Reductive dechlorination of octachlorodibenzo-p-dioxin by nanosized zero-valent zinc : Modeling of rate kinetics and congener profile

Polychlorinated dibenzo-p-dioxins (PCDDs), a group of recalcitrant toxic compounds, are ubiquitous in nature. Amongst them, octachlorodibenzo-p-dioxin (OCDD) is not only prevalent in soil and sediment due to its high lipophilicity and hydrophobicity, but also detected in ground water and surface water. The present study examined the degradation of OCDD in aqueous solutions using four different zero-valent metal nanoparticles; zero-valent aluminum (nZVAL), zero-valent zinc (nZVZ), zero-valent iron (nZVI) and zero-valent nickel (nZVN). Only nZVZ was found to efficiently degrade OCDD into lower chlorinated congeners [OCDD -> 1,2,3,4,6,7,9-HxCDD (63%) -> 1,2,3,6,8,9-HpCDD (21%) -> 1,2,4,7,8-PeCDD (46%) -> 1,2,4,7-TeCDD (19%)] under ambient conditions. Simulations were also performed to predict the OCDD dechlorination pathway using a linear free energy relationship (LFER) model. Additionally, toxic equivalent quantity (TEQ) and homologue patterns were calculated by LFER modeling. The experimentally observed congener profiles were in excellent agreement with the model-predicted results, especially considering the complexity of the OCDD dechlorination pathway (256 theoretically possible reactions). This study proposes nZVZ as a suitable candidate for OCDD dechlorination and constitutes the first report on OCDD degradation using zero-valent metal nanoparticles under ambient conditions. (C) 2013 Elsevier B.V. All rights reserved.X111212sciescopu

Integrated hybrid treatment for the remediation of 2,3,7,8-tetrachlorodibenzo-p-dioxin

The dioxin isomer 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TeCDD) has been reported as the deadliest compound known to science. Due to its highly recalcitrant nature and low bioavailability, it is stubborn toward bioremediation and chemical treatment. Efforts to degrade it using one single technique have not accomplished the desired results. In this study, we have tried to develop an integrated 2,3,7,8-TeCDD removal process using palladized iron nanoparticles (Pd/nFe) for initial reductive dechlorination under anoxic conditions and subsequent oxidative biomineralization. Using laboratory synthesized Pd/nFe, 2,3,7,8-TeCDD was completely dechlorinated to form the end product dibenzo-p-dioxin (DD). Oxidative degradation of DD was successfully achieved by growing active cells of a dioxin-degrading microorganism Sphingomonas wittichii RW1 (DSM 6014) under aerobic culture conditions. Metabolite identification was done by high performance liquid chromatography (HPLC) and whole cell protein was measured as the indicator for cell growth. To the best of our knowledge, this is the first report on integrated hybrid degradation method for 2,3,7,8-TeCDD. (C) 2012 Elsevier B.V. All rights reserved.X1158sciescopu

Predicting reductive debromination of polybrominated diphenyl ethers by nanoscale zerovalent iron and its implications for environmental risk assessment

The reductive debromination of polybrominated diphenyl ethers (PBDEs) by nanoscale zerovalent iron (nZVI) has proven to be a successful remediation approach. This study simulates the congener profiles and overall ecotoxicological impact of PBDE debromination by nZVI. The relationship between the calculated redox potential values and PBDE debromination rates was sufficiently strong to generate a satisfactory predictive capacity, which was further used to develop a quantitative structure-activity relationship (QSAR) model for the determination of the PBDE debromination patterns and dominant pathways. The predicted results of deca-BDE debromination showed that it would completely disappear within 30 days, but its lower brominated products, particularly tri- to penta-homologues, could exist in the environment even after 5 years. Formation and accumulation of more toxic, low brominated congeners through deca-BDE debromination suggest that deca-BDE may pose prolonged environmental risks. Changes in the toxic equivalent (TEQ) values during deca-BDE debromination parallel the occurrence and transformation of specific low brominated congeners with dioxin-like potency. (C) 2013 Elsevier B.V. All rights reserved.X11109sciescopu

Enhanced removal of chromate from aqueous solution by sequential adsorption-reduction on mesoporous iron-iron oxide nanocomposites

