Integrating plasmonic nanoparticles with TiO₂ photonic crystal for enhancement of visible-light-driven photocatalysis

Aimed at enhancing photocatalysis through intensifying light harvesting, a new photocatalyst was fabricated by infiltrating Au nanoparticles into TiO(2) photonic crystals (TiO(2) PC/Au NPs). Scanning electron microscopy (SEM) and transmission electron microscope (TEM) images showed that the Au NPs with average diameter around 15 nm were dispersed uniformly into the porous TiO(2) material. The results of the transmittance spectra demonstrated that the light absorption by Au NPs was amplified after they were infiltrated into TiO(2) 240, which was fabricated from 240 nm polystyrene spheres. In the photocatalytic experiments of 2,4-dichlorophenol degradation under visible light (λ > 420 nm) irradiation, the kinetic constant using TiO(2) 240/Au NPs was 2.3 fold larger than that using TiO(2) nanocrystalline/Au NPs (TiO(2) NC/Au NPs). The excellent photocatalysis benefited from the cooperatively enhanced light harvesting owing to the localized surface plasmon resonance of Au NPs, which extended the light response spectra and the photonic effect of the TiO(2) 240 which intensified the plasmonic absorption by Au NPs. The hydroxyl radicals originated from the electroreduction of dissolved oxygen with photogenerated electrons via chain reactions were the main reactive oxygen species responsible for the pollutant degradation.     

Systematic evaluation of land use regression models for NO₂

Land use regression (LUR) models have become popular to explain the spatial variation of air pollution concentrations. Independent evaluation is important. We developed LUR models for nitrogen dioxide (NO(2)) using measurements conducted at 144 sampling sites in The Netherlands. Sites were randomly divided into training data sets with a size of 24, 36, 48, 72, 96, 108, and 120 sites. LUR models were evaluated using (1) internal "leave-one-out-cross-validation (LOOCV)" within the training data sets and (2) external "hold-out" validation (HV) against independent test data sets. In addition, we calculated Mean Square Error based validation R(2)s. The mean adjusted model and LOOCV R(2) slightly decreased from 0.87 to 0.82 and 0.83 to 0.79, respectively, with an increasing number of training sites. In contrast, the mean HV R(2) was lowest (0.60) with the smallest training sets and increased to 0.74 with the largest training sets. Predicted concentrations were more accurate in sites with out of range values for prediction variables after changing these values to the minimum or maximum of the range observed in the corresponding training data set. LUR models for NO(2) perform less well, when evaluated against independent measurements, when they are based on relatively small training sets. In our specific application, models based on as few as 24 training sites, however, achieved acceptable hold out validation R(2)s of, on average, 0.60.     

Photocatalytic degradation efficiency and mechanism of microcystin-RR by mesoporous Bi₂WO₆ under near ultraviolet light

Microcystin-RR (MC-RR) is one of the most common cyanotoxin microcystins in fresh water and is of great concern due to its potential hepatotoxicity. In the present study, Bi(2)WO(6) was synthesized with a hydrothermal method by varying the pH of the reaction solution in the range of 1-11. The surface area of the catalysts decreased, but the crystallinity and crystal size increased with the pH. The adsorption and degradation capacities of the catalysts decreased with increasing the preparation solution pH. The Bi(2)WO(6) prepared at pH 1 (Bi(2)WO(6)-pH1) displayed the highest adsorption and degradation capacity to MC-RR even though it consisted of randomly aggregated particles. Nearly 100% of MC-RR at 10 mg L(-1) was removed after 30 min of irradiation of near-ultraviolet light (300-400 nm) in a solution with Bi(2)WO(6) concentration of 0.2 g L(-1). The photodegradation efficiency of Bi(2)WO(6)-pH1 was greater in acid medium than in basic solutions. Several intermediate products were observed and identified by liquid chromatography/mass spectrometry/mass spectrometry, and a unique photodegradation pathway was proposed. It was assumed that a photo-Kolbe process happened at the site carboxyl acid group of the d-Glu residue by the photogenerated holes, producing a hydroperoxyl product at m/z 513.8. This intermediate could be further decomposed to an alcohol product at m/z 505.8 and a ketone product at m/z 504.8. The aromatic ring and diene bond of the Adda chain could also be attacked by the holes and form phenol and diol products.     

