Selective and Efficient Photoextraction of Aqueous Cr(VI) as a Solid-State Polyhydroxy Cr(V) Complex for Environmental Remediation and Resource Recovery

Sustainable soil rehabilitation with multiple network structures of layered double hydroxide beads: Immobilization of heavy metals, fertilizer release, and water retention

The remediation of heavy metal-contaminated soils necessitated a holistic approach that encompassed water and fertilizer conservation alongside soil property restoration. This study introduced the synthesis of (poly)acrylamide-layered double hydroxide gel spheres (PAM-LDH beads), which were designed to simultaneously immobilize heavy metals, control the release of fertilizers, and enhance soil water retention. Laboratory soil experiments under diverse conditions highlighted the superior performance of PAM-LDH beads in the immobilization of hexavalent chromium (Cr(VI)). The layered double hydroxide (LDH) component was identified as the key player in Cr(VI) immobilization, with anion exchange being the predominant mechanism. Notably, the encapsulated urea within the beads was released independently of environmental influences, governed by a concentration gradient across the beads surface. This release process was characterized by an initial phase of absorptive swelling followed by a diffusive phase. The impact on plant growth was assessed, revealing that PAM-LDH beads significantly curtailed Cr(VI) accumulation and alleviated its phytotoxic effects. Changes in the carbon (C) and nitrogen (N) content of the plants suggested that the urea encapsulated within the beads served as a nutrient source, contributing to soil fertility. Moreover, the water-holding capacity and soil-water characteristic curves of PAM-LDH beads suggested that these superabsorbent beads could delay soil water evaporation. The observed shifts in microbial community structure provided evidence for the enhancement of soil carbon and nitrogen cycles, indicative of improved soil properties.

Photo-induced protonation assisted nano primary battery for highly efficient immobilization of diverse heavy metal ions

Highly-stable heavy metal ions (HMIs) appear long-term damage, while the existing remediation strategies struggle to effectively remove a variety of oppositely charged HMIs without releasing toxic substances. Here we construct an iron-copper primary battery-based nanocomposite, with photo-induced protonation effect, for effectively consolidating broad-spectrum HMIs. In FCPBN, Fe/Cu cell acts as the reaction impetus, and functional graphene oxide modified by carboxyl and UV-induced protonated 2-nitrobenzaldehyde serves as an auxiliary platform. Due to the groups and built-in electric fields under UV stimuli, FCPBN exhibits excellent affinity for ions, with a maximum adsorption rate constant of 974.26 g∙mg-1∙min-1 and facilitated electrons transfer, assisting to reduce 9 HMIs including Cr2O72-, AsO2-, Cd2+ in water from 0.03 to 3.89 ppb. The cost-efficiency, stability and collectability of the FCPBN during remediation, and the beneficial effects on polluted soil and the beings further demonstrate the splendid remediation performance without secondary pollution. This work is expected to remove multi-HMIs thoroughly and sustainably, which tackles an environmental application challenge.

Sustainable Biomass Acts as an Electron Donor for Cr(VI) Reduction during the Subcritical Hydrothermal Process: Molecular Insights into the Role of Hydrochar and Liquid Compounds

Heavy metal pollution is a critical environmental issue that has garnered significant attention from the international community. Subcritical hydrothermal liquefaction (HTL) as an emerging green technology has demonstrated remarkable promise in environmental remediation. However, there is limited research on the remediation of highly toxic Cr(VI) using HTL. This study reveals that the HTL reaction of biomass enables the simultaneous reduction and precipitation of Cr(VI). At 280 °C, the reduction of Cr(VI) was nearly complete, with a high reduction rate of 98.9%. The reduced Cr as Cr(OH)3 and Cr2O3 was primarily enriched in hydrochar, accounting for over 99.9% of the total amount. This effective enrichment resulted in the removal of Cr(VI) from the aqueous phase while simultaneously yielding clean liquid compounds like organic acids and furfural. Furthermore, the elevated temperature facilitated the formation of Cr(III) and enhanced its accumulation within hydrochar. Notably, the resulting hydrochar and small oxygenated compounds, especially aldehyde, served as electron donors for Cr(VI) reduction. Additionally, the dissolved Cr facilitated the depolymerization and deoxygenation processes of macromolecular compounds with lignin-like structures, leading to more small oxygenated compounds and subsequently influencing Cr(VI) reduction. These findings have substantial implications for green and sustainable development.

