Yanqing Cong

Synergistic photosensitized removal of Cr(VI) and Rhodamine B dye on amorphous TiO2 under visible light irradiation.

Amorphous TiO(2) (Am-TiO(2)) was prepared at room temperature by hydrolysis of Ti(OBu)(4) in water without addition of strong acids or organic solvents. Results from XRD and TEM revealed that the as-prepared Am-TiO(2) was composed of amorphous structure. For the simultaneous photosensitized removal of Cr(VI) and zwitterionic Rhodamine B (RhB) dye, Am-TiO(2) exhibited more significant synergistic effect than commercial P25-TiO(2). The removal efficiencies for RhB and Cr(VI) after 100 min visible light irradiation were 97.8% and 53.5% on Am-TiO(2), respectively. While 88.2% RhB and 42.1% Cr(VI) were removed on P25-TiO(2). Decreased synergistic activities as well as smaller surface areas were observed when Am-TiO(2) was pretreated at high temperatures (200-700°C). Thus, it was the larger specific surface area rather than better crystallinity dominated the synergistic degradation dynamics under visible light irradiation with lower pH (2), greater catalyst loading amount (2g/L), proper RhB/Cr(VI) ratios (1:8) and higher light intensity (500 W). Better synergistic performance was also obtained on Am-TiO(2) than P25-TiO(2) when Cr(VI) coexists with cationic dyes, while negligible difference was observed in the presence of anionic dyes. Superior stability and simplicity of Am-TiO(2) was also exhibited in the cyclic runs.

Highly enhanced photocatalytic reduction of Cr(VI) on AgI/TiO2 under visible light irradiation: Influence of calcination temperature.

AgI/TiO2 was prepared using a dissolution-precipitation method, followed by calcination at different temperatures (100-700°C). The as-prepared AgI/TiO2 powders were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis-DRS) and electrochemical impedance spectroscopy (EIS). The results revealed that calcination temperature significantly impacted the visible light absorption of AgI/TiO2 along with a shift from metastable γ-AgI to relatively stable β-AgI. We found that highest photocatalytic reduction rate of Cr(VI) and β-AgI content were obtained for a calcination temperature of 350°C. Furthermore, the pseudo-first order rate constant was five times that for a photocatalyst calcined at 100°C. The dramatically enhanced reduction rate of Cr(VI) was attributed to enhanced visible light absorption and greatly reduced charge transfer resistance, which eventually facilitates more efficient separation and easier transfer of photogenerated electron-hole pairs to the catalyst surface. Other experimental conditions were also carefully investigated and optimized with initial AgI loading percentage (5%), catalyst dosage (1.0g/L), coexisting organics (1.0mmol/L EDTA) and pH (1-2). The optimal AgI/TiO2 exhibited good stability with little change in activity after 5 cycles.

Synthesis of g-C3N4/Bi2O3/TiO2 composite nanotubes: enhanced activity under visible light irradiation and improved photoelectrochemical activity

g-C3N4 and Bi2O3 were successfully incorporated into TiO2 nanotubes (TiO2-NT) to produce a composite nanotube material designated as g-C3N4/Bi2O3/TiO2. The photoelectrochemical (PEC) activity under visible light irradiation with respect to bleaching of methylene blue (MB) and degradation of phenol were determined. The UV-vis absorption spectrum of the composite material, g-C3N4/Bi2O3/TiO2-NT, was red-shifted toward a narrower band gap energy (Eg). The valence band (VB) of g-C3N4/Bi2O3/TiO2-NT was shifted to a more positive potential resulting in an increased driving force for water oxidation. The photocurrent generated by g-C3N4/Bi2O3/TiO2-NTs was approximately 15 times higher and the incident photon-to-current efficiency (IPCE at λ = 400 nm) was higher than that of the naked TiO2-NTs. This response is attributed to increasing the lifetimes of photo-generated electron–hole pairs. A significantly higher PEC response in terms of the bleaching of MB was observed on g-C3N4/Bi2O3/TiO2-NT. The roles of improved charge separation and subsequent higher electron-transfer efficiencies for in g-C3N4/Bi2O3/TiO2-NT electrodes were examined.

The effect of C/N ratio on nitrogen removal in a bioelectrochemical system

The effect of C/N ratios of 2, 2.7, and 3.5 on nitrogen removal in a bioelectrochemical system (BES) was investigated. Starch was used as a carbon source for the electrogenesis phenomenon we observed in a previous study. The results showed that an increased C/N ratio helped the BES to remove nitrate and depress nitrite accumulation but did not increase autotrophic denitrification. Nitrate and total nitrogen removal were increased from 0.69±0.02gm(-3)h(-1) to 1.09±0.16gm(-3)h(-1), and from 0.52±0.08gm(-3)h(-1) to 0.97±0.06gm(-3)h(-1), respectively, when the C/N ratio was increased from 2.0 to 3.5. However, the autotrophic denitrification ratio decreased from 72.74% to 50.23% with the same increase in the C/N ratio. High C/N ratios postponed the excretion of soluble microbial products and increased electrogenesis, but did not improve the anode transformation efficiency.

Facile synthesis of AgI/BiOI-Bi2O3 multi-heterojunctions with high visible light activity for Cr(VI) reduction

AgI sensitized BiOI-Bi2O3 composite (AgI/BiOI-Bi2O3) with multi-heterojunctions was prepared using simple etching-deposition process. Different characterization techniques were performed to investigate the structural, optical and electrical properties of the as-prepared photocatalysts. It was found that the ternary AgI/BiOI-Bi2O3 composite exhibited: (1) improved photocurrent response, (2) smaller band gap, (3) greatly reduced charge transfer resistance and (4) negative shift of flat band potential, which finally led to easier generation and more efficient separation of photo-generated electron-hole pairs at the hetero-interfaces. Thus, for the reduction of Cr(VI), AgI/BiOI-Bi2O3 exhibited excellent photocatalytic activity under visible light irradiation at near neutral pH. AgI/BiOI-Bi2O3 was optimized when the initial molar ratio of KI to Bi2O3 and AgNO3 to Bi2O3 was 1:1 and 10%, respectively. The estimated kCr(VI) on optimized AgI/BiOI-Bi2O3 was about 16 times that on pure Bi2O3. Good stability was also observed in cyclic runs, indicating that the current multi-heterostructured photocatalyst is highly desirable for the remediation of Cr(VI)-containing wastewater.

