Ruthenium Catalysts for the Reduction of <i>N</i>‑Nitrosamine Water Contaminants

<i>N</i>-Nitrosamines have raised extensive concern due to their high toxicity and detection in treated wastewater and drinking water. Catalytic reduction is a promising alternative technology to treat <i>N</i>-nitrosamines, but to advance this technology pathway, there is a need to develop more-efficient and cost-effective catalysts. We have previously discovered that commercial catalysts containing ruthenium (Ru) are unexpectedly active in reducing nitrate. This study evaluated supported Ru activity for catalyzing reduction of <i>N</i>-nitrosamines. Experiments with <i>N</i>-nitrosodimethylamine (NDMA) show that contaminant is rapidly reduced on both commercial and in-house prepared Ru/Al₂O₃ catalysts, with the commercial material yielding an initial metal weight-normalized pseudo-first-order rate constant (<i>k</i>₀) of 1103 ± 133 L·g_Ru^–1·h^–1 and an initial turnover frequency (TOF₀) of 58.0 ± 7.0 h^–1. NDMA is reduced to dimethylamine (DMA) and ammonia end-products, and a small amount of 1,1-dimethylhydrazine (UDMH) was detected as a transient intermediate. Experiment with a mixture of five <i>N</i>-nitrosamines spiked into tap water (1 μg L^–1 each) demonstrates that Ru catalysts are very effective in reducing a range of <i>N</i>-nitrosamine structures at environmentally relevant concentrations. Cost competitiveness and high catalytic activities with a range of contaminants provide a strong argument for developing Ru catalysts as part of the water purification and remediation toolbox

Role of Orthophosphate As a Corrosion Inhibitor in Chloraminated Solutions Containing Tetravalent Lead Corrosion Product PbO₂

Addition of orthophosphate has been commonly employed to suppress lead levels in drinking water. Its detailed mechanism and time required for it to become effective, however, have not been fully elucidated. In this study, we investigated the mechanistic role of orthophosphate as a corrosion inhibitor in controlling lead release from tetravalent lead corrosion product PbO₂ in chloraminated solutions, a system representing distribution networks experiencing disinfectant changeover from free chlorine to monochloramine. In all experiments with orthophosphate addition of at least 1 mg/L as P, peaking of soluble Pb­(II) concentration within the first 24 h was observed before lead concentrations decreased and stabilized at levels lower than 15 μg/L. The variation of soluble Pb­(II) concentration could be attributed to the dynamics between the rate of PbO₂ reductive dissolution, primarily induced by monochloramine decomposition, and that of chloropyromorphite (Pb₅(PO₄)₃Cl) precipitation, which did not occur until a critical supersaturation ratio of about 2.36 was reached in the solution. Our findings provide insights to how orthophosphate reduces lead levels under drinking water conditions and highlight the potential risk of short-term elevated lead concentrations. Intensive monitoring following the disinfectant changeover may be required to determine the overall lead exposure when using orthophosphate as a corrosion inhibitor

Characterizing the Opportunity Space for Sustainable Hydrothermal Valorization of Wet Organic Wastes

Resource recovery from wet organic wastes can support circular economies by creating financial incentives to produce renewable energy and return nutrients to agriculture. In this study, we characterize the potential for hydrothermal liquefaction (HTL)-based resource recovery systems to advance the economic and environmental sustainability of wastewater sludge, FOG (fats, oils, and grease), food waste, green waste, and animal manure management through the production of liquid biofuels (naphtha, diesel), fertilizers (struvite, ammonium sulfate), and power (heat, electricity). From the waste management perspective, median costs range from −193 $·tonne–1 (FOG) to 251$·tonne–1 (green waste), and median carbon intensities range from 367 kg CO2 eq·tonne–1 (wastewater sludge) to 769 kg CO2 eq·tonne–1 (green waste). From the fuel production perspective, the minimum selling price of renewable diesel blendstocks are within the commercial diesel price range (2.37 to 5.81 $·gal–1) and have a lower carbon intensity than petroleum diesel (101 kg CO2 eq·MMBTU–1). Finally, through uncertainty analysis and Monte Carlo filtering, we set specific targets (i.e., achieve wastewater sludge-to-biocrude yield >0.440) for the future development of hydrothermal waste management system components. Overall, our work demonstrates the potential of HTL-based resource recovery systems to reduce the costs and carbon intensity of resource-rich organic wastes

Mechanism and Mitigation of the Decomposition of an Oxorhenium Complex-Based Heterogeneous Catalyst for Perchlorate Reduction in Water

