Changqiu Wang

Photocatalytically improved azo dye reduction in a microbial fuel cell with rutile-cathode.

Reductive decolorization of azo dye in wastewater was investigated in a dual-chamber microbial fuel cell (MFC) equipped with cathodes made of graphite or rutile-coated graphite. Rapid reduction of methyl orange (MO) with concomitant electricity production was achieved when the rutile-coated cathode was irradiated by visible light. The electrochemical impedance spectra (EIS) indicate that the polarization resistance (R(p)) of the rutile-cathode MFC decreased from 1378 Omega in dark to 443.4 Omega in light, demonstrating that photocatalysis of rutile can enhance the cathodic electron transfer process. The combination of the biologically active anode and photocatalysis-supported cathodic reduction of MO obeyed the pseudo-first-order kinetics. The analysis of decolorization products indicates that the azo bond of MO was probably cleaved by photoelectrons at the irradiated rutile-cathode, resulting in the products of colorless hydrazine derivatives. In addition, concurrently enhanced electricity generation in the MFCs involving photocatalyzed cathodic reduction of MO was observed throughout this study.

Growth of non-phototrophic microorganisms using solar energy through mineral photocatalysis

Phototrophy and chemotrophy are two dominant modes of microbial metabolism. To date, non-phototrophic microorganisms have been excluded from the solar light-centered phototrophic metabolism. Here we report a pathway that demonstrates a role of light in non-phototrophic microbial activity. In lab simulations, visible light-excited photoelectrons from metal oxide, metal sulfide, and iron oxide stimulated the growth of chemoautotrophic and heterotrophic bacteria. The measured bacterial growth was dependent on light wavelength and intensity, and the growth pattern matched the light absorption spectra of the minerals. The photon-to-biomass conversion efficiency was in the range of 0.13-1.90‰. Similar observations were obtained in a natural soil sample containing both bacteria and semiconducting minerals. Results from this study provide evidence for a newly identified, but possibly long-existing pathway, in which the metabolisms and growth of non-phototrophic bacteria can be stimulated by solar light through photocatalysis of semiconducting minerals.

Photo-reductive decolorization of an azo dye by natural sphalerite: Case study of a new type of visible light-sensitized photocatalyst

Natural sphalerite, which represents a new class of mineral-based catalyst, was characterized and investigated for photo-reduction of an azo dye methyl orange (MO) under visible light. After 2h of visible light irradiation, a complete decolorization of the MO solution was achieved. The degradation rate was related to the pH conditions. Spectra from FT-IR analysis indicate an initial adsorption of MO to sphalerite via its sulfonate group. Further reduction of the adsorbed MO by sphalerite under light irradiation led to the destruction of the azo structure, as indicated by the results from UV-vis, FT-IR and ESI-MS analyses. The visible light-induced photocatalytic reductive activity of natural sphalerite was mainly attributed to the distribution of foreign metal atoms in its crystal lattice, which reduces the intrinsic bandgap of sphalerite and also broadens its spectra responding range. In addition, the high conduction band potential of natural sphalerite may also enhance the photo-reduction of MO.

Synthesis, characterization, and catalytic activity of cryptomelane nanomaterials produced with industrial manganese sulfate.

Industrial manganese sulfate from manganese mines has been utilized to synthesize cryptomelane in a simple and feasible route. K-birnessite precursor was prepared by air oxidation of the mixture of MnSO4 and KOH solutions under alkaline conditions, and then transformed to cryptomelane under a heating process. The effects of OH- concentration, airflow rate, liquid reaction temperature, stirring rate, liquid reaction time, washing condition, calcination time, and temperature were investigated. X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) revealed that cryptomelane prepared under optimal conditions had a tetragonal symmetry and the particles were mostly in short lathy form with sizes of 20-30 nm. The average pore size and BET surface area of cryptomelane examined by N2 adsorption methods were 24.15 nm and 32.21 m2/g, respectively. X-ray photoelectron spectroscopy (XPS) studies demonstrated that the average oxidation state of manganese in cryptomelane was about 3.9 in comparison with prephase K-birnessite of 3.4. The synthesized cryptomelane sample showed improved catalytic activity for decomposition of hydrogen peroxide as compared with natural cryptomelane, but lower than those synthesized with hydrothermal and sol-gel methods. The results of this investigation will provide fundamental information for developing a large-scale production process for transforming manganese sulfate to cryptomelane.