Employing dolomite as magnesium source to prepare calcined layered double hydroxides for chromium contaminated soil treatment: Exploring the influence of temperature, bioavailability, and microbial diversity

Layered double hydroxides (LDH-D) and their calcined counterparts, using dolomite as a source of magnesium, were utilized for the immobilization of chromium (Cr(VI)) in soil. The results indicate that LDH-D, both with and without varying calcination temperatures, can effectively immobilize Cr(VI) in soil. Among the different calcination temperatures tested, LDH-D subjected to calcination at 500 °C (LDH-D-500) showed particularly high efficacy. Long-term TCLP experiments demonstrated the inhibition of soil-to-plant transmission of Cr(VI), thereby highlighting the long-lasting immobilization capacity of LDH-D and its calcined derivatives. Furthermore, the analysis of the microbial community's adaptation in post-remediation soil confirmed the durability and bioavailability of LDH-D-500 for Cr immobilization. Examination of the material's morphology and structure after immobilization shed light on the mechanism of immobilization in soil. The results revealed that interlayer anion exchange and surface adsorption were the main factors responsible for the effective immobilization of LDH-D and LDH-D-300. On the other hand, LDH-D-900, with a dominant spinel (MgAl2O4) structure, faced challenges in returning to its original layered configuration, making surface adsorption the primary mechanism for immobilization. LDH-D-500 primarily relied on the structure memory effects of LDHs to immobilize Cr(VI) through structural recovery processes, facilitated by electrostatic attraction and surface adsorption. It is also important to note that CaCO3 plays an important role in adsorption. Additionally, a portion of Cr(VI) was converted to Cr(III) through phenomena such as isomer substitution and complexation adsorption. The proficiency of LDH-D-500 in immobilizing Cr, its ability for instantaneous separation, and the potential for regeneration make it a promising material for remediation of heavy metal-contaminated soil. The investigations suggest that the use of dolomite to create hydrotalcite and calcining it at 500 °C could effectively render environmental Cr inactive, thereby optimizing resource utilization.

Controllable integration of nano zero-valent iron into MOFs with different structures for the purification of hexavalent chromium-contaminated water: Combined insights of scavenging performance and potential mechanism investigations

This work combined the stability of the porous structure of metal-organic frameworks with the strong reducibility of nano zero-valent iron, for the controllable integration of NZVI into MOFs to utilize the advantages of each component with enhancing the rapid decontamination and scavenging of Cr(VI) from wastewater. Hence, four kinds of MOFs/NZVI composites namely ZIF67/NZVI, MOF74/NZVI, MIL101(Fe)/NZVI, CuBTC/NZVI, were prepared for Cr(VI) capture. The results indicated that the stable structure of ZIF67, MOF74, MIL101(Fe), CuBTC, was beneficial for the dispersion of NZVI that could help more close contact between MOFs/NZVI reactive sites and Cr(VI), subsequently, MOFs/NZVI was proved to be better scavengers for Cr(VI) scavenging than NZVI alone. The Cr(VI) capture achieved the maximum adsorption capacity at pH ~ 4.0, which might be due to the participation of more H+ in the reaction and better corrosion of NZVI at lower pH. Mechanism investigation demonstrated synergy of adsorption, reduction and surface precipitation resulted in enhanced Cr(VI) scavenging, and Fe(0), dissolved and surface-bound Fe(II) were the primary reducing species. The findings of this investigation indicated that the as-prepared composites of ZIF67/NZVI, MOF74/NZVI, MIL101(Fe)/NZVI, CuBTC/NZVI, with high oxidation resistance and excellent reactivity, could provide reference for the decontamination and purification of actual Cr(VI)-containing wastewater.