A biomimetic heterogeneous catalyst combining palladium nanoparticles and an organic ligand-coordinated oxorhenium complex on activated carbon, Re­(<i>hoz</i>)₂–Pd/C, was previously developed and shown to reduce aqueous perchlorate (ClO₄^–) with H₂ at a rate ∼100 times faster than the first generation ReO_<i>x</i>–Pd/C catalyst prepared from perrhenate (ReO₄^–). However, the immobilized Re­(<i>hoz</i>)₂ complex was shown to partially decompose and leach into water as ReO₄^–, leading to an irreversible loss of catalytic activity. In this work, the stability of the immobilized Re­(<i>hoz</i>)₂ complex is shown to depend on kinetic competition between three processes: (1) Re^V(<i>hoz</i>)₂ oxidation by ClO₄^– and its reduction intermediates ClO_<i>x</i>^–, (2) Re^VII(<i>hoz</i>)₂ reduction by Pd-activated hydrogen, and (3) hydrolytic Re^VII(<i>hoz</i>)₂ decomposition. When Re^V(<i>hoz</i>)₂ oxidation is faster than Re^VII(<i>hoz</i>)₂ reduction, the Re^VII(<i>hoz</i>)₂ concentration builds up and leads to hydrolytic decomposition to ReO₄^– and free <i>hoz</i> ligand. Rapid Re^V(<i>hoz</i>)₂ oxidation is mainly promoted by highly reactive ClO_<i>x</i>^– formed from the reduction of ClO₄^–. To mitigate Re­(<i>hoz</i>)₂ decomposition and preserve catalytic activity, ruthenium (Ru) and rhodium (Rh) were evaluated as alternative H₂ activators to Pd. Rh showed superior activity for reducing the ClO₃^– intermediate to Cl^–, thereby preventing ClO_<i>x</i>^– buildup and lowering Re complex decomposition in the Re­(<i>hoz</i>)₂–Rh/C catalyst. In contrast, Ru showed the lowest ClO₃^– reduction activity and resulted in the most Re­(<i>hoz</i>)₂ decomposition among the Re­(<i>hoz</i>)₂–M/C catalysts. This work highlights the importance of using mechanistic insights from kinetic and spectroscopic tests to rationally design water treatment catalysts for enhanced performance and stability

Palladium Nanoparticles Encapsulated in Core–Shell Silica: A Structured Hydrogenation Catalyst with Enhanced Activity for Reduction of Oxyanion Water Pollutants

Noble metal nanoparticles have been applied to mediate catalytic removal of toxic oxyanions and halogenated hydrocarbons in contaminated water using H₂ as a clean and sustainable reductant. However, activity loss by nanoparticle aggregation and difficulty of nanoparticle recovery are two major challenges to widespread technology adoption. Herein, we report the synthesis of a core–shell-structured catalyst with encapsulated Pd nanoparticles and its enhanced catalytic activity in reduction of bromate (BrO₃^–), a regulated carcinogenic oxyanion produced during drinking water disinfection process, using 1 atm H₂ at room temperature. The catalyst material consists of a nonporous silica core decorated with preformed octahedral Pd nanoparticles that were further encapsulated within an ordered mesoporous silica shell (i.e., SiO₂@Pd@mSiO₂). Well-defined mesopores (2.3 nm) provide a physical barrier to prevent Pd nanoparticle (∼6 nm) movement, aggregation, and detachment from the support into water. Compared to freely suspended Pd nanoparticles and SiO₂@Pd, encapsulation in the mesoporous silica shell significantly enhanced Pd catalytic activity (by a factor of 10) under circumneutral pH conditions that are most relevant to water purification applications. Mechanistic investigation of material surface properties combined with Langmuir–Hinshelwood modeling of kinetic data suggest that mesoporous silica shell enhances activity by promoting BrO₃^– adsorption near the Pd active sites. The dual function of the mesoporous shell, enhancing Pd catalyst activity and preventing aggregation of active nanoparticles, suggests a promising general strategy of using metal nanoparticle catalysts for water purification and related aqueous-phase applications

Hydrothermal Destruction and Defluorination of Trifluoroacetic Acid (TFA)

