Photocatalytic Reduction of Aqueous Nitrate with Hybrid Ag/g-C3N4 under Ultraviolet and Visible Light

Novel ternary nanocomposite (TiO2@Fe3O4-chitosan) system for nitrate removal from water: an adsorption cum photocatalytic approach

Nitrate pollution of water emerging from various anthropogenic activities has become a major environmental concern because of its deleterious effects on natural water resources. The present work deals with the synthesis of the ternary nanocomposite based on chitosan, iron oxide (Fe3O4), and titanium dioxide (TiO2) and its application for the removal of nitrates from model-contaminated water. Fe3O4 derived through a coprecipitation method was incorporated into the chitosan matrix which was fabricated in the form of beads. The wet gel beads were then successfully coated with sol-gel-derived silver-doped titanium dioxide sol followed by drying under suitable conditions to get the functional nanocomposite beads. The synthesized functional materials were further characterized for their structural, morphological, and textural features using X-ray diffraction analysis, physical property measurement (PPMS), Fourier transform infrared (FTIR) analysis, UV visible spectroscopy analysis (UV-vis), BET surface area analysis (BET), field emission scanning electron microscopic (FESEM), and transmission electron microscopy (TEM) analysis. The ternary nanocomposites were further used for the removal of nitrates via adsorption cum photocatalytic reduction technique from the model contaminated water when subjected to an adsorption study under dark conditions and photocatalytic study under UV/visible/sunlight for a definite time. Fe3O4 in the nanocomposite provides enhanced adsorption features whereas the functional coating of titanium dioxide aids in the removal of nitrates through the photocatalytic reduction technique. The functional beads containing 3% Fe3O4 in the wet gel form (CTA-F3) have excellent nitrate removal efficiency of ~ 97% via adsorption cum solar photocatalysis towards the removal of nitrate ions from 50 ppm nitrate solution, whereas the dried nanocomposite beads have got a nitrate removal efficiency of ~ 68% in 1 h from 100 ppm nitrate solution. Continuous flow adsorption cum photocatalytic study was performed further using the oven-dried functional beads in which flow rate and bed height were varied while maintaining the concentration of feed solution as constant. A nitrate removal efficiency of 65% and an adsorption capacity of 4.1 mgg-1 were obtained for the CTA-F3 beads in the continuous flow adsorption cum photocatalysis experiment for up to 5 h when using an inlet concentration of 100 ppm, bed height 12 cm, and flow rate 5.0 ml min-1. A representative fixed-bed column adsorption experiment conducted using CTA-F3 beads for the treatment of a real groundwater sample shows reasonable results for nitrate removal (71.7% efficiency) along with a significant removal rate for the other anions as well. Thus, the novel adsorbent/photocatalyst developed is suitable for the removal of nitrates from water due to the synergistic effect between Fe3O4, chitosan, and titanium dioxide.

Sunlight-Driven Nitrate-to-Ammonia Reduction with Water by Iron Oxyhydroxide Photocatalysts

The photocatalytic reduction of harmful nitrates (NO3-) in strongly acidic wastewater to ammonia (NH3) under sunlight is crucial for the recycling of limited nitrogen resources. This study reports that a naturally occurring Cl--containing iron oxyhydroxide (akaganeite) powder with surface oxygen vacancies (β-FeOOH(Cl)-OVs) facilitates this transformation. Ultraviolet light irradiation of the catalyst suspended in a Cl--containing solution promoted quantitative NO3--to-NH3 reduction with water under ambient conditions. The photogenerated conduction band electrons promoted the reduction of NO3--to-NH3 over the OVs. The valence band holes promoted self-oxidation of Cl- as the direct electron donor and eliminated Cl- was compensated from the solution. Photodecomposition of the generated hypochlorous acid (HClO) produced O2, facilitating catalytic reduction of NO3--to-NH3 with water as the electron donor in the entire system. Simulated sunlight irradiation of the catalyst in a strongly acidic nitric acid (HNO3) solution (pH ∼ 1) containing Cl- stably generated NH3 with a solar-to-chemical conversion efficiency of ∼0.025%. This strategy paves the way for sustainable NH3 production from wastewater.

