Disentangling the size-dependent redox reactivity of iron oxides using thermodynamic relationships.

Nanoparticles often exhibit size-dependent redox reactivities, with smaller particles being more reactive in some cases, while less reactive in others. Predicting trends between redox reaction rates and particle sizes is often complicated because a particle's dimensions can simultaneously influence its aggregation state, reactive surface area, and thermodynamic properties. Here, we tested the hypothesis that interfacial redox reaction rates for nanoparticles with different sizes can be described with a single linear free-energy relationship (LFER) if size-dependent reactive surface areas and thermodynamic properties are properly considered. We tested this hypothesis using a well-known interfacial redox reaction: the reduction of nitrobenzene to aniline by iron-oxide-bound Fe2+. We measured the reduction potential (EH) values of nano-particulate hematite suspensions containing aqueous Fe2+ using mediated potentiometry and characterized the size and aggregation states of hematite samples at circumneutral pH values. We used the measured EH values to calculate surface energies and reactive surface areas using thermodynamic relationships. Nitrobenzene reduction rates were lower for smaller particles, despite their larger surface areas, due to their higher surface energies. When differences in surface areas and thermodynamic properties were considered, nitrobenzene reduction kinetics for all particle sizes was described with a LFER. Our results demonstrate that when Fe2+ serves as a reductant, an antagonistic effect exists, with smaller particles having larger reactive surface areas but also more positive reduction potentials. When Fe3+ serves as an oxidant, however, these two effects work in concert, which likely explains past discrepancies regarding how iron oxide particle sizes influence redox reaction rates.

Promotive Effects of Chloride and Sulfate on the Near-Complete Destruction of Perfluorocarboxylates (PFCAs) in Brine via Hydrogen-tuned 185-nm UV Photolysis: Mechanisms and Kinetics.

Hydrogen-tuned 185 nm vacuum ultraviolet (VUV/H2) photolysis is an emerging technology to destroy per- and polyfluoroalkyl substance (PFAS) in brine. This study discovered the promotive effects of two major brine anions, i.e., chloride and sulfate in VUV/H2 photolysis on the hydrated electron (eaq-) generation and perfluorocarboxylates (PFCAs) destruction and established a kinetics model to elucidate the promotive effects on the steady-state concentration of eaq- ([eaq-]ss). Results showed that VUV/H2 achieved near-complete defluorination of perfluorooctanoic acid (PFOA) in the presence of up to 1000 mM chloride or sulfate at pH 12. The defluorination rate constant (kdeF) of PFOA peaked with a chloride concentration at 100 mM and with a sulfate concentration at 500 mM. The promotive effects of chloride and sulfate were attributed to an enhanced generation of eaq- via their direct VUV photolysis and conversion of additionally generated hydroxyl radical to eaq- by H2, which was supported by a linear correlation between the predicted [eaq-]ss and experimentally observed kdeF. The kdeF value increased from pH 9 to 12, which was attributed to the speciation of the H·/eaq- pair. Furthermore, the VUV system achieved >95% defluorination and ≥99% parent compound degradation of a concentrated PFCAs mixture in a synthetic brine, without generating any toxic perchlorate or chlorate.

Enhancing Aqueous Chlorate Reduction Using Vanadium Redox Cycles and pH Control.

Chlorate (ClO3-) is a toxic oxyanion pollutant from industrial wastes, agricultural applications, drinking water disinfection, and wastewater treatment. Catalytic reduction of ClO3- using palladium (Pd) nanoparticle catalysts exhibited sluggish kinetics. This work demonstrates an 18-fold activity enhancement by integrating earth-abundant vanadium (V) into the common Pd/C catalyst. X-ray photoelectron spectroscopy and electrochemical studies indicated that VV and VIV precursors are reduced to VIII in the aqueous phase (rather than immobilized on the carbon support) by Pd-activated H2. The VIII/IV redox cycle is the predominant mechanism for the ClO3- reduction. Further reduction of chlorine intermediates to Cl- could proceed via VIII/IV and VIV/V redox cycles or direct reduction by Pd/C. To capture the potentially toxic V metal from the treated solution, we adjusted the pH from 3 to 8 after the reaction, which completely immobilized VIII onto Pd/C for catalyst recycling. The enhanced performance of reductive catalysis using a Group 5 metal adds to the diversity of transition metals (e.g., Cr, Mo, Re, Fe, and Ru in Groups 6-8) for water pollutant treatment via various unique mechanisms.

