Half-lives with and without

An aqueous solution (15 °C) of atrazine (10 mg/L) containing acetone (1% by volume) as a photosensitizer was exposed to UV light (). = 290 nm). The reported photolysis half-lives with and without the sensitizer were 25 and 4.9 h, respectively (Burkhard and Guth, 1976). Photoproducts formed were hydroxytriazines, two de-A-alkyls, and the de-A/A -dialkyl analogs. 

In a 5-m deep surface water body, the calculated half-lives for direct photochemical transformation at 40 °N latitude. In the midsummer during midday were 5.2 and 4.5 with and without sediment-water partitioning, respectively (Zepp and Schlotzhauer, 1979). 

Surface Water. In a 5-m deep surface water body, the calculated half-lives for direct photochemical transformation at 40 °N latitude, in the midsummer during midday were 3.2 and 13 d with and without sediment-water partitioning, respectively (Zepp and Schlotzhauer, 1979). The volatilization half-life of benzo [a] pyrene from surface water (1 m deep, water velocity 0.5 m/sec, wind velocity 1 m/sec) using experimentally determined Henry s law constants is estimated to be 1,500 h (Southworth, 1979). 

Surface Water. In a laboratory aquaria containing estuarine water, 43% of dissolved carbaryl was converted to 1-naphthol in 17 d at 20 °C (pH 7.5-8.1). The half life of carbaryl in estuarine water without mud at 8 °C was 38 d. When mud was present, both carbaryl and 1-naphthol decreased to <10% in the estuarine water after 10 d. Based on a total recovery of only 40%, it was postulated that the remainder was evolved as methane (Karinen et al, 1967). The rate of hydrolysis of carbaryl increased with an increase in temperature (Karinen et al., 1967) and in increases of pH values greater than 7.0 (Rajagopal et al, 1984). The presence of a micelle [hexadecyltrimethylammonium bromide (HDATB), 3 x 10 M] in natural waters greatly enhanced the hydrolysis rate. The hydrolysis half-lives in natural water samples with and without HDATB were 0.12-0.67 and 9.7-138.6 h, respectively (Gonzalez et al, 1992). In a sterilized buffer solution, a hydrolysis half-life of 87 h was observed (Ferreira and Seiber, 1981). In the dark. 

Miles et al. (1988) studied the rate of hydrolysis of carbaryl in phosphate-buffered water (0.01 M) at 26 °C with and without a chlorinating agent (10 mg/L hypochlorite solution). The hydrolysis half-lives at pH 7 and 8 with and without chlorine were 3.5 and 10.3 d and 0.05 and 1.2 d. 

Chemical/Physical. In an aqueous phosphate buffer solution (0.05 M) containing hydrogen sulfide ion, ethylene dibromide was transformed into 1,2-dithioethane and vinyl bromide. The hydrolysis half-lives for solutions with and without sulfides present ranged from 37 to 70 d and 0.8 to 4.6 yr, respectively (Barbash and Reinhard, 1989). Dehydrobromination of ethylene dibromide to vinyl bromide was observed in various aqueous buffer solutions (pH 7 to 11) in the temperature range of 45 to 90 °C. The estimated half-life for this reaction at 25 °C and pH 7 is 2.5... 

Li and Felbeck (1972) reported that the half-lives for atrazine at 25 °C and pH 4 with and without fulvic acid (2%) were 1.73 and 244 d, respectively. The hydrolysis half-lives in a 5 mg/L fulvic acid solution and 25 °C at pH values of 2.9, 4.5, 6.0, and 7.0 were 34.8, 174, 398, and 742 d, respectively. The only product identified was 2-(ethylamino)-4-hydroxy-6-isopropylamino-5-triazine (Khan, 1978). The primary degradative pathway appears to be chemical (i.e., hydrolysis) rather than microbial (Armstrong et al., 1967 Best and Weber, 1974 Gormley and Spalding, 1979 Geller, 1980 Lowder and Weber, 1982 Skipper et al, 1967). 

