Sunlight promoted removal of toxic hexavalent chromium by cellulose derived photoactive carbon dots

The efficient removal of high-toxic Cr(VI) by the simply modified sepiolite synergized with oxalic acid under UV

The modified sepiolite (H-Sep-OH) with high catalytic activity was simply prepared and used for the enhancement of hexavalent chromium(Cr(VI)) removal from water by oxalic acid (Ox) photoreduction. The effects of H-Sep-OH dosage, Cr (VI) concentration, initial concentration of Ox and initial pH on the Cr (VI) removal in the Ox + H-Sep-OH + UV system were investigated in detail. The mechanism of Cr (VI) removal of the Ox + H-Sep-OH + UV system was discussed by the effect of initial pH, the change of pH and Cr species in the solution during the process, the free-radical scavenging test, and the IR, XRD, UV-Vis diffuse reflectance and Mott-Schottky analysis. The results showed that H-Sep-OH could significantly synergize with Ox reduction to remove Cr(VI) under UV, and the concentration of Ox in the aqueous solution and the dosage of H-Sep-OH determined the amount of reducing substances and the speed of Cr(VI) reduction. Amorphous SiO2 in the H-Sep-OH played an important catalytic role in the photoreduction of Cr(VI). During the Cr(VI) removal process in the Ox + H-Sep-OH + UV system, the reductive removal of Cr(VI) from the water was achieved synergistically by the effectively separated electron(e-) (generated from the SiO2 in H-Sep-OH under UV irradiation), and the produced •CO2 - (derived from photolysis of Ox and the conversion of Ox by water and hole(h+).).

Highly sensitive electrochemical sensing of ascorbic acid (vitamin C) using Pd-doped MnO2 supported on carbon quantum dots (Pd-MnO2@CQD) in water and fruit juices

The Pd-MnO2 nanoparticles attached to carbon quantum dots nanocomposite were synthesized using the green synthesis and hydrothermal process. Characterization of the as-prepared nanocomposite was intensively performed by FT-IR, powder XRD, XPS, and HR-TEM analysis. The synthesized nanomaterial was further examined for its selective and sensitive ascorbic acid (ASA) sensing using a Pd-MnO2@CQD modified glassy carbon electrode (GCE). The Pd-MnO2@CQD nanocomposite exhibits a distinct and improved peak current of ASA when compared to electrodes treated with Pd-MnO2 and bare GCE. The designed sensor has excellent performance, with a linear range of 10-1500 μM, a low detection limit of 0.14 μM (S/N = 3), a high sensitivity of 1.9671 μAµM-1 cm-2. Furthermore, the constructed sensor demonstrates good sensitivity for detecting ASA in a variety of real samples.

Solar-based technologies for removing potentially toxic metals from water sources: a review

Technological advances have led to a proportional increase in the deposition of contaminants across various environmental compartments, including water sources. Heavy metals, also known as potentially toxic metals, are of particular concern due to their significant harmful impacts on environmental and human health. Among the available methods for mitigating the threat of these metals in water, solar radiation-based technologies stand out for their cleanliness, cost-effectiveness, and efficiency in removing or reducing the toxicity of heavy metals. The performance and productivity of these methods in removing heavy metals such as arsenic (As), chromium (Cr), mercury (Hg), and uranium (U) from water still need to be comprehensively synthesized. Thus, this work aims to address that gap. The performance, potential, and challenges of real-world applications of conventional solar stills (CSS), membrane-based solar stills, and solar heterogeneous photocatalysis are concisely summarized and critically reviewed. CSS and membrane-based stills are highly effective (efficacy > 98%) in removing and capturing heavy metals from water. However, structural and functional improvements are needed to enhance productivity (especially for CSS) and usability in real-world environmental remediation and drinking water supply scenarios. Solar heterogeneous photocatalysis is highly effective in removing and/or converting As, Cr, Hg, and U into their non-toxic or less toxic forms, which subsequent processes can easily remove. Further research is necessary to evaluate the safety of photocatalytic materials, their integration into scalable solar reactors, and their usability in real-world environmental remediation applications.

