl4.

The search for reactions where KOtBu and other tert-alkoxides might behave as single electron donors led us to explore their reactions with tetrahalomethanes, CX4, in the presence of adamantane. We recently reported the halogenation of adamantane under these conditions. These reactions appeared to mirror the analogous known reaction of NaOH with CBr4 under phase-transfer conditions, where initiation features single electron transfer from a hydroxide ion to CBr4. We now report evidence from experimental and computational studies that KOtBu and other alkoxide reagents do not go through an analogous electron transfer. Rather, the alkoxides form hypohalites upon reacting with CBr4 or CCl4, and homolytic decomposition of appropriate hypohalites initiates the halogenation of adamantane.

Contribution of alcohol radicals to contaminant degradation in quenching studies of persulfate activation process.

Alcohols such as ethanol (EtOH) and tert-butanol (TBA) are frequently used as quenching agents to identify the primary radical species in the persulfate (PS)-based oxidation processes. However, the contribution of alcohol radicals (ARs) to contaminant degradation in this process has rarely been assessed. In this study, trichloroacetic acid (TCA), phenol, and carbon tetrachloride were selected as probes to test the role of ARs in the thermally activated PS system. It was found that the degradation rates of these compounds were largely depended on their reactivities with ARs and the concentration of dissolved oxygen in the reaction system. In the PS/alcohol system, TCA was degraded efficiently under anaerobic conditions, while it was hardly degraded in the presence of oxygen. The results of electron paramagnetic resonance, reducing radical quenching studies, and the analysis of PS consumption suggested that ARs were the dominant reactive species contributing to TCA degradation in the PS/EtOH system under anaerobic conditions. Further studies indicated that ARs could significantly degrade CCl4 through dechlorination but not phenol. CCl4 was also degraded efficiently by ARs when oxygen in the reaction solution was completely consumed by ARs. This study highlights the important role of alcohol radicals in the degradation of contaminants during quenching studies in PS-activated processes.

Simultaneous anaerobic transformation of carbon tetrachloride to carbon dioxide and tetrachloroethene to ethene in a continuous flow column.

The simultaneous anaerobic transformation of tetrachloroethene (PCE) and carbon tetrachloride (CT) was evaluated in a continuous flow column. The column was packed with quartz sand and bioaugmented with the Evanite culture (EV) that is capable of transforming PCE to ethene. Azizian and Semprini (2016) reported that PCE and CT could be simultaneously transformed in the column, with PCE (0.1mM) transformed mainly to ethene and CT (0.015mM) to chloroform (CF) (20%) and an unknown transformation product, likely carbon dioxide (CO2). The fermentation of propionate, formed from lactate fermentation, was inhibited after the transformation of CT, likely from the exposure to CF. Reported here is the second phase of that study where a second bioaugmentation of the EV culture was made to reintroduce a lactate and propionate fermenting population to the column. Effective lactate and propionate fermentation were restored with a H2 concentration of ~25nM maintained in the column effluent. PCE (0.1mM) was effectively transformed to ethene (~98%) and vinyl chloride (VC) (~2%). Unlabeled CT (0.015 to 0.03mM) was completely transformed with a transient build-up of CF and chloromethane (CM), which were subsequently removed below their detection limits. A series of transient tests were initiated through the addition of carbon-13 labeled CT (13CT), with concentrations gradually increased from 0.03 to 0.10mM. GC-MS analysis of the column effluent showed that 13C labeled CO2 (13CO2) was formed, ranging from 82 to 93% of the 13CT transformed, with the transient increases in 13CO2 associated with the increased concentration of 13CT. A modified COD analysis indicated a lesser amount of 13CT (18%) was transformed to soluble products, while 13CO2 represented 82% the 13CT transformed. In a final transient test, the influent lactate concentration was decreased from 1.1 to 0.67mM. The transformation of both CT and PCE changed dramatically. Only 59% of the 13CT was transformed, primarily to CF. 13CO2 concentrations gradually decreased to background levels, indicating CO2 was no longer a transformation product. PCE transformation resulted in the following percentage of products formed: cDCE (60%), VC (36%), and ethene (4%). Incomplete propionate fermentation was also observed, consistent with the build-up of CF and the decrease in H2 concentrations to approximately 2nM. The results clearly demonstrate that high concentrations of CT were transformed to CO2, and effective PCE dehalogenation to ethene was maintained when excess lactate was fed and propionate was effectively fermented. However, when the lactate concentration was reduced, both PCE and CT transformation and propionate fermentation were negatively impacted.

