Tongxu Liu

Enhanced reductive dechlorination of DDT in an anaerobic system of dissimilatory iron-reducing bacteria and iron oxide

The transformation of DDT was studied in an anaerobic system of dissimilatory iron-reducing bacteria (Shewanella decolorationis S12) and iron oxide (alpha-FeOOH). The results showed that S. decolorationis could reduce DDT into DDD, and DDT transformation rate was accelerated by the presence of alpha-FeOOH. DDD was observed as the primary transformation product, which was demonstrated to be transformed in the abiotic system of Fe(2+)+alpha-FeOOH and the system of DIRB+alpha-FeOOH. The intermediates of DDMS and DBP were detected after 9 months, likely suggesting that reductive dechlorination was the main dechlorination pathway of DDT in the iron-reducing system. The enhanced reductive dechlorination of DDT was mainly due to biogenic Fe(II) sorbed on the surface of alpha-FeOOH, which can serve as a mediator for the transformation of DDT. This study demonstrated the important role of DIRB and iron oxide on DDT and DDD transformation under anaerobic iron-reducing environments.

Foliar application of two silica sols reduced cadmium accumulation in rice grains.

In the present study, pot experiments were conducted to investigate the effects of foliar application of two silica (Si) sols on the alleviation of cadmium (Cd) toxicity in contaminated soil to rice. Results showed that the foliar application of Si sols significantly increased the dry weight of grains (without husk) and shoots in rice grown in Cd contaminated soil, whereas the Cd concentration in the grains and shoots decreased obviously. The total accumulation of Cd in rice grains also decreased with the application of both of the Si sols, but no significant effect was found on the Cd accumulation in the shoots. For the optimal effect, Si-sol-B should be foliar applied at the tillering-stage during rice growth. The mechanism of Si foliar application to alleviate the toxicity and accumulation of Cd in grains of rice may be related to the probable Cd sequestration in the shoot cell walls.

Exogenous Electron Shuttle-Mediated Extracellular Electron Transfer of Shewanella putrefaciens 200: Electrochemical Parameters and Thermodynamics

Despite the importance of exogenous electron shuttles (ESs) in extracellular electron transfer (EET), a lack of understanding of the key properties of ESs is a concern given their different influences on EET processes. Here, the ES-mediated EET capacity of Shewanella putrefaciens 200 (SP200) was evaluated by examining the electricity generated in a microbial fuel cell. The results indicated that all the ESs substantially accelerated the current generation compared to only SP200. The current and polarization parameters were linearly correlated with both the standard redox potential (E(ES)(0)) and the electron accepting capacity (EAC) of the ESs. A thermodynamic analysis of the electron transfer from the electron donor to the electrode suggested that the EET from c-type cytochromes (c-Cyts) to ESs is a crucial step causing the differences in EET capacities among various ESs. Based on the derived equations, both E(ES)(0) and EAC can quantitatively determine potential losses (ΔE) that reflect the potential loss of the ES-mediated EET. In situ spectral kinetic analysis of ES reduction by c-Cyts in a living SP200 suspension was first investigated with the E(ES), E(c-Cyt), and ΔE values being calculated. This study can provide a comprehensive understanding of the role of ESs in EET.

Depassivation of Aged Fe0 by Inorganic Salts: Implications to Contaminant Degradation in Seawater

In this study, aged (iron oxide coated) Fe(0) was applied to the degradation of trichloroethylene (TCE) in seawater. It was found that while the aged Fe(0) was inactive with regard to TCE degradation in Milli-Q water, more than 95% of the TCE present was degraded in real and synthetic seawater solutions after exposure to aged Fe(0) for 21 days. Results with individual salts from the synthetic seawater revealed that no significant TCE degradation was observed in the presence of Na2SO4, CaCl2, and NaHCO3. Partial TCE degradation (28.4%) was observed in 500 mM NaCl after 21 days, while a similar extent of degradation to that found in the seawater solutions was observed in 50 mM solutions of magnesium salts (MgCl2 and MgSO4). Results of open circuit potential analysis suggested that the Fe(0) corrosion potential was not a key determinant of extent of TCE reduction since the corrosion potential decreased to levels similar to that of Fe(0)/Fe(2+) in the presence of all salts examined. Lower final pH values and higher dissolved Fe(II) concentrations were observed in the presence of magnesium salts compared to other salts. Formation of the surface complex >FeOMg(+) was identified as being critical to protonation of surface sites, reductive dissolution of the passivating Fe(III) oxyhydroxide layer coating the underlying Fe(0) and enhancement in extent of TCE reduction. These findings provide insight into the molecular-scale mechanism of depassivation of aged Fe(0) by inorganic salts with particular implications for the Fe(0)-mediated degradation of contaminants in saline natural waters such as seawater and saline groundwaters.

