Product quantum yields for the 305-nm photodecomposition of nitrate in aqueous solution

Abstract

The radical scavenging technique with product analysis was employed to determine quantum yields for the two main processes (1) N03~+ hv = N02" + O and (2) N03~+ H+ + hv = N02 + OH in dilute, air-saturated solutions of sodium nitrate. Oxygen atoms were detected by their reaction with cyclopentene and the production of ethene. The quantum yield found was ( ) = (1.1 ±0.1) x 10~3. The generation of OH radicals was confirmed by their reactions with benzene to give phenol and with 2-propanol to produce acetone. The second reaction was used to establish an OH quantum yield of ( ) = (9.2 ± 0.4) X 10~3 5. Nitrite was formed with a similar yield in the pH region 5-12, whereas at lower pH the yield declined. Formate as a scavenger converts OH radicals toward 02~/H02 radicals which produce H202. The yield of H202 at pH 5.6 was lower than expected whereas that of NOf was higher. Reactions are postulated to explain this observation as well as the pH dependence of nitrite formation.

Effect of dissolved organic compounds on the photodegradation of the herbicide MCPA in aqueous solution

“…At pH 7.7, which is most significant for surface waters, the quantum yield of MCPA photodegradation without phenol was Φ MCPA = 0.54±0.05 (µ±σ). It compares well with the value of 0.59 obtained by Zertal et al (Warneck and Wurzinger, 1988), and the fact that it is not being formed suggests that negligible production of • OH takes place with MCPA under irradiation. Figure B-SM (i.e.…”

“…Under the adopted elution conditions the retention times were (min): phenol (3.60) and MCPA (5.55), the column dead time being 1.90 min. Acetone was determined by pre-column derivatisation with 2,4-dinitrophenylhydrazine (Warneck and Wurzinger, 1988), followed by elution with 60:40 A:B at 1.0 mL min −1 flow rate. The hydrazone derivative of acetone had a retention time of 7.20 min and the quantification wavelength was 368 nm.…”

The effect of cations on NO2 production from the photolysis of aqueous thin water films of nitrate salts

“…The corresponding average value of F(OH) is (1.1 AE 0.4) Â 10 À2 (see Chu and Anastasio 97 for details of the calculation), which agrees well with past studies reporting a value of B0.01 at room temperature. 82,[90][91][92][93]97 These OH experiments thus suggest that the nature of the cations is not influencing the production of nitrate photo-products in the bulk. However, the bulk-phase measurements were conducted at concentrations that were a factor of 600 (for RbNO 3 ) to 2040 (for NaNO 3 ) times lower than the thin film experiments due to the low solubility of benzene in aqueous solution (23 mM).…”

“…At room temperature the bulk-phase quantum yields are B0.01 for OH production and an order of magnitude lower (B0.001) for O( 3 P) formation (1b) at 305 nm. 82,[90][91][92][93] In smaller water clusters (32-300 water molecules), 86 NO 3 À ions are predicted to be present at the air-water interface. In the interface region, nitrate ions are less solvated than those in the bulk, 86 resulting in increased NO 2 production yields.…”

APEX (Aqueous Photochemistry of Environmentally occurring Xenobiotics): a free software tool to predict the kinetics of photochemical processes in surface waters

“…These data are included in tabular form in an input .csv file, of which some examples for different xenobiotics are provided in ESI with file names of the kind compoundname.csv. These files also contain the molar absorption coefficients of 25 nitrate and nitrite, the wavelength trend of the quantum yield of…”