Synergistic improvement of Cr(VI) reduction and RhB degradation using RP/g-C3N4 photocatalyst under visible light irradiation

Recent progress in chromium removal from wastewater using covalent organic frameworks - A review

Covalent organic frameworks (COFs) offer a pivotal solution to urgently address heavy metal removal from wastewater due to their exceptional attributes such as high adsorption capacity, tunable porosity, controllable energy band structures, superior photocatalytic performance, and high stability-reusability. Despite these advantages, COFs encounter certain challenges, including inefficient utilization of visible light, rapid recombination of photogenerated carriers, and limited access to active sites due to close stacking. To enhance the photocatalytic and adsorptive performance of COF-based catalysts, various modification strategies have been reported, with a particular focus on molecular design, structural regulation, and heterostructure engineering. This review comprehensively explores recent advancements in COF-based photocatalytic and adsorptive materials for chromium removal from wastewater, addressing kinetics, mechanisms, and key influencing factors. Additionally, it sheds light on the influence of chemical composition and functional groups of COFs on the efficiency of hexavalent chromium [Cr (VI)] removal.

In situ formation of red/black phosphorus-modified SiO2@g-C3N4 multi-heterojunction for the enhanced photocatalytic degradation of organic contaminants

A new heterojunction material BP/RP-g-C₃N₄/SiO₂ was obtained by a one-step ball milling method, and its photocatalytic capacity was researched by the degradation of Rhodamine B (RhB) and ofloxacin (OFL) in simulated sunlight. The construction of an in situ BP/RP heterojunction can achieve perfect interface contact between different semiconductors and effectively promote the separation of photogenerated carriers. The composite material was well characterized, which proved that the multi-heterogeneous structure was prepared. Furthermore, the type II heterojunction was formed between the g-C₃N₄ and BP/RP interface, playing an important role in the degradation and promoting electron transfer. The degradation effect of BP/RP-g-C₃N₄/SiO₂ on RhB reached 90% after 26 min of simulated solar irradiation, which was 1.8 times that of g-C₃N₄/SiO₂. The degradation of OFL by BP/RP-g-C₃N₄/SiO₂ reached 85.3% after illumination for 50 min, while the degradation of g-C₃N₄/SiO₂ was only 35.4%. The mechanisms were further discussed, and ˙O₂ ^- and h⁺ were found to be the main active substances to degrade RhB. The catalyst also revealed distinguished stability of catalyst and recyclability, and the degradation effect of RhB can still realize 85% after 4 runs of experiment. Thus, this study provided a novel method for the design and preparation of multi-heterojunction catalysts in the removal of organic pollutants from wastewater.

Immobilization of ZnIn2S4 on sodium alginate foam for efficient hexavalent chromium removal

Photocatalytic technology has been extensively studied in the removal of toxic Cr(VI) from wastewater. However, common powdery photocatalysts suffer from poor recyclability and secondly pollution. Herein, the zinc indium sulfide (ZnIn₂S₄) particles were integrated onto the sodium alginate foam(SA) matrix through a facile way to obtain foam-shape catalyst. Diverse characterization techniques including X-ray diffraction(XRD), Fourier transform infrared(FT-IR), scanning electron microscope(SEM) and X-ray photoelectron spectroscopy(XPS) were employed to reveal the composite compositions, organic-inorganic interface interactions, mechanical property, and pore morphology of the foams. Results demonstrated that the ZnIn₂S₄ crystals wrapped on SA skeleton tightly and constructed a flower-like structure. As-prepared hybrid foam with lamellar structure showed great potential in Cr(VI) treatment due to the presence of macropores and highly available active sites. A maximum Cr(VI) photoreduction efficiency of 93 % were observed over the optimal sample of ZS-1 (with a ZnIn₂S₄:SA mass ratio of 1:1) under visible irradiation. When tested with mixed pollutants (Cr(VI)/dyes), the ZS-1 sample displayed an enhanced removal efficiency of 98 % for Cr(VI) and 100 % for Rhodamine B(RhB). Moreover, the composite maintained prominent photocatalytic performance and a relatively integral 3D structure scaffold after continuous six runs, revealing its superior reusability and durability.