Two-stage hydrothermal oxygenation for efficient removal of Cr(VI) by starch-based polyporous carbon: Wastewater application and removal mechanism

Cr(VI) is of concern because of its high mobility and toxicity. In this work, a two-stage hydrothermal strategy was used to activate the O sites of starch, and by inserting K-ion into the pores, starch-based polyporous carbon (S-PC) adsorption sites was synthesized for removal of Cr(VI). Physicochemical characterization revealed that the O content of the S-PC reached 20.66 % after activation, indicating that S-PC has excellent potential for adsorption of Cr(VI). The S-PC removal rate for 100 mg/L Cr(VI) was 96.29 %, and the adsorption capacity was 883.86 mg/g. Moreover, S-PC showed excellent resistance to interference, and an equal concentration of hetero-ions reduced the activity by less than 5 %. After 8 cycles of factory wastewater treatment, the S-PC maintained 81.15 % of its original activity, which indicated the possibility of practical application. Characterization and model analyses showed that the removal of Cr(VI) from wastewater by the S-PC was due to CC, δ-OH, ν-OH, and C-O-C groups, and the synergistic effect of adsorption and reduction was the key to the performance. This study provides a good solution for treatment of Cr(VI) plant wastewater and provides a technical reference for the use of biological macromolecules such as starch in the treatment of heavy metals.

Covalent organic framework modified vermiculite for total Cr removal and subsequent recycling for efficient ciprofloxacin and NO photooxidation

Design and fabrication of feasible remediation composites for total Cr (Cr(T)) removal is still challenging but urgently required. Herein, eco-friendly expanded vermiculite (VE) is integrated with a photoactive covalent organic framework (COF) polymer, in which photoinduced electrons of surface anchored COF can freely transfer to Cr(VI) for chemical reduction, and layered expanded VE allows ion exchange between resultant Cr(III) cations and interlayered K+, Ca2+, Mg2+, Na+, etc. The Cr(T) removal capacities of the surface-modified VE with important parameters (solution pH value, initial Cr(VI) concentration, etc.) are discussed extensively to understand how to select the best conditions for optimum Cr(T) removal performance. More interestingly, from a circular economy view point, spent Cr-loading VE-based waste can serve as a photocatalyst towards oxidation conversion of ciprofloxacin and NO gas subsequently. Explanations for different effects on physicochemical properties as well as catalytic activities of the reused Cr-loading waste are given. This strategy could provide valuable and promising contribution towards the development of sustainable low-cost mineral materials for Cr(T) removal. These findings also shed new light on the research of recycling spent photocatalyst for resource and reutilization.

Enhancement of Cr(VI) adsorption on lignin-based carbon materials by a two-step hydrothermal strategy: Performance and mechanism

Cr(VI) is a carcinogenic heavy metal that forms an oxygen-containing anion, which is difficult to remove from water by adsorbents. Here, industrial alkali lignin was transformed into a Cr(VI) adsorbent (N-LC) by using a two-step hydrothermal strategy. The characterization results of the adsorbent showed that O and N were uniformly distributed on the surface of the adsorbent, resulting in a favorable morphology and structure. The Cr(VI) adsorption of N-LC was 13.50 times that of alkali lignin, and the maximum was 326.10 mg g-1, which confirmed the superiority of the two-step hydrothermal strategy. After 7 cycles, the adsorption of N-LC stabilized at approximately 62.18 %. In addition, in the presence of coexisting ions, N-LC showed a selective adsorption efficiency of 85.47 % for Cr(VI), which is sufficient to support its application to actual wastewaters. Model calculations and characterization showed that N and O groups were the main active factors in N-LC, and CO, -OH and pyridinic-N were the main active sites. This study provides a simple and efficient method for the treatment of heavy metals and the utilization of waste lignin, which is expected to be widely applied in the environmental, energy and chemical industries.

Using Multistage Energy Barrier of Heterojunctions in Improving Cr(VI) Detection

Detecting heavy metals in seawater is challenging due to the high salinity and complex composition, which cause strong interference. To address this issue, we propose using a multistage energy barrier as an electrochemical driver to generate electrochemical responses that can resist interference. The Ni-based heterojunction foams with different types of barriers were fabricated to detect Cr(VI), and the effects of the energy barriers on the electrochemical response were studied. The single-stage barrier can effectively drive the electrochemical response, and the multistage barrier is even more powerful in improving sensing performance. A prototype Ni/NiO/CeO2/Au/PANI foam with multistage barriers achieved a high sensitivity and recovery rate (93.63-104.79%) in detecting seawater while resisting interference. The use of multistage barriers as a driver to resist electrochemical interference is a promising approach.

