Photocatalytic reduction of nitrate in seawater using C/TiO2 nanoparticles

A systematic review of strategies to overcome barrier for nitrate separation systems from drinking water: Focusing on waste streams treatment processes

By 2030, the UN General Assembly issued the Sustainable Development Goal 6, which calls for the provision of safe drinking water. However, water resources are continuously decreasing in quantity and quality. NO3- is the most widespread pollutant worldwide, threatening both human health and ecosystems. NO3- separation systems (NSS) using IX and membrane-based techniques (MBT) are considered practical and efficient technologies, but the management of IX waste brine (IXWB) and concentrate streams for MBT (CSM), as well as the high salt requirements for IX regeneration, are challenging from both economic and environmental perspectives. It is essential to classify the different waste management strategies in order to examine the current state of research and identify the best option to address these issues. This review provides harmonized information on IXWB/CSM management strategies. This study is the first systematic review of all papers available in the Web of Science, Scopus, and PubMed databases published until February 2023. 75% of the studies focused on the use of biological denitrification (BD) and catalytic denitrification (CD). Although innovative technologies (bio-regeneration and direct CD) have advantages over indirect processes, they are not yet practical for large-scale plants because their reliability is unknown. Moreover, the generation of NH4+ is the major challenge for application large-scale of chemical reduction. An innovative work flow diagram, challenges, and future prospects are presented. The review shows that integrating modified NSS with IXWB/CSM treatment is a promising sustainable solution, as the combination could be economically and environmentally beneficial and remove barriers to NNS application.

Optimum anatase/rutile ratios of TiO2 for photocatalytic denitrification from IX brine waste and real RO concentrate: RSM-CCD model and the use of an economical and efficient hole scavenger study

Both ion exchange (IX) and reverse osmosis (RO) technologies are effective in removing NO3- from drinking water, but the disposal of waste streams and the large amount of salt needed for to prepare fresh brine in IX have become economic and environmental challenges. To overcome these barriers, photocatalytic denitrification (PD) using TiO2 nanoparticles in different anatase/rutile (A/R) ratios was applied to IX brine waste (IXWB) and real RO concentrate (real ROC). The synthesized samples were characterized by XRD, FESEM-EDX, and elemental mapping, BET, and UV-Vis absorption spectra. Experiments design, process optimization, and confirmation of results were performed using CCD-RSM. The study also investigated the use of glycerol, a by-product of biodiesel production, as an economic hole scavenger. The effect of different concentrations of SO4-2 on the removal efficiency of NO3- and the N2 selectivity was also investigated. The anatase phase converts to rutile with increasing calcination temperature, resulting in larger crystallites and particle sizes and narrower optical band gaps of TiO2 nanoparticles. Under optimal conditions, the mixed A (79%)/R (21%) phase of TiO2 with FA showed the highest photoactivity in conversion NO3- (89% and 95%) with N2 selectivity (83% and 85% for IXWB and real ROC, respectively). For real ROC, the use of glycerol as an economical hole scavenger resulted in 100% NO3- reduction. A possible mechanism involving glycerol and FA is discussed. Finally, optimized (A/R) ratios of TiO2 nanoparticles were successfully supported on the surface of GAC (GAC/TiO2). The composite sample can be easily recycled and reused from solution and exhibits high photoactivity even after five cycles.

Simultaneous removal of fluoride and nitrate from synthetic aqueous solution and groundwater by the electrochemical process using non-coated and coated anode electrodes: A human health risk study

