Microbial fuel cells (MFCs) have the dual advantage of mitigating Cr(Ⅵ) wastewater ecological threats while generating electricity. However, the low electron transfer efficiency and the limited enrichment of active electrogens are barriers to MFCs advancement. This study describes the synthesis of the TP-PDA-RGO@CC negative electrode using tea polyphenol as a reducing agent and polydopamine-doped graphene, significantly enhances the roughness and hydrophilicity of the anode. The charge transfer resistance was reduced by 94%, and the peak MFC power was 1375.80 mW m-2. Under acidic conditions, the Cr(Ⅵ) reduction rate reached 92% within 24 h, with a 52% increase in coulombic efficiency. Biodiversity analysis shows that the TP-PDA-RGO@CC anode could enrich electrogens, thereby boosting the electron generation mechanism at the anode and enhancing the reduction efficiency of Cr(Ⅵ) in the cathode chamber. This work emphasizes high-performance anode materials for efficient pollutant removal, energy conversion, and biomass reuse.

Nanoscale zerovalent iron synthesized using borohydride (B-NZVI) has been widely applied in environmental remediation in recent decades. However, the contribution of boron in enhancing the inherent reactivity of B-NZVI and its effectiveness in removing hexavalent chromium [Cr(VI)] have not been well recognized and quantified. To the best of our knowledge, herein, a core-shell structure of B-NZVI featuring an Fe-B alloy shell beneath the iron oxide shell is demonstrated for the first time. Alloyed boron can reduce H+, contributing to more than 35.6% of H2 generation during acid digestion of B-NZVIs. In addition, alloyed B provides electrons for Fe3+ reduction during Cr(VI) removal, preventing in situ passivation of the reactive particle surface. Meanwhile, the amorphous oxide shell of B-NZVI exhibits an increased defect density, promoting the release of Fe2+ outside the shell to reduce Cr(VI), forming layer-structured precipitates and intense Fe-O bonds. Consequently, the surface-area-normalized capacity and surface reaction rate of B-NZVI are 6.5 and 6.9 times higher than those of crystalline NZVI, respectively. This study reveals the importance of alloyed B in Cr(VI) removal using B-NZVI and presents a comprehensive approach for investigating electron pathways and mechanisms involved in B-NZVIs for contaminant removal.

Recently, the reaction speed and cycle performance of hexavalent chromium reduction over microsized zero-valent iron (ZVI) with an Fe0 core and iron oxide (FeOx) shell structure have been improved by activating the Fe0-core electrons through electromagnetic coupling between Fe0-core electrons and charges (hexavalent chromium in solution, double-charge layers of the ZVI/solution interface). Herein, the abovementioned electromagnetic coupling was greatly increased by adding salt (CH3COONa, NaCl, NaNO3, and Na2SO4) in the hexavalent chromium solution to increase the charge response. Adding salt greatly improved the reaction speed and cycle performance of hexavalent chromium reduction. It took 8 min to reduce hexavalent chromium with CH3COONa to below the discharge standard of wastewater in the first cycle and 20 min after reducing for 20 cycles. The best apparent rate of constant value (0.416 (min)-1) is nearly four times larger than those without salts. X-ray diffraction and X-ray photoelectron spectroscopy revealed the production of amorphous iron oxide shell with salt. The salt improves the hexavalent chromium reduction speed and cycle performance and impedes the Fe0-core-electron transfer via the produced Fe2O3, resulting in existence of an optimized salt dosage. This work aims to provide an effective route for enhancing the removal efficiency and cycle performance of heavy-metal-ion reduction via Fe0. And this work also proposes a novel viewpoint that adding salt in waste water would increase the electromagnetic coupling between the charges in solution and Fe0-core electrons which could finally activate the redox reaction.

Photosensitized biohybrid system (PBS) enables bacteria to exploit light energy harvested by semiconductors for rapid pollutants transformation, possessing a promising future for water reclamation. Maintaining a biocompatible environment under photocatalytic conditions is the key to developing PBS-based treatment technologies. Natural microbial cells are surrounded by extracellular polymeric substances (EPS) that either be tightly bound to the cell wall (i.e., tightly bound EPS, tbEPS) or loosely associated with cell surface (i.e., loosely bound EPS, lbEPS), which provide protection from unfavorable environment. We hypothesized that providing EPS fractions can enhance bacterial viability under adverse environment created by photocatalytic reactions. We constructed a model PBS consisting of Shewanella oneidensis and CdS using Cr(VI) as the target pollutant. Results showed complete removal of 25 mg/L Cr(VI) within 90 min without an electron donor, which may mainly rely on the synergistic effect of CdS and bacteria on photoelectron transfer. Long-term cycling experiment of pristine PBS and PBS with extra EPS fractions (including lbEPS and tbEPS) for Cr(VI) treatment showed that PBS with extra lbEPS achieved efficient Cr(VI) removal within five consecutive batch treatment cycles, compared to the three cycles both in pristine PBS and PBS with tbEPS. After addition of lbEPS, the accumulation of reactive oxygen species (ROS) was greatly reduced via the EPS-capping effect and quenching effect, and the toxic metal internalization potential was lowered by complexation with Cd and Cr, resulting in enhanced bacterial viability during photocatalysis. This facile and efficient cytoprotective method helps the rational design of PBS for environmental remediation.