Integrated enzyme hydrolysis assisted cellulose nanofibril (CNF) fabrication: A sustainable approach to paper mill sludge (PMS) management

The landfilling of paper mill sludge (PMS) has been restricted or even banned in many countries due to the raised concern about greenhouse gas (GHG) emissions and contamination of the soil and water, calling for a sustainable PMS management approach. The potential valorization of PMS to nanomaterials combined with traditional biorefinery was examined in this work. Three types of PMS-derived cellulose nanofibrils (CNFs) were prepared and evaluated: enzymatically assisted CNF (AU: with in-house produced enzyme and CT: with commercial enzyme), mechanically pretreated CNF (BT), and chemically pretreated CNF by TEMPO oxidation (TEMPO). It was found that enzyme-assisted mechanical fibrillation-derived CNFs had a comparable average diameter (27.9 nm for AU and 22.7 nm for CT) with that produced from mechanical pretreatment (26.5 nm for BT) and TEMPO oxidation pretreatment (20.0 nm for TEMPO), and they showed the best drainage properties among the three types of CNF. The CNFs resulting from enzymatic pretreatment reduced 15% of energy consumption compared to the mechanical method and had better thermostability than TEMPO oxidation method. In addition, the on-site produced enzyme showed similar performance to the commercial enzymes towards the CNF properties. These findings provide new insights into a promising integrated strategy in engineering CNF from PMS with on-site enzyme production as a novel and sustainable approach for PMS management and valorization.

Well-dispersed titanium dioxide and silver nanoparticles on external and internal surfaces of asymmetric alumina hollow fibers for enhanced chromium (VI) photoreductions

Heterogenous photocatalysis is a suitable alternative for wastewater treatment. The supporting of the solid catalyst in a porous material is suggested to facilitate catalyst recovery and reuse. Here we propose for the first time the evaluation of supporting silver (Ag)-decorated titanium dioxide (TiO₂) catalysts on internal and external surfaces of alumina hollow fibers with asymmetric pore size distribution. The produced catalysts were considered for Cr(VI) photoreductions. The ultrasound-assisted process potentialized the distribution of Ag nanoparticles on the TiO₂ surface. The loading of Ag nanoparticles at concentrations greater than 5 wt% was necessary to improve the TiO₂ activity for Cr(VI) photoreduction. The loading of Ag nanoparticles at 30 wt% improved the Cr(VI) photoreduction of the single TiO₂ catalyst from 40.49 ± 0.98 to 55.00 ± 0.83% after 180 min of reaction. Suspended and supported Ag-decorated TiO₂ catalysts achieved total Cr(VI) photoreduction after 21 h of reaction. The adjusted reaction rate constant with the externally supported Ag-TiO₂ catalyst was 3.57 × 10^-3 ± 0.18 × 10^-3 min^-1. Similar reaction rate constants were achieved with suspended and internally supported catalysts (approximately 2.70 × 10^-3 min^-1). After 10 sequential reuses, all catalysts presented similar Cr(VI) photoreductions of approximately 66%. Nevertheless, the use of the externally supported catalyst is suggested for Cr(VI) photoreductions due to its superior catalyst activity at least in the first reuse cycles.

Applications of Fluorescent Carbon Dots as Photocatalysts: A Review

Carbon dots (CDs) have attracted considerable interest from the scientific community due to their exceptional properties, such as high photoluminescence, broadband absorption, low toxicity, water solubility and (photo)chemical stability. As a result, they have been applied in several fields, such as sensing, bioimaging, artificial lighting and catalysis. In particular, CDs may act as sole photocatalysts or as part of photocatalytic nanocomposites. This study aims to provide a comprehensive review on the use of CDs as sole photocatalysts in the areas of hydrogen production via water splitting, photodegradation of organic pollutants and photoreduction and metal removal from wastewaters. Furthermore, key limitations preventing a wider use of CDs as photocatalysts are pointed out. It is our hope that this review will serve as a basis on which researchers may find useful information to develop sustainable methodologies for the synthesis and use of photocatalytic CDs.

Recent Progress of Carbon Dots for Air Pollutants Detection and Photocatalytic Removal: Synthesis, Modifications, and Applications

Rapid industrialization has inevitably led to serious air pollution problems, thus it is urgent to develop detection and treatment technologies for qualitative and quantitative analysis and efficient removal of harmful pollutants. Notably, the employment of functional nanomaterials, in sensing and photocatalytic technologies, is promising to achieve efficient in situ detection and removal of gaseous pollutants. Among them, carbon dots (CDs) have shown significant potential due to their superior properties, such as controllable structures, easy surface modification, adjustable energy band, and excellent electron-transfer capacities. Moreover, their environmentally friendly preparation and efficient capture of solar energy provide a green option for sustainably addressing environmental problems. Here, recent advances in the rational design of CDs-based sensors and photocatalysts are highlighted. An overview of their applications in air pollutants detection and photocatalytic removal is presented, especially the diverse sensing and photocatalytic mechanisms of CDs are discussed. Finally, the challenges and perspectives are also provided, emphasizing the importance of synthetic mechanism investigation and rational design of structures.