Per- and polyfluoroalkyl substances (PFAS) have received increased attention due to their environmental prevalence and threat to public health. Trifluoroacetic acid (TFA) is an ultrashort-chain PFAS and the simplest perfluorocarboxylic acid (PFCA). While the US EPA does not currently regulate TFA, its chemical similarity to other PFCAs and its simple molecular structure make it a suitable model compound for studying the transformation of PFAS. We show that hydrothermal processing in compressed liquid water transforms TFA at relatively mild conditions (T = 150–250 °C, P < 30 MPa), initially yielding gaseous products, such as CHF3 and CO2, that naturally aspirate from the solution. Alkali amendment (e.g., NaOH) promotes the mineralization of CHF3, yielding dissolved fluoride, formate, and carbonate species as final products. Fluorine and carbon balances are closed using Raman spectroscopy and fluoride ion selective electrode measurements for experiments performed at alkaline conditions, where gas yields are negligible. Qualitative FTIR gas analysis allows for establishing the transformation pathways; however, the F-balance could not be quantitatively closed for experiments without NaOH amendment. The kinetics of TFA transformation under hydrothermal conditions are measured, showing little to no dependency on NaOH concentration, indicating that the thermal decarboxylation is a rate-limiting step. A proposed TFA transformation mechanism motivates additional work to generalize the hydrothermal reaction pathways to other PFCAs

Bioinspired Complex-Nanoparticle Hybrid Catalyst System for Aqueous Perchlorate Reduction: Rhenium Speciation and Its Influence on Catalyst Activity

A highly active catalyst for reduction of the inert water contaminant perchlorate (ClO₄^–) to Cl^– with 1 atm H₂ at 25 °C is prepared by noncovalently immobilizing the rhenium complex Re^V(O)­(hoz)₂Cl (hoz = 2-(2′-hydroxyphenyl)-2-oxazoline) together with Pd⁰ nanoparticles on a porous carbon support. Like the Mo complex centers in biological oxyanion reductases, the immobilized Re complex serves as a single site for oxygen atom transfer from ClO₄^– and ClO_<i>x</i>^– intermediates, whereas Pd⁰ nanoparticles provide atomic hydrogen reducing equivalents to sustain redox cycling of the immobilized Re sites, replacing the more complex chain of electron transfer steps that sustain Mo centers within oxyanion reductases. An in situ aqueous adsorption method of immobilization was used to preserve the active Re^V(O)­(hoz)₂ structure during bimetallic catalyst preparation and enable study of Re redox cycling and reactions with ClO₄^–. Heterogeneous reaction kinetics, X-ray photoelectron spectroscopy, and experiments with homogeneous model Re complexes are combined to obtain insights into the catalytic reaction mechanisms and the influence of Re speciation on catalyst reactivity with ClO₄^–. Redox cycling between hoz-coordinated Re^V and Re^VII species serves as the main catalytic cycle for ClO₄^– reduction. Under reducing conditions, approximately half of the immobilized hoz-coordinated Re^V is further reduced to Re^III, which is not directly reactive with ClO₄^–. A small fraction of the hoz-coordinated Re^VII species can dissociate to ReO₄^– and free hoz, which are then reductively reimmobilized as a less reactive mixture of Re^V, Re^III, and Re^I species. This study provides an example wherein highly active metal complexes that were originally developed for homogeneous organic phase catalysis can be incorporated into heterogeneous catalysts for practical environmental applications. Findings suggest a general blueprint for developing hybrid catalysts combining single-site transition metal complexes with hydrogen-activating metal nanoparticles

Inactivation of Bacteriophage MS2 with Potassium Ferrate(VI)

Ferrate [Fe­(VI); FeO₄^2–] is an emerging oxidizing agent capable of controlling chemical and microbial water contaminants. Here, inactivation of MS2 coliphage by Fe­(VI) was examined. The inactivation kinetics observed in individual batch experiments was well described by a Chick–Watson model with first-order dependences on disinfectant and infective phage concentrations. The inactivation rate constant <i>k</i>_<i>i</i> at a Fe­(VI) dose of 1.23 mgFe/L (pH 7.0, 25 °C) was 2.27(±0.05) L/(mgFe × min), corresponding to 99.99% inactivation at a <i>Ct</i> of ∼4 (mgFe × min)/L. Measured <i>k</i>_i values were found to increase with increasing applied Fe­(VI) dose (0.56–2.24 mgFe/L), increasing temperature (5–30 °C), and decreasing pH conditions (pH 6–11). The Fe­(VI) dose effect suggested that an unidentified Fe byproduct also contributed to inactivation. Temperature dependence was characterized by an activation energy of 39(±6) kJ mol^–1, and <i>k</i>_<i>i</i> increased >50-fold when pH decreased from 11 to 6. The pH effect was quantitatively described by parallel reactions with HFeO₄^– and FeO₄^2–. Mass spectrometry and qRT-PCR analyses demonstrated that both capsid protein and genome damage increased with the extent of inactivation, suggesting that both may contribute to phage inactivation. Capsid protein damage, localized in the two regions containing oxidant-sensitive cysteine residues, and protein cleavage in one of the two regions may facilitate genome damage by increasing Fe­(VI) access to the interior of the virion

