Catalytic air oxidation of biogas slurry using Cu sub-nanocluster supported by mesoporous TiZrO4 and protected by SiO2 shell.

Biogas slurry, an inevitable outcome of anaerobic digestion (AD), is a treatment burden for urban environmental management. In this study, two kinds of biogas slurry (slurry J and slurry C), collected from the AD plants in Japan and China, were treated using novel TiZrO4 @Cu and TiZrO4 @Cu@SiO2 multilayered hollow spheres containing Cu sub-nanoclusters as the catalyst. The results showed that the chemical oxygen demand (COD) was removed by 63 % for slurry J and 44 % for slurry C after 5 h. The Cu sub-nanoclusters acted as co-catalysts and active centers, facilitating rapid electron transfer to oxygen molecules and forming highly reactive •O2- and •OH species (Use slurry J as the based solution). These free radicals cleaved the interconnecting bonds between benzene rings, disintegrated the ring structure, formed intermediate compounds such as n-hexylic acid, and ultimately mineralized organic pollutants in biogas slurry into CO2 and H2O. At the same time, TiZrO4 @Cu@SiO2 had excellent stability due to the protection of the SiO2 shell and reduced threefold Cu leaching than TiZrO4 @Cu. The COD removal rate was always 60 % in six cycles in the slurry J. The new catalyst ensured the high performance of catalytic air oxidation at low temperatures, which has significant potential as an environmentally friendly and energy-saving method for organic wastewater treatment.

Biogas slurry recirculation regulates food waste fermentation: Effects and mechanisms.

Regularly adding biogas slurry into fermentation reactors is an effective way to enhance hydrogen or methane production. However, how this method affects the production of valuable organic acids and alcohols is still being determined. This study investigated the effects of different addition ratios on semi-continuous fermentation reactors using food waste as a substrate. The results showed that an addition ratio of 0.2 increased lactic acid production by 30% with a yield of 0.38 ± 0.01 g/g VS, while a ratio of 0.4 resulted in mixed acid fermentation dominated by n-butyric acid (0.07 ± 0.01 g/g VS) and n-caproic acid (0.06 ± 0.00 g/g VS). The introduction of Bifidobacteriaceae by biogas slurry played a crucial role in increasing lactic acid production. In contrast, exclusive medium-chain fatty acid producers enhanced the synthesis of caproic acid and heptanoic acid via the reverse ß-oxidation pathway. Mechanism analyses suggested that microbial community structure and activity, substrate hydrolysis, and cell membrane transport system and structure changed to varying degrees after adding biogas slurry.

Atomically dispersed copper sites on titanium zirconium oxide accelerate the simultaneous oxidative removal of organic carbon and ammonia from landfill leachate.

Landfill leachate is a refractory wastewater. Low-temperature catalytic air oxidation (LTCAO) has shown considerable potential for leachate treatment owing to its green and simple operation, but the simultaneous removal of chemical oxygen demand (COD) and ammonia from leachate remains challenging. Herein, TiZrO4 @CuSA hollow spheres with high-loading single-atom Cu were synthesized using isovolumic vacuum impregnation and co-calcination methods, and the catalyst was applied to the LTCAO treatment of real leachate. Consequently, the removal rate of UV254 reached 66% at 90 °C within 5 h, while that for COD was 88%. Simultaneously, the NH3/NH4+ (33.5 mg/L, 100 wt%) in the leachate was oxidized to N2 (88.2 wt%), NO2--N (11.0 wt%), and NO3--N (0.3 wt%) owing to the effect of free radicals. The single-atom Cu co-catalyst in TiZrO4 @CuSA exhibited a localized surface plasmon resonance effect at the active center, which could quickly transfer electrons to O2 in water to form O2.- with a high activation efficiency. The degradation products were determined and the deduced pathway was as follows: the bonds joining benzene rings were first broken, and then the ring structure was further opened to produce acetic acid and other simple organic macromolecules, which were finally mineralized to CO2 and H2O.

Enhanced methane production from the co-digestion of food waste and thermally hydrolyzed sludge filtrate.