Role of Carbonate in Thermodynamic Relationships Describing Pollutant Reduction Kinetics by Iron Oxide-Bound Fe2.

The reduction of environmental pollutants by Fe2+ bound to iron oxides is an important process that determines pollutant toxicities and mobilities. Recently, we showed that pollutant reduction rates depend on the thermodynamic driving force of the reaction in a linear free energy relationship that was a function of the solution pH value and the reduction potential, EH, of the interfacial Fe3+/Fe2+ redox couple. In this work, we studied how carbonate affected the free energy relationship by examining the effect that carbonate has on nitrobenzene reduction rates by Fe2+ bound to goethite (α-FeOOH). Carbonate slowed nitrobenzene reduction rates by inducing goethite particle aggregation, as evidenced by surface charge and particle size measurements. We observed no evidence for carbonate affecting Fe3+/Fe2+ reduction potentials or the mechanism of nitrobenzene reduction. The linear free energy relationship accurately described the data collected in the presence of carbonate when we accounted for the effect it had on the reactive surface area of goethite. The findings from this work provide a framework for determining why common groundwater constituents affect the EH-dependence of reaction rates involving oxide-bound Fe2+ as a reductant.

Nitrate Removal via a Formate Radical-Induced Photochemical Process.

Removal of excess nitrate is critical to balance the nitrogen cycle in aquatic systems. This study investigated a novel denitrification process by tailoring photochemistry of nitrate with formate. Under UV light irradiation, short-lived radicals (i.e., HO•, NO2•, and CO3•-) generated from nitrate photolysis partially oxidized formate to highly reductive formate radical (CO2•-). CO2•- further reduced nitrogen intermediates generated during photochemical denitrification (mainly NO•, HNO, and N2O) to gas-phase nitrogen (i.e., N2O and N2). The degradation kinetics of total dissolved nitrogen was mainly controlled by the photolysis rates of nitrate and nitrite. The distribution of final products was controlled by the reaction between CO2•- and N2O. To achieve a simultaneous and complete removal of dissolved nitrogen (i.e., nitrate, nitrite, and ammonia) and organic carbon, the formate-to-nitrate stoichiometry was determined as 3.1 ± 0.2 at neutral pH in deionized water. Solution pH impacted the removal rates of nitrate and nitrite but not that of total dissolved nitrogen or formate. The presence of dissolved organic matter at levels similar to those in groundwater had a negligible impact on the photochemical denitrification process. A high denitrification efficiency was also achieved in a synthetic groundwater matrix. Outcome from this study provides a potential denitrification technology for decentralized water treatment and reuse facilities to abate nitrate in local water resources.

Understanding the Reduction Kinetics of Aqueous Vanadium(V) and Transformation Products Using Rotating Ring-Disk Electrodes.

Vanadium(V) is an emerging contaminant in the most recent Environmental Protection Agency's candidate contaminant list (CCL4). The redox chemistry of vanadium controls its occurrence in the aquatic environment, but the impact of vanadium(V) speciation on the redox properties remains largely unknown. This study utilized the rotating ring-disk electrode technique to examine the reduction kinetics of four pH- and concentration-dependent vanadium(V) species in the presence and the absence of phosphate. Results showed that the reduction of VO2+, HxV4O12+x(4+x)- (V4), and HVO42- proceeded via a one-electron transfer, while that of NaxHyV10O28(6-x-y)- (V10) underwent a two-electron transfer. Koutecky-Levich and Tafel analyses showed that the intrinsic reduction rate constants followed the order of V10 > VO2+ > V4 > HVO42-. Ring-electrode collection efficiency indicated that the reduction product of V10 was stable, while those of VO2+, HVO42-, and V4 had short half-lives that ranged from milliseconds to seconds. With molar ratios of phosphate to vanadium(V) varying from 0 to 1, phosphate accelerated the reduction kinetics of V10 and V4 and enhanced the stability of the reduction products of VO2+, V4, and HVO42-. This study suggests that phosphate complexation could enhance the reductive removal of vanadium(V) and inhibit the reoxidation of its reduction product in water treatment.

Degradation of nitrilotris-methylenephosphonic acid (NTMP) antiscalant via persulfate photolysis: Implications on desalination concentrate treatment.