The effectiveness and adsorption mechanism of iron-carbon nanotube composites for removing phosphate from aqueous environments

This study successfully employed iron-carbon nanotubes (Fe-CNT) to recover phosphate (P) from water. We examined the effects of various iron concentrations denoted by Fe-CNT-1 and Fe-CNT-2 on P removal and compared them with pristine carbon nanotubes (CNTs). The adsorption capacity of Fe-CNTs was much better than pristine CNTs. According to the high adsorption capacity, Fe-CNT-2 sample was very effective for P recovery and exhibits ∼7 times higher P removal efficiency than that of pristine CNTs. The characterization of the as-obtained adsorbent (Fe-CNT-2) and pristine CNTs were performed using X-ray diffraction, Brunauer-Emmett-Teller method, Field emission scanning electron microscope coupled with energy-dispersive spectroscopy detector (FESEM-EDS), X-ray photoelectron spectroscopy and Transmission electron microscopy. Results demonstrated that iron oxide nanoparticles were successfully deposited on the surface of CNT. The adsorption kinetics and isotherm studies for P removal showed pseudo-second-order rate constants (R² > 0.99) and the Langmuir isotherm (R² > 0.99) respectively, thus revealing that the nature of adsorption was chemisorption. The estimated Langmuir adsorption capacity of Fe-CNT-2 was 36.5 mgP/g or 112 mg PO₄/g at an equilibrium time of 3 h. The ionic strength provided by SO₄^2-, NO₃^-, and Cl^- demonstrated no considerable influence on phosphate adsorption. Moreover, the P adsorbed Fe-CNT-2 was efficiently recovered with different concentrations of desorbing reagents, such as NaOH and NaCO₃^2-. Moreover, the findings of X-ray photoelectron spectroscopy (XPS) analysis demonstrated that OH group played a major role in the P removal by Fe-CNT-2. The findings of this study demonstrate that Fe-CNT-2 had a great deal of application as an effective and stable adsorbent for the P recovery from aquatic environments.

Efficient all-in-one removal of total chromium over nonconjugated polymer-inorganic ZnIn2S4 semiconductor hybrid

Chromium (Cr)-containing wastewater has caused a serious threat to the environment due to its high toxicity and mobility. The traditional Cr removal methods are generally based on an inconvenient two-step process with the first transformation of Cr(VI) to Cr(III) and the consecutive removal of Cr(III) by precipitation. Herein, we demonstrate the efficient all-in-one removal of total Cr through the simultaneous photocatalytic reduction of Cr(VI) to Cr(III) and in-situ fixation of Cr(III) over the nonconjugated polymer engineered ZnIn₂S₄ (P-ZIS) photocatalyst. By in-situ polyvinylpyrrolidone (PVP) modification of ZIS during the preparation process, the resulted P-ZIS can completely reduce Cr(VI) within 60 min under visible light irradiation. The kinetics of Cr(VI) reduction over P-ZIS is 2.8 times as that of pure ZIS, which is proved to be benefited from the enhanced light absorption, uplifted conduction band for strengthening reducibility, and accelerated charge carrier transfer. Moreover, as compared to ZIS, P-ZIS also exhibits significantly improved in-situ adsorption ability for Cr(III), thus resulting in efficient all-in-one elimination of total Cr within a single system. We show that this polymer engineered strategy could be a facile and versatile protocol for modulating the electronic structure and surface chemistry of the semiconductor photocatalysts towards complete, safe, and cost-efficient removal of Cr.

Synthesis of novel CuNb2O6/g-C3N4 binary photocatalyst towards efficient visible light reduction of Cr (VI) and dyes degradation for environmental remediation

The further development of an efficient and sustainable water treatment requires the development of a very active and controllable photocatalyst. The heterojunction is a promising site where the activity of such a photocatalyst can be enhanced. Organic dyes have become a severe concern in recent years owing to their significant presence in wastewater. Hexavalent Chromium (Cr (VI)) is a potential carcinogen also exhibiting great persistence in wastewater. So, a low-waste, high-performance materials is required to eliminate organic dyes and Cr (VI) from wastewater. In this study, CNO/g-CN (CuNb₂O₆/g-C₃N₄) photocatalyst synthesized via co-precipitation, followed by calcination which were characterized using physiochemical and photo-electrochemical approaches to identify their structural, photochemical and optical traits. The uniqueness of the synthesized photocatalyst is due to both its efficient photo-reduction of Cr (VI) and photo-degradation of Rhodamine B (RhB), Methylene Blue (MB) and Methyl Orange (MO) under visible light. The CNO/g-CN composite with 30% CNO heterojunctions exhibited the highest photocatalytic activity with Cr (VI) 92.80% photoreduction and efficiency degradation for RhB, MB, MO of 99.6%, 98.50%, 99.0%, respectively, with constant rate (k). This efficient photocatalytic activity is attributed to the lower recombination rate of electron-hole pairs. Free radical trapping experiments showed that ^•O₂^- and h⁺ play an important role in the photodegradation. The study, therefore, opens an alternative route in the synthesis of very efficient binary photocatalysts for application in environmental remediation.

