Ligand-Enhanced Reduction of Perchlorate in Water with Heterogeneous Re−Pd/C Catalysts

Electrochemical reduction of wastewater by non-noble metal cathodes: From terminal purification to upcycling recovery

A shift beyond conventional environmental remediation to a sustainable pollutant upgrading conversion is extremely desirable due to the rising demand for resources and widespread chemical contamination. Electrochemical reduction processes (ERPs) have drawn considerable attention in recent years in the fields of oxyanion reduction, metal recovery, detoxification and high-value conversion of halogenated organics and benzenes. ERPs also have the potential to address the inherent limitations of conventional chemical reduction technologies in terms of hydrogen and noble metal requirements. Fundamentally, mechanisms of ERPs can be categorized into three main pathways: direct electron transfer, atomic hydrogen mediation, and electrode redox pairs. Furthermore, this review consolidates state-of-the-art non-noble metal cathodes and their performance comparable to noble metals (e.g., Pd, Pt) in electrochemical reduction of inorganic/organic pollutants. To overview the research trends of ERPs, we innovatively sort out the relationship between the electrochemical reduction rate, the charge of the pollutant, and the number of electron transfers based on the statistical analysis. And we propose potential countermeasures of pulsed electrocatalysis and flow mode enhancement for the bottlenecks in electron injection and mass transfer for electronegative pollutant reduction. We conclude by discussing the gaps in the scientific and engineering level of ERPs, and envisage that ERPs can be a low-carbon pathway for industrial wastewater detoxification and valorization.

Electrocatalytic Perchlorate Reduction Using an Oxorhenium Complex Supported on a Ti4O7 Reactive Electrochemical Membrane

An organometallic rhenium catalyst was deposited on a Ti₄O₇ reactive electrochemical membrane (Re/REM) for the electrocatalytic reduction of aqueous ClO₄^- to Cl^-. Results showed increasing ClO₄^- reduction upon increasing cathodic potential (i.e., -0.4 to-1.7 V/SHE). A 5 mM ClO₄^- solution was reduced by ∼21% in a single pass (residence time ∼0.2 s) through the Re/REM at a pH of 7, with >99% Cl^- selectivity and a current efficiency of ∼100%. Kinetic analysis indicated that the reaction rate constant increased from 3953 to 7128 L h^-1 g_Re^-1 at pH values of 9 to 3, respectively, and was mass transport-limited at pH < 5. The rate constants were 2 orders of magnitude greater than reported values for an analogous catalytic system using hydrogen as an electron donor. A continuous flow Re/REM system reduced 1 ppm ClO₄^- in a groundwater sample by >99.9% for the first 93.5 h, and concentrations were lower than the EPA ClO₄^- guideline (56 ppb) for 374 h of treatment. The fast ClO₄^- reduction kinetics and high chloride selectivity without the need for acidic conditions and a continual hydrogen electron donor supply for catalyst regeneration indicate the promising ability of the Re/REM for aqueous electrocatalytic ClO₄^- treatment.

Catalytic enhancement of hydrogenation reduction and oxygen transfer reaction for perchlorate removal: A review

Perchlorate is the main contaminant in surface water and groundwater, and it is of current urgency to remove due to its high water solubility, mobility, and endocrine-disrupting properties. The conversion of perchlorate into harmless chloride ions by using appropriate catalysts is the most promising and effective route to overcome its high activation energy and kinetic stability. Perchlorate is usually reduced in two ways: (1) indirect reduction via oxygen atom transfer (OAT) reaction or (2) hydrodeoxygenation through highly active reducing H atoms. This paper discusses the mechanisms underlying both the OAT reaction catalyzed by homogenous rhenium-oxo complexes or biological Mo-based enzymes and the heterogeneous hydrogenation for perchlorate reduction. Particular emphasis is placed on the factors affecting the catalytic process and the synergy between the (1) and (2) reactions. For completeness, the applicability of different electrolysis devices, electrodes, and bioreactors is also illustrated. Finally, this article gives prospects for the synthesis and application of catalysts in different pathways.

Rhenium Nanostructures Loaded into Amino-Functionalized Resin as a Nanocomposite Catalyst for Hydrogenation of 4-Nitrophenol and 4-Nitroaniline

The present work presents a new nanocomposite catalyst with rhenium nanostructures (ReNSs) for the catalytic hydrogenation of 4-nitrophenol and 4-nitroaniline. The catalyst, based on an anion exchange resin with functionality derived from 1,1'-carboimidazole, was obtained in the process involving anion exchange of ReO4- ions followed by their reduction with NaBH4. The amino functionality present in the resin played a primary role in the stabilization of the resultant ReNSs, consisting of ≈1% (w/w) Re in the polymer mass. The synthesized and capped ReNSs were amorphous and had the average size of 3.45 ± 1.85 nm. Then, the obtained catalyst was used in a catalytic reduction of 4-nitrophenol (4-NP) and 4-nitroaniline (4-NA). Following the pseudo-first-order kinetics, 5 mg of the catalyst led to a 90% conversion of 4-NP with the mass-normalized rate constant (km1) of 6.94 × 10-3 min-1 mg-1, while the corresponding value acquired for 4-NA was 7.2 × 10-3 min-1 mg-1, despite the trace amount of Re in the heterogenous catalyst. The obtained material was also conveniently reused.

