Is Nitrate Anion Photodissociation Mediated by Singlet-Triplet Absorption?

Photoinduced Reactions of Nitrate in Aqueous Microdroplets by Triplet Energy Transfer

In-situ Raman spectroscopy of single levitated charged aqueous microdroplets irradiated by dual-beam (266 and 532 nm) lasers demonstrates that the nitrate anion (NO3-) can be depleted in the droplet through an energy transfer mechanism following excitation of sulfanilic acid (SA), a UV-absorbing aromatic organic compound. Upon 266 nm irradiation, a fast decrease of the NO3- concentration was observed when SA is present in the droplet. This photoinduced reaction occurs without the direct photolysis of NO3-. Instead, the rate of NO3- depletion was found to depend on the initial concentration of SA and the pH of the droplet. Based on absorption-emission spectral analysis and excited-state energy calculations, triplet-triplet energy transfer between SA and NO3- is proposed as the underlying mechanism for the depletion of NO3- in aqueous microdroplets. These results suggest that energy transfer mechanisms initiated by light-absorbing organic molecules may play a significant role in NO3- photochemistry.

Spectroscopy and photochemistry of copper nitrate clusters

The investigation of copper nitrate cluster anions Cu(ii)n(NO3)2n+1-, n ≤ 4, in the gas phase using ultraviolet/visible/near-infrared (UV/vis/NIR) spectroscopy provides detailed insight into the electronic structure of the copper salt and its intriguing photochemistry. In the experimentally studied region up to 5.5 eV, the spectra of copper(ii) nitrate exhibit a 3d-3d band in the vis/NIR and well-separated bands in the UV. The latter bands originate from Ligand-to-Metal Charge Transfer (LMCT) as well as n-π* transitions in the nitrate ligands. The clusters predominantly decompose by loss of neutral copper nitrate in the electronic ground state after internal conversion or via the photochemical loss of a neutral NO3 ligand after a LMCT. These two decomposition channels are in direct competition on the ground state potential energy surface for the smallest copper nitrate cluster, Cu(ii)(NO3)3-. Here, copper nitrate evaporation is thermochemically less favorable. Population of π* orbitals in the nitrate ligands may lead to N-O bond photolysis. This is observed in the UV region with a small quantum efficiency, with photochemical loss of either nitrogen dioxide or an oxygen atom.

Energy partitioning and spin-orbit effects in the photodissociation of higher chloroalkanes

We investigate the photodissociation dynamics of the C-Cl bond in chloroalkanes CH3Cl, n-C3H7Cl, i-C3H7Cl, n-C5H11Cl, combining velocity map imaging (VMI) experiment and direct ab initio dynamical simulations. The Cl fragment kinetic energy distributions (KEDs) from the VMI experiment exhibit a single peak with maximum close to 0.8 eV, irrespective of the alkyl chain length and C-Cl bond position. In contrary to CH3Cl, where less than 10% of the available energy is deposited into the internal excitation of the CH3 fragment, for all higher chloroalkanes around 40% to 60% of the available energy goes into the alkyl fragment excitation. We apply the classical hard spheres and spectator model to explain the energy partitioning, and compare the classical approach with direct ab initio dynamics simulations. The alkyl chain appears to be a soft, energy absorbing unit. We further investigate the role of the spin-orbit effects on the excitation and dynamics. Combining our experimental data with theory allows us to derive the probability of the direct absorption into the triplet electronic state as well as the probabilities for intersystem crossing. The results indicate an increasing direct absorption into the triplet state with increasing alkyl chain length.

