Xin Song

Impurity band conduction in the thermoelectric material ZnSb

ZnSb is favourable as a thermoelectric material, from both an environmental and a global resources point of view. Its efficiency can possibly be improved by the reduction of the thermal conductivity through nanostructuring and optimization of doping. These tasks require a better understanding of the material and, in particular, of the interplay between preparation techniques and material properties. We have prepared bulk polycrystalline samples and report on low-temperature electrical transport measurements (6 K to room temperature). The data have been interpreted in terms of hole impurity band conduction: intrinsic acceptor defects creating bands that are conducting when there are also compensating donors. Modelling the transport reveals qualitatively good agreement. Quantitative differences are discussed in terms of the structure of the samples, which has been studied by using x-ray diffraction, a scanning electron microscope and an electron microprobe. The systematics of adding different amounts of Mn and Cr to ZnSb has been studied and has the effect of varying the density of states in impurity bands and varying the hole concentrations.

Enhancement of thermoelectric properties by energy filtering: Theoretical potential and experimental reality in nanostructured ZnSb

Energy filtering has been suggested by many authors as a means to improve thermoelectric properties. The idea is to filter away low-energy charge carriers in order to increase Seebeck coefficient without compromising electronic conductivity. This concept was investigated in the present paper for a specific material (ZnSb) by a combination of first-principles atomic-scale calculations, Boltzmann transport theory, and experimental studies of the same system. The potential of filtering in this material was first quantified, and it was as an example found that the power factor could be enhanced by an order of magnitude when the filter barrier height was 0.5 eV. Measured values of the Hall carrier concentration in bulk ZnSb were then used to calibrate the transport calculations, and nanostructured ZnSb with average grain size around 70 nm was processed to achieve filtering as suggested previously in the literature. Various scattering mechanisms were employed in the transport calculations and compared with the ...

Nanostructuring of Undoped ZnSb by Cryo-Milling

We report the preparation of nanosized ZnSb powder by cryo-milling. The effect of cryo-milling then hot-pressing of undoped ZnSb was investigated and compared with that of room temperature ball-milling and hot-pressing under different temperature conditions. ZnSb is a semiconductor with favorable thermoelectric properties when doped. We used undoped ZnSb to study the effect of nanostructuring on lattice thermal conductivity, and with little contribution at room temperature from electronic thermal conductivity. Grain growth was observed to occur during hot-pressing, as observed by transmission electron microscopy and x-ray diffraction. The thermal conductivity was lower for cryo-milled samples than for room-temperature ball-milled samples. The thermal conductivity also depended on hot-pressing conditions. The thermal conductivity could be varied by a factor of two by adjusting the process conditions and could be less than a third that of single-crystal ZnSb.

A study of transport properties in Cu and P doped ZnSb

ZnSb samples have been doped with copper and phosphorus and sintered at 798 K. Electronic transport properties are interpreted as being influenced by an impurity band close to the valence band. At low Cu dopant concentrations, this impurity band degrades the thermoelectric properties as the Seebeck coefficient and effective mass are reduced. At carrier concentrations above 1 × 1019 cm−3, the Seebeck coefficient in Cu doped samples can be described by a single parabolic band.

Activated Persulfate Oxidation of Perfluorooctanoic Acid (PFOA) in Groundwater under Acidic Conditions.

Perfluorooctanoic acid (PFOA) is an emerging contaminant of concern due to its toxicity for human health and ecosystems. However, successful degradation of PFOA in aqueous solutions with a cost-effective method remains a challenge, especially for groundwater. In this study, the degradation of PFOA using activated persulfate under mild conditions was investigated. The impact of different factors on persulfate activity, including pH, temperature (25 °C–50 °C), persulfate dosage and reaction time, was evaluated under different experimental conditions. Contrary to the traditional alkaline-activated persulfate oxidation, it was found that PFOA can be effectively degraded using activated persulfate under acidic conditions, with the degradation kinetics following the pseudo-first-order decay model. Higher temperature, higher persulfate dosage and increased reaction time generally result in higher PFOA degradation efficiency. Experimental results show that a PFOA degradation efficiency of 89.9% can be achieved by activated persulfate at pH of 2.0, with the reaction temperature of 50 °C, molar ratio of PFOA to persulfate as 1:100, and a reaction time of 100 h. The corresponding defluorination ratio under these conditions was 23.9%, indicating that not all PFOA decomposed via fluorine removal. The electron paramagnetic resonance spectrometer analysis results indicate that both SO4−• and •OH contribute to the decomposition of PFOA. It is proposed that PFOA degradation occurs via a decarboxylation reaction triggered by SO4−•, followed by a HF elimination process aided by •OH, which produces one-CF2-unit-shortened perfluoroalkyl carboxylic acids (PFCAs, Cn−1F2n−1COOH). The decarboxylation and HF elimination processes would repeat and eventually lead to the complete mineralization all PFCAs.