A Bioinspired Molybdenum Catalyst for Aqueous Perchlorate Reduction

Perchlorate (ClO₄^-) is a pervasive, harmful, and inert anion on both Earth and Mars. Current technologies for ClO₄^- reduction entail either harsh conditions or multicomponent enzymatic processes. Herein, we report a heterogeneous (L)Mo-Pd/C catalyst directly prepared from Na₂MoO₄, a bidentate nitrogen ligand (L), and Pd/C to reduce aqueous ClO₄^- into Cl^- with 1 atm of H₂ at room temperature. A suite of instrument characterizations and probing reactions suggest that the Mo^VI precursor and L at the optimal 1:1 ratio are transformed in situ into oligomeric Mo^IV active sites at the carbon-water interface. For each Mo site, the initial turnover frequency (TOF₀) for oxygen atom transfer from ClO_x^- substrates reached 165 h^-1. The turnover number (TON) reached 3840 after a single batch reduction of 100 mM ClO₄^-. This study provides a water-compatible, efficient, and robust catalyst to degrade and utilize ClO₄^- for water purification and space exploration.

Ultrafine Re/Pd nanoparticles on polydopamine modified carbon nanotubes for efficient perchlorate reduction and reusability

This study synthesized nanocomposite catalysts via a modification of Re/Pd codoped carbon nanotubes (CNTs) with different concentrations of polydopamine (PDA), which were used for perchlorate (ClO₄^-) reduction. The loads, dispersion and reducibility of Re/Pd nanoparticles increased yet their particle sizes significantly decreased with the increase of PDA concentrations. The average diameter of Re/Pd codoped D₂CNT (CNT modified by 2 mg/mL PDA) with a narrow size distribution was measured to be 2 nm. The ultrafine Re/Pd codoped D₂CNT catalysts represented outstanding catalytic reduction activity for the conversion of ClO₄^- to Cl^- with TOF of 17.34 h^-1 under the room H₂ atmospheric pressure, which was about 8 times than that of the unmodified catalysts. Furthermore, PDA modification minimized the dissociation of Re by chemical bonding between Re and CNTs carrier and maintained good stability of nanocomposite. This study inspires us to apply green bionic methods to enhance the catalytic reduction of perchlorate by changing the physical properties of Re/Pd nanoparticles.

Rational design of nanomaterials for water treatment

The ever-increasing human demand for safe and clean water is gradually pushing conventional water treatment technologies to their limits. It is now a popular perception that the solutions to the existing and future water challenges will hinge upon further developments in nanomaterial sciences. The concept of rational design emphasizes on 'design-for-purpose' and it necessitates a scientifically clear problem definition to initiate the nanomaterial design. The field of rational design of nanomaterials for water treatment has experienced a significant growth in the past decade and is poised to make its contribution in creating advanced next-generation water treatment technologies in the years to come. Within the water treatment context, this review offers a comprehensive and in-depth overview of the latest progress in rational design, synthesis and applications of nanomaterials in adsorption, chemical oxidation and reduction reactions, membrane-based separation, oil-water separation, and synergistic multifunctional all-in-one nanomaterials/nanodevices. Special attention is paid to the chemical concepts related to nanomaterial design throughout the review.

“Click” post-functionalization of a metal–organic framework for engineering active single-site heterogeneous Ru(iii) catalysts

A terpyridyl NNN-chelator has been successfully incorporated into MIL-101(Cr) by using a click post-synthetic modification method, offering a useful platform for metalation to prepare highly active and stable single-site heterogeneous catalysts.

Critical review of electrochemical advanced oxidation processes for water treatment applications

Electrochemical advanced oxidation processes (EAOPs) have emerged as novel water treatment technologies for the elimination of a broad-range of organic contaminants. Considerable validation of this technology has been performed at both the bench-scale and pilot-scale, which has been facilitated by the development of stable electrode materials that efficiently generate high yields of hydroxyl radicals (OH˙) (e.g., boron-doped diamond (BDD), doped-SnO2, PbO2, and substoichiometic- and doped-TiO2). Although a promising new technology, the mechanisms involved in the oxidation of organic compounds during EAOPs and the corresponding environmental impacts of their use have not been fully addressed. In order to unify the state of knowledge, identify research gaps, and stimulate new research in these areas, this review critically analyses published research pertaining to EAOPs. Specific topics covered in this review include (1) EAOP electrode types, (2) oxidation pathways of select classes of contaminants, (3) rate limitations in applied settings, and (4) long-term sustainability. Key challenges facing EAOP technologies are related to toxic byproduct formation (e.g., ClO4(-) and halogenated organic compounds) and low electro-active surface areas. These challenges must be addressed in future research in order for EAOPs to realize their full potential for water treatment.