Pd sub-nanolayer on Au core for enhanced catalytic hydrogenation reduction of oxyanions pollutants: Synergistic effect of Pd and Au

Oxyanion pollutants in industrial wasterwater, such as (Cr(VI)), BrO3- (Br(V)) and SeO32- (Se(IV)) have detrimental or toxic effects on individual health when their concentrations accumulated to a certain level. The conversion of these oxyanions into harmless/industrial-valuable products or removal from wastewater is of significance. Herein, we designed Pd sub-nanolayer on Au core catalysts supported on Al2O3 (sub-Pd-Au/Al2O3) for highly effective catalytic hydrogenation reduction of oxyanions under ambient conditions. The sub-Pd(0.049)-Au(0.927)/Al2O3 catalyst exhibited the highest catalytic activity and TOF value for Cr(VI), Br(V) and Se(IV) reduction, respectively, by optimizing the Pd loading amount. The synergistic effect between Pd sub-nanolayer and Au core enhanced catalytic activity by regulating the Pd dispersion and site property, according to thorough characterizations that included high-angle annular dark-field transmission electron microscopy (HAADF-TEM) image, in-situ CO-IR adsorption, CO chemisorption, and X-ray photoelectron spectroscopy (XPS). This work might provide some new lights on design of highly efficient catalysts for the elimination of oxyanion pollutants.

Selective removal of total Cr from a complex water matrix by chitosan and biochar modified-FeS: Kinetics and underlying mechanisms

Cr(VI) is difficult to remove from wastewater via a one-step method because it is a type of oxyanion. Developing ARPs to selectively remove total Cr is critical for Cr(VI) remediation, including Cr(VI) adsorption-reduction and Cr(III) complexation. Hereon, chitosan and biochar modified-FeS (CTS-FeS@BC) was prepared to apply in the selective removal of total Cr from wastewaters. The results showed that the activity of amorphous FeS on CTS-FeS@BC for Cr(VI) removal (110.0 mg/g FeS) was significantly enhanced by CTS and BC, and efficiency was inhibited slightly by many anions and humic acid (HA). Meanwhile, the removal of total Cr by CTS-FeS@BC (99.1 mg/g FeS) via ARPs was improved by 1.2 and 40.3 times when compared with CTS-FeS and raw FeS, respectively. Besides, CTS-FeS@BC exhibited an outstanding selectivity for total Cr removal in metal cations-Cr binary solutions and in a complex water matrix. The mechanism of ARPs on CTS-FeS@BC demonstrated by the results of the 1,10-phenanthroline experiment and the distribution of Cr species was that Cr(VI) was first adsorbed by outer-sphere complexation for reduction, and then adsorbed Cr(III) combined with Fe(III) species to generate Fe(III)-Cr(III) complex for total Cr removal. Overall, this study provides an ARP to effectively solve Cr pollution in wastewaters.

Physicochemical Effects of Sulfur Precursors on Sulfidated Amorphous Zero-Valent Iron and Its Enhanced Mechanisms for Cr(VI) Removal

Amorphous zerovalent iron (AZVI) has gained considerable attention due to its remarkable reactivity, but there is limited research on sulfidated amorphous zerovalent iron (SAZVI) and the influence of different sulfur precursors on its reactivity remains unclear. In this study, SAZVI materials with an amorphous structure were synthesized using various sulfur precursors, resulting in significantly increased specific surface area and hydrophobicity compared to AZVI. The Cr(VI) removal efficiency of SAZVI-Na₂S, which exhibited the most negative free corrosion potential (-0.82 V) and strongest electron transfer ability, was up to 8.5 times higher than that of AZVI. Correlation analysis revealed that the water contact angle (r = 0.87), free corrosion potential (r = -0.92), and surface Fe(II) proportion (r = 0.98) of the SAZVI samples played crucial roles in Cr(VI) removal. Furthermore, the enhanced elimination ability of SAZVI-Na₂S was analyzed, primarily attributed to the adsorption of Cr(VI) by the FeS_x shell, followed by the rapid release of internal electrons to reduce Cr(VI) to Cr(III). This process ultimately led to the precipitation of FeCr₂O₄ and Cr₂S₃ on the surface of SAZVI-Na₂S, resulting in their removal from the water. This study provides insights into the influence of sulfur precursors on the reactivity of SAZVI and offers a new strategy for designing highly active AZVI for efficient Cr(VI) removal.

