Theodore Mill

Development and evaluation of sunlight actinometers

rn We have developed and used two binary chemical actinometers for sunlight measurements from 300 to 370 nm that have the great advantage of possessing adjustable quantum yields to provide half-lives ranging from a few minutes to several weeks in sunlight. One actinometer is the known p-nitroanisole (PNA)/pyridine (pyr) system in water, the half-life of which in sunlight can be varied from a few minutes to 12 h by changing the concentration of the pyridine. The other actinometer is p-nitroacetophenone (PNAP)/pyr, which has a half-life in sunlight ranging from several hours to 2 months and also can be adjusted by varying the concentration of pyr. Both systems are well behaved kinetically and have quantum yields invariant with wavelength from 313 to 366 nm. Neither PNA nor PNAP is volatile from water or readily sorbed, and each is easily analyzed by UV or HPLC. For PNA/pyr, 4 = 0.44[pyr] + 0.00028 and for PNAP/pyr, 4 = O.O169[pyr].

Free-Radical Oxidants in Natural Waters

Photooxidation of cumene (isopropylbenzene) and pyridine in dilute solution in natural waters gives products characteristic of reactions with alkylperoxy (RO2�) and hydroxyl (HO�) radicals. On the basis of the rates of formation of the products, the average concentrations of RO2� and HO� are estimated to be about 10–9 and 10–17 mole per liter, respectively. The concentration of RO2� is large enough that, for some classes of reactive chemicals, oxidation can be an important process in natural waters.

Photolysis of polycyclic aromatic hydrocarbons in water

Abstract We have studied the photochemical reactions of eight polycyclic aromatics and heteroatom analogs (PAHs). Quantum yields and rate constants in sunlight and at single wavelengths were measured and half-lives of the PAHs as a function of the time of year were calculated. Product studies and the effect of humic acid and oxygen on rates are reported.

Products and quantum yields for photolysis of chloroaromatics in water

Photolysis of chlorobenzene, 2- and 4-chlorobiphenyl, and 2- and 4-chlorobiphenyl ethers in water with 250-300-nm light produce corresponding phenols or, in the case of 2-chlorobiphenyl ether, dibenzofuran exclusively. Quantum yields in most cases are very similar to those reported in hexane for the reduction process. 1,2,4-Trichlorobenzene and 2,3,7,8-tetrachlorodibenzodioxin (TC-DD) photolyze much less efficiently in water than in hexane. A common pathway for photolysis of monochloroaromatics involving aryl cations accounts well for the experimental observations. C-O rather than C-Cl cleavage in TCDD may be a major pathway for its loss. Half-lives for photolysis of these chloroaromatics in sunlight in water range from 460 years to 5 days; TCDD photolyzes in water with a half-life of about 4-5 days in summer at 40/sup 0/ latitude. 40 references, 1 figure, 8 tables.

Predicting photoreaction rates in surface waters

Predicting the rates of photolysis of organic compounds in natural waters is limited by several factors. For direct photolysis of dilute solutions of organic compounds in surface waters, the product of the quantum yield (efficiency) and light absorption rate are needed, but can rarely be calculated from structure activity relations (SARs); direct kinetic constants can be estimated reliably using measured quantum yields and solar models or simple spread sheet data bases to calculate rates as a function of latitude and season. Indirect photoreaction constants can be calculated from measured or estimated rate constants and the average transient photooxidant concentration. SARs for photooxidant rate constants are available for some classes of compounds. The overall indirect photolysis rate for a specific compound is the sum of all significant oxidation reactions for the compound. Quantitation of the rate process requires that a reaction profile be developed for each compound.

Photolysis of nitroaromatics in aquatic systems. I. 2,4,6-trinitrotoluene

Abstract Photolysis of 2,4,6-trinitrotoluene (TNT) occurs rapidly in pure and natural waters irradiated with sunlight, with half-lives of less than a half-hour in some natural waters. Experiments to study the TNT photolysis products and to obtain evidence for complexation of TNT with natural substances are discussed.

Direct observation of phototransients in natural waters

Abstract Nanosecond laser spectroscopic measurements were made on a variety of natural waters containing humic substances. Both short-lived (nanosecond) and long-lived (microsecond) transient absorptions were observed. The long-lived transients exhibited maxima at 475 and 720 nm. The former is tentatively assigned to a triplet state transition while the latter is due to solvated electrons formed on laser excitation. Water samples from widely diverse sources showed similar transient absorption properties as well as fluorescence properties. All transients were shown to depend linearly on laser intensity, indicating that the photoproducts observed are likely to be formed in water sources illuminated by solar radiation having much lower intensities than used here.

RATE CONSTANTS FOR INTERACTION OF 1O21Δg) WITH AZIDE ION IN WATER

Abstract— The rate constant for quenching of 1O2 by azide ion in water was determined to be (5.0 ± 0.4) × 108M−1 s−1 using a variety of sensitizers (including humic acids) and 1O2 acceptors. The apparent second‐order rate constant decreases with pH below pH 5.5 in accordance with the protonation of azide ion to form hydrazoic acid (pKa= 4.6). Quenching by hydrazoic acid is at least 2 orders of magnitude slower than by azide ion. Greater than 99% of all interactions between 1O2 and azide ion involve physical quenching rather than chemical reaction. Humic acid triplets are not significantly quenched by azide ion at concentrations less than 2 mM, allowing azide ion quenching to be used as a diagnostic test for the intermediacy of 1O2 in photosensitized oxidations in natural surface waters.

Structure activity relationships for environmental processes 1: Hydrolysis of esters and carbamates

Abstract Structure activity relationships (SAR) for base-promoted hydrolysis of esters and carbamates have been developed using Hammett and Taft parameters. A master SAR equation correlates k B values for 103 esters with a range of reactivity of 10 10 . Two SAR equations are needed to correlate k B value for 80 carbamates.

E-beam treatment of trichloroethylene-air mixtures: Products and rates

Abstract Electron beam (E-beam) treatment of 3000 ppmv trichloroethylene (TCE) vapor in dry and wet air led to rapid, nearly quantitative, conversion of TCE to dichloroacetyl chloride, plus small amounts of phosgene. Higher E-beam doses, up to 110 kGy, led to oxidation of the initial products to CO, CO 2 , HCl and Cl 2 . The results parallel results found for photo- and Cl-atom initiated oxidation of TCE vapor, and are accounted for by an efficient Cl-atom chain oxidation. Lack of effect of 28,000 ppmv water vapor (90% RH) on rates or products reflects a very high efficiency for the Cl-atom chain oxidation and the very slow reaction of vapor phase water with acyl halides. Irradiation experiments conducted with TCE dissolved in aerated and deaerated water at 10 and 300 ppm showed marked differences in radiolytic products from those found in the vapor phase. A preliminary cost estimate indicates that E-beam treatment of TCE vapor is very competitive with conventional activated carbon treatment and catalytic oxidation.