Photoreduction of chromium(VI) in microstructured ceramic hollow fibers impregnated with titanium dioxide and coated with green algae Chlorella vulgaris

Harnessing Solar Energy for the Photocatalytic Reduction of Hexavalent Chromium: A High-Performance Yarrowia lipolytica-CdS Biohybrid System

Photosynthetic semiconductor biohybrids, which combine the light-harvesting capacity of semiconductors and catalytic activity of whole-cell microorganisms, show substantial potential for advancing bioremediation technology. However, few yeast-based biohybrid systems for pollutant removal were reported. In this study, we have constructed a whole-cell biohybrid system based on Yarrowia lipolytica featuring in situ synthesized biocompatible cadmium sulfide (CdS) nanoparticles (NPs) for the photocatalytic reduction of hexavalent chromium [Cr(VI)] under UV irradiation. The integration of these CdS NPs onto the surface of modified Y. lipolytica cells endowed the system with excellent photocatalytic performance, achieving 100% Cr(VI) reduction within 2 h. The system exhibited a higher kinetic constant (0.03 min-1). In the trapping experiments, the reactive oxygen species (ROS) generated photochemically, specifically the superoxide anion (•O2-), which were identified as crucial mediators that facilitate the reduction of Cr(VI). The enhanced activity of the Y. lipolytica-CdS biohybrid was attributed to efficient electron transfer. Additionally, through transcriptome analysis, we found that the differentially expressed genes are associated with membrane transport, oxidation-reduction process, energy metabolism, and electron transfer. This whole-cell biohybrid catalytic strategy holds promise as an innovative approach for the reduction of Cr(VI) and has the potential to enhance our understanding of the interactions among light, inorganic material, and microorganisms.

Perspectives on nanomaterial-empowered bioremediation of heavy metals by photosynthetic microorganisms

Environmental remediation of heavy metals (HMs) is a crucial aspect of sustainable development, safeguarding natural resources, biodiversity, and the delicate balance of ecosystems, all of which are critical for sustaining life on our planet. The bioremediation of HMs by unicellular phototrophs harnesses their intrinsic detoxification mechanisms, including biosorption, bioaccumulation, and biotransformation. These processes can be remarkably effective in mitigating HMs, particularly at lower contaminant concentrations, surpassing the efficacy of conventional physicochemical methods and offering greater sustainability and cost-effectiveness. Here, we explore the potential of various engineered nanomaterials to further enhance the capacity and efficiency of HM bioremediation based on photosynthetic microorganisms. The critical assessment of the interactions between nanomaterials and unicellular phototrophs emphasised the ability of tailored nanomaterials to sustain photosynthetic metabolism and the defence system of microorganisms, thereby enhancing their growth, biomass accumulation, and overall bioremediation capacity. Key factors that could shape future research efforts toward sustainable nanobioremediation of HM are discussed, and knowledge gaps in the field have been identified. This study sheds light on the potential of nanobioremediation by unicellular phototrophs as an efficient, scalable, and cost-effective solution for HM removal.

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.

Decoration of alumina hollow fibers with zinc oxide: improvement of the photocatalytic system for methylene blue degradation

The photocatalytic degradation of methylene blue (MB) in aqueous solutions and under visible light was investigated with dispersed and supported zinc oxide (ZnO) as catalysts. The ZnO catalyst was successfully impregnated in asymmetric alumina hollow fibers by the simple vacuum-assisted dip-coating method. According to energy-dispersive analyses, the photocatalyst was homogenously distributed in the substrate. A strong correlation was observed between the initial dye concentration and the efficiency of the supported photocatalyst. For the initial MB concentration of 5 mg L^-1 and catalyst dosage of 1 g L^-1, the photocatalytic system with both dispersed and supported catalysts reached almost 100% of MB degradation. The photocatalytic process followed the pseudo-first-order kinetic model, and, for the initial MB concentration of 5 mg L^-1, the apparent constants were 0.05415 and 0.00642 min^-1 for suspended and supported catalysts, respectively. The treated MB solutions presented low phytotoxicity to the germination Lactuca sativa seeds with germination indexes greater than 80% after irrigation with the treated MB solutions. The produced supported ZnO catalyst showed suitable photocatalytic stability even after several reuse cycles.

Chlorella vulgaris meets TiO2 NPs: Effective sorbent/photocatalytic hybrid materials for water treatment application

A new class of bio-nano hybrid catalyst useable in downstream wastewater treatment was developed. We combined the sorption potentialities of Chlorella vulgaris microalgae with the photocatalytic properties of TiO₂ NPs in order to investigate unexplored synergistic effects that could push the algal remediation technology toward a more promising cost-effective balance. We exploited non-living C. vulgaris, which keeps the biosorption properties of the living microalgae, but greatly enhancing the overall processability. C. vulgaris biomass was coupled with TiO₂ NPs and the nanosols were then dried by means of a spray freeze drying (SFD) process able to produce highly reactive granules. A widespread physicochemical characterization supported the preparation and the performance evaluation, so highlighting the key-role of C. vulgaris/TiO₂ interaction at the colloidal state. Heavy metal adsorption, tested for copper ions, and photocatalytic activity, assessed for Rhodamine B (RhB) photodegradation, were evaluated as key performances. The results pointed out a positive synergistic effect for hybrid samples consistent with the enhancement of metal biosorption which ranges from 103 mg g^-1, for pristine C. vulgaris, to about 4000 mg g^-1, when the biomass was coupled with the inorganic nanophase. The photocatalytic activity was well preserved with a complete RhB conversion after 1 h and even advanced in presence of SiO₂NPs into the inorganic counterpart, so increasing the kinetic constant from 8.70 to 10.7 10^-2 min^-1. The results pave the way for the integration of these sorbent/photocatalytic hybrid materials into water remediation systems in an innovative sustainable design perspective.

