Enhanced dechlorination of carbon tetrachloride by Geobacter sulfurreducens in the presence of naturally occurring quinones and ferrihydrite.

The effect of naturally occurring quinones including lawsone (LQ), ubiquinone (UQ), juglone (JQ), and 1,4-naphthoquinone (NQ) on the biotransformation of carbon tetrachloride (CT) in the presence of Geobacter sulfurreducens and ferrihydrite was investigated. AQDS was used as the model compound for comparison. The reductive dissolution of ferrihydrite by G. sulfurreducens was enhanced by AQDS, NQ, and LQ. However, addition of UQ and JQ had little enhancement effect on Fe(II) production. The bioreduction efficiency and rate of ferrihydrite was highly dependent on the natural property and concentration of quinone compounds and the addition of low concentrations of LQ and NQ significantly accelerated the biotransformation rate of CT. The pseudo-first-order rate constants for CT dechlorination (kobsCT) in AQDS-, LQ- and NQ-amended batches were 5.4-5.8, 4.6-7.4 and 2.4-5.8 times, respectively, higher than those in the absence of quinone. A good relationship between kobsCT for CT dechlorination and bioreduction ratio of ferrihydrite was observed, indicating the important role of biogenic Fe(II) in dechlorination of CT under iron-reducing conditions. Spectroscopic analysis showed that AQDS and NQ could be reduced to semiquinones and hydroquinones, while only hydroquinones were generated in LQ-amended batches.

[Kinetic study of 4-chloronitrobenzene degradation by zero-valent iron].

4-chloronitrobenzene as a representative material of nitroaromatic compounds was used in this study to investigate the degradation reaction rate and products by different concentrations of zero-valent iron (ZVI) under anoxic condition. According to stoichiometry, different reaction rates of products were obtained by fitting the experimental data. Products of ZVI were measured by Mössbauer technique. The results show that reduction of 4-chloronitrobenzene corresponds to the concentration of ZVI. The production and transformation rates of intermediate products, 4-chloronitrosobenzene and 4-chloro phenyl hydroxylamine, can be achieved. The 4-chloronitrobenzene reduction reaction is the fastest when the ZVI concentration is 1.04 g x L(-1). The reaction rate constant is 0.189 min(-1). Ferrous iron ions generated during reaction are sorbed on the ZVI surfaces in the early age of the reaction. Formation and reduction reaction rates of different products depend on the reactive sites of ZVI and the mass transfer between each other.

Enhanced dechlorination of tetrachloroethylene by zerovalent silicon in the presence of polyethylene glycol under anoxic conditions.

The combination of zerovalent silicon (Si(0)) with polyethylene glycol (PEG) is a novel technique to enhance the dechlorination efficiency and rate of chlorinated hydrocarbons. In this study, the dechlorination of tetrachloroethylene (PCE) by Si(0) in the presence of various concentrations of PEG was investigated under anoxic conditions. Several surfactants including cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and Tween 80 were also selected for comparison. Addition of SDS and Tween 80 had little effect on the enhancement of PCE dechlorination, while CTAB and PEG significantly enhanced the dechlorination efficiency and rate of PCE by Si(0) under anoxic conditions. The Langmuir-Hinshelwood model was used to describe the dechlorination kinetics of PCE and could be simplified to pseudo-first-order kinetics at low PCE concentration. The rate constants (k(obs)) for PCE dechlorination were 0.21 and 0.36 h(-1) in the presence of CTAB and PEG, respectively. However, the reaction mechanisms for CTAB and PEG are different. CTAB could enhance the apparent water solubility of PCE in solution containing Si(0), leading to the enhancement of dechlorination efficiency and rate of PCE, while PEG prevented the formation of silicon dioxide, and significantly enhanced the dechlorination efficiency and rate of PCE at pH 8.3 ± 0.2. In addition, the dechlorination rate increased upon increasing PEG concentration and then leveled off to a plateau when the PEG concentration was higher than 0.2 µM. The k(obs) for PCE dechlorination by Si(0) in the presence of PEG was 106 times higher than that by Si(0) alone. Results obtained in this study would be helpful in facilitating the development of processes that could be useful for the enhanced degradation of cocontaminants by zerovalent silicon.

