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Waterborne pathogenic viruses, with an ultra-small particle size of tens of nanometers, a high risk of causing disease, and strong persistence in natural environment and water treatment processes, pose a serious threat to human health. Here, a new class of metal-free heterojunction photocatalysts was developed by integrating oxygen-doped graphitic carbon nitride microspheres (O-g-C3N4) with hydrothermal carbonation carbon (HTCC) via a facile low-temperature solvothermal-hydrothermal approach for the inactivation of human adenovirus type 2 (HAdV-2). The sample of O-g-C3N4/HTCC-2 with a uniform coverage of HTCC, strong visible light absorption, and a narrow band gap exhibited the higher virucidal activity against highly resistant HAdV-2 under visible light irradiation, compared to HTCC, bulk g-C3N4, and O-g-C3N4. A titer of 105 MPN/mL viruses was completely inactivated within 120 min of photocatalysis, and viral inactivation efficiency was enhanced with the increase of water temperature from 4 to 37 °C, the decrease of pH from 8 to 5, or the presence of salinity (NaCl) and hardness (Ca2+). Furthermore, the effectiveness for HAdV-2 inactivation in real drinking water and excellent photocatalyst stability of O-g-C3N4/HTCC-2 highlighted its promising potential for water disinfection in practice. The mechanism of enhanced virucidal performance of O-g-C3N4/HTCC was revealed, and it was because of enhanced charge separation by the formation of heterojunction in the photocatalyst. Besides, Z-scheme heterojunction was proposed to enable the production of OH as a strong antiviral agent in photocatalysis, in contrast to Type II heterojunction. Interestingly, OH rather than O2− dominated HAdV-2 inactivation, and it led to the rupture, distortion, and hole formation of viral capsid. In addition to the excellent photocatalytic performance of O-g-C3N4/HTCC for HAdV-2 inactivation, the photocatalyst exhibited negligible toxicity to a human cell line, suggesting the material is safe for water purification. Our study not only highlights the promising future of an emerging metal-free visible-light-responsive heterostructure of O-g-C3N4/HTCC for viral disinfection and an effective, sustainable, and safe water purification process, but also sheds light on the key photocatalyst properties and mechanisms that improve photocatalytic performance.

Microbial contamination in drinking water is of great concern around the world because of high pathogenic risks to humans. Semiconductor photocatalysis has aroused an increasing interest as a promising environmental remediation technology for water disinfection and microbial control. Among various photocatalysts, graphitic carbon nitride (g-C3N4), as a fascinating two-dimensional conjugated polymer consisting of low-cost, earth-abundant elements, has drawn broad attention as a robust, metal-free, and visible-light-active material in the fields of both environmental remediation and solar energy conversion. Photocatalytic applications of g-C3N4-based nanomaterials for water splitting, hydrogen production, carbon dioxide reduction, and pollutant degradation have been extensively investigated and systematically reviewed. In contrast, their antimicrobial properties have been explored more recently due to the complex structure and unique metabolism of living microorganisms compared with chemicals. The corresponding rapidly increasing research efforts in the last five years have inspired us to conduct the review. This review is the first to comprehensively summarize the progress in design and antimicrobial performance of g-C3N4-based photocatalysts for water disinfection and microbial control, involving not only bacteria but also viruses and microalgae. Moreover, the underlying inactivation mechanisms of photocatalysts for microorganisms are evaluated to provide further understanding of g-C3N4-based advanced disinfection processes. In addition, some exciting future opportunities and challenges at the forefront of this research platform are pointed out. It is expected that this review can pave a new avenue for the development of a facile, cost-effective, environmental-friendly, and sustainable disinfection alternative.