Sorption kinetics, isotherms and mechanisms of PFOS on soils with different physicochemical properties

Perfluorooctane sulfonate (PFOS), an emerging contaminant, is environmentally persistent, bioaccumulative and toxic to human health and ecosystems. It has been widely detected in groundwater, surface water, soil and sediment. So far, very few research has reported on the PFOS sorption behaviors onto soils, one of the primary processes that influence its fate and transport in the subsurface. In this study, the sorption and desorption of PFOS onto six soils with different physicochemical properties were investigated. Kinetic and equilibrium studies of PFOS sorption onto six soils were carried out in batch experiment. The sorption kinetics of PFOS on the six soils demonstrated that PFOS sorption reached equilibrium within 48h, and the well-fitted pseudo-second-order kinetic model to experimental data suggested that chemisorption was involved in PFOS sorption on soils. The intraparticle diffusion model results indicated that both film diffusion and intraparticle diffusion were the rate-limiting steps for five of the six soil samples, while the intraparticle diffusion was the only limiting step in the PFOS sorption on the sixth soil. PFOS sorption isotherms can be described by the Freundlich model well for all six soils (R2=0.979-0.999). The correlation analysis between KF of PFOS and the physicochemical properties of the soils showed that a positive correlation between KF and Al2O3, SOC and Fe2O3. The FTIR data demonstrated hydrophobic interaction, ion exchange, surface complexing and hydrogen bonding might all play a role in the PFOS sorption onto soil samples. PFOS sorption onto soil minerals, especially iron oxide minerals, needs to be further explored in future.

Simultaneous Decolorization and Biohydrogen Production from Xylose by Klebsiella oxytoca GS-4-08 in the Presence of Azo Dyes with Sulfonate and Carboxyl Groups

ABSTRACT Biohydrogen production from the pulp and paper effluent containing rich lignocellulosic material could be achieved by the fermentation process. Xylose, an important hemicellulose hydrolysis product, is used less efficiently as a substrate for biohydrogen production. Moreover, azo dyes are usually added to fabricate anticounterfeiting paper, which further increases the complexity of wastewater. This study reports that xylose could serve as the sole carbon source for a pure culture of Klebsiella oxytoca GS-4-08 to achieve simultaneous decolorization and biohydrogen production. With 2 g liter−1 of xylose as the substrate, a maximum xylose utilization rate (URxyl) and a hydrogen molar yield (HMY) of 93.99% and 0.259 mol of H2 mol of xylose−1, respectively, were obtained. Biohydrogen kinetics and electron equivalent (e− equiv) balance calculations indicated that methyl red (MR) penetrates and intracellularly inhibits both the pentose phosphate pathway and pyruvate fermentation pathway, while methyl orange (MO) acted independently of the glycolysis and biohydrogen pathway. The data demonstrate that biohydrogen pathways in the presence of azo dyes with sulfonate and carboxyl groups were different, but the azo dyes could be completely reduced during the biohydrogen production period in the presence of MO or MR. The feasibility of hydrogen production from industrial pulp and paper effluent by the strain if the xylose is sufficient was also proved and was not affected by toxic substances which usually exist in such wastewater, except for chlorophenol. This study offers a promising energy-recycling strategy for treating pulp and paper wastewaters, especially for those containing azo dyes. IMPORTANCE The pulp and paper industry is a major industry in many developing countries, and the global market of pulp and paper wastewater treatment is expected to increase by 60% between 2012 and 2020. Such wastewater contains large amounts of refractory contaminants, such as lignin, whose reclamation is considered economically crucial and environmentally friendly. Furthermore, azo dyes are usually added in order to fabricate anticounterfeiting paper, which further increases the complexity of the pulp and paper wastewater. This work may offer a better understanding of biohydrogen production from xylose in the presence of azo dyes and provide a promising energy-recycling method for treating pulp and paper wastewater, especially for those containing azo dyes.