A review on synergistic coexisting pollutants for efficient photocatalytic reaction in wastewater remediation

With the tremendous development of the economy and industry, the pollution of water is becoming more serious due to the excessive chemical wastes that need to remove thru reduction or oxidation reactions. Simultaneous removal of dual pollutants via photocatalytic redox reaction has been tremendously explored in the last five years due to effective decontamination of pollutants compared to a single pollutants system. In a photocatalysis mechanism, the holes in the valence band can remarkably promote the oxidation of a pollutant. At the same time, photoexcited electrons are also consumed for the reduction reaction. The synergistic between the reduction and oxidation inhibits the recombination of electron-hole pairs extending their lifetime. In this review, the binary pollutants that selectively removed via photocatalysis reduction or oxidation are classified according to heavy metal-organic pollutant (HM/OP), heavy metal-heavy metal (HM/HM) and organic-organic pollutants (OP/OP). The intrinsic between the pollutants was explained in three different mechanisms including inhibition of electron-hole recombination, ligand to metal charge transfer and electrostatic attraction. Several strategies for the enhancement of this treatment method which are designation of catalysts, pH of mixed pollutants and addition of additive were discussed. This review offers a recent perspective on the development of photocatalysis system for industrial applications.

Recent advances and perspectives of g-C3N4-based materials for photocatalytic dyes degradation

Photocatalytic degradation technology is regarded as a promising technology for dye-contained wastewater treatment due to its superior efficiency and recycling. The key to the implementation of photocatalytic degradation technology is the selection of sunlight-active photocatalyst. Graphitic carbon nitride (g-C₃N₄) photocatalyst has been put into a lot of research in the field of organic pollutant degradation because of its low cost, suitable electronic structure and high chemical stability. In this perspective review, we comprehensively discuss the recent advance of photocatalytic dyes degradation over g-C₃N₄-based materials. The properties, structure and preparation methods of g-C₃N₄ are briefly introduced. Furthermore, the progress in improving the degradation efficiency of g-C₃N₄-based photocatalyst is highlighted in the article. The possible pathways and different active species for dyes decomposition are also summarized. We expect this review can provide instructive application of g-C₃N₄-based catalysts for environmental remediation.

Recent developments in architecturing the g-C3N4 based nanostructured photocatalysts: Synthesis, modifications and applications in water treatment

Water pollution is becoming an inevitable problem in today's world. Tons and tons of wastewater with hazardous pollutants are getting discharged into the clean water bodies every day. In this regard, photocatalytic environmental remediation using nanotechnology such as the use of organic, metal and non-metal based semiconductor photocatalysts for photodegradation of pollutants has gained enormous attention in the past few decades. This review is focused particularly on graphitic carbon nitride (g-C₃N₄) which is a cheap, metal-free, polymeric photoactive compound and it is used as a potential photocatalyst in wastewater treatment. Though, pristine g-C₃N₄ is a good photocatalyst, it has certain drawbacks such as poor visible light absorption capacity, quicker recombination of photoelectrons and holes, delayed mass and charge transfer, etc. As a result, the pristine g-C₃N₄ catalyst is modified into novel 0D, 1D, 2D and 3D morphologies such as nano-quantum dots, nanorods, nanotubes, nanowires, nanosheets, nanoflakes, nanospheres, nanoshells, etc. It was also tailored into novel composites along with various compounds through doping, metal deposition, heterojunction formation, etc., to enhance the photocatalytic property of pure g-C₃N₄. The modified catalysts showed promising photocatalytic performance such as degradation of majority of pollutants in the environment. It also showed excellent results in the removal or reduction of heavy metals. This review provides a detailed record of g-C₃N₄ and its diverse photocatalytic applications in the past years and it provides knowledge for the development of such similar novel compounds in the future.