Enhanced nitrate reduction by metal deposited g-C3N4/rGO/TiO2 Z-schematic photocatalysts: Performance and mechanism comparison of Pd-Cu and Ag

Deposition of bimetals on Z-scheme photocatalysts has been reported to improve the nitrate nitrogen (NO3-) reduction properties. However, it is not clear whether bimetal deposition possesses advantage over single metal deposition and what is the different reaction mechanisms. In this work, the g-C3N4(Pd-Cu)/rGO/TiO2 and g-C3N4(Ag)/rGO/TiO2 composites with bimetallic Pd-Cu and single metal Ag deposited on the graphitic carbon nitride/reduced graphene oxide/titanium dioxide (g-C3N4/rGO/TiO2) Z-scheme photocatalyst were prepared, and their photocatalytic NO3- reduction properties and the mechanisms under visible light irradiation were studied. The results showed that the NO3- and total nitrogen (TN) removal efficiencies of g-C3N4(Pd-Cu)/rGO/TiO2 were 57.78% and 20.1%, respectively, 1.15 and 1.72 times higher than those of g-C3N4(Ag)/rGO/TiO2. This can be ascribed to that Pd-Cu enriched more electrons and absorbed more NO3- molecules due to the different charge densities, and the NO3- reduction process were enhanced by the staged NO3-→NO2- and NO2-→N2/NH4+ processes on Cu and Pd. The effects of reductive species were demonstrated to be photogenerated electrons > ·OH (·CO2-) > ·O2- in g-C3N4(Ag)/rGO/TiO2, while it was photogenerated electrons > ·O2- > ·OH (·CO2-) in g-C3N4(Pd-Cu)/rGO/TiO2, which may be caused by the better O2 reduction property of the latter. Finally, the cyclic experiment proved the good stability of both materials. This work provided some reference for design of metal deposited Z-scheme photocatalysts for various reduction reactions.

Ultrafast Charge Injection in Silver-Modified Graphitic Carbon Nitride

Graphitic carbon nitride (gCN) is a promising organic platform for driving light-activated charge-transfer reactions in a number of valuable photocatalytic cycles. A primary limitation of gCN as a photocatalyst is its short excited-state lifetime, which is mediated by a high density of trap and defect sites that result in rapid excited-state decay and low photocatalytic efficiency. To enhance the catalytic activity, gCN is often functionalized with a metal co-catalyst; however, the mechanism by which metal co-catalysts enhance the reactivity has not been clearly established. In this work, the excited-state dynamics of gCN and silver-modified gCN are compared using ultrafast transient absorption and time-resolved photoluminescence spectroscopies. In silver-modified gCN, an ultrafast spectral shift in the silver plasmon resonance provides direct spectral evidence of electron transfer from gCN to the silver nanoparticles. The electron-transfer rate is competitive with other non-radiative relaxation pathways, with electron-transfer yields approaching 50%, thus providing an effective strategy for mitigating losses associated with defects and trap sites.

Efficient nitrate reduction in water using an integrated photocatalyst adsorbent based on chitosan-titanium dioxide nanocomposite

Globally, there exists a huge concern on the increased discharge of nitrates to the natural water resources out of various anthropogenic activities as it causes serious environmental pollution and associated harmful effects. In the present work, sol-gel-derived functional nanocomposites based on silver (Ag) and nitrogen (N)-doped titanium dioxide (TiO2)-coated chitosan nanocomposites were successfully synthesized in the form of beads, and their application for the reduction of nitrates in water was studied. The synthesized nanocomposite beads were characterized for their structural, textural, and morphological features using X-ray diffraction analysis, Fourier transform infrared spectroscopy, UV-visible spectroscopy, BET surface area analysis, Scanning electron microscopy, Transmission electron microscopy, and X-ray photoelectron spectroscopy. A uniform coating of doped titania species on the chitosan porous structure was achieved through electrostatic interaction. Adsorption/photocatalytic reduction of nitrates was further carried out using functional nanocomposite beads by monitoring the nitrate concentration of the model contaminated water, in an adsorption study under dark condition and photocatalytic study under UV/sunlight for a definite time period. Drying conditions of the nanocomposite beads were found to have a significant effect on the adsorption cum photocatalytic efficiencies of the nanocomposite. The freeze-dried chitosan-titania nanocomposite beads containing 0.5 mol% Ag exhibited an adsorption efficiency of ~ 43.5% (under dark for 30 min) and photocatalytic reduction capability of ~ 95% (under sunlight for 2 h), whereas the oven dried beads of the same composition exhibits adsorption and photocatalytic efficiencies of 40% (under dark for 30 min) and 70% (under UV light for 2 h) respectively, towards the reduction of nitrate ions in an aqueous solution. Continuous flow adsorption cum photocatalytic study using the oven-dried nanocomposite beads was also carried out with the help of an experimental setup fabricated in-house and under varying experimental conditions such as flow rate, bed height, and concentration of feed solution. Nitrate reduction efficiency of 87.6% and an adsorption capacity of 7.9 mg g-1 were obtained for the nanocomposite beads in the continuous flow adsorption cum photocatalysis experiment for up to 8 h when using an inlet concentration of 100 ppm, bed height 12 cm, and flow rate 5.0 mL min-1. A representative fixed-bed column adsorption experiment performed with oven dried nanocomposite beads in a real groundwater sample collected from the Palakkad District of Kerala shows promising results for nitrate reduction (85.9% efficiency) along with a significant removal rate for the other anions as well. Thus, the adsorption cum photocatalytic nitrate reduction efficiency of the functional nanocomposite material makes them suitable for the reduction of nitrates from water/wastewater through an integrated nanocomposite approach.

