Radical Chemistry in a Femtosecond Laser Plasma: Photochemical Reduction of Ag⁺ in Liquid Ammonia Solution

Plasmas with dense concentrations of reactive species such as hydrated electrons and hydroxyl radicals are generated from focusing intense femtosecond laser pulses into aqueous media. These radical species can reduce metal ions such as Au³⁺ to form metal nanoparticles (NPs). However, the formation of H₂O₂ by the recombination of hydroxyl radicals inhibits the reduction of Ag⁺ through back-oxidation. This work has explored the control of hydroxyl radical chemistry in a femtosecond laser-generated plasma through the addition of liquid ammonia. The irradiation of liquid ammonia solutions resulted in a reaction between NH₃ and OH·, forming peroxynitrite and ONOO^-, and significantly reducing the amount of H₂O₂ generated. Varying the liquid ammonia concentration controlled the Ag⁺ reduction rate, forming 12.7 ± 4.9 nm silver nanoparticles at the optimal ammonia concentration. The photochemical mechanisms underlying peroxynitrite formation and Ag⁺ reduction are discussed.

Effects of pH and H2O2 on ammonia, nitrite, and nitrate transformations during UV254nm irradiation: Implications to nitrogen removal and analysis

In order to achieve better removal and analyses of three dissolved inorganic nitrogen (DIN) species via ultraviolet-activated hydrogen peroxide (UV/H₂O₂) process, this study systematically investigated the rates of photo-oxidations of ammonia/ammonium (NH₃/NH₄⁺) and nitrite (NO₂^-) as well as the photo-reduction of nitrate (NO₃^-) at varying pH and H₂O₂ conditions. The results showed that the mass balances of nitrogen were maintained along irradiation despite of interconversions of DIN species, suggesting that no nitrogen gas (N₂) or other nitrogen-containing compound was formed. NH₃ was more reactive than NH₄⁺ with hydroxyl radical (OH), and by a stepwise H₂O₂ addition method NH₃/NH₄⁺ can be completely converted to NO_x^-; NO₂^- underwent rapid oxidation to form NO₃^- when H₂O₂ was present, suggesting that it is an intermediate compound linking NH₃/NH₄⁺ and NO₃^-; but once H₂O₂ was depleted, NO₃^- can be gradually photo-reduced back to NO₂^- at high pH conditions. Other than H₂O₂, the transformation kinetics of DINs were all dependent on pH, but to varying aspects and extents: the NH₃ photo-oxidation favored a pH of 10.3, which fell within the pK_a values of NH₄⁺ (9.24) and H₂O₂ (11.6); the NO₃^- photo-reduction increased with increasing pH provided that it exceeds the pK_a of peroxynitrous acid (6.8); while the NO₂^- photo-oxidation remained stable unless the pH neared the pK_a of H₂O₂ (11.6). The study thereby demonstrates a picture of the evolutions of DIN species together during UV/H₂O₂ irradiation process, and for the first time presents a method to achieve complete conversion of NH₄⁺ to NO₃^- with UV/H₂O₂ process.

Removal of several pesticides in a falling water film DBD reactor with activated carbon textile: Energy efficiency

Bio-recalcitrant micropollutants are often insufficiently removed by modern wastewater treatment plants to meet the future demands worldwide. Therefore, several advanced oxidation techniques, including cold plasma technology, are being investigated as effective complementary water treatment methods. In order to permit industrial implementation, energy demand of these techniques needs to be minimized. To this end, we have developed an electrical discharge reactor where water treatment by dielectric barrier discharge (DBD) is combined with adsorption on activated carbon textile and additional ozonation. The reactor consists of a DBD plasma chamber, including the adsorptive textile, and an ozonation chamber, where the DBD generated plasma gas is bubbled. In the present paper, this reactor is further characterized and optimized in terms of its energy efficiency for removal of the five pesticides α-HCH, pentachlorobenzene, alachlor, diuron and isoproturon, with initial concentrations ranging between 22 and 430 μg/L. Energy efficiency of the reactor is found to increase significantly when initial micropollutant concentration is decreased, when duty cycle is decreased and when oxygen is used as feed gas as compared to air and argon. Overall reactor performance is improved as well by making it work in single-pass operation, where water is flowing through the system only once. The results are explained with insights found in literature and practical implications are discussed. For the used operational conditions and settings, α-HCH is the most persistent pesticide in the reactor, with a minimal achieved electrical energy per order of 8 kWh/m³, while a most efficient removal of 3 kWh/m³ or lower was reached for the four other pesticides.

Computational study on the negative electron affinities of NO2−∙(H2O)n clusters (n=0–30)

Here we report negative electron affinities of NO(2)(-).(H2O)n clusters (n=0-30) obtained from density functional theory calculations and a simple correction to Koopmans' theorem. The method relies on the calculation of the detachment energy of the monoanion and its highest occupied molecular orbital and lowest unoccupied molecular orbital energies, and explicit calculations on the dianion itself are avoided. A good agreement with resonances in the cross section for neutral production in electron scattering experiments is found for n=0, 1, and 2. We find several isomeric structures of NO(2)(-).(H2O)2 of similar energy that elucidate the interplay between water-water and ion-water interactions. The topology is predicted to influence the electron affinity by 0.5 and 0.4 eV for NO(2)(-).(H2O) and NO(2)(-).(H2O)2, respectively. The electron affinity of larger clusters is shown to follow a (n+delta)-1/3 dependence, where delta=3 represents the number of water molecules that in volume, could replace NO(2) (-).