This study reports the simultaneous oxidation of As(III) and reduction of the Cr(VI) strain Ensifer adhaerens M8 screened from soils around abandoned gold tailings contaminated with highly complex metals (loids). Physiological, biochemical, and genomic techniques were used to explore the mechanism. The strain M8 could simultaneously oxidize 1 mM As(III) and reduce 45.3 % 0.1 mM Cr(VI) in 16 h, and the Cr(VI) reduction rate was increased by 5.8 % compared with the addition of Cr(VI) alone. Cellular debris was the main site of M8 arsenic oxidation. Chromium reduction was dominated by the reduction of extracellular hexavalent chromium (23.80-35.67 %). The genome of M8 included one chromosome and four plasmids, and a comparison of the genomes showed that M8 had two more plasmids than strains of the same genus, which may be related to strong environmental adaptations. M8 had 10 heavy metal resistance genes (HMRs), and plasmid D had a complete cluster of arsenic resistance-oxidation-transport genes (arsOHBCCR-aioSR-aioBA-cytCmoeA-phoBBU-PstBACS-phnCDEE). The genes involved in Cr(VI) detoxification include DNA repair (RecG, ruvABC, and UvrD), Cr(VI) transport (chrA, TonB, and CysAPTW) and Cr(VI) reduction. In summary, this study provides a molecular basis for As (III) and Cr (VI) remediation.

Chromium(Ⅵ) and arsenic(Ⅲ) as typical anionic heavy metal pollutants normally coexist in the environment, greatly aggravating their environmental risks and elevating the difficulty of remediation. Here, a novel polyethyleneimine modified magnetic mesoporous polydopamine nanocomposite (Fe3O4 @mesoPDA/PEI) with abundant active functional groups was exploited as the synchronous adsorbent of Cr(Ⅵ) and As(Ⅲ). The results showed that Cr(Ⅵ) and As(Ⅲ) could mutually promote their conversions and adsorptions on Fe3O4 @mesoPDA/PEI. The adsorption mechanisms of Fe3O4 @mesoPDA/PEI were primarily redox chemistry and also involved electrostatic interactions and coordination. Cr(Ⅵ) was mainly reduced by reductive catechol, while As(Ⅲ) was oxidized to As(Ⅴ) by oxidative active substances (e.g., H2O2, •OH, and quinone). Meanwhile, active intermediate (semiquinone radicals) generated during the Cr(Ⅵ) reduction and As(Ⅲ) oxidation could constitute redox microcirculation with Cr(Ⅵ) and As(Ⅲ) to further accelerate redox reactions of Cr(Ⅵ) and As(Ⅲ) on Fe3O4 @mesoPDA/PEI, thereby exhibiting a synergistic effect. Moreover, newly immobilized Cr(Ⅲ) onto Fe3O4 @mesoPDA/PEI became extra active sites for As adsorption through cation bridges and then recovered by magnetic separation in favor of diminishing the environmental hazards of Cr and As. These findings also provide new inspirations for the roles of redox-active functional groups in the remediation of multiple redox-sensitive heavy metals including Cr(Ⅵ) and As(Ⅲ).

Novel g-C3N4 functionalized yttrium-doped ZrO2 hybrid heterostructured (g-YZr) nanoparticles have been synthesized to investigate photocatalytic Cr(VI) reduction as well as electrochemical energy storage applications. The nanoparticles have been characterized to examine their structural, optical, and photocatalytic properties. XRD confirmed the incorporation of dopant ions and heterostructure development between g-C3N4 and doped ZrO2. When g-C3N4 was doped with ZrO2, the ability of light adsorption was greatly enhanced due to the narrow band gap. The distinctive structure of g-YZr exhibited outstanding photocatalytic Cr(VI) reduction owing to its superior surface area, which greatly prevented the charge carriers\' recombination rate and exhibited superior photocatalytic performance within 90 min of solar light irradiation. Furthermore, these catalysts demonstrated similar catalytic Cr(VI) reduction activity following four repeatability tests, indicating the exceptional structural stability of g-YZr catalysts. The electrochemical performance of the electrodes revealed that g-YZr exhibited superior specific capacitance over the other electrodes owing to extra energetic sites and robust synergic effect. Enhanced specific capacitance and long cyclic stability of the hybrid heterostructures displayed their usefulness for energy storage applications.