Uranyl-sorption properties of amorphous and crystalline TiO₂/ZrO₂ millimeter-sized hierarchically porous beads

Hierarchically porous TiO(2)/ZrO(2) millimeter-sized beads were synthesized using a sol-gel templating technique, and investigated for suitability as radionuclide sorbents using uranyl as a radionuclide-representative probe. The bead properties were varied by altering either composition (22, 36, and 82 wt % Zr in the Ti/Zr composite) or calcination temperature (500 or 700 °C). Uranyl adsorption was higher for the crystalline beads (surface area: 52-59 m(2) g(-1)) than the amorphous beads (surface area: 95-247 m(2) g(-1)), reaching a maximum of 0.170 mmol g(-1) for the 22 wt % Zr sample. This was attributed to the higher surface hydroxyl density (OH nm(-2)), presence of limited microporosity, and larger mesopores in the crystalline beads. Mass transport properties of the crystalline beads were not compromised by the large bead diameter: sorption rates comparable to those reported for powders were achieved and rates were higher than exclusively mesoporous reported systems, thereby highlighting the importance of pore hierarchy in designing materials with improved kinetics. Chemical stability of the sorbent, an important property for processes involving corrosive effluents (e.g., radioactive waste), was also assessed. Crystalline beads displayed superior resistance against matrix leaching in HNO(3). Stability varied with composition: the 22 wt % Zr sample demonstrated the highest stability.     

Inhibitory effect of dissolved silica on H₂O₂ decomposition by iron(III) and manganese(IV) oxides: implications for H₂O₂-based in situ chemical oxidation

The decomposition of H(2)O(2) on iron minerals can generate ?OH, a strong oxidant that can transform a wide range of contaminants. This reaction is critical to In Situ Chemical Oxidation (ISCO) processes used for soil and groundwater remediation, as well as advanced oxidation processes employed in waste treatment systems. The presence of dissolved silica at concentrations comparable to those encountered in natural waters decreases the reactivity of iron minerals toward H(2)O(2), because silica adsorbs onto the surface of iron minerals and alters catalytic sites. At circumneutral pH values, goethite, amorphous iron oxide, hematite, iron-coated sand, and montmorillonite that were pre-equilibrated with 0.05-1.5 mM SiO(2) were significantly less reactive toward H(2)O(2) decomposition than their original counterparts, with the H(2)O(2) loss rates inversely proportional to SiO(2) concentrations. In the goethite/H(2)O(2) system, the overall ?OH yield, defined as the percentage of decomposed H(2)O(2) producing ?OH, was almost halved in the presence of 1.5 mM SiO(2). Dissolved SiO(2) also slowed H(2)O(2) decomposition on manganese(IV) oxide. The presence of dissolved SiO(2) results in greater persistence of H(2)O(2) in groundwater and lower H(2)O(2) utilization efficiency and should be considered in the design of H(2)O(2)-based treatment systems.     

TiO₂-photocatalytic reduction of pentavalent and trivalent arsenic: production of elemental arsenic and arsine

Heterogeneous photocatalytic reduction of As(V) and As(III) at different concentrations over TiO(2) under UV light in deoxygenated aqueous suspensions is described. For the first time, As(0) was unambiguously identified together with arsine (AsH(3)) as reaction products. As(V) reduction requires the presence of an electron donor (methanol in the present case) and takes place through the hydroxymethyl radical formed from methanol oxidation by holes or hydroxyl radicals. On the contrary, As(III) reduction takes place through direct reduction by the TiO(2)-conduction band electrons. Detailed mechanisms for the photocatalytic processes are proposed. Although reduction to solid As(0) is convenient for purposes of As removal from water as a deposit on TiO(2), attention must be paid to formation of AsH(3), one of the most toxic forms of As, and strategies for AsH(3) treatment should be considered.     