Modulation of the Bifunctional CrVI to CrIII Photoreduction and Adsorption Capacity in ZrIV and TiIV Benchmark Metal-Organic Frameworks

The presence of hexavalent chromium water pollution is a growing global concern. Among the currently applied technologies to remove CrVI, its adsorption and photocatalytic reduction to CrIII less mobile and toxic forms are the most appealing because of their simplicity, reusability, and low energy consumption. However, little attention has been paid to bifunctional catalysts, that is, materials that can reduce CrVI to CrIII and retain both hexavalent and trivalent chromium species at the same time. In this work, the dual CrVI adsorption–reduction capacity of two iconic photoactive water-stable zirconium and titanium-based metal–organic frameworks (MOFs) has been investigated: UiO-66-NH2 and MIL-125. The bifunctionality of photoactive MOFs depends on different parameters, such as the particle size in MIL-125 or organic linker functionalization/defective positions in UiO-66 type sorbents. For instance, the presence of organic linker defects in UiO-66 has shown to be detrimental for the chromium photoreduction but beneficial for the retention of the CrIII phototransformed species. Both compounds are able to retain from 90 to 98% of the initial chromium present at acidic solutions as well as immobilize the reduced CrIII species, demonstrating the suitability of the materials for CrVI environmental remediation. In addition, it has been demonstrated that adsorption can be carried out also in a continuous flux mode through a diluted photoactive MOF/sand chromatographic column. The obtained results open the perspective to assess the bifunctional sorption and photoreduction ability of a plethora of MOF materials that have been applied for chromium capture and photoreduction purposes. In parallel, this work opens the perspective to develop specific chemical encoding strategies within MOFs to transfer this bifunctionality to other related water remediation applications.

Morphology-Controlled Synthesis of α-Fe2O3 Nanocrystals Impregnated on g-C3N4-SO3H with Ultrafast Charge Separation for Photoreduction of Cr (VI) Under Visible Light

Surface functionalization and shape modifications are the key strategies being utilized to overcome the limitations of semiconductors in advanced oxidation processes (AOP). Herein, the uniform α-Fe₂O₃ nanocrystals (α-Fe₂O₃-NCs) were effectively synthesized via a simple solvothermal route. Meanwhile, the sulfonic acid functionalization (SAF) and the impregnation of α-Fe₂O₃-NCs on g-C₃N₄ (α-Fe₂O₃-NCs@CN-SAF) were achieved through complete solvent evaporation technique. The surface functionalization of the sulfonic acid group on g-C₃N₄ accelerates the faster migration of electrons to the surface owing to robust electronegativity. The incorporation of α-Fe₂O₃-NCs with CN-SAF significantly enhances the optoelectronic properties, ultrafast spatial charge separation, and rapid charge transportation. The α-Fe₂O₃-HPs@CN-SAF and α-Fe₂O₃-NPs@CN-SAF nanocomposites attained 97.41% and 93.64% of Cr (VI) photoreduction in 10 min, respectively. The photocatalytic efficiency of α-Fe₂O₃-NCs@CN-SAF nanocomposite is 2.4 and 2.1 times higher than that of pure g-C₃N₄ and α-Fe₂O₃, respectively. Besides, the XPS, PEC and recycling experiments confirm the excellent photo-induced charge separation via Z-scheme heterostructure and cyclic stability of α-Fe₂O₃-NCs@CN-SAF nanocomposites.

Multi-component design and in-situ synthesis of visible-light-driven SnO2/g-C3N4/diatomite composite for high-efficient photoreduction of Cr(VI) with the aid of citric acid

A novel ternary SnO₂/g-C₃N₄/diatomite (SCN/DE) nanocomposite was rationally designed and successfully synthesized via a two-step method with in-situ polymerization and self-assembling. Under visible light illumination, the resulting SCN/DE composite exhibited superior photocatalytic performance and good reusability for the photoreduction of Cr(VI) to Cr(III) in the presence of citric acid, the apparent rate constant of SCN/DE composite was up to around 22.68 times, 13.53 times and 8.65 times as much as those of g-C₃N₄ (CN), g-C₃N₄/diatomite (CN/DE) and SnO₂/g-C₃N₄ (SCN) composites, respectively. The citric acid is a co-catalyst (chelating agent) rather than a reactant during the reactive process. Besides, the intimate interface contact and ternary heterogeneous structure were established among the SnO₂, g-C₃N₄ and diatomite. The induced positive charged surface of diatomite should be the key factor in enhancing photoactivity of the resultant SCN/DE composite, which significantly accelerated the charge separation of photogenerated electron-hole pairs as well as improved the adsorption performance towards Cr (VI). In particular, a possible reduction pathway of Cr(VI) to Cr(III) by SCN/DE composite with the assistance of citric acid was first investigated and proposed. This work provides a novel strategy for synthesizing highly efficient mineral-based photocatalysts with great promising application foreground for Cr(VI)-containing wastewater treatment.