Titanium dioxide surface modified with both palladium and fluoride as an efficient photocatalyst for the degradation of urea

Effects of surface fluoride modification on TiO2 for the photocatalytic oxidation of toluene

In the present study, we investigated the influence of surface fluorine (F) on TiO2 for the photocatalytic oxidation (PCO) of toluene. TiO2 modified with different F content was prepared and tested. It was found that with the increasing of F content, the toluene conversion rate first increased and then decreased. However, CO2 mineralization efficiency showed the opposite trend. Based on the characterizations, we revealed that F substitutes the surface hydroxyl of TiO2 to form the structure of Ti-F. The presence of the appropriate amount of surface Ti-F on TiO2 greatly enhanced the separation of photogenerated carriers, which facilitated the generation of ·OH and promoted the activity for the PCO of toluene. It was further revealed that the increase of only ·OH promoted the conversion of toluene to ring-containing intermediates, causing the accumulation of intermediates and then conversely inhibited the ·OH generation, which led to the decrease of the CO2 mineralization efficiency. The above results could provide guidance for the rational design of photocatalysts for toluene oxidation.

Visible light-induced photocatalytic degradation of tetrabromobisphenol A on platinized tungsten oxide

In this study, we investigated the degradation of the flame retardant tetrabromobisphenol A (TBBPA) using platinized tungsten oxide (Pt/WO3), synthesized via a simple photodeposition method, under visible light. The results of degradation experiments show a significant enhancement in TBBPA degradation upon surface platinization of WO3, with the degradation rate increasing by 13.4 times compared to bare WO3. The presence of Pt on the WO3 surface stores conduction band electrons, which facilitates the two-electron reduction of oxygen and enhances the production of valence band holes (hVB+) and hydroxyl radicals (●OH). Both hVB+ and ●OH are significantly involved in the degradation of TBBPA in the visible light-irradiated Pt/WO3 system. This was verified through fluorescence spectroscopy employing coumarin as a chemical probe and oxidizing species-quenching experiments. The analysis of degradation products and their toxicity assessment demonstrate that the toxicity of TBBPA-contaminated water is significantly reduced after Pt/WO3 photocatalysis. The degradation rate of TBBPA increased with increasing Pt/WO3 dosage, reached an optimum at a Pt content of 0.5 wt%, but decreased with increasing TBBPA concentration. The decrease in degradation efficiency of Pt/WO3 was minor, both in the presence of various anions and after repeated use. This study proposes that Pt/WO3 is a viable photocatalyst for the degradation of TBBPA in water under visible light.

Urea catalytic oxidation for energy and environmental applications

Urea is one of the most essential reactive nitrogen species in the nitrogen cycle and plays an indispensable role in the water-energy-food nexus. However, untreated urea or urine wastewater causes severe environmental pollution and threatens human health. Electrocatalytic and photo(electro)catalytic urea oxidation technologies under mild conditions have become promising methods for energy recovery and environmental remediation. An in-depth understanding of the reaction mechanisms of the urea oxidation reaction (UOR) is important to design efficient electrocatalysts/photo(electro)catalysts for these technologies. This review provides a critical appraisal of the recent advances in the UOR by means of both electrocatalysis and photo(electro)catalysis, aiming to comprehensively assess this emerging field from fundamentals and materials, to practical applications. The emphasis of this review is on the design and development strategies for electrocatalysts/photo(electro)catalysts based on reaction pathways. Meanwhile, the UOR in natural urine is discussed, focusing on the influence of impurity ions. A particular emphasis is placed on the application of the UOR in energy and environmental fields, such as hydrogen production by urea electrolysis, urea fuel cells, and urea/urine wastewater remediation. Finally, future directions, prospects, and remaining challenges are discussed for this emerging research field. This critical review significantly increases the understanding of current progress in urea conversion and the development of a sustainable nitrogen economy.