Cadmium removal from simulated groundwater using alumina nanoparticles: behaviors and mechanisms

Cadmium (Cd), one of the most toxic contaminants in groundwater, can cause a severe threat to human health and ecological systems. In this study, alumina nanoparticles were synthesized and tested for high-efficiency Cd removal from simulated groundwater. Furthermore, the synthesized alumina nanoparticles were successfully modified using negatively charged glycerol, to alleviate the challenge of its low mobility in groundwater for the Cd removal. The maximum removal efficiency of both synthesized and glycerol-modified alumina nanoparticles were more than 99%. The sorption isotherm and kinetic data of both synthesized and glycerol-modified alumina nanoparticles were best fitted to the Freundlich model and the pseudo-second-order model, respectively, indicating that the sorption of Cd ions occurs on heterogeneous surfaces of both alumina nanoparticles via the chemisorption mechanism. X-ray photoelectron spectroscopy and energy dispersive X-ray analysis revealed the presence of Cd peak in both sorbents after contact with Cd. In addition, the FTIR analyses demonstrated that hydroxyl group participated in the sorption of Cd on both synthesized and glycerol-modified alumina nanoparticles, while other glycerol associated groups contributed to the removal of Cd ions by the glycerol-modified alumina nanoparticles. It was concluded that Cd removal by synthesized and glycerol-modified alumina nanoparticles were mainly due to ion exchange and electrostatic attraction, respectively. Desorption experiment suggested that both alumina nanoparticles are effective and practically significant sorbents to remediate Cd from contaminated groundwater. However, the stronger bond between Cd and glycerol-modified alumina, plus its potential of higher mobility due to the negative charge on the surface, warrant glycerol-modified alumina nanoparticles a better performance in remediating Cd contaminated groundwater than that of the synthesized alumina nanoparticles.

Distribution, source identification and health risk assessment of PFASs and two PFOS alternatives in groundwater from non-industrial areas.

Little research has been carried out for the per- and polyfluoroalkyl substances (PFASs) in groundwater from non-industrial areas, even though it has been proved that PFASs can transport for long distance. In this study, the concentration profiles and geographical distribution of 14 PFASs, including two alternatives of perfluorooctane sulfonate (PFOS), 6:2 fluorotelomer sulfonate (6:2 FTS) and potassium 9-chlorohexadecafluoro-3-oxanonane-1-sulfonate (F-53B), were analyzed in groundwater samples (n = 102) collected from water wells in non-industrial areas. The total concentrations of PFASs (Σ14PFASs) in groundwater samples ranged from 2.69 to 556 ng/L (mean 43.1 ng/L). The detection rates of shorter chain (C4-C9) PFASs were 62.75-100%, higher than those of long chain (> C10) PFASs with detection rates of less than 40%. The source identification using hierarchical cluster analysis and Spearman rank correlation analysis suggested that domestic sewage and atmospheric deposition may contribute significantly to the PFAS occurrence in groundwater in non-industrial areas, while the nearby industrial parks may contribute some, but not at a significant level. Furthermore, the human health risk assessment analysis shows that the health hazards associated with perfluorooctanoic acid (PFOA) and PFOS, two of the main PFAS constituents in groundwater from non-industrial areas, were one or two orders of magnitude higher than those in a previous study, but were unlikely to cause long-term harm to the residents via the drinking water exposure pathway alone.

Characterization of a thermo-alkali-stable laccase from Bacillus subtilis cjp3 and its application in dyes decolorization

ABSTRACT In this work, a novel bacterial strain exhibiting laccase activity was isolated from black liquor and identified as Bacillus subtilis cjp3. The CotA-laccase gene was cloned from strain cjp3 and expressed in Escherichia coli. The purified recombinant laccase has a maximum activity of 7320 U/L, maintaining high stabilities under a wide pH range and high temperature conditions. Nearly no loss of laccase activity was observed even at pH 9.0 after 10 h of incubation. Reactive blue 19, reactive black 5 and indigo carmine could be efficiently decolorized by the purified laccase in the presence of a mediator ABTS. More than 86% of tested dyes were removed in 4 h at pH = 9.0. The recombinant laccase can work well in a broad range of temperatures of 20–80°C(>80% relative activity). These special properties indicated the potential use of the CotA-laccase in treating wastewater containing synthetic dyes.