Achieving efficient disinfection of waterborne pathogens with minimized harmful disinfection byproducts demands a facile, cost-effective, and environmentally friendly technology. Recently, photocatalytic water disinfection has attracted an ever-growing worldwide attention due to its powerful oxidative capability and promising potential in solar energy utilization. Among waterborne pathogens, viruses, which have been found with very small sizes, high risks of illness, and resistant to environmental inactivation/decomposition, pose a great threat to public health. Over the past a few decades, efforts have been made to employ photocatalysis to achieve effective viral inactivation. Though photocatalysis has been comprehensively reviewed for bacterial disinfection, photocatalytic disinfection of viruses with quite different compositions, structures, and resistance to oxidative stress compared to bacteria was not systematically documented. Here, we present an overview of antiviral effects of a wide range of photocatalysts, including TiO2-based, metal-containing (other than TiO2), and metal-free photocatalysts. Moreover, the development of photocatalytic reactors for viral inactivation is summarized to promote practical engineering applications for water disinfection. In addition, key mechanisms that determine the performance of photocatalytic viral disinfection are reviewed. Future perspectives of research opportunities and challenges in photocatalytic viral disinfection are also included. This review will shed light on the development and implementation of sustainable disinfection strategies for controlling waterborne viruses in the future.

Solar-energy-enabled photocatalysis is a sustainable process to destruct persistent environmental pollutants via the attack of photogenerated reactive species (e.g., holes, reactive oxygen species such as OH, O2− /HO2 , H2O2, and 1O2). Graphitic carbon nitride (g-C3N4) has emerged as a promising polymeric photocatalyst, and its highly tunable properties allow the material with an enhanced photocatalytic activity for environmental remediation. In this study, by taking advantage of simulation and experimental tools, we systematically evaluated the production of reactive species of undoped and carbon (C)-doped g-C3N4 samples, and identify the role of these species in contaminant oxidation under simulated visible sunlight irradiation (xenon lamp, λ > 400 nm). Both g-C3N4 samples produced negligible OH or triplet-excited states (3g-C3N4*), but the C-doped g-C3N4 sample generated more 1O2, O2− , and H2O2 compared to the undoped counterpart. Surprisingly, all these oxidative species did not contribute substantially for the degradation kinetics of contaminant phenol and atrazine, at least for the initial oxidation of the parent compounds; and the results otherwise highlighted the important role of the holes for contaminant transformation. The surface-mediated hole oxidation of the contaminants, including the adsorption of the contaminants on the photocatalyst surface (quantified by the binding free energy) and electron transfer kinetics from the contaminants to the excited photocatalysts (quantified by the concerted proton-coupled electron transfer (CPCET) rate) were investigated by molecular dynamics (MD) and density functional theory (DFT) simulations. The simulation results indicated that C-doping could favor the binding of atrazine but not of phenol on g-C3N4. The C-doping also significantly decreased the CPCET rate of phenol on g-C3N4 but could have little to no adverse impact for the CPCET rate of atrazine. These simulation and experimental results could explain the selective oxidation of phenol and atrazine on undoped and C-doped g-C3N4 in our previous study, and the results also highlight the dominant role of the holes for contaminant transformation that was largely overlooked before. This study generates mechanistic insights of photocatalytic oxidation, and will provide guidelines for the rational design of g-C3N4 as an effective visible-light-responsive photocatalyst for many applications, including contaminant degradation, chemical synthesis, and beyond.

Waterborne pathogens, especially bacteria and viruses, pose significant health risks to the public, calling for the development of a sustainable, efficient, and robust disinfection strategy with reduced energy footprint and minimized byproduct formation. Here, we developed a sustainable photocatalytic composite for antimicrobial applications by integrating visible-light-responsive graphitic carbon nitride (g-C3N4) with low-density porous expanded perlite (EP) mineral, and g-C3N4/EP-520 showed a high specific surface area of 45.3 m2/g and optimum performance for disinfection. g-C3N4/EP-520 achieved 8-log inactivation of E. coli and MS2 under 180 and 240 min visible-light irradiation without stirring, respectively. Water quality parameters were found to influence the disinfection performance of g-C3N4/EP-520: MS2 inactivation was promoted with the increase of dissolved oxygen (DO), proton concentration, salinity (NaCl), and hardness (Ca2+). Importantly, g-C3N4/EP-520 could fully inactivate MS2 in a real source water sample with prolonged light irradiation, and negligible activity loss was observed in recycle use, demonstrating its viability and robustness for waterborne pathogen removal. Antimicrobial mechanisms of g-C3N4/EP-520 were systemically evaluated by radical scavenger addition, and revealed that the inactivation behavior was dependent on the type of microorganisms. Microscopic analyses confirmed that the destruction of bacterial cells and viral particles, leading to the inactivation of microorganisms.