Bubble-propelled micromotors for ammonia generation

Micromotors have emerged as promising tools for environmental remediation, thanks to their ability to autonomously navigate and perform specific tasks at the microscale. In this study, we present the development of MnO2 tubular micromotors modified with laccase for enhanced oxidation of organic pollutants by providing an additional oxidative catalytic pathway for pollutant removal. These modified micromotors exhibit efficient ammonia generation through the catalytic decomposition of urea, suggesting their potential application in the field of green energy generation. Compared to bare micromotors, the MnO2 micromotors modified with laccase exhibit a 20% increase in rhodamine B degradation. Moreover, the generation of ammonia increased from 2 to 31 ppm in only 15 min, evidencing their high catalytic activity. To enable precise tracking of the micromotors and measurement of their speed, a deep-learning-based tracking system was developed. Overall, this work expands the potential applicability of bio-catalytic tubular micromotors in the energy field.

Enhanced oxidation of urea by pH swing during chlorination: pH-dependent reaction mechanism

Urea reacts with chlorine to form chlorinated ureas (chloroureas), and fully chlorinated urea (tetrachlorourea) is further hydrolyzed into CO2 and chloramines. This study found that the oxidative degradation of urea by chlorination was enhanced by the pH swing, wherein the reaction proceeded under an acidic pH (e.g., pH = 3) in the first stage, and the solution pH was subsequently increased to a neutral or alkaline value (e.g., pH > 7) in the second-stage reaction. The degradation rate of urea by pH-swing chlorination increased with increasing chlorine dose and pH during the second-stage reaction. The pH-swing chlorination was based on the opposite pH dependence of sub-processes comprising urea chlorination. The formation of monochlorourea was favored under acidic pH conditions; however, the subsequent conversion into di- and trichloroureas was favored under neutral or alkaline pH conditions. The deprotonation of monochlorourea (pKa = 9.7 ± 1.1) and dichlorourea (pKa = 5.1 ± 1.4) was suggested to be responsible for the accelerated reaction in the second stage under increased pH conditions. pH-swing chlorination was also effective in degrading urea at low concentrations (micromolar levels). In addition, the total nitrogen concentration significantly decreased during the degradation of urea because of the volatilization of chloramines and the release of other gaseous nitrogen compounds.

Solar-Driven Catalytic Urea Oxidation for Environmental Remediation and Energy Recovery

The water-energy nexus is highly related to sustainable societal development. As one of the most abundant biowastes discharged into the environment, mild abatements and green conversions of urea wastewater have been widely investigated. Due to abundant sources, global distribution, and easy control, light-based catalytic strategies have become alternative on-site treatment approaches. After comprehensively surveying the recent progress, recent achievements of urea oxidation under light irradiation are reviewed herein. Several typical light-promoted systems employed in urea conversion, including photocatalysis, photo-electrocatalysis, photo-biocatalysis, and photocatalytic fuel cells, are meticulously introduced and discussed, from catalyst designs and medium conditions to established mechanisms. To realize the goal of sustainability, the chemical energy in urea-rich water could be utilized for the value-added production of hydrogen fuel and electricity. Finally, based on current developments, existing challenges are enumerated and developmental prospects in the future of light-driven urea conversion technologies are proposed.

Enhancement of urea removal from reclaimed water using thermally modified spent coffee ground biochar activated by adding peroxymonosulfate for ultrapure water production

To enhance the degradation of urea in reclaimed water for producing ultrapure water (UPW), thermally modified biochar (TBC) was prepared by secondary pyrolysis using spent coffee biochar with the function as an activator of peroxymonosulfate (PMS). Results showed that 94.4% of urea can be degraded effectively by the TBC-PMS system at the dosage of 0.4 g/L TBC and 2 g/L PMS under neutral and weak acid conditions. Moreover, urea removal mainly depended on the free radical pathway (SO₄^{• -} and OH^•), especially OH^•. The inorganic anions of TBC increased via secondary pyrolysis, especially carbonate and phosphate, resulting in higher electrical conductance (EC) value than the original biochar. It was conducive to activating PMS. As well, C-O, -OH worked as an active site in the TBC-PMS system, providing electrons and activating PMS. This work provides a novel strategy for UPW production using TBC-PMS system.