Co-presence of fluoride (F^-) and nitrate (NO₃^-) in water causes numerous health complications. Thus, they should be eliminated by an appropriate method like the EC process. In this research, simultaneous removal of F^- and NO₃^- from synthetic aqueous solution and groundwater has been considered by the EC technique under operational parameters like anode materials (un-coated (Al and Fe) and synthesized coated (Ti/TiRuSnO₂ and Ti/PbO₂)), cathode materials (Cu, St, and Gr), current density (12, 24, and 36 mA/cm²), inter-electrode distance (0.5, 1, and 2 cm), pH (5.5, 7, and 8.5), NaCl concentrations (0.5, 1, and 1.5 g/L), electrolysis time (15, 30, 45, 60, 90, and 120 min), NO₃^- concentrations (75, 150, and 225 mg/L), and F^- concentrations (2, 4, 6, and 8 mg/L) for the first time in this research. The results proved that Al as non-coated anode and Cu as cathode electrodes were more effective in the co-removal of F^- and NO₃^-. The maximum removal efficiencies of 94.19 and 95% were observed at the current density of 36 mA/cm², 1 cm of inter-electrode distance, pH 7, 1 g/L of NaCl, and 90 min electrolysis time by Al-Cu electrode for F^- (2 mg/L) and NO₃^- (75 mg/L), respectively. The higher efficiency of Al-Cu electrodes was due to the simultaneous occurrence of electrocoagulation, electroreduction, and electrooxidation processes. Al-Cu electrode application considerably diminished f- and NO3- concentrations in the groundwater. Health risk assessment proved that HQ of F^- was significantly decreased after treatment by the Al-Cu electrode. Thus, the EC process using an appropriate and effective electrode is a promising technique for treating aqueous solutions containing F^- and NO₃^-.

Anatase quantum dots decorated silica/carbon lamellas for removal of antipsychotic drugs via adsorption-photocatalysis and toxicity evaluation

In this work, discrete quantum dots of crystallized anatase TiO₂ are successfully anchored on carbon nanosheets containing amorphous SiO₂ via templated self-assembly and pyrolysis routes. The novel hybrid photocatalyst of TiO₂/C/SiO₂ exhibits well coupled adsorption and visible light photocatalysis on chlorpromazine (CPZ) and the rate constants are 0.0223 and 0.0198 min^-1, respectively. The direct photocatalytic degradation of CPZ under static conditions reaches 91.1% within 3 h while a removal rate of 31.4% for CPZ could be retained under dynamic flow conditions, and the improved performance could be attributed to enhanced adsorption via SiO₂/C and highly exposure of TiO₂ QDs surface. Based on the trapping experiments, ESR, LC-MS, and toxicity evaluation, O₂^- free radicals are identified as main reactive species for CPZ degradation along three possible pathways, with reduced toxicities for its intermediates. The cell viability tests of photocatalytic-degraded solutions and the catalyst exhibit negligible toxicities for both intermediates and the material, suggesting the novel composite of TiO₂/C/SiO₂ as an environmental friendly photocatalyst for pharmaceutical wastewater treatment.

Sunlight-mediated photocatalytic removal of phenanthrene from wastewater using carbon-doped zinc oxide (C-ZnO) nanoparticles

In an effort for efficient solar energy harvesting, carbon-doped zinc oxide (C-ZnO) nanoparticles with intriguing properties were synthesized by sonicated sol-gel technique with the aid of activated charcoal. Compared to pure ZnO, the incorporation of carbon has drastically promoted the photocatalytic activity of C-ZnO towards the degradation of phenanthrene under illumination of both UV and sunlight. The characterization of the as-synthesized nanoparticles by scanning electron microscope (SEM), UV-vis spectra, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectroscopy (EDS) confirmed the carbon doping of C-ZnO. The highest degradation rate of phenanthrene was obtained at pH 7 and C-ZnO loading of 0.5 g L^-1. Finally, the kinetic studies of the photocatalytic degradation of phenanthrene by using C-ZnO were well-fitted with the Langmuir-Hinshelwood model and followed the pseudo-first-order rate expression.