Influence of high-temperature steam on the reactivity of CaO sorbent for CO₂ capture

Calcium looping is a high-temperature CO(2) capture technology applicable to the postcombustion capture of CO(2) from power station flue gas, or integrated with fuel conversion in precombustion CO(2) capture schemes. The capture technology uses solid CaO sorbent derived from natural limestone and takes advantage of the reversible reaction between CaO and CO(2) to form CaCO(3); that is, to achieve the separation of CO(2) from flue or fuel gas, and produce a pure stream of CO(2) suitable for geological storage. An important characteristic of the sorbent, affecting the cost-efficiency of this technology, is the decay in reactivity of the sorbent over multiple CO(2) capture-and-release cycles. This work reports on the influence of high-temperature steam, which will be present in flue (about 5-10%) and fuel (～20%) gases, on the reactivity of CaO sorbent derived from four natural limestones. A significant increase in the reactivity of these sorbents was found for 30 cycles in the presence of steam (from 1-20%). Steam influences the sorbent reactivity in two ways. Steam present during calcination promotes sintering that produces a sorbent morphology with most of the pore volume associated with larger pores of ～50 nm in diameter, and which appears to be relatively more stable than the pore structure that evolves when no steam is present. The presence of steam during carbonation reduces the diffusion resistance during carbonation. We observed a synergistic effect, i.e., the highest reactivity was observed when steam was present for both calcination and carbonation.     

Can degradation products be used as documentation for natural attenuation of phenoxy acids in groundwater?

In situ indicators of degradation are important tools in the demonstration of natural attenuation. A literature survey on the production history of phenoxy acids and degradation pathways has shown that metabolites of phenoxy acid herbicides also are impurities in the herbicide products, making the bare presence of these compounds useless as in situ indicators. These impurities can make up more than 30% of the herbicides. Degradation of phenoxy acids was demonstrated in microcosm experiments using groundwater and sediment contaminated with MCPP, dichlorprop, and related compounds such as other phenoxypropionic acids and chlorophenols. Field observations at two phenoxy acid-contaminated sites showed the occurrence of several impurities including metabolites in the groundwater. Neither the microcosm experiments nor the field observations verified that metabolites were actually produced or accumulated in situ. However, it was demonstrated that the impurity/parent herbicide ratios can be useful in situ indicators of degradation.     

Determination of intermediates and mechanism for soot combustion with NOx/O₂ on potassium-supported Mg-Al hydrotalcite mixed oxides by in situ FTIR

The soot combustion with NO(x) and/or O(2) on potassium-supported Mg-Al hydrotalcite mixed oxides under tight contact condition was studied using temperature-programmed oxidation (TPO), isothermal reaction and in situ FTIR techniques. The presence of NO(x) in O(2) favors the soot combustion at lower temperatures (<300 °C). However, a little suppression was observed at higher temperatures (>300 °C), which was accompanied by a substantial NO(x) reduction. The ketene (C═C═O) and isocyanate (NCO(-)) species were determined as the reaction intermediates. In NO(x) + O(2), NO(2) directly interacts with the free carbon sites (C═C*) through two parallel reactions: (1) NO(2) + C═C* → C═C═O + NO; (2) NO(2) + C═C* → NCO(-) + CO(2). The two reactions can proceed easily, which accounts for the promotion effect of NO(x) on soot combustion at lower temperatures. The further oxidation of NCO(-) by NO(2) or O(2) is responsible for the simultaneous reduction of NO(x). However, the reactions between NO(2) and C═C* are limited by the amount of free carbon sites, which can be provided by the oxidation of soot by O(2) at higher temperatures. The interaction of NO(x) and catalyst results in the formation of nitrates and nitrites, which poisoned the active K sites.     

Development of a solid phase microextraction protocol for the GC–MS determination of volatile off-flavour compounds from citral degradation in oil-in-water emulsions

The conditions for headspace solid phase microextraction (HS-SPME) analysis of volatile off-flavour compounds in citral emulsion were determined. Type of SPME phase (65 μm PDMS/DVB, 100 μm PDMS and 75 μm CAR/PDMS), adsorption temperature and salt concentration were significant factors affecting total peak area in the gas chromatogram and optimised in one factor experiments. Then, adsorption temperature (30–50 °C), adsorption time (20–40 min), and salt concentration (0–6 M) were studied to develop HS-SPME condition for obtaining the highest extraction efficiency. PDMS/DVB in 65 μm was the optimum fiber because of high adsorption efficiency and good reproducibility. The optimal condition was adsorption at 50 °C for 40 min and 6 M salt added to sample. Good Linearity, high recovery, good reproducibility and low limit of detection (LOD) for all off-odour compounds according to the optimised SPME conditions indicated that the SPME procedure was applicable for the analysis of the degraded citral products in headspace volatile of emulsion.