Fe(III) oxides accelerate microbial nitrate reduction and electricity generation by Klebsiella pneumoniae L17

Klebsiella pneumoniae L17 is a fermentative bacterium that can reduce iron oxide and generate electricity under anoxic conditions, as previously reported. This study reveals that K. pneumoniae L17 is also capable of dissimilatory nitrate reduction, producing NO2(-), NH4(+), NO and N2O under anoxic conditions. The presence of Fe(III) oxides (i.e., α-FeOOH, γ-FeOOH, α-Fe2O3 and γ-Fe2O3) significantly accelerates the reduction of nitrate and generation of electricity by K. pneumoniae L17, which is similar to a previous report regarding another fermentative bacterium, Bacillus. No significant nitrate reduction was observed upon treatment with Fe(2+) or α-FeOOH+Fe(2+), but a slight facilitation of nitrate reduction and electricity generation was observed upon treatment with L17+Fe(2+). This result suggests that aqueous Fe(II) or mineral-adsorbed Fe(II) cannot reduce nitrate abiotically but that L17 can catalyze the reduction of nitrate and generation of electricity in the presence of Fe(II) (which might exist as cell surface-bound Fe(II)). To rule out the potential effect of Fe(II) produced by L17 during microbial iron reduction, treatments with the addition of TiO2 or Al2O3 instead of Fe(III) oxides also exhibited accelerated microbial nitrate reduction and electricity generation, indicating that cell-mineral sorption did account for the acceleration effect. However, the acceleration caused by Fe(III) oxides is only partially attributed to the cell surface-bound Fe(II) and cell-mineral sorption but may be driven by the iron oxide conduction band-mediated electron transfer from L17 to nitrate or an electrode, as proposed previously. The current study extends the diversity of bacteria of which nitrate reduction and electricity generation can be facilitated by the presence of iron oxides and confirms the positive role of Fe(III) oxides on microbial nitrate reduction and electricity generation by particular fermentative bacteria in anoxic environments.

Electron transfer capacity dependence of quinone-mediated Fe(III) reduction and current generation by Klebsiella pneumoniae L17.

Quinone groups in exogenous electron shuttles can accelerate extracellular electron transfer (EET) from bacteria to insoluble terminal electron acceptors, such as Fe(III) oxides and electrodes, which are important in biogeochemical redox processes and microbial electricity generation. However, the relationship between quinone-mediated EET performance and electron-shuttling properties of the quinones remains incompletely characterized. This study investigates the effects of a series of synthetic quinones (SQs) on goethite reduction and current generation by a fermenting bacterium Klebsiella pneumoniae L17. In addition, the voltammetric behavior and electron transfer capacities (ETCs) of SQ, including electron accepting (EAC) and donating (EDC) capacities, is also examined using electrochemical methods. The results showed that SQ can significantly increase both the Fe(III) reduction rates and current outputs of L17. Each tested SQ reversibly accepted and donated electrons as indicated by the cyclic voltammograms. The EAC and EDC results showed that Carmine and Alizarin had low relative capacities of electron transfer, whereas 9,10-anthraquinone-2,6-disulfonic acid (AQDS), 2-hydroxy-1,4-naphthoquinone (2-HNQ), and 5-hydroxy-1,4-naphthoquinone (5-HNQ) showed stronger relative ETC, and 9,10-anthraquinone-2-carboxylic acid (AQC) and 9,10-anthraquinone-2-sulfonic acid (AQS) had high relative ETC. Enhancement of microbial goethite reduction kinetics and current outputs by SQ had a good linear relationship with their ETC, indicating that the effectiveness of quinone-mediated EET may be strongly dependent on the ETC of the quinones. Therefore, the presence of quinone compounds and fermenting microorganisms may increase the diversity of microbial populations that contribute to element transformation in natural environments. Moreover, ETC determination of different SQ would help to evaluate their performance for microbial EET under anoxic conditions.

Depassivation of aged Fe 0 by divalent cations: correlation between contaminant degradation and surface complexation constants.