Photocatalytic membrane filtration has emerged as a promising technology for water purification because it integrates both physical rejection and chemical destruction of contaminants in a single unit, and also largely mitigates membrane fouling by natural organic matter (NOM). In this study, we evaluated the performance of a photocatalytic membrane system for mitigating fouling by a humic acid, which is representative NOM, and identified critical properties of the humic acid that determined membrane fouling. We prepared a partially oxidized humic acid (OHA) through the photocatalysis of a purified humic acid (PHA), and the OHA showed reduced fouling for polyvinylidene fluoride (PVDF) ultrafiltration membranes compared to PHA. Molecular-level characterizations indicated that OHA had a reduced molecular size, an increased oxygen content, and increased hydrophilicity. OHA also formed smaller aggregates on the fouled membrane surfaces than PHA. The introduction of oxygen-containing, hydrophilic functional groups, e.g., -OH and -COOH, to the humic acid and the depolymerization or mineralization of the humic acid in photocatalysis could result in the reduction of the foulant-membrane and foulant-foulant interactions, as characterized by atomic force microscopy (AFM), thereby mitigating membrane fouling. Foulant-membrane adhesion forces were always larger than foulant-foulant adhesion forces in our study, irrespective of the humic acid before or after photocatalytic oxidation, which may suggest that the reduction of foulant-membrane interactions is critical for membrane fouling control. In summary, this study sheds light into humic acid fouling in a photocatalytic membrane system through a systematic and comprehensive research approach, and provides insights for the design of novel membrane materials and processes with improved performance for water purification.

Carbon-based nanomaterials have shown great promise in regenerative medicine because of their unique electrical, mechanical, and biological properties; however, it is still difficult to engineer 2D pure carbon nanomaterials into a 3D scaffold while maintaining its structural integrity. In the present study, we developed novel carbon nanofibrous scaffolds by annealing electrospun mats at elevated temperature. The resultant scaffold showed a cohesive structure and excellent mechanical flexibility. The graphitic structure generated by annealing renders superior electrical conductivity to the carbon nanofibrous scaffold. By integrating the conductive scaffold with biphasic electrical stimulation, neural stem cell proliferation was promoted associating with upregulated neuronal gene expression level and increased microtubule-associated protein 2 immunofluorescence, demonstrating an improved neuronal differentiation and maturation. The findings suggest that the integration of the conducting carbon nanofibrous scaffold and electrical stimulation may pave a new avenue for neural tissue regeneration.

In this work, we demonstrate the production of reactive, robust, sustainable catalysts for water treatment created through Natural Fiber Welding (NFW) of lignocellulose-supported palladium-indium (Pd-In) nanoparticles onto linen yarns. First, Pd-In catalysts were synthesized by incipient wetness onto ball-milled powders of linen. Our process preserved the lignocellulose, yielding small (5–10 nm), near-spherical crystalline nanoparticles of Pd-In alloy and a uniform Pd-In metal composition throughout the fibers. Nitrate reduction tests identified the existence of an optimum Pd-In catalyst composition (5 wt% Pd and 1.2 wt% In with respect to lignocellulose) for maximum reactivity; the most reactive Pd-In catalyst was 10 times more reactive than the best performing Pd-Cu nanoparticles deposited on lignocellulose using the same approach. This improved performance was most likely due to more uniform distribution of alloyed Pd-In nanoparticles throughout the support. Nitrate reduction tests and X-ray photoelectron spectroscopy depth profiling of aged Pd-In catalysts showed that they remained stable and lost no reactivity during extended storage in air at room temperature. Next, the optimized Pd-In catalyst was fiber-welded onto linen yarns, using a custom-built yarn-coating system and a novel, scalable process that controlled catalyst loading, delivering a Pd-In catalyst coating onto the yarn surface. This fiber-welded Pd-In catalyst yarn was integrated into a novel water treatment reactor and evaluated during four months and more than 180 h of nitrate reduction tests in ultrapure water. During this evaluation, the fiber-welded catalysts maintained their reactivity with negligible metal leaching. When tested in raw or (partially) treated drinking water and wastewater, the fiber-welded catalysts were robust and stable, and their performance was not significantly impacted by constituents in the complex waters (e.g. alkalinity, organic matter). Our research demonstrates an innovative, scalable approach through NFW to design and implement robust, sustainable lignocellulose-supported catalysts with enhanced reactivity capable of water purification in complex water chemistries.