The effect of carbon sources on nitrogen removal performance in bioelectrochemical systems.

In order to ascertain the effects of different carbon sources (methanol, glucose, starch and NaHCO(3)) on denitrification in BESs, the experiment was conducted in a constant current, 3.5 of chemical oxygen demand to nitrate ratio in a greenhouse. Among the four carbon sources investigated in BESs, NaHCO(3) showed the highest nitrite accumulation and the ratio of soluble microbial products to soluble chemical oxygen demand (SMP/SCOD) with a value of 3.68 ± 0.68 mg/L and 94%, respectively. And the addition of organic substrates could reduce SMP production and enhance the denitrification process. In the constant voltage experiment, it was observed that the organics could be used by microbes to generate electrons at the anode. And a maximal current value of 11.0 mA in the BESs fed with starch indicated that the complex carbon source was easier to be used by microorganisms to generate electricity than the simple carbon source.

Synergistic photoelectrochemical reduction of Cr(VI) and oxidation of organic pollutants by g-C3N4/TiO2-NTs electrodes.

The g-C3N4/TiO2-NTs electrodes were synthesized by a dip-coating procedure followed by high-temperature annealing used in photoelectrochemical process. From the results, a simultaneous and rapid reduction of Cr(VI) and degradation of phenol in Cr(VI)/phenol system was observed with photoelectrocatalytic activity under UV-visible light irradiation than photocatalytic and electrocatalytic activities. The different kinds of Cr(VI)/organic pollutants systems were also investigated systematically. In addition, different scavengers were also added in Cr(VI)/phenol and Cr(VI)/benzyl alcohol systems to indicate that the hydroxyl radicals and superoxide radicals were the most major active species for the denomination of Cr(VI) and organic pollutants. The intermediates of phenol and benzyl alcohol were also detected during the reaction in order to deduce the photoelectrocatalysis mechanism underg-C3N4/TiO2-NTs electrodes that the charge separation was improved and subsequently electron-transfer efficiency was higher.

Phenol degradation by TiO2 photocatalysts combined with different pulsed discharge systems.

Films of TiO2 nanotubes distributed over the inner surface of a discharge reactor cylinder (CTD) or adhered to a stainless steel electrode surface (PTD) in a discharge reactor were compared with a single-discharge (SD) system to investigate their efficiencies in phenol degradation. Morphology studies indicated that the TiO2 film was destroyed in the PTD system, but that there was no change in the CTD system after discharge. X-ray diffraction results revealed that the anatase phase of the original sample was preserved in the CTD system, but that an anatase-to-rutile phase transformation occurred in the PTD system after discharge. The highest efficiencies of phenol degradation and total organic carbon (TOC) mineralization were observed in the CTD system, and there was no decrease in phenol degradation efficiency upon reuse of a TiO2 film, indicating high catalysis activity and stability of the TiO2 photocatalysts in the combined treatment. TiO2 photocatalysts favored the formation of hydrogen peroxide and disfavored the formation of ozone. A greater degree of oxidation of intermediates and higher energy efficiency in phenol oxidation were observed with the TiO2-plasma systems, especially in the CTD system, compared to those with the SD system.

Preparation of AgI sensitized amorphous TiO2 as novel high-performance photocatalyst for environmental applications

A novel visible-light-active material was prepared by dispersion of AgI on amorphous TiO2 through simple one-pot process (AgI/Am-TiO2-S). For comparison, AgI sensitized TiO2 (amorphous, anatase and P25) were also prepared via traditional deposition-precipitation method. The samples were characterized by XRD, XPS, TGA-DSC, UV-Vis-DRS, BET, and etc. Larger specific surface area, negative shift of flat band potential, as well as greatly reduced charge transfer resistance were observed for AgI/Am-TiO2-S comparing to other samples. Moreover, with the same molar of initial Ti and Ag, the weight of AgI/Am-TiO2-S obtained was the heaviest, due to large amount of surface titania hydrate. The photocatalytic activity of the as-prepared AgI/titania samples were evaluated by the reduction of Cr(VI) in the absence or presence of organic pollutants (dyes, phenol). AgI/Am-TiO2-S always presented the highest photocatalytic activity. The estimated k(Cr(VI)) on AgI/Am-TiO2-S was about 2 times that on AgI/P25-TiO2 in the absence/presence of RhB. Superior stability was also observed in the cyclic runs indicating that the as-prepared AgI/Am-TiO2-S is highly desirable for the remediation of Cr(VI)-organic co-contaminated wastewaters.

How to ascertain the importance of autotrophic denitrification process in a bioelectrochemical system

The ratio of autotrophic denitrification (Rauto) is proposed as a metric to better evaluate the nitrate removal efficiency of bioelectrochemical systems (BESs). Two methods for computing Rauto were compared, one based on electron charge transfer and the other on a controlled reaction. The experiment was conducted in a greenhouse with a temperature of 30±2 °C. A reaction with a constant voltage supply was used as a BES and open circuit control was used for the controlled reaction. The results indicated that calculating the Rauto based on electron charge transfer was more suitable for evaluating the performance of BESs. Further discussion about the factors influencing the result of this computing method is presented.