Inward-to-outward assembly of amine-functionalized carbon dots and polydopamine to Shewanella oneidensis MR-1 for high-efficiency, microbial-photoreduction of Cr(VI)

A novel photosensitized living biohybrid was fabricated by inward-to-outward assembly of amine-functionalized carbon dots (NCDs) and polydopamine (PDA) to Shewanella oneidensis MR-1 and applied for high-efficiency, microbial-photoreduction of Cr(VI). Within a 72 h test period, biohybrids achieved a pronounced catalytic reduction capacity (100%) for 100 mg/L Cr(VI) under visible illumination, greatly surpassing the poor capacity (only 2.5%) displayed by the wild strain under dark conditions. Modular configurations of NCDs and PDA afforded biohybrids with a large electron flux by harvesting extracellular photoelectrons generated from illuminated NCDs and increasing reducing equivalents released from an enlarged intracellular NADH/NAD⁺ pool. Further, increased production of intracellular c-type cytochromes and extracellular flavins resulting from the modular configuration enhanced the biohybrid electron transport ability. The enhancement of electron transport was also attributed to more conductive conduits at NCDs-PDA junction interfaces. Moreover, because NCDs are highly reductive, the enhanced Cr(VI) reduction was also attributed to direct reduction by the NCDs and the direct Cr(VI) reduction by sterile NCDs-assembled biohybrid was up to 20% in the dark. Overall, a highly efficient strategy for removal/transformation of Cr(VI) by using NCD-assembled photosensitized biohybrids was proposed in this work, which greatly exceeded the performance of Cr(VI)-remediation strategies based on conventional microbial technologies.

Reusable composite membranes for highly efficient chromium removal from real water matrixes

Natural or industrial hexavalent chromium water pollution continues to be a worldwide unresolved threat. Today, there is intense research on new active and cost-effective sorbents for Cr(VI), but most still exhibit a critical limitation: their powdered nature makes their recovery from water cost and energy consuming. In this work, Al(OH)₃, MIL-88-B(Fe), and UiO-66-NH₂ Cr(VI) sorbents were immobilized into a poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) polymeric substrate to develop an easily reactivable and reusable water filtering technology. The immobilization of the sorbents into the PVDF-HFP porous matrix modified the macro and meso-porous structure of the polymeric matrix, tuning in parallel its wettability. Although a partial blocking of the Cr(VI) adsorptive capacity was observed for of Al(OH)₃ and MIL-88-B(Fe) when immobilized into composite membranes, PVDF-HFP/UiO-66-NH₂ filter (i) exceeded the full capacity of the non-immobilized sorbent to trap Cr(VI), (ii) could be reactivated and reusable, and (iii) it was fully functional when applied in real water effluents.

Non-aqueous onion like nano-carbons from waste diesel-soot used as FRET-based sensor for sensing of nitro-phenols

Herein, a simple-functionalization method is described to prepare the oleylamine functionalized non-aqueous version of onion-like nanocarbons (ONC-OA), where ONC was isolated from the waste pollutant soot exhausted from the diesel engine. The surface group analysis of ONC-OA has been investigated via Nuclear Magnetic Resonance and X-ray Photoelectron Spectroscopy. ONC-OA shows blue fluorescence with a quantum yield of ∼6% in tetrahydrofuran (THF). The fluorescence-based sensing applications of ONC-OA has been investigated for selective sensing of toxic aromatic nitro-phenols compounds (para-nitro, dinitro, and trinitro phenols) from the tested many nitro organic compounds. Based on the limit of detection values, ONC-OA shows much better results for tri-nitro phenol compared to di and mono nitrophenol. To understand the quenching mechanism, a time-resolved photoluminescence analysis of the sensor with and without the addition of quenchers is performed. The effective lowering in fluorescence lifetime of the sensor after the addition of quenchers concludes that the quenching observed is majorly due to the Förster Resonance Energy Transfer (FRET) mechanism. The real-life application of ONC-OA was analyzed by external spiking of N-PhOHs in soil samples.

Rapid chromium reduction by metal-free organic polymer photocatalysis via molecular engineering

The conversion of hexavalent chromium (Cr(VI)), a highly poisonous heavy metal found in natural environment, to less poisonous trivalent chromium (Cr(III)) has attracted a lot of interest. However, little interest has been paid to the development of metal-free catalysts. Here, we demonstrate for the first time a molecular engineering strategy to synthesize a range of donor-acceptor conjugated polymer photocatalysts, which can significantly increase the reduction efficiency of Cr(VI) by a factor of 5.2, corresponding to a significant change in the reduction reaction rate constant (from 0.0337 to 0.1740 min^-1). In addition, the apparent quantum efficiency (AQE) of Cr(VI) removal was obtained, and the optimized photocatalyst (Py-SO₁) could achieve the highest apparent quantum efficiency at wavelength of 420 nm in those samples. Despite the narrow light absorption of Py-SO₁ polymer, its excellent exciton separation efficiency and efficient electron output enabled it to achieve excellent performance in photoreduction of Cr(VI), surpassing that of the reported metal-free photocatalysts. The results show that the present work provides a new perspective for designing suitable environmental remediation catalysts based on molecular engineering strategies.