Continuous photocatalysis via photo-charging and dark-discharging presents a paradigm shift in conventional photocatalysis with the requirement of continuous illumination to maintain the catalytic activity. This is expected to meet the ever-increasing demand for sustainable development of energy and environment driven by natural day-night cycles. Substantial advances in continuous photocatalysis for various environmental applications under light-dark cycles have been witnessed during the last decade. However, there lacks a systematic and critical review on basic but important information of continuous photocatalysis for environmental remediation, challenging robust scientific progress of this technology towards potential practical use. Here, the general description of continuous photocatalysis involving energy storage mechanisms (hole and electron storage) and characterizations (electron storage behaviors, release behaviors and storage capacity) has been first introduced. Importantly, the remediation performance and mechanism of continuous photocatalysis for environmental applications are qualitatively and quantitatively demonstrated, including chemical pollutant oxidation and reduction, microbial pathogen inactivation, and multifunctional treatment. In addition, key factors influencing its remediation performance are analyzed, for the first time, from both operational and environmental views. The ample opportunities in the field of continuous photocatalysis for sustainable environmental remediation are also pointed out, calling for more efforts to fill current knowledge gaps in the future.

Catalytic reduction of 4-nitrophenol (4-NP) by palladium-resin catalysts is promising for 4-NP detoxification, in situ regeneration of the catalysts, and recycling of the products. In this study, we examined the catalytic reduction of 4-NP by Pd 0 -based catalysts supported on three neutral resins and two anion exchange resins, using hydrogen gas as the electron donor. The reduction kinetics followed the same trend as the adsorption kinetics, i.e., faster adsorption and reduction were observed at acidic pHs for the neutral resins and at basic pHs for the anionic resins (4-NP pK a = 7.15). The Langmuir-Hinshelwood model based on surface reaction as the rate determining step fitted the kinetic data well. These findings point to the significant role of adsorption in the overall catalytic reaction. Cl − inhibited the reactivity of the neutral resins less than that of the anion exchangers due to the hydrophobic nature of the former. The longevity of the catalysts was examined in two tests: (1) 1 mM 4-NP was repeatedly spiked into the same catalysts for eight times, where the accumulation of 4-NP and the sole reduction product (4-aminophenol) on the resin surfaces resulted in the gradual loss in the catalytic activity, which was largely restored after regeneration, and (2) the catalysts were subjected to fresh influent in each adsorption and catalytic reduction cycle for seven times, where the catalysts sustained their reactivity through the seven cycles, significantly eliminating the need for frequent regeneration.