Piezoelectric materials and techniques for environmental pollution remediation

With the rapid development of industrialization and agriculture, a series of critical imminent environmental problems and water pollution have caught wide attention from the public and society. Piezoelectric catalysis technology with piezoelectric materials is a green and environmental method that can efficiently improve the separation of electron-hole pairs, then generating the active substances such as OH, H₂O₂ and O₂^-, which can degrade water pollutants. Therefore, we firstly surveyed the piezoelectric catalysis in piezoelectric materials and systematically concluded and emphasized the relationship between piezoelectric materials and the piezoelectric catalytic mechanism, the goal to elucidate the effect of polarization on piezoelectric catalytic performance and enhance piezoelectric catalytic performance. Subsequently, the applications of piezoelectric materials in water treatment and environmental pollutant remediation were discussed including degradation of organic pollutants, removal of heavy mental ions, radionuclides, bacteria disinfection and water splitting for H₂ generation. Finally, the development prospects and future outlooks of piezoelectric catalysis were presented in detail.

Simultaneous detection and removal of 2,4,6-trinitrophenyl phenol and dichromate by metal-organic framework

2,4,6-trinitrophenyl phenol (TNP) and dichromate (Cr₂O₇^2-) have serious toxicological effects on environment. Therefore, it is very important to detect and remove TNP and Cr₂O₇^2- in environmental matrix. In this work, a dual-functional UiO type metal-organic framework (Zr-Sti) was synthesized for simultaneous detection and removal of those pollutants in aqueous solution. As for detection, Zr-Sti exhibited sensitive and selective fluorescence response to TNP and Cr₂O₇^2- with detection limit below μM level, and possible mechanism behind was proposed and partially supported by experiment data. In addition, adsorption capacity of the prepared Zr-Sti for TNP and Cr₂O₇^2- was further investigated to evaluate its performance in pollutant removal from aqueous solution, and the mechanism behinds the obtained high removal efficiency was proposed. These results together with the satisfied recovery for simultaneous detection of TNP and Cr₂O₇^2- in real sample, indicate the potential of the prepared Zr-Sti material in the field of environment monitoring and remediation.

Preparation and evaluation of celite decorated iron nanoparticles for the sequestration performance of hexavalent chromium from aqueous solution

The increasing usage of an important heavy metal chromium for industrial purposes, such as metallurgy, electroplating, leather tanning, and other fields, has contributed to an augmented level of hexavalent chromium (Cr(VI)) in watercourses negatively impacting the ecosystems and significantly making Cr(VI) pollution a serious environmental issue. In this regard, iron nanoparticles exhibited great reactivity in remediation of Cr(VI)-polluted waters and soils, but, the persistence and dispersion of the raw iron should be improved. Herein, this article utilized an environment-friendly celite as a modifying reagent and described the preparation of a novel composites namaly celite decorated iron nanoparticles (C-Fe⁰) and evaluation of C-Fe⁰ for the sequestration performance of Cr(VI) from aqueous solution. The results indicated that initial Cr(VI) concentration, adsorbent dosage, and especially solution pH are all critical factors to control C-Fe⁰ performance in Cr(VI) sequestration. We demonstrated that C-Fe⁰ could achieve a high Cr(VI) sequestration efficiency with an optimized adsorbent dosage. Fitness of the pseudo-second-order kinetics model with data indicated that adsorption was the rate-controlling step and chemical interaction controlled Cr(VI) sequestration on C-Fe⁰. The adsorption isotherm of Cr(VI) could be the best depicted by Langmuir model with a monolayer adsorption. The underlying sequestration path of Cr(VI) by C-Fe⁰ was then put forward, and the combined effect of adsorption and reduction implied the potentials of C-Fe⁰ in Cr(VI) removal.