Pitaya-Structured Microspheres with Dual Laser Wavelength Responses for Polymer Laser Direct Writing

A unique pitaya-structured graphene/TiO₂@PS microsphere with dual laser wavelength responses is designed and prepared via a facile approach of polymer melt blending. The graphene/TiO₂ particles ("pitaya seeds") are homogeneously distributed in the polystyrene ("pitaya pulp") of the microspheres with an average size of 1.5 μm. The graphene in microspheres serves not only as a laser absorber that has responses to both 355 nm UV and 1064 nm NIR lasers but also as a reducing agent of TiO₂ during laser direct writing (LDW). As expected, benefiting from the unique pitaya-structured structure, the graphene/TiO₂@PS microsphere can remarkably improve the performance of both NIR and UV LDW of polymers. The results of characterizations reveal that the black color caused by NIR LDW is due to the generation of the amorphous carbon and the color change after UV LDW is owing to the formation of black sp/sp² carbon compounds. Meanwhile, some TiO₂ in microspheres is reduced into the black/gray titanium oxides of Ti²⁺ and Ti³⁺ after NIR and UV LDW, respectively. The above co-contribution endows the graphene/TiO₂@PS microspheres with an outstanding color-changing ability. This pitaya-structured microsphere will have a profound effect on polymers' laser direct writing.

Degradation of Tetracycline on SiO2-TiO2-C Aerogel Photocatalysts under Visible Light

SiO₂-TiO₂-C aerogel photocatalysts with different carbon loadings were synthesized by using sol-gel chemistry. The anatase crystal and nonmetal carbon dopant were introduced during the sol preparation and formed by hydrothermal treatment, which can simultaneously enhance the adsorption ability and visible light photo-activity. A high surface area (759 g cm⁻³) SiO₂-TiO₂-C aerogel composite can remove up to 80% tetracycline hydrochloride within 180 min under visible light. The characterization of the gel structures shows that the homogeneous dispersion of O, Si, Ti and C in the skeleton, indicating that hydrothermal synthesis could provide a very feasible way for the preparation of composite materials. n(C):n(Ti) molar ratio of 3.5 gives the best catalytic performance of the hybrid aerogel, and the cyclic test still confirms over 60% degradation activity after seven use cycles. All catalysis reaction followed the pseudo-first-order rate reaction with high correlation coefficient. The electrons and holes in the compound could be effectively restrained with doping proper amount of C, and ESR results indicate that the oxidation process was dominated by the hydroxyl radical (•OH) and superoxide radical (•O₂⁻) generated in the system.

Selective reduction of nitrate into N2 by novel Z-scheme NH2-MIL-101(Fe)/BiVO4 heterojunction with enhanced photocatalytic activity

Nitrate and its metabolites as common pollutants in water had attracted widespread attentions. Converting nitrate to nontoxic and harmless nitrogen via photocatalysis was a promising approach. In this study, a novel Z-scheme NH₂-MIL-101(Fe)/BiVO₄ heterojunction was successfully prepared. As-prepared Z-scheme heterojunction along with built-in electric field facilitated the charge separation and enhanced the photocatalytic activity in nitrate reduction. The results showed that 0.10-MBiVO photocatalyst exhibited the highest nitrate removal rate of 94.8% (initial concentration 100 mgN/L) and final selectivity to N₂ of 93.4% in 50 min under ultraviolet irradiation. Moreover, formic acid was proved as better hole scavenger compared with methanol and oxalic acid. And the concentration of formic acid had significant influence on the process of nitrate photocatalytic reduction. 0.10-MBiVO photocatalyst exhibited excellent reusability in the recycling tests, indicating its great potential in practical application of nitrate photocatalytic removal. The mechanism of the enhancement as well as reaction pathways for nitrate photocatalytic reduction on NH₂-MIL-101(Fe)/BiVO₄ were comprehensively explored and described at the end.

Strategic management of nitrate pollution from contaminated water using viable adsorbents: An economic assessment-based review with possible policy suggestions