Polymeric resins have been widely used for the removal of halogenated compounds, however, the sustainability of resin adsorption is compromised by frequent resin regeneration and further treatment/disposal of concentrated brine. Palladium (Pd)-based catalytic hydrogenation is promising to treat various classes of contaminants, including halogenated compounds, and the development of Pd-polymeric resin composites is advantageous because the composites can convert halogenated compounds to less toxic chemicals in situ and mitigate challenges associated with resin regeneration. In this work, three neutral resins (MN200, MN100, and XAD4) and two anion exchange resins (IRA910 and IRA96) were used as Pd supports to evaluate 4-chlorophenol (4-CP) hydrodechlorination reactivity. Similar 4-CP adsorption and reduction kinetics were observed, i.e., adsorption and reduction were both faster at acidic pHs for the neutral resins but faster at basic pHs for the anionic resins. The developed Langmuir–Hinshelwood kinetic model based on surface reaction as the rate determining step also suggested an enhancing effect of adsorption on the catalytic reactivity. When adsorption was constant, the reactivity of the catalysts increased with increasing solution pH. This is because higher pH mitigates the adverse impacts of the dehalogenation products (i.e., H + and Cl − ) on the catalytic reaction. The inhibition effect of Cl − was more pronounced on IRA910 than on MN200. IRA910 with anion exchange functional groups facilitates Cl − adsorption and promotes the production of PdCl 3 − and PdCl 4 2− species, which are inactive for catalytic reduction. The accumulation of phenol, the dominant product in 4-CP reduction, on resin resulted in catalytic activity loss over eight cycles when 4-CP was repeatedly spiked into the same reactor; the catalytic activity was largely restored after resin regeneration.

The challenge to achieve effective water disinfection of pathogens, especially viruses, with minimized harmful disinfection byproducts calls for a cost-effective and environmentally benign technology. Here, polymeric graphitic carbon nitride (g-C3N4), as a metal-free robust photocatalyst, was explored for the first time for its ability to inactivate viruses under visible light irradiation. MS2 with an initial concentration of 1 × 108 PFU/mL was completely inactivated by g-C3N4 with a loading of 150 mg/L under visible light irradiation of 360 min. g-C3N4 was a robust photocatalyst, and no decrease in its virucidal performance was observed over five cycles of sequential MS2 photocatalytic inactivation. The reactive oxygen species (ROSs) were measured by a range of scavengers, and photo-generated electrons and its derived ROSs (O- 2) were found to be the leading contributor for viral inactivation. TEM images indicated that the viral particle shape was distorted and the capsid shell was ruptured after photocatalysis. Viral surface proteins, particularly replicase proteins and maturation proteins, were damaged by photocatalytic oxidation. The loss of proteins would result in the leakage and rapid destruction of interior components (four main types of RNA genes), finally leading to viral death without regrowth. Our work opens a new avenue for the exploration and applications of a low-cost, high-efficient, and robust metal-free photocatalyst for green/sustainable viral disinfection.

Decontamination of water polluted with recalcitrant chemicals, such as phenolic and aromatic substances, for which conventional wastewater treatment processes are largely not effective, remains a major challenge all over the world. The simultaneous combination of photocatalytic and biological processes in a single system, either with or without the support of porous carriers, exhibits superior synergistic performance for removing refractory contaminants with the advantages of both photocatalysis and biotransformation. This promising emerging alternative, defined as simultaneous coupling here, has attracted increased attention and has been substantially developed over the last five years. To our best knowledge, this is the first critical review systematically focusing on the simultaneous coupling of photocatalytic and biological processes in enhancing decontamination of recalcitrants from water. The current review includes not only the synergy for pollutant oxidation/reduction removal and corresponding key factors affecting coupled systems, but also the underlying mechanisms of two different couplings in the interaction view of photocatalytic and biological responses. Last but not least, the challenges and opportunities faced by the coupling are pointed out. This review can provide useful information on the design and application of the synergistic coupling of chemical and biological processes for efficient and complete decontamination of recalcitrant compounds in water.