Concentration effect of copper loading on the reductive dechlorination of tetrachloroethylene by zerovalent silicon.

The dechlorination of tetrachloroethylene (PCE) by zerovalent silicon (Si(0)) in the presence of low concentration of Cu(II) ion was investigated under anaerobic conditions. The mass loadings of Cu(II) in the Si(0)-H(2)O system were in the range 0.06-3 wt% (0.02-1 mM). In addition, the X-ray photoelectron spectroscopy (XPS) and electron probe microanalysis (EPMA) were used to characterize the change in chemical species and distribution patterns of metals, respectively. Results showed that the pre-incubation time of 3 d was needed to activate the reactive sites of Si(0) before the dechlorination of PCE. Addition of low concentration of Cu(II) at 0.06 wt% significantly enhanced the dechlorination of PCE, while high concentration of Cu(II) would occupy the reactive sites of Si(0), and subsequently decreased the dechlorination efficiency and rate of PCE. The pseudo first-order rate constant (k(obs)) for PCE dechlorination by 0.06 wt% Cu/Si was 0.028 h(-1), which was 2.8 times higher than that by Si(0) alone. However, the k(obs) for PCE dechlorination decreased to 0.0016 h(-1) when the loading of Cu(II) increased to 3 wt%. The EPMA results showed that the distribution of 0.06 wt% Cu on the Si(0) surface was homogeneous without any aggregation, which means that the maximum rate constant was observed before the total coverage of the active sites on the reductive metal by the catalytic metal layer. The surface coverage of Cu to Si(0) can theoretically calculate by estimation of the lowest energy fcc(111) crystallographic orientation. The calculated surface coverage of 0.06 wt% Cu onto Si(0) was approximately 43%, which is consistent with the experimental results obtained in this study.

Dechlorination of tetrachloroethylene in aqueous solutions using metal-modified zerovalent silicon.

The combination of zerovalent metal with a catalytic second metal ion (bimetallic materials) to enhance the dechlorination efficiency and rate of chlorinated compounds has received much attention. Bimetallic materials not only enhance the dechlorination process but also alter the reduction pathway and product distribution. In this study, the efficiency and rate of tetrachloroethylene (PCE) dechlorination by metal-modified zerovalent silicon was investigated as a potential reductant for chlorinated hydrocarbons under anoxic conditions. The X-ray photoelectron spectroscopic (XPS) results showed that metal ions including Ni(II), Cu(II), and Fe(II) could be reduced to their zerovalent forms on the Si surface. The dechlorination of PCE obeyed the pseudo-first-order kinetics, and the pseudo-first-order rate constants (k(obs)) for PCE dechlorination followed the order Ni/Si > Fe/Si > Cu/Si. Addition of Cu(II) lowered the dechlorination efficiency and rate of PCE by Si, while the k(obs) values for PCE dechlorination in the presence of 0.1 mM Fe(II) and Ni(II) were 1.5-3.8 times higher than that by Si alone. In addition, the efficiency and rate of PCE dechlorination increased upon increasing the mass loading of Ni(II) ranging between 0.05 and 0.5 mM and then decreased when the Ni(II) loading was further increased to 1 mM. The scanning electron microscopic (SEM) images and electron probe microanalytical (EPMA) maps showed that the Ni nanoparticles deposited on the Si surface and aggregated to a large particle at 1 mM Ni(II), which clearly depicts that the Ni(II) loading of 0.5 mM is the optimal value to enhance the efficiency and rate of PCE dechlorination by Si. Also, the reaction pathways for PCE dechlorination changed from hydrogenolysis in the absence of Ni(II) to hydrodechlorination when Ni(II) concentrations were higher than 0.05 mM. Results obtained in this study reveal that the metal-deposited zerovalent silicon can serve as an environmentally friendly reductant for the enhanced degradation of chlorinated hydrocarbons for long-term performance.