Groundwater contaminated with nitrate has prompted a flurry of research studies around the world in the recent years to address this burning environmental issue. The common presence of nitrates in groundwater, wastewater, and surface waters has thrown an enormously critical challenge to the global research communities to provide safe and clean drinking water to municipalities. As per WHO, the maximum permissible limit of nitrate in drinking water is 10 mg/L and in groundwater is 50 mg/L; exceeding the limits, several human health problems are observed. Adsorption, ion-exchange processes, membrane-based approaches, electrochemical and chemical procedures, biological methods, filtration, nanoparticles, etc. have been well investigated and reviewed to reduce nitrate levels in water samples in the recent years. Process conditions, as well as the efficacy of various approaches, were discovered to influence different techniques for nitrate mitigation. But, because of low cost, simple operation, easy handling, and high removal effectiveness, adsorption has been found to be the most suitable and efficient approach. The main objectives of this review primarily focuses on the creation of a naturally abundant, cost-effective innovative abundant material, such as activated clay particles combined with iron oxide. Oxide-clay nanocomposite materials, effectively remove nitrate with higher removal efficiency along with recovery of nitrate concentrated sludge. Such methods stand out as flexible and economic ways for capturing stabilized nitrate in solid matrices to satisfy long-term operations. A techno-economic assessment along with suitable policy suggestions have been reported to justify the viability of the brighter processes. Indeed, this kind of analytical review appears ideal for municipal community recommendations on abatement of excess nitrate to supply of clean water.

A review on water treatment technologies for the management of oxoanions: prospects and challenges

Oxoanions are a class of contaminants that are easily released into the aquatic systems either through natural or anthropogenic activities. Depending on their oxidation states, they are highly mobile, resulting in the contamination of underground water. Above the permissible level in groundwater, they pose as threats to mammals when the contaminated water is consumed. Some of the health challenges caused are cancer, neurological, cardiac, gastrointestinal, and skin disorders. Several treatment technologies have been adopted over the years for the management of these oxoanions present in the aquatic systems. However interesting these treatment technologies might be, they also have their limitations such as cost-effectiveness, the complexity of the process, and generation of secondary pollutants. This work focused on some of the water treatment technologies applied for the removal of oxoanions. Some of the advantages and disadvantages of these treatment technologies are also highlighted. Amongst all the treatment technologies, adsorption is the most applied method for the removal of oxoanions. However, photocatalysis has a higher prospect since it is non-selective and secondary pollutants are not generated after the treatment process. Also, photocatalysis can simultaneously reduce and oxidise oxoanions as well as organic pollutants respectively.

Formic acid motivated photocatalytic reduction of Cr(VI) to Cr(III) with ZnFe2O4 nanoparticles under UV irradiation

UV-light irradiated photocatalytic reduction of Cr(VI) to Cr(III) in aqueous solution using ZnFe₂O₄ nanoparticles in the presence of formic acid was reported. X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and UV-Vis diffuse reflectance spectroscopy (DRS) were employed to characterize ZnFe₂O₄ nanoparticles. The photocatalytic activity of pure ZnFe₂O₄ under UV irradiation was significantly low. However, the Cr(VI) reduction efficiency on nano-sized ZnFe₂O₄ in the presence of 0.40% formic acid reached 95.4% within 4 h. Herein, the effect of pH, photocatalyst amount, initial concentration of Cr(VI) and formic acid concentration on the photocatalytic reduction of Cr(VI) was investigated. The results indicated that the photocatalytic reduction of Cr(VI) decreased with increase in the initial concentration of Cr(VI), photocatalyst dosage and pH. The reduction rate constant declined from 0.017 min^-1 to 0.0023 min^-1 with the increase in initial concentration of Cr(VI) from 5 to 25 mg L^-1. However, the reduction rate constant sharply increased from 0.000075 min^-1 to 0.0127 min^-1 with the increase in formic acid concentration from 0.05% to 0.40%. The formic acid could capture the photogenerated holes, and eventually formate (HCOO^-) ions could be converted into carbon dioxide radicals (•CO₂^-). Because of more negative redox potential for •CO₂^- radicals, Cr(VI) species could easily be reduced to Cr(III) under UV irradiation. The pseudo-first-order kinetic reaction was confirmed for this reduction process. A tenable mechanism for the photocatalytic Cr(VI) reduction has also been demonstrated.