Chemical/Physical. Decomposes at elevated temperatures forming methyl isocyanate (Windholz et al., 1983) and nitrogen oxides (Lewis, 1990). Hydrolyzes in water to 1-naphthol and 2-iso-propoxyphenol (Miles et al., 1988). At pH 6.9, half-lives of 78 and 124 d were reported under aerobic and anaerobic conditions, respectively (Kanazawa, 1987). Miles et al. (1988) studied the rate of hydrolysis of propoxur in phosphate-buffered water (0.01 M) at 26 °C with and without a chlorinating agent (10 mg/L hypochlorite solution). The hydrolysis half-lives at pH 7 and 8 with and without chlorine were 3.5 and 10.3 d and 0.05 and 1.2 d, respectively. The reported hydrolysis half-lives of propoxur in water at pH 8, 9, and 10 were 16.0 d, 1.6 d, and 4.2 h, respectively (Aly and ELDib, 1971). In a 0.50 N sodium hydroxide solution at 20 °C, the hydrolysis half-life was reported to be 3.0 d (ELDib and Aly, 1976). 

Kelly et al. intravenously administered 5 mg of THC to eight males and periodically monitored THC, THCCOOH, and THCCOOH-glucuronide conjugates by GC/MS [limit of detection (LOD) 1 ng/ml for THC and THCCOOH] in plasma with and without alkaline hydrolysis for up to 10 h, and then once daily for up to 12 days (Kelly and Jones 1992). The elimination half-lives of THC, THCCOOH, and THCCOOH-glucuronide in the plasma of frequent cannabis users were 116.8 min, 5.2 days, and 6.8 days, respectively, and 93.3 min, 6.2 days and 3.7 days in infrequent users. Conjugated THCCOOH was detected in the plasma of 75% of the frequent and 25% of the infrequent users at day 12. 

Artificial radionuclides intentionally produced for specific applications are usually strictly controlled and confined to safe storage facilities. Unintentional disposal of small quantities is usually without any lasting impact because of their short half-lives with the possible exception of °Co such isotopes are not expected outside controlled areas. 

Table 2. Half-lives (h) of fumigants (0.5 mmol kg ) in Arlington Sandy Loam with and without addition of ammonium thiosulfate (ATS) at 1.0...

Since the radioactive half-lives of the known transuranium elements and their resistance to spontaneous fission decrease with increase in atomic number, the outlook for the synthesis of further elements might appear increasingly bleak. However, theoretical calculations of nuclear stabilities, based on the concept of closed nucleon shells (p. 13) suggest the existence of an island of stability around Z= 114 and N= 184. Attention has therefore been directed towards the synthesis of element 114 (a congenor of Pb in Group 14 and adjacent superheavy elements, by bombardment of heavy nuclides with a wide range of heavy ions, but so far without success. 

Half-lives span a very wide range (Table 17.5). Consider strontium-90, for which the half-life is 28 a. This nuclide is present in nuclear fallout, the fine dust that settles from clouds of airborne particles after the explosion of a nuclear bomb, and may also be present in the accidental release of radioactive materials into the air. Because it is chemically very similar to calcium, strontium may accompany that element through the environment and become incorporated into bones once there, it continues to emit radiation for many years. About 10 half-lives (for strontium-90, 280 a) must pass before the activity of a sample has fallen to 1/1000 of its initial value. Iodine-131, which was released in the accidental fire at the Chernobyl nuclear power plant, has a half-life of only 8.05 d, but it accumulates in the thyroid gland. Several cases of thyroid cancer have been linked to iodine-131 exposure from the accident. Plutonium-239 has a half-life of 24 ka (24000 years). Consequently, very long term storage facilities are required for plutonium waste, and land contaminated with plutonium cannot be inhabited again for thousands of years without expensive remediation efforts. 

---