Uptake of BF Dye from the Aqueous Phase by CaO-g-C3N4 Nanosorbent: Construction, Descriptions, and Recyclability

Removing organic dyes from contaminated wastewater resulting from industrial effluents with a cost-effective approach addresses a major global challenge. The adsorption technique onto carbon-based materials and metal oxide is one of the most effective dye removal procedures. The current work aimed to evaluate the application of calcium oxide-doped carbon nitride nanostructures (CaO-g-C3N4) to eliminate basic fuchsine dyes (BF) from wastewater. CaO-g-C3N4 nanosorbent were obtained via ultrasonication and characterized by scanning electron microscopy, X-ray diffraction, TEM, and BET. The TEM analysis reveals 2D nanosheet-like nanoparticle architectures with a high specific surface area (37.31 m2/g) for the as-fabricated CaO-g-C3N4 nanosorbent. The adsorption results demonstrated that the variation of the dye concentration impacted the elimination of BF by CaO-C3N4 while no effect of pH on the removal of BF was observed. Freundlich isotherm and Pseudo-First-order adsorption kinetics models best fitted BF adsorption onto CaO-g-C3N4. The highest adsorption capacity of CaO-g-C3N4 for BF was determined to be 813 mg. g−1. The adsorption mechanism of BF is related to the π-π stacking bridging and hydrogen bond, as demonstrated by the FTIR study. CaO-g-C3N4 nanostructures may be easily recovered from solution and were effectively employed for BF elimination in at least four continuous cycles. The fabricated CaO-g-C3N4 adsorbent display excellent BF adsorption capacity and can be used as a potential sorbent in wastewater purification.

Efficient electrocatalytic nitrate reduction in neutral medium by Cu/CoP/NF composite cathode coupled with Ir-Ru/Ti anode

In this work, a three-dimensional self-supporting copper/cobalt phosphide/nickel foam (Co/CoP/NF) composite was fabricated and employed as the cathode for electrochemical nitrate removal from surface water with the assistance of a commercial Ir-Ru/Ti anode. The experimental results demonstrate that the introduction of Cu nanoparticles on CoP nanosheets is favorable for the electrocatalytic nitrate reduction. The influences of operating parameters (pH value, current density and initial nitrate concentration) on the nitrate reduction were assessed with the presence of Cl^-. At the optimized conditions, the removal of nitrate exhibits an efficiency ca. 100% via the coupling electrochemical reduction and oxidation processes. Moreover, the nitrogen selectivity is found to be as high as 98.8% within 210 min, accompanied with a promising test endurance (>94.0% for total nitrogen (TN) and NO₃^- removal efficiencies after an electrochemical run of 24.5 h). Importantly, as for the treated actual surface water, the concentration of TN is smaller than 1.5 mg L^-1, in accordance with the limit of Ⅳ-level standard of the surface water environmental quality in China (GB 3838-2002). The efficient removal of nitrate can be attributed to the synergistic effect of Cu and CoP microparticles to enhance the reduction activity, as well as the subsequent chloride oxidation for the major intermediate of ammonium.