The dechlorination of trichloroethylene (TCE) by aged Fe(0) in the presence of a series of divalent cations was investigated with the result that while no significant degradation of TCE was observed in Milli-Q water or in solutions of Ba(2+), Sr(2+), or Ca(2+), very effective TCE removal was observed in solutions containing Mg(2+), Mn(2+), Co(2+), Fe(2+), Ni(2+), Zn(2+), Cu(2+), or Pb(2+). The rate constants of TCE removal in the presence of particular cations were positively correlated to the log K representing the affinity of the cations for hydrous ferric oxide (HFO) surface sites though the treatments with Co(2+) and Ni(2+) were found to provide particularly strong enhancement in TCE degradation rate. The extent of Fe(II) release to solution also increased with increase in log K, while the solution pH from both experimental measurement and thermodynamic calculation decreased with increasing log K. While the peak areas of Fe and O XPS spectra of the passivated ZVI in the presence of Ba(2+), Sr(2+), and Ca(2+) were very close to those in Milli-Q water, very significant increases in surface Fe and O (and OH) were observed in solutions of Mg(2+), Mn(2+), Co(2+), Fe(2+), Ni(2+), Zn(2+), Cu(2+) and Pb(2+), revealing that the surface oxide layer dissolution is consistent with the recovery of aged Fe(0) with respect to TCE degradation. The depassivation process is proposed to involve (i) surface complexation of cations on surface coatings of aged Fe(0), (ii) dissolution of the hydrated surface as a consequence of magnetite exposure, and (iii) transport of electrons from underlying Fe(0) via magnetite to TCE, resulting in TCE dechlorination and, for some cations (Co(2+), Ni(2+), Cu(2+), and Pb(2+)), reduction to their zero or +1 valence state (with potential for these reduced metals to enhance TCE degradation).

Effect of Aeromonas hydrophila on reductive dechlorination of DDTs by zero-valent iron.

This study presents a reductive transformation method that combines zerovalent iron (ZVI) and Aeromonas hydrophila HS01 with iron oxide reduction property to degrade DDT (1,1-trichloro-2,2-bis(4-chlorophenyl)ethane) under anoxic conditions. The results suggest that HS01 has weak capability in terms of reducing DDT to DDD (1,1-dichloro-2,2-bis(p-chlorophenyl)ethane) and nearly failed to reduce DDD or its transformed intermediates. The coexistence of ZVI and HS01 results in a slight enhancement of DDT degradation compared with the ZVI system alone. The reduction of intermediates by ZVI, however, can be obviously accelerated in the presence of HS01, and the addition of anthraquinone-2,6-disulfonic disodium salt (AQDS) can accelerate the transformation rates further, especially for intermediate reduction. The analysis of the amount and electrochemical properties of Fe(III)/Fe(II) indicates that the presence of HS01 with or without AQDS is beneficial to the reduction of Fe(III) to Fe(II), resulting in the removal of passivating ferric precipitates on the ZVI surface. A mechanism and pathway that clarify the roles of ZVI, HS01, and AQDS in the ZVI + HS01 + AQDS system for DDT transformation are proposed. The quick removal of surface ferric precipitates is thought to be the reason for the enhancement of the transformation of DDT and its intermediates.

Effect of iron oxides and carboxylic acids on photochemical degradation of bisphenol A

Abstractγ-FeOOH was initially prepared by hydrothermal process and then sintered at 280°C, 310°C, and 420°C, and four kinds of iron oxides were obtained and named as lepidocrocite, IO-280, IO-310, and IO-420. They were characterized by XRD, SEM, and BET techniques to disclose the crystal composition, morphology, and surface area. The XRD results show that IO-280 and IO-310 should consist of maghemite and hematite, while IO-420 should be pure hematite. With the increase of temperature, the specific surface area significantly decreased. To test the photocatalytic activity of iron oxides, bisphenol A (BPA) was selected as a model chemical. The results show that BPA photocatalytic degradation should depend strongly on pH value, light source, and the crystal structure of iron oxides and that IO-310 had the highest activity in the absence of oxalic acid under UV or visible light illumination. The dependence of BPA photodegradation on carboxylic acids in lepidocrocite-carboxylate systems was investigated. BPA degradation was promoted greatly by the addition of oxalic and citric acid, and slightly by tartaric, malonic, and malic acid. The first-order kinetic constant (k) of BPA degradation follows the order oxalic% MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX! % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbbjxAHX % garmWu51MyVXgatuuDJXwAK1uy0HwmaeHbfv3ySLgzG0uy0Hgip5wz % aebbnrfifHhDYfgasaacH8qrps0lbbf9q8WrFfeuY-Hhbbf9v8qqaq % Fr0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qq % Q8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeWaeaaakeaatu % uDJXwAK1uy0HMmaeXbfv3ySLgzG0uy0HgiuD3BaGqbaiab-LTincaa % !4341!