Fabrication of bimetallic Ag/Fe immobilized on modified biochar for removal of carbon tetrachloride.

As an effective conventional absorbent, biochar exhibited limited adsorption ability toward small hydrophobic molecules. To enhance the adsorption capacity, a novel adsorbent was prepared by immobilizing nanoscale zero-valent iron onto modified biochar (MB) and then the elemental silver was attached to the surface of iron (Ag/Fe/MB). It's noted that spherical Ag/Fe nanoparticles with diameter of 51nm were highly dispersed on the surface of MB. As the typical hydrophobic contaminant, carbon tetrachloride was selected for examining the removal efficiency of the adsorbent. The removal efficiencies of carbon tetrachloride by original biochar (OB), Ag/Fe, Ag/Fe/OB and Ag/Fe/MB were fully investigated. It's found that Ag/Fe/MB showed higher carbon tetrachloride removal efficiency, which is about 5.5 times higher than that of the OB sample due to utilizing the merits of high adsorption and reduction. Thermodynamic parameters revealed that the removal of carbon tetrachloride by Ag/Fe/MB was a spontaneous and exothermic process, which was affected by solution pH, initial carbon tetrachloride concentration and temperature. The novel Ag/Fe/MB composites provided a promising material for carbon tetrachloride removal from effluent.

Amino Acid-Assisted Dehalogenation of Carbon Tetrachloride by Green Rust: Inhibition of Chloroform Production.

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.

Iron-Sulfide-Associated Products Formed during Reductive Dechlorination of Carbon Tetrachloride.

This paper investigated the mackinawite (FeS)-associated products formed during reaction between FeS and carbon tetrachloride (CT) at pH 7 and 8. At pH 8, reaction of FeS with CT led to formation of abundant spherical particles with diameters between 50 and 400 nm on the FeS surface and in solution; far fewer such particles were observed at pH 7. Analysis of the FeS surface by energy dispersive X-ray spectroscopy after reaction with CT at pH 8 showed decreased sulfur and elevated oxygen compared to unreacted FeS. The spherical particles that formed upon FeS reaction with CT were mostly amorphous with localized areas of poorly crystalline two-line ferrihydrite. X-ray photoelectron spectroscopy indicated that the predominant Fe surface species after reaction with CT at pH 8 was Fe(III)-O, consistent with ferrihydrite and other amorphous iron (hydr)oxides as major products. Powder X-ray diffraction analysis suggested formation of greigite upon reaction of FeS with CT at pH 7. Both ferrihydrite and Fe(2+), which is a product of greigite dissolution, can react with dissolved HS(-) to form FeS, suggesting that, after oxidation by chlorinated aliphatics, FeS can be regenerated by addition or microbial generation of sulfide.

Chemical Reactivity Probes for Assessing Abiotic Natural Attenuation by Reducing Iron Minerals.

Increasing recognition that abiotic natural attenuation (NA) of chlorinated solvents can be important has created demand for improved methods to characterize the redox properties of the aquifer materials that are responsible for abiotic NA. This study explores one promising approach: using chemical reactivity probes (CRPs) to characterize the thermodynamic and kinetic aspects of contaminant reduction by reducing iron minerals. Assays of thermodynamic CRPs were developed to determine the reduction potentials (ECRP) of suspended minerals by spectrophotometric determination of equilibrium CRP speciation and calculations using the Nernst equation. ECRP varied as expected with mineral type, mineral loading, and Fe(II) concentration. Comparison of ECRP with reduction potentials measured potentiometrically using a Pt electrode (EPt) showed that ECRP was 100-150 mV more negative than EPt. When EPt was measured with small additions of CRPs, the systematic difference between EPt and ECRP was eliminated, suggesting that these CRPs are effective mediators of electron transfer between mineral and electrode surfaces. Model contaminants (4-chloronitrobenzene, 2-chloroacetophenone, and carbon tetrachloride) were used as kinetic CRPs. The reduction rate constants of kinetic CRPs correlated well with the ECRP for mineral suspensions. Using the rate constants compiled from literature for contaminants and relative mineral reduction potentials based on ECRP measurements, qualitatively consistent trends were obtained, suggesting that CRP-based assays may be useful for estimating abiotic NA rates of contaminants in groundwater.