Nitrilotris-methylenephosphonic acid (NTMP) has been widely used as an antiscalant in reverse osmosis (RO) desalination and other industrial processes to inhibit scaling from calcium and other hardness ions. Removal of NTMP from RO concentrate can induce the precipitation of oversaturated scale-forming substances, enable additional water recovery from RO concentrate, and reduce the risk of eutrophication after brine disposal. This study investigated the kinetics and mechanisms of oxidative degradation of NTMP by UV photolysis of persulfate at 254 nm. Results showed that NTMP was effectively degraded by persulfate photolysis and the reaction followed pseudo first-order kinetics. The degradation of NTMP was favorable at circumneutral pHs but significantly inhibited in highly alkaline conditions (e.g., pH of 11.5), mainly due to the reduced concentration of SO4•-. Using a competition reaction kinetics approach, the second-order rate constants of NTMP with SO4•- and HO• were determined to be (2.9 ±â€¯0.6) × 107 M-1s-1 and (1.1 ±â€¯0.1) × 108 M-1s-1, respectively. SO4•- had a predominant contribution to NTMP degradation (62%-95%), because the steady-state concentration of SO4•- was 11-54 times higher than that of HO• at pHs between 4 and 11.5. NTMP degradation rate increased with an increase in persulfate dosage and a decrease in NTMP concentration. In the real RO concentrate, NTMP degradation rate was impacted by the presence of chloride and bicarbonate. The degradation of NTMP started with the cleavage of C-N bonds, and then generated intermediates including iminodi(methylene)phosphonate, hydroxymethylphosphonic acid and aminotris(methylenephosphonic acid), which were eventually mineralized into ammonia, phosphate and carbon dioxide. This study demonstrated that UV/persulfate is a promising technology to remove phosphonate antiscalants from RO concentrate.

M13 bacteriophage spheroids as scaffolds for directed synthesis of spiky gold nanostructures.

The spherical form (s-form) of a genetically-modified gold-binding M13 bacteriophage was investigated as a scaffold for gold synthesis. Repeated mixing of the phage with chloroform caused a 15-fold contraction from a nearly one micron long filament to an approximately 60 nm diameter spheroid. The geometry of the viral template and the helicity of its major coat protein were monitored throughout the transformation process using electron microscopy and circular dichroism spectroscopy, respectively. The transformed virus, which retained both its gold-binding and mineralization properties, was used to assemble gold colloid clusters and synthesize gold nanostructures. Spheroid-templated gold synthesis products differed in morphology from filament-templated ones. Spike-like structures protruded from the spherical template while isotropic particles developed on the filamentous template. Using inductively coupled plasma-mass spectroscopy (ICP-MS), gold ion adsorption was found to be comparatively high for the gold-binding M13 spheroid, and likely contributed to the dissimilar gold morphology. Template contraction was believed to modify the density, as well as the avidity of gold-binding peptides on the scaffold surface. The use of the s-form of the M13 bacteriophage significantly expands the templating capabilities of this viral platform and introduces the potential for further morphological control of a variety of inorganic material systems.

Photocatalytic removal of hexavalent chromium by newly designed and highly reductive TiO2 nanocrystals.

Hexavalent chromium Cr(VI), a highly toxic oxyanion, widely occurs in drinking water supplies. This study designed and synthesized a new type of highly reductive TiO2 nanocrystals for photochemical Cr(VI) removal, via the thermal hydrolysis of TiCl4 in the presence of diethylene glycol (DEG). Surface analyses and hydroxyl radical measurements suggested that DEG was chemically bonded on TiO2 surface that resulted in an internal hole-scavenging effect and a high electron-releasing capacity, making it advantageous to conventional TiO2 materials. Upon UV irradiation, the synthesized TiO2 photocatalyst exhibited fast Cr(VI) reduction kinetics in diverse water chemical conditions. Fast elimination of Cr(VI) was achieved on a time scale of seconds in drinking water matrices. The removal of Cr(VI) by reductive TiO2 exhibited a three-stage kinetic behavior: an initial fast-reaction phase, a lag phase resulting from surface precipitation of Cr(OH)3(s), and a final reaction phase due to surface regeneration from oxidation-reduction induced ripening process. The lag phase disappeared in acidic conditions that prevented the formation of Cr(OH)3(s). The catalyst exhibited extremely high electron-releasing capacity that can be reused for multiple cycles of Cr(VI) removal in drinking water treatment.