STUDY OF THE INTERACTION BETWEEN BENTONITE AND A STRAIN OF BACILLUS MUCILAGINOSUS

Mineral-microbe interactions are widespread in a number of environmental processes such as mineral weathering, decomposition, and transformation. Both clay minerals and silicate-weathering bacteria are widely distributed in nature, and the latter contribute to weathering, diagenesis, and mineralization of major rock-forming minerals. The purpose of this study was to observe changes in the chemical composition and structure, especially the phase transformation, of smectite after processing by a silicate-weathering bacterium. The interaction between Bacillus mucilaginosus and bentonite was studied using custom culture media. Results from Inductively Coupled Plasma-Atomic Emission Spectrometry revealed that the bacterium promoted release of Si and Al from solid bentonite to solution. Concomitantly, the Ka nd Fe contents of the mineral increased as shown by X-ray photoelectron spectroscopy results. After interaction with the bacterium, the montmorillonite underwent a possible structure transformation to smectite, as indicated by the emergence of a new weak peak (d = 9.08 Å) shown by X-ray diffraction patterns. The mineralogical changes were also demonstrated by the decrease in the specific surface area of the mineral from 33.0 to 24.0 m2/g (these lower values for SSA of bentonite are related to the particle size of the smectite examined (120-160 mesh) and the weakened absorption bands in Al-O-H and Si-O-Si vibrations by Micro Fourier-transform infrared spectroscopy. The morphology changes in the bacteria observed by environmental scanning electron microscopy and atomic force microscopy revealed an obvious growth of the flagella in the presence of bentonite.

Enhanced Alcaligenes faecalis Denitrification Rate with Electrodes as the Electron Donor.

ABSTRACT The utilization by Alcaligenes faecalis of electrodes as the electron donor for denitrification was investigated in this study. The denitrification rate of A. faecalis with a poised potential was greatly enhanced compared with that of the controls without poised potentials. For nitrate reduction, although A. faecalis could not reduce nitrate, at three poised potentials of +0.06, −0.06, and −0.15 V (versus normal hydrogen electrode [NHE]), the nitrate was partially reduced with −0.15- and −0.06-V potentials at rates of 17.3 and 28.5 mg/liter/day, respectively. The percentages of reduction for −0.15 and −0.06 V were 52.4 and 30.4%, respectively. Meanwhile, for nitrite reduction, the poised potentials greatly enhanced the nitrite reduction. The nitrite reduction rates for three poised potentials (−0.06, −0.15, and −0.30 V) were 1.98, 4.37, and 3.91 mg/liter/h, respectively. When the potentials were cut off, the nitrite reduction rate was maintained for 1.5 h (from 2.3 to 2.25 mg/liter/h) and then greatly decreased, and the reduction rate (0.38 mg/liter/h) was about 1/6 compared with the rate (2.3 mg/liter/h) when potential was on. Then the potentials resumed, but the reduction rate did not resume and was only 2 times higher than the rate when the potential was off.

Synergistic Interaction between Electricigens and Natural Pyrrhotite to Produce Active Oxygen Radicals

In this work we demonstrated that the active oxygen radicals could be produced by the synergistic interaction between electricigens and natural pyrrhotite. The identification of such an interactive pathway was conducted by using a fuel cell-type design, in which the electricigen-attached carbon felt electrode was used as the anode and the pyrrhotite-coated graphite electrode was used as the corresponding cathode. Current density, polarization and power density curves obtained at different treatments demonstrated the synergistic effects of electricigens and pyrrhotite improved the electrons transfer rate between them. Cyclic voltammetry (CV) analysis showed the reductive peaks of O2/H2O2 at 0.88 V (vs. SCE, saturated calomel electrode) and ionic and structural Fe(III)/Fe(II) at 0.31 V (vs. SCE) and 0 V (vs. SCE), respectively. The electrochemical results indicated the electricigen-assisted pyrrhotite photoelectrochemical reactions gave rise to Fentons reagents: Fe2+ and H2O2, which underwent a further reaction to generate active oxygen radical ·OH. By using N, N-dimethyl-p-nitrosoaniline discoloration as a model reaction, the ·OH production rate at the pyrrhotite-cathode was found to follow the first-order kinetics. Practical application of the synergistic interaction between the electricigen and natural pyrrhotite to a real old-aged landfill leachate degradation resulted in 78% chemical oxygen demand (COD) removal and 77% decolourization efficiency. The current generation lasted more than 45 days verified the validity of such system in long-term operation. The proposed interactive pathway would be expected as an alternative cost-effective technology for future wastewater treatment.