Elemental red phosphorus-based photocatalysts for environmental remediation: A review

The low-cost and environmentally benign elemental red phosphorus (RP) is a new class of photocatalysts with tunable bandgaps (ca. 1.5-2.4 eV) and has a strong visible-light response. It has been considered as a promising metal-free photocatalyst for solving the energy crisis and environmental problems. Unfortunately, due to the low-charge carrier mobility, and serve charge trapping effects, its photocatalytic activity is still restricted in comparison with the traditional compound photocatalysts. Considerable efforts, such as morphology modification, cocatalysts addition, heterostructure construction, charge trapping mitigation, have been conducted to improve the photocatalytic activity of the RP photocatalysts. In this review, the physical and chemical properties and the synthetic strategies of the RP photocatalysts were summarized along with the application in environmental remediation accompanied by the photocatalytic reaction mechanism. Finally, an overview and outlook on the problems and future avenues in designing and constructing advanced RP photocatalysts were also proposed.

Sustainable Synthesis of MOF-5@GO Nanocomposites for Efficient Removal of Rhodamine B from Water

Herein, a metal-organic framework (MOF-5) is synthesized by a solvothermal process and graphene oxide (GO) is prepared from the improved Hummer's method. The synthesis of MOF-5@GO nanocomposites is one-pot process via a grinding method and employed for the removal of Rhodamine B (RhB) dye. The removal efficiency of RhB is found to be 60.64% (151.62 mg·g-1) at 500 ppm. About 98.88% of RhB is removed within 5 min of contact time and increased up to 99.68% up to 10 min. The removal rate of MOF-5@GO nanocomposites is much better than that of pristine MOF-5. Equilibrium adsorption capacity is determined by a series of different experimental conditions such as pH, time, and concentration of dye solution. Although the results also showed that dye removal on MOF-5@GO nanocomposites is well described by the Langmuir isotherm (R 2 = 0.9703), the adsorption kinetics data reveals pseudo-second-order (R 2 = 0.9908). The synthesized nanocomposite is efficient for removal of dye, cost-effective, and reusable. Additionally, stability and self-degradation studies of pure RhB are reported in aqueous solution for up to 120 days at different pH values (pH 1-12).

Elemental red phosphorus-based materials for photocatalytic water purification and hydrogen production

Semiconductor-based photocatalysis is a renewable and sustainable technology to solve global environmental pollution and energy shortage problems. It is essential to exploit highly efficient photocatalyst materials. Recently, Earth-abundant elemental red phosphorus (RP) with broader light-harvesting and appropriate band structure characteristics has been widely studied in photocatalysis. In this review, the crystal and electronic structures of RP (e.g., amorphous, Hittorf's and fibrous phosphorus) materials are firstly summarized along with the current advancement in the synthesis strategies of RP and RP-based materials in photocatalysis accompanied by a thorough discussion of the applications of RP-based materials in photocatalytic pollutant degradation, bacterial inactivation, and water splitting. Finally, this review also offers some guidance and perspectives for the future design of efficient visible-light-driven photocatalysts.

The Roles of Nanomaterials in Conventional and Emerging Technologies for Heavy Metal Removal: A State-of-the-Art Review

Heavy metal (HM) pollution in waterways is a serious threat towards global water security, as high dosages of HM poisoning can significantly harm all living organisms. Researchers have developed promising methods to isolate, separate, or reduce these HMs from water bodies to overcome this. This includes techniques, such as adsorption, photocatalysis, and membrane removal. Nanomaterials play an integral role in all of these remediation techniques. Nanomaterials of different shapes have been atomically designed via various synthesis techniques, such as hydrothermal, wet chemical synthesis, and so on to develop unique nanomaterials with exceptional properties, including high surface area and porosity, modified surface charge, increment in active sites, enhanced photocatalytic efficiency, and improved HM removal selectivity. In this work, a comprehensive review on the role that nanomaterials play in removing HM from waterways. The unique characteristics of the nanomaterials, synthesis technique, and removal principles are presented. A detailed visualisation of HM removal performances and the mechanisms behind this improvement is also detailed. Finally, the future directions for the development of nanomaterials are highlighted.