Interfacial Coordinational Bond Triggered Photoreduction Membrane for Continuous Light-Driven Precious Metals Recovery

Chemical/electric energy-driven processes dominate the traditional precious metal (PM) recovery market. The renewable energy-driven selective PM recycling approach crucial for carbon neutrality is under exploration. Herein, via an interfacial structure engineering approach, coordinational-active pyridine groups are covalently integrated onto the photoactive semiconductor SnS₂ surface to construct Py-SnS₂. Triggered by the preferred coordinational binding force between PMs and pyridine groups, together with the photoreduction capability of SnS₂, Py-SnS₂ shows significantly enhanced selective PM-capturing performance toward Au³⁺, Pd⁴⁺, and Pt⁴⁺ with recycling capacity up to 1769.84, 1103.72, and 617.61 mg/g for Au³⁺, Pd⁴⁺, and Pt⁴⁺, respectively. Further integrating the Py-SnS₂ membrane into a homemade light-driven flow cell, 96.3% recovery efficiency was achieved for continuous Au recycling from a computer processing unit (CPU) leachate. This study reported a novel strategy to fabricate coordinational bonds triggered photoreductive membranes for continuous PM recovery, which could be expanded to other photocatalysts for broad environmental applications.

Application of covalent organic frameworks and metal–organic frameworks nanomaterials in organic/inorganic pollutants removal from solutions through sorption-catalysis strategies

With the fast development of agriculture, industrialization and urbanization, large amounts of different (in)organic pollutants are inevitably discharged into the ecosystems. The efficient decontamination of the (in)organic contaminants is crucial to human health and ecosystem pollution remediation. Covalent organic frameworks (COFs) and metal–organic frameworks (MOFs) have attracted multidisciplinary research interests because of their outstanding physicochemical properties like high stability, large surface areas, high sorption capacity or catalytic activity. In this review, we summarized the recent works about the elimination/extraction of organic pollutants, heavy metal ions, and radionuclides by MOFs and COFs nanomaterials through the sorption-catalytic degradation for organic chemicals and sorption-catalytic reduction-precipitation-extraction for metals or radionuclides. The interactions between the (in)organic pollutants and COFs/MOFs nanomaterials at the molecular level were discussed from the density functional theory calculation and spectroscopy analysis. The sorption of organic chemicals was mainly dominated by electrostatic attraction, π-π interaction, surface complexation and H-bonding interaction, whereas the sorption of radionuclides and metal ions was mainly attributed to surface complexation, ion exchange, reduction and incorporation reactions. The porous structures, surface functional groups, and active sites were important for the sorption ability and selectivity. The doping or co-doping of metal/nonmetal, or the incorporation with other materials could change the visible light harvest and the generation/separation of electrons/holes (e−/h+) pairs, thereby enhanced the photocatalytic activity. The challenges for the possible application of COFs/MOFs nanomaterials in the elimination of pollutants from water were described in the end.

Enhancing the extracellular electron transfer ability via Polydopamine@S. oneidensis MR-1 for Cr(VI) reduction

Microbial reduction of hexavalent chromium (Cr (VI)) shows better efficiency and cost-effectiveness. However, immobilization of Cr (III) remains a challenge as there is a limited supply of electron donors. A greener and cleaner option for donating external electrons was using bioelectrochemical systems to perform the microbial reduction of Cr(VI). In this system, we constructed a polydopamine (PDA) decorated Shewanella oneidensis MR-1 (S. oneidensis MR-1) bioelectrode with bidirectional electron transport, abbreviated as PDA@S. oneidensis MR-1. The conjugated PDA distributed on the intracellular and extracellular of individual S. oneidensis MR-1 has been shown to accelerate electron transfer by outer membrane C-type cytochromes and flavin-bound MtrC/OmcA pathway by various electrochemical analyses. As expected, the PDA@S. oneidensis MR-1 biofilm achieved 88.1% Cr (VI) removal efficiency (RE) and 58.1% Cr (III) immobilization efficiency (IE) within 24 h under the autotrophic conditions at the optimal voltage (-150 mV) compared with the control potential (0 mV). The PDA@S. oneidensis MR-1 biofilm showed increased RE activity was attributed to the shortening of the distance between individual bacteria by PDA. This research provides a viable strategy for in situ bioremediation of Cr(VI) polluted aquatic environment.