Although many technologies can be applied to sewage sludge (SS) and food waste (FW) treatment, high investment and operational costs, high land occupation, and the "not-in-my-backyard" effect pose many challenges in practice. Thus, it is important to develop and utilize low-carbon or negative-carbon technologies to tackle the carbon problem. This paper proposes a method of anaerobic co-digestion of FW and SS, thermally hydrolyzed sludge (THS), or THS filtrate (THF) to enhance their methane potential. Compared to the co-digestion of SS with FW, the methane yield of the co-digestion of THS and FW was 9.7-69.7% higher, and that of the co-digestion of THF and FW was 11.1-101.1% higher. The synergistic effect was weakened with the addition of THS but enhanced with the addition of THF, potentially owing to the change in humic substances. Filtration removed most humic acids (HAs) from THS but retained fulvic acids (FAs) in THF. Moreover, THF produced 71.4% of the methane yield of THS, although only 25% of the organic matter permeated from THS to THF. This indicated that hardly biodegradable substances remained in the dewatering cake and were removed from anaerobic digestion systems. The results indicate that the co-digestion of THF and FW is an effective way to enhance methane production.

Remove humic acid from water quickly using only oxygen and sulfite at nickel cobalt spinel catalyst.

The typical refractory organic pollutant, humic acid (HA), causes many water and wastewater treatment obstacles. In this study, a novel method was proposed to degrade HA based on the low-temperature (<100 °C) catalytic air oxidation technology (LTCAO) using the NiCo-spinel (NCO) as a catalyst and the sulfite as a promoter. Sulfite enhanced the quantity of mineralized HA to 2.4 times that without sulfite assistance, and the removal rate of total organic carbon reached 93.1% within 60 min at 90 °C. HA gradually degrades into small organic molecules and is mineralized through interfacial reactions and radical paths. Sulfite plays a triple role in these reactions. Sulfite sulfonated HA destroyed its pseudomicellar structure, making HA easily oxidized. Sulfite also coordinated with NCO and promoted the internal electronic hopping conduction of NCO because of the fast electron transfer between SO32- and the h+sites, thus accelerating the electron transfer between HA and O2 mediated by NCO. In addition, the coordinated SO32- was activated to form the radical ∙SO3-, which strengthened the oxidation of HA. This study supports a simple and green method for efficiently cleaning water and wastewater rich in HA.

Embedding ruthenium nanoparticles in the shell layer of titanium zirconium oxide hollow spheres to catalyze the degradation of alkali lignin under mild condition.

To catalyze the degradation of lignin in refractory wastewater efficiently, a new nanocomposite with Ru nanoparticles embedded on the surface of TiZrO4 hollow spheres was fabricated with three method a "sol-gel + calcination + vacuum-impregnation" template method, and the unique binary composition of TiZrO4/Ru prevented the aggregation of Ru and keep its high activity. During 3-h catalytic-oxidation at 160 °C and 2.0 MPa O2, 98% alkali lignin was degraded and 70% organic carbon was mineralized with the catalysis of TiZrO4/Ru, while the values were only 50% and 25% without analysts. The catalyst increased the catalytic-oxidation rate constant k1 (h-1) of alkali lignin from 0.282 h-1 to 1.175 h-1 because of high-efficiency hydroxyl radical production, as determined by EPR. LC-OCD showed that the catalyst decomposed alkali lignin with molecular weight 1-2 kDa to small molecules. Butyl acetate was the main intermediate product, which should be derived from the auto synthesis of butanol and acetic acid. In addition to high conversion efficiency, the catalyst had good stability with 95% capability after five cycles. In real biogas slurry treatment, an increase of biochemical to COD ratio from 0.28 to 0.51, with obvious decoloration, indicated TiZrO4/Ru enhanced the biodegradability of the refractory wastewater significantly.

Construction of a turn off-on fluorescent nanosensor for cholesterol based on fluorescence resonance energy transfer and competitive host-guest recognition.

Using chloroauric acid as precursor and ß-cyclodextrin (ß-CD) as reducing agent and stabilizer, ß-CD@AuNPs with negative charge were synthesized by one-step colloidal synthesis method. The positive charged carbon quantum dots (CQDs) were synthesized by one-step of sonication of cetylpyridinium chloride. Under the role of static electricity, the fluorescence resonance energy transfer (FRET) occurred between CQDs and ß-CD@AuNPs. CQDs and ß-CD@AuNPs served as the fluorescence energy donors and receptors, respectively, i.e., the fluorescence of CQDs was turned off by ß-CD@AuNPs. Based on the specific host-guest recognition between the inner cavity of ß-CD and cholesterol, CQDs was replaced by cholesterol, the FRET could be interrupted, and then the fluorescence of CQDs was turned on. A good linear relationship between cholesterol concentration (10-210 µmol L-1) and fluorescence intensity was obtained and the LOD was 343.48 nmol L-1. Because of excellent fluorescence quenching ability of FRET, the analytical performance (including LOD and linear scope) of such a turn off-on fluorescent nanosensor (e.g., CQDs/ß-CD@AuNPs) was better than nanosensor only via competitive host-guest recognition (e.g., ß-CD functionalized CQDs). The synergistic effect of competitive host-guest recognition and FRET was proved. Because of selective recognition, ultrasensitive, wide linear range, and strong anti-interference ability, CQDs/ß-CD@AuNPs as a turn off-on fluorescent nanosensor was developed to determine cholesterol in porcine serum.