Recent advances in waste-derived functional materials for wastewater remediation

Water pollution is a major concern for public health and a sustainable future. It is urgent to purify wastewater with effective methods to ensure a clean water supply. Most wastewater remediation techniques rely heavily on functional materials, and cost-effective materials are thus highly favorable. Of great environmental and economic significance, developing waste-derived materials for wastewater remediation has undergone explosive growth recently. Herein, the applications of waste (e.g., biowastes, electronic wastes, and industrial wastes)-derived materials for wastewater purification are comprehensively reviewed. Sophisticated strategies for turning wastes into functional materials are firstly summarized, including pyrolysis and combustion, hydrothermal synthesis, sol-gel method, co-precipitation, and ball milling. Moreover, critical experimental parameters within different design strategies are discussed. Afterward, recent applications of waste-derived functional materials in adsorption, photocatalytic degradation, electrochemical treatment, and advanced oxidation processes (AOPs) are analyzed. We mainly focus on the development of efficient functional materials via regulating the internal and external characteristics of waste-derived materials, and the material's property-performance correlation is also emphasized. Finally, the key future perspectives in the field of waste-derived materials-driven water remediation are highlighted.

Recent Advances in Synthesis, Modification, Characterization, and Applications of Carbon Dots

Although there is significant progress in the research of carbon dots (CDs), some challenges such as difficulty in large-scale synthesis, complicated purification, low quantum yield, ambiguity in structure-property correlation, electronic structures, and photophysics are still major obstacles that hinder the commercial use of CDs. Recent advances in synthesis, modification, characterization, and applications of CDs are summarized in this review. We illustrate some examples to correlate process parameters, structures, compositions, properties, and performances of CDs-based materials. The advances in the synthesis approach, purification methods, and modification/doping methods for the synthesis of CDs are also presented. Moreover, some examples of the kilogram-scale fabrication of CDs are given. The properties and performance of CDs can be tuned by some synthesis parameters, such as the incubation time and precursor ratio, the laser pulse width, and the average molar mass of the polymeric precursor. Surface passivation also has a significant influence on the particle sizes of CDs. Moreover, some factors affect the properties and performance of CDs, such as the polarity-sensitive fluorescence effect and concentration-dependent multicolor luminescence, together with the size and surface states of CDs. The synchrotron near-edge X-ray absorption fine structure (NEXAFS) test has been proved to be a useful tool to explore the correlation among structural features, photophysics, and emission performance of CDs. Recent advances of CDs in bioimaging, sensing, therapy, energy, fertilizer, separation, security authentication, food packing, flame retardant, and co-catalyst for environmental remediation applications were reviewed in this article. Furthermore, the roles of CDs, doped CDs, and their composites in these applications were also demonstrated.

Application of polymer-coated Macadamia integrifolia nutshell biomass impregnated with palladium for chromium(VI) remediation

Freely suspended and porous basket restrained granules of palladium nanoparticles supported on polymer-grafted Macadamia nutshell biomass (Pd@Polym-MNS) composite were used for the treatment chromium(VI)-containing water. In the presence of formic acid, the Pd@Polym-MNS demonstrated its activity in the adsorption-reduction-based conversion of noxious chromium(VI) to less toxic chromium(III) with a low activation energy of 13.4 kJ mol-1, ΔH0 (+ 10.8 kJ mol-1), ΔS0 (-270.0 J mol-1 K-1), and ΔG0 (+ 91.3 to + 98.0 kJ mol-1) indicated the exothermic, endergonic and non-spontaneous nature of the catalytic redox reaction. In addition to facilitating easy recovery, rinsing, and reuse, restraining the Pd@Polym-MNS in the basket reactor helped maintain the integrity of the catalysts by preventing violent collisions of suspended granules with the mixing apparatus and the walls of the reaction vessel. Whereas the pseudo-first-order rate constant was recorded as 0.157 min-1 upon initial use, values of the mean and relative standard deviation for the second, third and fourth consecutive uses were found to be 0.219 min-1 and 1.3%, respectively. According to a response surface methodological approach to batch experimentation, the initial concentration of chromium(VI) and catalyst dosage had the greatest impact on the redox reaction rate, accounting for 85.7% and 11.6% of the variability in the value of the pseudo-first-order rate constant, respectively. Mutually beneficial effects of the combinations of high formic acid and low chromium(VI) concentration, high temperature and catalyst dosage as well as high formic acid and catalyst dosage were recorded.