Copper(II) phthalocyanine (CuPc) and non-peripheral octamethyl-substituted copper(II) phthalocyanine (N-CuMe2Pc) were combined with reduced graphene oxide (rGO) via a precipitation method to form CuPc/rGO and N-CuMe2Pc/rGO nanocomposites, respectively. CuPc nanorods are distributed on rGO, and N-CuMe2Pc exists as nanorods and nanoparticles on rGO. The Cr(VI) removal ratio of N-CuMe2Pc/rGO exposed in simulated sunlight is 99.0% with a fast photocatalytic reaction rate of 0.0320 min-1, which is approximately 1.5 times faster than that of CuPc/rGO (0.0215 min-1) and far surpasses that of pristine phthalocyanine and rGO. As an electron acceptor, rGO can suppress the recombination of photo-induced electron-hole pairs and also can provide a large surface area for Cr(VI) removal, both of which are beneficial to the reducing capacity of the nanocomposites. The higher removal efficiency of N-CuMe2Pc/rGO compared with that of CuPc/rGO is attributed to the higher specific surface area, higher light harvesting, higher conductivity and more negative lowest unoccupied molecular orbital level of N-CuMe2Pc/rGO. The N-CuMe2Pc/rGO nanocomposite shows excellent photochemical recyclability which is essential for application in wastewater treatment.

The present study demonstrates the potential utilization of urea/cow urine as anolyte for Cr(VI) reduction via a simple three-chambered electrolytic cell. The inherent chemical energy in the dual-waste stream (Cr(VI)-urea/urine) is employed for its self-oxidation-reduction without the need for any external energy supply. Ni foam as electroactive anode and catalyst-free carbon felt as cathode, along with the appropriate positioning of ion-selective separators, indirectly improved the cell performance by impeding electrolyte crossover. A fundamental study involving five different membrane configurations was conducted herein to improve Cr(VI) reduction efficiency. The Cr(VI) reduction efficiencies were 11.84 ± 0.27%, 10.55 ± 0.17%, 77.24 ± 0.38% at 24 h, 13.57 ± 0.25% at 72 h with glass frit, cation exchange membrane (CEM), sandwiched membrane, and anion exchange membrane (AEM) as separators in a dual-chambered H-cell, respectively, with an initial Cr(VI) concentration of 100 mg/L. The fifth configuration, consisting of a middle chamber between the anode and cathode with the CEM close to the anode and the AEM close to the cathode resulted in a reduction efficiency of 79.98 ± 2.24% within 45 min for an initial Cr(VI) concentration of 400 mg/L. The first order rate constants were determined to be 0.024, 0.018, and 0.013 min-1 for Cr(VI) concentrations of 100, 200, and 400 mg/L, respectively. Moreover, when urea was replaced with cow urine as anolyte, a reduction efficiency of 98.94 ± 1.28% was achieved at pH 2 in 45 min with 400 mg/L as initial Cr(VI) concentration. Furthermore, the XPS spectra of reduced Cr corresponding to binding energies of 579.4 eV and 589.3 eV, respectively, confirmed the presence of low-toxic Cr(III). The effect of applied load, initial Cr(VI) and urea concentration, Cr(VI) reduction under different initial H2SO4 concentrations were succinctly investigated to evaluate the performance of the electrolytic cell. The redox electrolytic cell can thus be an alternative to the conventional chemical or energy intensive processes for the reduction of hexavalent chromium.

The current work presents different approaches to overcome mass and photon transfer limitations in heterogeneous photocatalytic processes applied to the reduction of hexavalent chromium to its trivalent form in the presence of a sacrificial agent. Two reactor designs were tested, a monolithic tubular photoreactor (MTP) and a micro-meso-structured photoreactor (NETmix), both presenting a high catalyst surface area per reaction liquid volume. In order to reduce photon transfer limitations, the tubular photoreactor was packed with transparent cellulose acetate monolithic structures (CAM) coated with the catalyst by a dip-coating method. For the NETmix reactor, a thin film of photocatalyst was uniformly deposited on the front glass slab (GS) or on the network of channels and chambers imprinted in the back stainless steel slab (SSS) using a spray system. The reaction rate for the NETmix photoreactor was evaluated for two illumination sources, solar light or UVA-LEDs, using the NETmix with the front glass slab or/and back stainless steel slab coated with TiO2-P25. The reusability of the photocatalytic films on the NETmix walls was also evaluated for three consecutive cycles using fresh Cr(VI) solutions. The catalyst reactivity in combination with the NETmix-SSS photoreactor is almost 70 times superior to one obtained with the MTP.