Highly crystalline graphitic carbon nitride quantum dots as a fluorescent probe for detection of Fe(III) via an innner filter effect.

Bulk g-C3N4 was transformed into water-soluble graphitic carbon nitride quantum dots (g-CNQDs) via a chemical oxidation and liquid exfoliation process. The g-CNQDs possess a size distribution ranging from 1 to 5 nm (centered at 3 nm), excellent crystallinity, and are water soluble. It is found that Fe(III) ions are adsorbed on the surface of the g-CNQDs via electrostatic interaction, and that the blue fluorescence of the g-CNQDs is reduced by Fe(III) via an inner filter effect. By using the g-CNQDs as a fluorescent probe, Fe(III) can be determined at excitation/emission wavelengths of 241/368 nm in spiked natural water samples within 1 min and with good selectivity over other ions. Response is linear in the 0.2-60 µmol·L-1 Fe(III) concentration range, and the detection limit is 23 nmol·L-1. Graphical abstract Graphitic carbon nitride quantum dots (g-CNQDs) emit blue fluorescence at an excitation wavelength of 241 nm. Fe(III) ions are quickly adsorbed on the g-CNQDs via electrostatic interaction, and fluorescence is quenched due to an inner filter effect.

Noble metal sandwich-like TiO2@Pt@C3N4 hollow spheres enhance photocatalytic performance.

The goal of this work was to assess the performance of Pt nanoparticles (NPs) as co-catalysts on the photocatalytic activity of TiO2@Pt@C3N4 hollow spheres, which was tested by photodegrading rhodamine B and methyl blue under visible light irradiation. TiO2@Pt@C3N4 composites were fabricated by using modified polystyrene balls as templates, hydrothermal reactions, and calcination. Under simulated sunlight irradiation, photocatalytic activity was in the following of TiO2@Pt@C3N4 > TiO2@C3N4 > C3N4 > P25. The photo-conversion rate of the TiO2@Pt@C3N4 increased significantly relative to TiO2@C3N4 and the others. The combination of TiO2 and C3N4, as well as the sandwiched of Pt NPs reduce electron-hole recombination as a result of having an electron trap site, which can store and shuttle photo-generated electrons, and enhance photo-generation of active radicals. Electron paramagnetic resonance (EPR) spectroscopy, as well as photo-luminescence (PL), and electrochemical measurements were taken to verify this conclusion. Considering the multi-functional combination of precious metals and semiconductor materials, this work may provide new ideas for the design of high-performance catalysts.

Constituting fully integrated colorimetric analysis system for Fe(III) on multifunctional nitrogen-doped MoO3/cellulose paper.

Using ammonium molybdate and thiourea as precursors, nitrogen-doped MoO3 was produced by a one-step carbonization and then fixed into the cellulose filter paper (NMCP) with acrylic resin as a fixative. NMCP was designed as a multifunctional nanocomposite, i.e., solid phase adsorbent for Fe(III) preconcentration, photocatalyst for iron species transformation and color interference removal, and colorimetric sensor for Fe(III) determination. After photocatalysis, the complex of Fe-humic substances could be transformed into Fe(III) ions, the interference of colored organic matter (e.g., aqueous humic substance) was removed, Fe(III) was enriched selectively onto NMCP with the coexistence of interference metal ions (e.g. Co(II) and Cd(II)) and then transformed into Fe(II) by hydroxylamine and photoreduction and for colorimetric analysis. The obstacle of o-phenanthroline colorimetric method was overcome. The photodegradation activity of MoO3 was improved 2.02 times by nitrogen doping with the optimal mass ratio, which was also 5.11 times of P25-TiO2. The concentration of Fe(III) on NMCP was quantified by the gray-scale using smart phones and image processing software, without complicated equipment. Based on multifunctional NMCP, a fully integrated visual analysis system was proposed and suitable for the field detection of Fe(III) in natural water. The log-linear calibration curve for Fe(III) was in the range of 0.05-5mg/L with a determination coefficient (R2) of 0.976 and detection limit of 15µg/L.