X‑ray Spectroscopic Characterization of Immobilized Rhenium Species in Hydrated Rhenium–Palladium Bimetallic Catalysts Used for Perchlorate Water Treatment

Carbon-supported rhenium–palladium catalysts (Re–Pd/C) effectively transform aqueous perchlorate, a widespread drinking water pollutant, via chemical reduction using hydrogen as an electron donor at ambient temperature and pressure. Previous work demonstrated that catalyst activity and stability are heavily dependent on solution composition and Re content in the catalyst. This study relates these parameters to changes in the speciation and molecular structure of Re immobilized on the catalyst. Using X-ray spectroscopy techniques, we show that Re is immobilized as Re^VII under oxic solution conditions, but transforms to a mixture of reduced, O-coordinated Re species under reducing solution conditions induced by H₂ sparging. Under oxic solution conditions, extended X-ray absorption fine structure (EXAFS) analysis showed that the immobilized Re^VII species is indistinguishable from the dissolved tetrahedral perrhenate (ReO₄^–) anion, suggesting outer-sphere adsorption to the catalyst surface. Under reducing solution conditions, two Re species were identified. At low Re loading (≤1 wt %), monomeric Re^I species form in direct contact with Pd nanoclusters. With increased Re loading, speciation gradually shifts to oxidic Re^V clusters. The identified Re structures support a revised mechanism for catalytic reduction of ClO₄^– involving oxygen atom transfer reactions between odd-valence oxorhenium species and the oxyanion (Re oxidation steps) and atomic hydrogen species (Re reduction steps) formed by Pd-catalyzed dissociation of H₂

A New Bioinspired Perchlorate Reduction Catalyst with Significantly Enhanced Stability via Rational Tuning of Rhenium Coordination Chemistry and Heterogeneous Reaction Pathway

Rapid reduction of aqueous ClO₄^– to Cl^– by H₂ has been realized by a heterogeneous Re­(<i>hoz</i>)₂–Pd/C catalyst integrating Re­(O)­(<i>hoz</i>)₂Cl complex (<i>hoz</i> = oxazolinyl-phenolato bidentate ligand) and Pd nanoparticles on carbon support, but ClO_<i>x</i>^– intermediates formed during reactions with concentrated ClO₄^– promote irreversible Re complex decomposition and catalyst deactivation. The original catalyst design mimics the microbial ClO₄^– reductase, which integrates Mo­(MGD)₂ complex (MGD = molybdopterin guanine dinucleotide) for oxygen atom transfer (OAT). Perchlorate-reducing microorganisms employ a separate enzyme, chlorite dismutase, to prevent accumulation of the destructive ClO₂^– intermediate. The structural intricacy of MGD ligand and the two-enzyme mechanism for microbial ClO₄^– reduction inspired us to improve catalyst stability by rationally tuning Re ligand structure and adding a ClO_<i>x</i>^– scavenger. Two new Re complexes, Re­(O)­(<i>htz</i>)₂Cl and Re­(O)­(hoz)­(<i>htz</i>)Cl (<i>htz</i> = thiazolinyl-phenolato bidentate ligand), significantly mitigate Re complex decomposition by slightly lowering the OAT activity when immobilized in Pd/C. Further stability enhancement is then obtained by switching the nanoparticles from Pd to Rh, which exhibits high reactivity with ClO_<i>x</i>^– intermediates and thus prevents their deactivating reaction with the Re complex. Compared to Re­(<i>hoz</i>)₂–Pd/C, the new Re­(<i>hoz</i>)­(<i>htz</i>)–Rh/C catalyst exhibits similar ClO₄^– reduction activity but superior stability, evidenced by a decrease of Re leaching from 37% to 0.25% and stability of surface Re speciation following the treatment of a concentrated “challenge” solution containing 1000 ppm of ClO₄^–. This work demonstrates the pivotal roles of coordination chemistry control and tuning of individual catalyst components for achieving both high activity and stability in environmental catalyst applications