TiO2-based catalysts for photocatalytic reduction of aqueous oxyanions: State-of-the-art and future prospects

Nowadays, an increasing discharge of oxyanions to the natural environment has been attracting worldwide attention. TiO₂-based photocatalysis is regarded as one of the most promising technologies for the conversion of toxic oxyanions (such as chromate, nitrate, nitrite, bromate, perchlorate and selenate) to harmless and/or less toxic substances in contaminated waters. Various types of TiO₂-based catalysts have been developed, and each of them exhibits its own advantages in catalytic reduction of oxyanions. However, the application of these nanostructured TiO₂ in real water bodies remains a challenge, with limitations associated with sunlight harvesting abilities, production costs, reuse stability and exposure risks. Herein, we aim to present a critical review on reported TiO₂-based photocatalytic reduction of aqueous oxyanions, provide a comprehensive understanding of the possible reaction pathways of formed active species, and evaluate the reduction performance of different types of TiO₂-based catalysts. In addition, the impact of operating parameters (such as solution pH, temperature, dissolved oxygen and coexisting substances) on catalytic reduction performance is discussed. Furthermore, the perspectives of TiO₂-based photocatalytic reduction of oxyanions are also proposed.

Recent Progress in Biochar-Based Photocatalysts for Wastewater Treatment: Synthesis, Mechanisms, and Applications

Biochar (BC) is a carbon-rich material produced from pyrolysis of biomass. In addition to its low toxicity, environmental compatibility, and low cost, BC has the desired advantages of well-developed mesoporous structure and abundant surface functional groups. In recent years, BC-based photocatalysts (BCPs) have played a significant role in many environmental fields. In this paper, we highlight the current progress and several exciting results of BCPs by focusing on their synthesis, characterization, mechanisms, and applications in wastewater treatment. Details on various preparation methods include sol–gel, hydrothermal/solvothermal, ultrasound, calcination, and in situ methods are summarized and discussed. The underlying mechanisms and the applications of BCPs for different semiconductors are reviewed. Furthermore, some future trends and potentials are outlined.

Alcohol solvothermal reduction for commercial P25 to harvest weak visible light and fabrication of the resulting floating photocatalytic spheres

In this study, to fabricate stable floating photocatalytic spheres, facile alcohol solvothermal reduction was first employed to modify commercial TiO₂ (P25) photocatalysts to harvest visible light and improve their performances for photodegrading phenol in seawater exciting by visible light. Floating photocatalytic spheres were then prepared by loading reduced P25 photocatalysts on inner and outer surfaces of acrylic hollow spheres. The structural characterizations showed that reduction of P25 introduced disorder–crystalline shell–core structures with present Ti³⁺ in reduced P25 photocatalysts. These features facilitated visible light response and phenol degradation in seawater under visible light irradiation. As reduction time or temperature of alcohol solvothermal process rose, more Ti³⁺ and shell–core structures were introduced into reduced P25, resulting in higher performances towards phenol degradation in seawater. However, extended periods of time and elevated temperatures decreased disordered layer of reduced P25, deteriorating the photocatalytic performances. Thanks to good light transmission of the hollow spheres and the high performance of the reduced P25, the photocatalytic performances of spheres loaded with reduced P25 could effectively degrade phenol in seawater even at low concentrations. The removal rate of phenol by floating spheres reached more than 95% after 8 h. In addition, the floating spheres displayed good stability and convenient reusability after six repeated photocatalytic degradation for phenol in seawater, promising features for future treatment of organic pollutants in oceans.