TiO2@Pt@CeO2 nanocomposite as a bifunctional catalyst for enhancing photo-reduction of Cr (VI) and photo-oxidation of benzyl alcohol.

An solar-light-driven and bifunctional photocatalyst was designed for photo-reduction of Cr(VI) and selective photo-oxidation of benzyl alcohol into benzaldehyde in the presence of water under ambient conditions. Double-shelled and sandwiched TiO2@Pt@CeO2 hollow spheres were prepared by using functionalized polystyrene spheres, sol-gel, hydrothermal reaction, and calcination. The Pt nanoparticles (NPs) were controllably loaded between the TiO2 shell and CeO2 shell. Under solar-light irradiation, the photo-reduction rate of Cr(VI) (µmol h-1) was in the order of TiO2@Pt@CeO2 (1.901) > TiO2@CeO2 (1.424) > TiO2 (1.040) > CeO2 (0.992). Among the above-mentioned photocatalysts, the conversion rate of benzyl alcohol for TiO2@Pt@CeO2 was also the best. These results were attributed to the combination of TiO2 and CeO2 as photocatalyst and oxygen buffer, the double-shelled and sandwiched nanostructure, and the addition of Pt NPs as cocatalyst and electron trap site, which could store and shuttle photo-generated electrons, reduce the recombination of the electron-hole, and then enhance photo-generation of active radicals. This conclusion was verified by the electron paramagnetic resonance (EPR) spectroscopy. Considering the versatile combination of photocatalyst, oxygen buffer and cocatalyst, this work could provide new insights into the design of high-performance bifunctional photocatalysts for heavy metal removal and selective synthesis.

Influence of TiO2 hollow sphere size on its photo-reduction activity for toxic Cr(VI) removal.

After polystyrene@titanium dioxide (PS@TiO2) composite with different size was calcined at designated temperature, TiO2 hollow sphere with controllable size was obtained for high efficient photo-reduction of Cr(VI). The feature of the TiO2 hollow sphere was investigated by SEM, TEM, XRD, UV-Vis, and photoluminescence. The photo-reduction of Cr(VI) were measured for the performance assessment of the TiO2 hollow sphere, Cr(VI) was used as an electron acceptor. After irradiation for 2h, the photo-reduction rate of Cr(VI) (pH=2.82) for TiO2(450nm) was 96%, which exhibited an increase of 5% and 8% compared with TiO2(370nm) and TiO2(600nm). The absorption edges of TiO2 hollow sphere (450nm) was largest with the increasing of hollow sphere size from 370 to 600nm. The optimal hollow sphere size of TiO2 was 450nm for the photo-reduction of Cr(VI), because the light-harvesting efficiency (the best of absorption edge) and photo-generated electron-hole separation rate (the best of photo-reduction rate) of TiO2 hollow sphere were controlled by its hollow sphere size. In addition, we find that the behavior of the hydrogen production was inhibited by the coexistence Cr(VI) solution. This study can improve our understanding of the mechanism for the activity enhancement by the optimal hollow sphere size of TiO2.

Au@Cu2O stellated polytope with core-shelled nanostructure for high-performance adsorption and visible-light-driven photodegradation of cationic and anionic dyes.

Au nanoparticles were covered by Cu2O nanoparticles shell and then Au@Cu2O stellated polytope was synthesized by a facile aqueous solution approach. The samples were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction patterns, X-ray photoelectron spectroscopy, Brunner-Emmet-Teller measurements, and Ultraviolet-visible spectroscopy analysis. With good aqueous dispersibility, surface positive charge, and high chemisorption capacity, Au@Cu2O could be used for anionic dyes removal. Compared with Degussa P25-TiO2, the adsorption of anionic dyes (acid violet 43 or methyl blue, 5.0 mg L(-1)) onto Au@Cu2O was increased by 90.12% and 50.8%, respectively. The photodegradation activity of methyl orange and methyl violet were in the declining order: Au@Cu2O>Cu2O-Au nanocomposites>Cu2O>P25-TiO2. The synergistic effect of coupling Au core with Cu2O shell on the dyes photodegradation was observed. The photoexcited electrons from Cu2O conduction band could be captured by Au nanoparticles, resulting in an improved electron-hole separation. Moreover, a Schottky barrier was assumed to form at the Cu2O-Au interface and Au NPs as electron sink could reduce the recombination of photoinduced electrons and holes, facilitating the photocatalytic interface reaction. The geometry of core-shell and stellated polytope is effective in the design of Cu2O-Au nanocomposites for adsorption and photocatalysis.

