Valeria B. Arce

Photoinduced Degradation of the Herbicide Clomazone Model Reactions for Natural and Technical Systems

The photodegradation of the herbicide clomazone in the presence of S2O82− or of humic substances of different origin was investigated. A value of (9.4 ± 0.4) × 108 m−1 s−1 was measured for the bimolecular rate constant for the reaction of sulfate radicals with clomazone in flash‐photolysis experiments. Steady state photolysis of peroxydisulfate, leading to the formation of the sulfate radicals, in the presence of clomazone was shown to be an efficient photodegradation method of the herbicide. This is a relevant result regarding the in situ chemical oxidation procedures involving peroxydisulfate as the oxidant. The main reaction products are 2‐chlorobenzylalcohol and 2‐chlorobenzaldehyde. The degradation kinetics of clomazone was also studied under steady state conditions induced by photolysis of Aldrich humic acid or a vermicompost extract (VCE). The results indicate that singlet oxygen is the main species responsible for clomazone degradation. The quantum yield of O2(a1Δg) generation (λ = 400 nm) for the VCE in D2O, ΦΔ = (1.3 ± 0.1) × 10−3, was determined by measuring the O2(a1Δg) phosphorescence at 1270 nm. The value of the overall quenching constant of O2(a1Δg) by clomazone was found to be (5.7 ± 0.3) × 107 m−1 s−1 in D2O. The bimolecular rate constant for the reaction of clomazone with singlet oxygen was kr = (5.4 ± 0.1) × 107 m−1 s−1, which means that the quenching process is mainly reactive.

EXAFS and DFT study of the cadmium and lead adsorption on modified silica nanoparticles

Silica nanoparticles of 7 nm diameter were modified with (3-aminopropyl) triethoxysilane (APTES) and characterized by CP-MAS (13)C and (29)Si NMR, FTIR, zeta potential measurements, and thermogravimetry. The particles were shown to sorb successfully divalent lead and cadmium ions from aqueous solution. Lead complexation with these silica nanoparticles was clearly confirmed by EXAFS (Extended X-ray Absorption Fine Structure) with synchrotron light measurements. Predicted Pb-N and Pb-C distances obtained from quantum-chemical calculations are in very good agreement with the EXAFS determinations. The calculations also support the higher APTES affinity for Pb(2+) compared to Cd(2+).

Triplet state of 4-methoxybenzyl alcohol chemisorbed on silica nanoparticles

The knowledge of photochemical kinetics in colloidal systems is important in understanding environmental photochemistry on dispersed solid surfaces. As model materials for the chemically sorbed organic compounds present in natural environments, modified silica nanoparticles (NPs) were obtained here by condensation of the silanol groups of fumed silica nanoparticles with 4-methoxybenzyl alcohol. These particles were characterized by different techniques. To evaluate their toxicity, the inhibition of the natural luminescence emission of the marine bacterium Vibrio fischeri in suspensions of the particles was measured. Laser flash-photolysis experiments (λ(exc) = 266 nm) performed with NP suspensions in acetonitrile-aqueous phosphate buffer mixtures showed the formation of the lowest triplet excited state of the chemisorbed organic groups (λ(max) = 390 nm). DFT calculations of the absorption spectrum of this radical support the assignment. From the calculated triplet energy, a thermodynamically favorable energy transfer from these triplet states to oxygen to yield singlet molecular oxygen is predicted. A value of 0.09 was measured for the quantum yield of singlet molecular oxygen generation by air-saturated suspensions of the nanoparticles in the mixture of solvents acetonitrile-aqueous phosphate buffer. The quantum yield of singlet molecular oxygen generation by the free 4-methoxybenzyl alcohol in the same solvent is 0.31.

The use of molecular probes for the characterization of dispersions of functionalized silica nanoparticles

Butoxylated silica nanoparticles (BSN) were prepared by esterification of the silanol groups of fumed silica nanoparticles with butanol and characterized by 13C and 29Si NMR and thermogravimetry. The molecular probes benzophenone (BP) and safranine-T were used to investigate the BSN suspensions in water:acetonitrile. Laser flash-photolysis experiments at lambda(exc)=266 nm performed with BSN suspended in acetonitrile:aqueous phosphate buffer supported previous results of our group obtained by time-resolved phosphorescence experiments and showed that only free and adsorbed excited triplet states of BP and diphenylketyl radicals contribute to the signals. The UV-vis spectroscopic and photophysical properties of safranine-T are strongly solvent-dependent. Thus, the analysis of the emission spectra and fluorescence lifetimes yielded information on the localization of this probe molecule in suspensions of BSN and of the bare silica nanoparticles. The values of the equilibrium constant for the adsorption of the ground-state safranine-T on the particles were found to be (9.2+/-0.8)x10(4), (7.2+/-0.8)x10(5), and (3.0+/-0.1)x10(4) for the BSN in 1:1 acetonitrile:water, SiO2 in 1:1 acetonitrile:water, and SiO2 in acetonitrile, respectively.

Generation of Chemisorbed Benzyl Radicals on Silica Nanoparticles

Functionalized silica nanoparticles (NP) were obtained by esterification of the silanol groups of fumed silica nanoparticles with benzyl alcohol. These particles were characterized by Fourier transform infrared spectroscopy, 13C and 29Si NMR spectroscopy, thermogravimetry, total organic carbon, Brunauer–Emmett–Teller analysis, UV‐visible spectroscopy, and transmission electron microscopy. NP suspensions in water/acetonitrile mixtures were used as quenchers of benzophenone (BP) phosphorescence in time‐resolved experiments at the excitation wavelength of 266 nm. The phosphorescence signals obtained in the presence of the nanoparticles were fitted to biexponential decays. Both decays were accelerated in the presence of increasing amounts of NP. A model, including the reversible adsorption of BP on the NP, which was supported by computer simulations accounts for the observed results. Laser flash‐photolysis experiments with excitation at 266 nm of NP suspensions in water/acetonitrile in the presence of BP generated benzyl radicals that were attached to the silica surface. These radicals were detected at their absorption maxima (320 nm) by transient optical techniques.