Reduction of nitrate by zero valent iron (ZVI)-based materials: A review

Zero valent iron (ZVI) and ZVI-based materials have been widely used for the reduction of nitrate, a major contaminant commonly detected in groundwater and surface water. The reduction of nitrate by ZVI is influenced by various factors, such as the physical and chemical characteristics of ZVI and the operational parameters. There are some problems for the nitrate reduction by ZVI alone, for example, the formation of iron oxides on the surface of ZVI at high pH condition, which will inhibit the further reduction of nitrate; in addition, the end reduction product is mainly ammonium, which itself needs to be concerned. Several strategies, such as the optimization of the structure of ZVI composites and the addition of reducing assistants, have been proposed to increase the reduction efficiency and the selectivity of end product of nitrate reduction in a wide range of pH, especially under neutral pH condition. This review will mainly focus on the high efficient reduction of nitrate by ZVI-based materials. Firstly, the reduction of nitrate by ZVI alone was briefly introduced and discussed, including the influence of physical and chemical characteristics of ZVI and some operational parameters on the reduction efficiency of nitrate. Then, the strategies for enhancing the reduction efficiency and the N₂ selectivity of the reductive products of nitrate were systematically analyzed and evaluated, especially the optimization of the structure of ZVI composites (e.g., doped ZVI composite, supported ZVI composite and premagnetized ZVI), and the addition of reducing assistants (e.g., metal cations, ligand, hydrogen gas and light) were highlighted. Thirdly, the mechanisms and pathways of nitrate reduction were discussed. Finally, concluding remarks and some suggestions for the future research were proposed.

Photocatalytic Reduction of Hexavalent Chromium with Nanosized TiO2 in Presence of Formic Acid

Nanosized titanium dioxide (TiO2) nanoparticles were used for the photocatalytic reduction of hexavalent chromium in the presence of formic acid. The photoreduction of Cr(VI) in the absence of formic acid was quite slow. When formic acid was added in the chromium solution as the hole scavenger, a rapid photocatalytic reduction of Cr(VI) was observed, owing to the consumption of hole and the acceleration of the oxidation reaction. Furthermore, three commercial TiO2 nanoparticles (AEROXIDE® P25; Ishihara Sangyo ST-01; FUJIFILM Wako Pure Chemical Corp.) were evaluated for the photoactivity of reduction of Cr(VI).

Microwave synthesised Pd-TiO2 for photocatalytic ammonia production

Palladium doped anatase TiO2 nanoparticles were synthesised by a rapid (3 min) one-pot microwave synthesis technique at low temperature and pressure. After being fully characterised by SEM, XRD, Raman, XPS and EDX, photocatalytic nitrate reduction and ammonia production were studied over various dopant levels between 0-3.97 wt% Pd and compared to similar previous literature. Improved yields of ammonia were observed with most dopant levels when compared to non-doped microwave synthesised TiO2 with 2.65 wt% found to be the optimum dopant level producing 21.2 μmol NH3. Electrochemical impedance spectroscopy of TiO2 and Pd-TiO2 photoelectrodes revealed improvements in charge transfer characteristics at high Pd dopant levels.

Challenges in photocatalytic reduction of nitrate as a water treatment technology

Management of ubiquitous nitrate contamination in drinking water sources is a major engineering challenge due to its negative impacts from eutrophication to immediate risk to human health. Several water treatment technologies exist to manage nitrate pollution in water sources. However, the most widely used technologies are phase separation treatments. In this context, nanoscale photocatalysis emerges as a highly promising transformative technology capable of reducing nitrate to innocuous nitrogen with noticeable selectivity. This critical review describes the photocatalytic reduction mechanisms of nitrate towards undesirable products (nitrite, ammonium) and the more desirable product (dinitrogen). The mechanisms are based on the standard reduction potential of each individual species and highlight the contribution of reducing species (e.g. CO₂^-) radicals formed from different hole scavengers. The strategic use of different pure, doped, and composite nanoscale photocatalysts is discussed on the basis of reduction mechanisms' overall conversion, kinetic rates, and selectivity towards N₂. The choice of light source affects pathways and influences by-product selectivity because direct photolysis of N-intermediates, which has been overlooked in the literature. In addition, the re-oxidation of nitrite and ammonia as drawback process is explained. Finally, an exhaustive analysis presents the photocatalytic reduction applications for treating real water matrices and the competing effect of other species. Overall, this critical review aims to contribute to the understanding of the potential application/constraints of photocatalysis in inorganic nitrogen management, and guide researchers towards future efforts required for widespread implementation.

Enhanced photocatalytic activity for degrading phenol in seawater by TiO2-based catalysts under weak light irradiation

Given the small size and distribution of TiO₂, catalysts with strong phenol adsorption showed high photodegradation efficiency in seawater.