A critical review on challenges and trend of ultrapure water production process

The recent and vigorous developments in semiconductor technology strictly request better quality and large quantity of ultrapure water (UPW) for their production. It is crucial to secure a large amount of raw water for the future development of UPW production. Using reclaimed water as alternative raw water source to produce UPW is therefore considered the feasible trend and solution for sustainable use of water resources towards a common future practice in UPW production. The challenge of using reclaimed water is due to its higher content of organic pollutants, especially small molecule organic pollutants such as urea, which are difficult to remove through traditional UPW production process. Consequently, improving the existing UPW production process to meet the water standard desired in the semiconductor industry is essential. This paper reviewed the current traditional processes for removing organic matters in UPW production, including ion-exchange (IX) adsorption, granular activated carbon (GAC) adsorption, reverse osmosis (RO) and ultraviolet (UV) irradiation. The potential problems in the actual UPW production process were identified when using reclaimed water as raw water source. A new strategy of applying the advanced oxidation process (AOPs) to UPW production as a supplementary unit to guarantee UPW quality was proposed. Its feasibility and research focus were then analyzed and discussed in obtaining a new solution for a future development of the UPW production process.

Photocatalytic exoskeleton: Chitin nanofiber for retrievable and sustainable TiO2 carriers for the decomposition of various pollutants

Loading a photocatalytic TiO₂ to organic carriers has been desired for volumetric TiO₂ incorporation, facile retrieval, and sustainable utilization. Traditionally, suspended TiO₂ nanoparticles or its thin film on two-dimensional substrate are popularly fabricated for pollutants decomposition without carriers; due to poor thermomechanical properties of the organic carriers. Herein, a combination of the chitin nanofiber carrier and atomic layer deposition proves relevance for formation of anatase TiO₂ thin layer so that photocatalytic decomposition in three-dimensional surface. Moreover, chitin nanofiber is capable of holding the TiO₂ nanoparticles for multiple cycles of photocatalysis. Those types of TiO₂ show characteristic degradation performance for gaseous (acetaldehyde) and aqueous pollutants (4-chlorophenol and rhodamine B). After catalytic reaction, chitin/TiO₂ is retrievable owing to carrier's robustness even in water without TiO₂ aggregation and loss. This work suggests that chitin-based photocatalyst is applicable to numerous pollutants through chitin's relatively high chemical resistance and stably wedged TiO₂ during photocatalytic reaction.

Enhanced visible-light photocatalytic activity of perylene diimide (PDI) supramolecular nanorods with Pt QDs deposited in situ

Well-dispersed Pt quantum dots (QDs) were the first to be successfully deposited onto a PDI supramolecular nanorods surface via a simple in situ chemical reduction. Under visible light irradiation, Pt QDs/PDI composites displayed excellent photocatalytic property in the degradation of phenol. The optimum 1 wt% Pt QDs/PDI composite was found to be 6.2 times greater than pure PDI supramolecular nanorods for the degradation rate constant (k). The enhanced photocatalytic performance can be attributed to the rapid transfer and efficient separation of photogenerated carriers, originating from the effective trapping and transporting of electrons by Pt QDs. At the same time, Pt QDs were also loaded as active sites during the photocatalytic reaction. Moreover, the 1 wt% Pt QDs/PDI composite was found to have high photocatalytic stability and cycle utilization, suggesting its great potential in the area of water environmental purification.

Electrochemical mineralization of uric acid with boron-doped diamond electrode: Factor analysis and degradation mechanism

In the present study, the mineralization performance and pathway of uric acid (UA) on boron-doped diamond (BDD) anodes were investigated. The oxidation behavior of UA on BDD surface was firstly tested through cyclic voltammetry measurements. The individual and joint effects of four quantitative parameters (applied current density, NaHCO₃ concentration, NaCl concentration and flow rate) on UA mineralization were then examined with Doehlert experimental design. The results acquired by statistical analysis revealed that NaCl concentration and applied current density displayed the most dominant roles on UA degradation, while the influences of NaHCO₃ concentration and flow rate were statistically insignificant. As a result, the following optimal conditions were reached: applied current density of 7.80 mA cm^-2, NaHCO₃ concentration of 6.0 mM, NaCl concentration of 9.0 mM and flow rate of 600 mL min^-1, which gave a TOC decay of 89.4%, a specific energy consumption of 125.36 KWh kg^-1 TOC, a combustion current efficiency of 15.0% and an electrical energy per order of 35.79 KWh m^-3 order^-1 within 30 min of electrolysis. Further results from LC/MS analysis confirmed the ring rupture of UA during the electrolysis, due to the attack of hydroxyl radicals and active chlorine species. Accordingly, two plausible degradation pathways of UA in bicarbonate and chloride media on BDD anode were proposed respectively.