The adsorption behavior of mesoporous iron nanocomposites was investigated with respect to chromate [Cr(VI)] removal from aqueous solutions to consider its application for purifying chromate-contaminated wastewaters. These nanocomposites were prepared by borohydride reduction in aqueous solutions containing varying concentrations of acetone as co-solvent. Using batch methods, enhanced adsorption of Cr(VI) on the nanocomposite surface was achieved at neutral pH conditions, which subsequently resulted in Cr(VI) reduction to Cr(III). The Langmuir model was found to excellently describe the adsorption process, offering a maximum adsorptive capacity of 34.1 mg/g for composites prepared with 50 % acetone concentration. The Cr(VI) removal efficiency of these iron nanocomposites is strongly dependent on the acetone concentration, as evident from their (1) increased surface area (141.1 m(2)/g) compared to conventional iron nanoparticles (33.2 m(2)/g), and (2) highly porous and acicular structure, which efficiently traps Cr(VI) through adsorption. X-ray photoelectron spectroscopy analysis of Cr(III) on the nanocomposite surface confirmed that Cr(VI) removal from solution was achieved by sequential adsorption-reduction.X11611sciescopu

Effect of Fe-Pd bimetallic nanoparticles on Sphingomonas sp PH-07 and a nano-bio hybrid process for triclosan degradation

In this study, we have evaluated the effect of palladium-iron bimetallic nanoparticles (nFe-Pd) on diphenyl ether (DE) degrading bacterial strain Sphingomonas sp. PH-07 as well as a sequential nano-bio hybrid process with nFe-Pd as catalytic reductant and PH-07 as biocatalyst for degradation of triclosan. Strain PH-07 grew well in the presence of nFe-Pd up to 0.1 g/L in minimal salts medium with DE as carbon source. In aqueous system, TCS (17.3 mu M) was completely dechlorinated within 2 h by nFe-Pd (0.1 g/L) with concomitant release of 2-phenoxyphenol (16.8 mu M) and chloride ions (46 mu M). All possible dichloro-and monochloro-2-phenoxyphenol intermediates were identified by HPLC and GC-MS analyses, and the dechlorination pathway was proposed. Addition of PH-07 cells into the reactor effectively degraded the 2-phenoxyphenol. Our results reveal that strain PH-07 survives well in the presence of nFe-Pd and nFe-Pd/PH-07 hybrid treatment could be a potential strategy for degradation of TCS. (C) 2011 Elsevier Ltd. All rights reserved.X113031sciescopu

Degradation of triclosan by an integrated nano-bio redox process

In this study, a sequential reduction-oxidation method was developed for complete degradation of triclosan (2,4,4'-trichloro-2'-hydroxydiphenyl ether, TCS) in aqueous solution. Rapid reductive dechlorination of TCS was achieved with palladized zero-valent iron nanoparticles (Pd/nFe), under anaerobic conditions, with generation of 2-phenoxyphenol as the sole dechlorination product. Sequentially, 2-phenoxyphenol was transformed into a non-toxic polymer using laccase (EC:1.10.3.2) derived from Trametes versicolor in the presence of natural redox mediator syringaldehyde (SYD). High performance liquid chromatography combined with electrospray ionization mass spectroscopy (HPLC-ESI-MS) revealed the formation of dimer and trimer products during the laccase-mediated transformation process. The efficiency of the integrated method is critically dependent on the Fe2+ concentration, which was effectively controlled by optimizing the solution pH. To the best of our knowledge, this is the first report of a redox two-step hybrid system for the complete transformation of TCS into non-toxic products. (C) 2010 Elsevier Ltd. All rights reserved.X113837sciescopu

Triclosan susceptibility and co-metabolism – a comparison for three aerobic pollutant-degrading bacteria

The antimicrobial agent triclosan is an emerging and persistent environmental pollutant. This study evaluated the susceptibility and biodegradation potential of triclosan by three bacterial strains (Sphingomonas wittichii RW1, Burkholderia xenovorans LB400 and Sphingomonas sp. PH-07) that are able to degrade aromatic pollutants (dibenzofuran, biphenyl and diphenyl ether, respectively) with structural similarities to triclosan. These strains showed less susceptibility to triclosan when grown in complex and mineral salts media. Biodegradation experiments revealed that only strain PH-07 was able to catabolize triclosan to intermediates that included hydroxylated compounds (monohydroxy-triclosan, and dihydroxy-triclosan) and the ether bond cleavage products (4-chlorophenol and 2,4-dichlorophenol), indicating that the initial dihydroxylation occurred on both aromatic rings of triclosan. Additional growth inhibition tests demonstrated that the main intermediate, 2,4-dichlorophenol, was less toxic to strain PH-07 than was triclosan. Our results indicate that ether bond cleavage might be the primary mechanism of avoiding triclosan toxicity by this strain. (C) 2010 Elsevier Ltd. All rights reserved.X115958sciescopu