Recombination of geminate (OH,eaq−) pairs in concentrated alkaline solutions: lack of evidence for hydroxyl radical deprotonation

Response of nitrifiers to gradually increasing pH conditions in a membrane nitrification bioreactor: Microbial dynamics and alkali-resistant mechanism

Nitrification and nitrifiers are pH-sensitive especially under the alkaline environment in the activated sludge system. However, it is unclear how nitrifiers and nitrification respond to long-term alkaline environment. This study employed a continuous flow membrane nitrification bioreactor to investigate the dynamics of nitrification efficiency and microbial community adaptation under a 320-day alkaline operation. Results showed that activated sludge adapted remarkably to a progressive increase in pH from 7.5 to 10.0, achieving robust nitrification with average ammonia removal efficiencies of 96.6 ± 2.2%. Subsequently, an integrated alkali-resistant mechanism of nitrifiers was proposed. Specifically, under the long-term operation of pH 10.0, certain bacteria secreted enhanced extracellular acidic polysaccharides (i.e., up to 10.95 ± 0.27 mg·g-1 MLVSS in soluble extracellular polymeric substances (EPS)) and acidic organic compounds (e.g., humic acids increased by 1.47-fold in tightly bounded EPS) to neutralize external alkalinity. Moreover, significant enrichments in both the ammonia oxidizing bacteria Nitrosomonas (by 1.3%) and the nitrite oxidizing bacteria Nitrospira (by 5.4%) were observed in a 170-day operation of pH 10.0 condition. Meanwhile, norank_f__JG30-KF-CM45 (2.0%) and Rhodobacter (0.9%) also contributed to ammonia removal at pH 10.0. On the cellular-level, bacteria enabled to maintain intracellular pH stabilization primarily through cation/proton antiporters, evidenced by significant increases in NhaA, TrkA and KefB activities by 98.0%, 151.7% and 115.2%, respectively. A 43.1% increase in carbonic anhydrase activity also facilitated consumption of aqueous OH- ions through biomineralization, leading to CaCO3 deposition on microbial surface. These findings further enhanced understandings of physiological adaptation of nitrifiers in the long-term alkaline activated sludge system.

Exploring Autoionization and Photoinduced Proton-Coupled Electron Transfer Pathways of Phenol in Aqueous Solution

The excited state dynamics of phenol in water have been investigated using transient absorption spectroscopy. Solvated electrons and vibrationally cold phenoxyl radicals are observed upon 200 and 267 nm excitation, but with formation time scales that differ by more than 4 orders of magnitude. The impact of these findings is assessed in terms of the relative importance of autoionization versus proton-coupled electron transfer mechanisms in this computationally tractable model system.

Reactions of Hydrated Electron with Various Radicals: Spin Factor in Diffusion-Controlled Reactions

The reactions of hydrated electron (eaq-) with various radicals have been studied in pulse radiolysis experiments. These radicals are hydroxyl radical (*OH), sulfite radical anion (*SO3-), carbonate radical anion (CO3*-), carbon dioxide radical anion (*CO2-), azidyl radical (*N3), dibromine radical anion (Br2*-), diiodine radical anion (I2*-), 2-hydroxy-2-propyl radical (*C(CH3)2OH), 2-hydroxy-2-methyl-1-propyl radical ((*CH2)(CH3)2COH), hydroxycyclohexadienyl radical (*C6H6OH), phenoxyl radical (C6H5O*), p-methylphenoxyl radical (p-(H3C)C6H4O*), p-benzosemiquinone radical anion (p-OC6H4O*-), and phenylthiyl radical (C6H5S*). The kinetics of eaq- was followed in the presence of the counter radicals in transient optical absorption measurements. The rate constants of the eaq- reactions with radicals have been determined over a temperature range of 5-75 degrees C from the kinetic analysis of systems of multiple second-order reactions. The observed high rate constants for all the eaq- + radical reactions have been analyzed with the Smoluchowski equation. This analysis suggests that many of the eaq- + radical reactions are diffusion-controlled with a spin factor of 1/4, while other reactions with *OH, *N3, Br2*-, I2*-, and C6H5S* have spin factors significantly larger than 1/4. Spin dynamics for the eaq-/radical pairs is discussed to explain the different spin factors. The reactions with *OH, *N3, Br2*-, and I2*- have also been found to have apparent activation energies less than that for diffusion control, and it is suggested that the spin factors for these reactions decrease with increasing temperature. Such a decrease in spin factor may reflect a changing competition between spin relaxation/conversion and diffusive escape from the radical pairs.

Electron Binding Energies of Aqueous Alkali and Halide Ions: EUV Photoelectron Spectroscopy of Liquid Solutions and Combined Ab Initio and Molecular Dynamics Calculations

Photoelectron spectroscopy combined with the liquid microjet technique enables the direct probing of the electronic structure of aqueous solutions. We report measured and calculated lowest vertical electron binding energies of aqueous alkali cations and halide anions. In some cases, ejection from deeper electronic levels of the solute could be observed. Electron binding energies of a given aqueous ion are found to be independent of the counterion and the salt concentration. The experimental results are complemented by ab initio calculations, at the MP2 and CCSD(T) level, of the ionization energies of these prototype ions in the aqueous phase. The solvent effect was accounted for in the electronic structure calculations in two ways. An explicit inclusion of discrete water molecules using a set of snapshots from an equilibrium classical molecular dynamics simulations and a fractional charge representation of solvent molecules give good results for halide ions. The electron binding energies of alkali cations computed with this approach tend to be overestimated. On the other hand, the polarizable continuum model, which strictly provides adiabatic binding energies, performs well for the alkali cations but fails for the halides. Photon energies in the experiment were in the EUV region (typically 100 eV) for which the technique is probing the top layers of the liquid sample. Hence, the reported energies of aqueous ions are closely connected with both structures and chemical reactivity at the liquid interface, for example, in atmospheric aerosol particles, as well as fundamental bulk solvation properties.

Ultrafast dynamics for electron photodetachment from aqueous hydroxide

Charge-transfer-to-solvent reactions of hydroxide induced by 200 nm monophotonic or 337 and 389 nm biphotonic excitation of this anion in aqueous solution have been studied by means of pump-probe ultrafast laser spectroscopy. Transient absorption kinetics of the hydrated electron, e(aq) (-), have been observed, from a few hundred femtoseconds out to 600 ps, and studied as function of hydroxide concentration and temperature. The geminate decay kinetics are bimodal, with a fast exponential component ( approximately 13 ps) and a slower power "tail" due to the diffusional escape of the electrons. For the biphotonic excitation, the extrapolated fraction of escaped electrons is 1.8 times higher than for the monophotonic 200 nm excitation (31% versus 17.5% at 25 degrees C, respectively), due to the broadening of the electron distribution. The biphotonic electron detachment is very inefficient; the corresponding absorption coefficient at 400 nm is <4 cm TW(-1) M(-1) (assuming unity quantum efficiency for the photodetachment). For [OH(-)] between 10 mM and 10 M, almost no concentration dependence of the time profiles of solvated electron kinetics was observed. At higher temperature, the escape fraction of the electrons increases with a slope of 3x10(-3) K(-1) and the recombination and diffusion-controlled dissociation of the close pairs become faster. Activation energies of 8.3 and 22.3 kJ/mol for these two processes were obtained. The semianalytical theory of Shushin for diffusion controlled reactions in the central force field was used to model the geminate dynamics. The implications of these results for photoionization of water are discussed.