The abatement of major pollutants in air and water by environmental catalysis

This review reports the research progress in the abatement of major pollutants in air and water by environmental catalysis. For air pollution control, the selective catalytic reduction of NO x (SCR) by ammonia and hydrocarbons on metal oxide and zeolite catalysts are reviewed and discussed, as is the removal of Hg from flue gas by catalysis. The oxidation of Volatile organic compounds (VOCs) by photo- and thermal-catalysis for indoor air quality improvement is reviewed. For wastewater treatment, the catalytic elimination of inorganic and organic pollutants in wastewater is presented. In addition, the mechanism for the procedure of abatement of air and water pollutants by catalysis is discussed in this review. Finally, a research orientation on environment catalysis for the treatment of air pollutants and wastewater is proposed.

Comparative assessment of the environmental sustainability of existing and emerging perchlorate treatment technologies for drinking water

Environmental impacts of conventional and emerging perchlorate drinking water treatment technologies were assessed using life cycle assessment (LCA). Comparison of two ion exchange (IX) technologies (i.e., nonselective IX with periodic regeneration using brines and perchlorate-selective IX without regeneration) at an existing plant shows that brine is the dominant contributor for nonselective IX, which shows higher impact than perchlorate-selective IX. Resource consumption during the operational phase comprises >80% of the total impacts. Having identified consumables as the driving force behind environmental impacts, the relative environmental sustainability of IX, biological treatment, and catalytic reduction technologies are compared more generally using consumable inputs. The analysis indicates that the environmental impacts of heterotrophic biological treatment are 2-5 times more sensitive to influent conditions (i.e., nitrate/oxygen concentration) and are 3-14 times higher compared to IX. However, autotrophic biological treatment is most environmentally beneficial among all. Catalytic treatment using carbon-supported Re-Pd has a higher (ca. 4600 times) impact than others, but is within 0.9-30 times the impact of IX with a newly developed ligand-complexed Re-Pd catalyst formulation. This suggests catalytic reduction can be competitive with increased activity. Our assessment shows that while IX is an environmentally competitive, emerging technologies also show great promise from an environmental sustainability perspective.

Application of a Re-Pd bimetallic catalyst for treatment of perchlorate in waste ion-exchange regenerant brine

Concentrated sodium chloride (NaCl) brines are often used to regenerate ion-exchange (IX) resins applied to treat drinking water sources contaminated with perchlorate (ClO(4)(-)), generating large volumes of contaminated waste brine. Chemical and biological processes for ClO(4)(-) reduction are often inhibited severely by high salt levels, making it difficult to recycle waste brines. Recent work demonstrated that novel rhenium-palladium bimetallic catalysts on activated carbon support (Re-Pd/C) can efficiently reduce ClO(4)(-) to chloride (Cl(-)) under acidic conditions, and here the applicability of the process for treating waste IX brines was examined. Experiments conducted in synthetic NaCl-only brine (6-12 wt%) showed higher Re-Pd/C catalyst activity than in comparable freshwater solutions, but the rate constant for ClO(4)(-) reduction measured in a real IX waste brine was found to be 65 times lower than in the synthetic NaCl brine. Through a series of experiments, co-contamination of the IX waste brine by excess NO(3)(-) (which the catalyst reduces principally to NH(4)(+)) was found to be the primary cause for deactivation of the Re-Pd/C catalyst, most likely by altering the immobilized Re component. Pre-treatment of NO(3)(-) using a different bimetallic catalyst (In-Pd/Al(2)O(3)) improved selectivity for N(2) over NH(4)(+) and enabled facile ClO(4)(-) reduction by the Re-Pd/C catalyst. Thus, sequential catalytic treatment may be a promising strategy for enabling reuse of waste IX brine containing NO(3)(-) and ClO(4)(-).

Critical review of Pd-based catalytic treatment of priority contaminants in water

Catalytic reduction of water contaminants using palladium (Pd)-based catalysts and hydrogen gas as a reductant has been extensively studied at the bench-scale, but due to technical challenges it has only been limitedly applied at the field-scale. To motivate research that can overcome these technical challenges, this review critically analyzes the published research in the area of Pd-based catalytic reduction of priority drinking water contaminants (i.e., halogenated organics, oxyanions, and nitrosamines), and identifies key research areas that should be addressed. Specifically, the review summarizes the state of knowledge related to (1) proposed reaction pathways for important classes of contaminants, (2) rates of contaminant reduction with different catalyst formulations, (3) long-term sustainability of catalyst activity with respect to natural water foulants and regeneration strategies, and (4) technology applications. Critical barriers hindering implementation of the technology are related to catalyst activity (for some contaminants), stability, fouling, and regeneration. New developments overcoming these limitations will be needed for more extensive field-scale application of this technology.