TREATMENT OF MUNICIPAL LANDFILL LEACHATE WITH ORGANICALLY MODIFIED BENTONITE

Landfill leachate is one of the most difficult effluents with which to deal from an environmental perspective because of its concentration and complex composition, including refractory and toxic components such as heavy metals or xenobiotic organic compounds. The objective of the present study was to use organically modified bentonite (OMB) to dispose of landfill leachate >10 y old. The OMB was synthesized using a new method, which removed four steps (filtering, washing, drying, and grinding) from the traditional process. After treatment using OMB, the chemical oxygen demand concentration (COD concentration, an index of the organic pollutants in the landfill leachate, was determined using the potassium dichromate method) of the landfill leachate sample decreased from 2400 to 245 mg/L in 5 h, i.e. the organic pollutants reduction efficiency was as high as 90%. Gas chromatography-mass spectrometry results indicated that most of the organic compounds were removed during the process. The modified and unmodified bentonite contained in the OMB deal with the hydrophobic and hydrophilic organic pollutants, respectively, resulting in significant degradation of the leachate. The study results have provided a new cost-effective method for treatment of landfill leachate.

A cost-effective birnessite–silicon solar cell hybrid system with enhanced performance for dye decolorization

A cost-effective and simple configuration of birnessite–silicon solar cell (Bir–SSC) hybrid system is reported in this study. Birnessite, with a band gap of 2.1 eV as determined by UV-vis spectroscopy, was electrochemically deposited on a fluorine-doped tin oxide (FTO) for usage as the anode. It was thoroughly characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and Raman spectroscopy, and its prompt response to visible light was further tested by linear sweep voltammetry (LSV). When birnessite electrode was connected with a silicon solar cell in a hybrid system, a remarkably enhanced methyl orange (MO) decolorization from 47.1% (with a bare SSC) to 95.8% was observed. The results indicated the synergistic effects of photoelectrochemical and electrochemical reactions in the hybrid system. In addition, the electron utilization efficiency was 15.29% and 8.54% with and without light irradiation on birnessite respectively. When applied with three different rated voltage SSC, 2.0 V SSC showed the best fit in the hybrid system. Cycling experiments exhibited the stable performance of birnessite electrode, where the MO color removal ratio in ten cycles remained stable at 90.1 ± 2.5%, which is close to the first cycle (95.8%). The hybrid system possesses the merits of cost-effectiveness, low-power consumption, and “green” fabrication strategy, which exbihits promising potential in solar energy utilization and wastewater treatment.

Photoelectrochemical performance of birnessite films and photoelectrocatalytic activity toward oxidation of phenol

Birnessite films on fluorine-doped tin oxide (FTO) coated glass were prepared by cathodic reduction of aqueous KMnO4. The deposited birnessite films were characterized with X-ray diffraction, Raman spectroscopy, scanning electron microscopy and atomic force microscopy. The photoelectrochemical activity of birnessite films was investigated and a remarkable photocurrent in response to visible light was observed in the presence of phenol, resulting from localized manganese d-d transitions. Based on this result, the photoelectrocatalytic oxidation of phenol was investigated. Compared with phenol degradation by the electrochemical oxidation process or photocatalysis separately, a synergetic photoelectrocatalytic degradation effect was observed in the presence of the birnessite film coated FTO electrode. Photoelectrocatalytic degradation ratios were influenced by film thickness and initial phenol concentrations. Phenol degradation with the thinnest birnessite film and initial phenol concentration of 10mg/L showed the highest efficiency of 91.4% after 8hr. Meanwhile, the kinetics of phenol removal was fit well by the pseudofirst-order kinetic model.