Efficient algal harvesting, cell pretreatment and lipid extraction are the major steps challenging the algal biofuel industrialization. To develop sustainable solutions for economically viable algal biofuels, our research aims at devising innovative reactive electrochemical membrane (REM) filtration systems for simultaneous algal harvesting and pretreatment for lipid extraction. The results in this work particularly demonstrated the use of the Ti4O7-based REM in algal pretreatment and the positive impacts on lipid extraction. After REM treatment, algal cells exhibited significant disruption in morphology and photosynthetic activity due to the anodic oxidation. Cell lysis was evidenced by the changes of fluorescent patterns of dissolved organic matter (DOM) in the treated algal suspension. The lipid extraction efficiency increased from 15.2 ± 0.6 g-lipidg-algae(-1) for untreated algae to 23.4 ± 0.7 g-lipidg-algae(-1) for treated algae (p

Solar-energy-enabled photocatalysis is promising for sustainable water purification. However, photoreactor design, especially immobilizing nano-sized photocatalysts, remains a major barrier preventing industrial-scale application of photocatalysis. In this study, we immobilized photocatalytic graphitic carbon nitride on chitosan to produce g-C3N4/chitosan hydrogel beads (GCHBs), and evaluated GCHB photoreactivity for degrading phenol and emerging persistent micropollutants in a 3D printed compound parabolic collector (CPC) reactor. The CPC photocatalytic system showed comparable performance with slurry reactors for sulfamethoxazole and carbamazepine degradation under simulated sunlight, and it maintained the performance for contaminant removal in real water samples collected from water/wastewater treatment plants or under outdoor sunlight irradiation. Global drinking water production was estimated for the CPC system, and it holds promise for small-scale sustainable water treatment, including, but not limited to, the production of high-quality potable water for single houses, small communities, rural areas, and areas impacted by natural disasters in both developed and developing countries.

Natural Fiber Welding (NFW) is a green engineering process that uses Ionic Liquids (IL) to prepare functional biocomposites for applications including water purification, energy storage, sensing, and thermal enhancement. The composition of the welding solution can significantly impact welding efficiency as well as the structure and composition of fiber-welded biocomposites. This study evaluates how NFW solvent composition influences metal leaching from lignocellulose fiber-supported Pd–In nanoparticles. Solutions were prepared from pure, dry 1-ethyl-3-methylimidazolium acetate, and controlled amounts of IL contaminants (water, acetic acid), and one molecular co-solvent (acetonitrile). Lignocellulose fibers with embedded Pd–In nanoparticles were welded in each solution and reconstituted in water; the recovered precipitates/filtrates were evaluated using ICP-MS to determine how much palladium and indium leached from each catalyst into the welding solvent. Our data show how excess acetic acid in the NFW solvent causes indium metal to dealloy from Pd–In nanoparticles. Because Pd–In nanoparticles are proven hydrogenation catalysts for water purification, nitrate reduction tests were performed with each fiber-welded product and compared to unwelded control samples. These data show how preferential leaching of indium directly impacted the catalysts nitrate reduction reactivity, and provide insight into how one might control this phenomenon to optimize development of fiber-welded catalyst systems.

The electrocatalytic hydrodechlorination of 4-chlorobiphenyl on palladized nickel foam with high porous structure in an aqueous solution containing MeOH, bromide of hexadecyltrimethylammonium (CTAB), sodium acetate, and acetic acid were investigated in a membrane-separated flow-through cell. The Pd/Ni foam electrode was prepared by electroless deposition method, on which the Pd particles dispersed finely over Ni foam surface indicated by SEM–EDX analysis. The effects of current density, organic cosolvent, initial concentration, temperature, and flow rate on the hydrodechlorination of 4-chlorobiphenyl were examined. Methanol was among the best cosolvents and was used in preferential concentration of 50 vol%. Moderate current density (e.g., 2.23 mA cm−2), relatively high initial concentration, temperature, and flow rate were beneficial to improve the hydrodechlorination of 4-chlorobiphenyl. The current efficiencies for the conversion of 1 mM 4-MCB decreased with increasing current density and range from 37.2% at 0.74 mA cm−2 to 14.1% at 5.21 mA cm−2 after 20 min electrolysis cut. Under the optimized conditions, 1 mM of 4-MCB could be removed rapidly with the rate of 94.6% after 2 h electrolysis, which gave current efficiencies and energy consumptions in range of 8.1–24.6% and 1.7–5.2 kW h kg−1, respectively.