Synergistic effect of double-shelled and sandwiched TiO2@Au@C hollow spheres with enhanced visible-light-driven photocatalytic activity.

A novel approach for the fabrication of double-shelled, sandwiched, and nanostructured hollow spheres was proposed, using hydrotherm reaction and calcination. The negatively charged nanoparticles (e.g., Au, Ag, and Pt) could be adsorbed successively onto the positively charged hollow spheres (e.g., TiO2, ZnO, and ZrO2). The resulted nanocomposites (TiO2@Au, as a proof-of-concept) were dispersed in glucose solution under hydrothermal conditions. After calcination, uniform double-shelled and sandwiched TiO2@Au@C hollow spheres were obtained and Au nanoparticles were sandwiched between the shell wall of TiO2 and C. The samples were characterized by SEM, TEM, XRD, XPS, BET, and UV-vis DRS. The photocatalytic activity for the degradation of 4-nitroaniline was in the order of TiO2@Au@C > TiO2@C > TiO2/Au > P25. The visible-light photodegradation rate of 92.65% for 4-nitroaniline was achieved by TiO2@Au@C, which exhibited an increase of 75% compared to Degussa P25 TiO2. Furthermore, no deactivation occurred during catalytic reaction for three times, i.e., the TiO2@Au@C microspheres exhibited superior photocatalytic stability. TiO2@Au@C microspheres could also enhance the photocatalytic activity for hydrogen generation from methanol/water solutions. The synergistic effect of coupling TiO2 hollow spheres with Au nanoparticles and C shell on photocatalytic performance was proved by us. The photoexcited electrons from Au nanoparticles could be captured by the conduction band of TiO2 and then the electron-hole separation was improved. Moreover, both the visible light absorption and the affinity between TiO2 and pollutants could be improved by the coexistence of carbonaceous materials, which could facilitate the photocatalytic interface reaction.

Ascorbic acid surface modified TiO2-thin layers as a fully integrated analysis system for visual simultaneous detection of organophosphorus pesticides.

TiO2 photocatalysis and colorimetric detection are coupled with thin layer chromatography (TLC) for the first time to develop a fully integrated analysis system. Titania@polystyrene hybrid microspheres were surface modified with ascorbic acid, denoted AA-TiO2@PS, and used as the stationary phase for TLC. Because the affinity between AA-TiO2@PS and organophosphorus pesticides (OPs) was different for different species of OPs (including chlopyrifos, malathion, parathion, parathion-methyl, and methamidophos), OPs could be separated simultaneously by the mobile phase in 12.0 min with different Rf values. After surface modification, the UV-vis wavelength response range of AA-TiO2@PS was expanded to 650 nm. Under visible-light irradiation, all of the OPs could be photodegraded to PO4(3-) in 25.0 min. Based on the chromogenic reaction between PO4(3-) and chromogenic agents (ammonium molybdate and ascorbic acid), OPs were quantified from color intensity images using a scanner in conjunction with image processing software. So, AA-TiO2@PS was respectively used as the stationary phase of TLC for efficient separation of OPs, as a photocatalyst for species transformation of phosphorus, and as a colorimetric probe for on-field simultaneous visual detection of OPs in natural water. Linear calibration curves for each OP ranged from 19.3 nmol P L(-1) to 2.30 µmol P L(-1). This integrated analysis system was simple, inexpensive, easy to operate, and sensitive.

Constituting fully integrated visual analysis system for Cu(II) on TiO2/cellulose paper.

As a cheap and abundant porous material, cellulose filter paper was used to immobilize nano-TiO2 and denoted as TiO2/cellulose paper (TCP). With high adsorption capacity for Cu(II) (more than 1.65 mg), TCP was used as an adsorbent, photocatalyst, and colorimetric sensor at the same time. Under the optimum adsorption conditions, i.e., pH 6.5 and 25 °C, the adsorption ratio of Cu(II) was higher than 96.1%. Humic substances from the matrix could be enriched onto TCP but the interference of their colors on colorimetric detection could be eliminated by the photodegradation. In the presence of hydroxylamine, neocuproine, as a selective indicator, was added onto TCP, and a visual color change from white to orange was generated. The concentration of Cu(II) was quantified by the color intensity images using image processing software. This fully integrated visual analysis system was successfully applied for the detection of Cu(II) in 10.0 L of drinking water and seawater with a preconcentration factor of 10(4). The log-linear calibration curve for Cu(II) was in the range of 0.5-50.0 µg L(-1) with a determination coefficient (R(2)) of 0.985 and its detection limit was 0.073 µg L(-1).