Susceptibility of atrazine photo-degradation in the presence of nitrate: Impact of wavelengths and significant role of reactive nitrogen species

The role of reactive nitrogen species (RNS) formed from nitrate photolysis has aroused interests in transformation of contaminants of emerging concern. This study investigated the influence of UV wavelengths (255, 285 and 365 nm) on photolysis of nitrate for degradation of atrazine (ATZ). The UV₂₈₅/nitrate system showed the fastest rate constant for degradation of ATZ with k_obs of 0.0022 cm² mJ^-1. UV photolysis, RNS, and hydroxyl radical (HO) were identified as main contributors to ATZ degradation in UV/nitrate system. Among the contributors, RNS made the major contribution to degradation of ATZ in UV₂₈₅/nitrate system, while HO is the predominant specie in UV₂₅₅/nitrate system. Variance decomposition analysis showed that degradation of ATZ was slightly impacted by natural organic matter and carbonate/bicarbonate in UV₂₈₅/nitrate system but was dramatically affected in UV₂₅₅/nitrate system. Main transformation products of ATZ in UV₂₈₅/nitrate system were identified and possible pathways were proposed. RNS were confirmed to be favorable for acceleration of ATZ photolysis through further reaction of RNS with hydroxyatrazine (with electron-rich moieties). Our study provides deep insights on the influence of UV wavelength on nitrate photolysis and ATZ degradation, and provides a novel method for remediation of water co-contaminated by nitrate and organic contaminants.

The riddle of the forbidden UV absorption of aqueous nitrate: the oscillator strength of the n → π* transition in NO3− including second order vibronic coupling

The environmentally relevant n → π* transition in the nitrate anion is doubly forbidden by symmetry. A simple scheme for including second order vibronic coupling is presented.

Challenges in photocatalytic reduction of nitrate as a water treatment technology

Management of ubiquitous nitrate contamination in drinking water sources is a major engineering challenge due to its negative impacts from eutrophication to immediate risk to human health. Several water treatment technologies exist to manage nitrate pollution in water sources. However, the most widely used technologies are phase separation treatments. In this context, nanoscale photocatalysis emerges as a highly promising transformative technology capable of reducing nitrate to innocuous nitrogen with noticeable selectivity. This critical review describes the photocatalytic reduction mechanisms of nitrate towards undesirable products (nitrite, ammonium) and the more desirable product (dinitrogen). The mechanisms are based on the standard reduction potential of each individual species and highlight the contribution of reducing species (e.g. CO₂^-) radicals formed from different hole scavengers. The strategic use of different pure, doped, and composite nanoscale photocatalysts is discussed on the basis of reduction mechanisms' overall conversion, kinetic rates, and selectivity towards N₂. The choice of light source affects pathways and influences by-product selectivity because direct photolysis of N-intermediates, which has been overlooked in the literature. In addition, the re-oxidation of nitrite and ammonia as drawback process is explained. Finally, an exhaustive analysis presents the photocatalytic reduction applications for treating real water matrices and the competing effect of other species. Overall, this critical review aims to contribute to the understanding of the potential application/constraints of photocatalysis in inorganic nitrogen management, and guide researchers towards future efforts required for widespread implementation.

Phototransformation of Acesulfame K in surface waters: Comparison of two techniques for the measurement of the second-order rate constants of indirect photodegradation, and modelling of photoreaction kinetics

By use of photochemical modelling we show that acesulfame K (ACE) can undergo photodegradation in surface waters, mainly by reaction with OH and possibly ³CDOM* (the triplet states of chromophoric dissolved organic matter). With the possible exception of shallow water bodies containing low dissolved organic carbon, we predict ACE to be a refractory compound in environmental waters which agrees well with many literature reports. We used two methods to measure the photoreactivity parameters of ACE, of which one is based on the monitoring of the time evolution of ACE alone and the other is based on the monitoring of both ACE and a reference compound (hereafter, they are referred to as substrate-only and substrate + reference method, respectively). The substrate + reference method can be time-saving, but it is potentially prone to interferences. In this work, ibuprofen and atrazine were used as reference compounds of known behaviour to study the photoreactivity of ACE by competition kinetics in the substrate + reference method. The two methods gave overall comparable results, partially because two different reference compounds instead of only one were used in the substrate + reference method. By so doing, however, one loses part of the time-saving advantage of the substrate + reference method.