Hydroxyl radicals soils

Most of the trichloroethylene used in the United States is released into the atmosphere by evaporation primarily from degreasing operations. Once in the atmosphere, the dominant trichloroethylene degradation process is reaction with hydroxyl radicals the estimated half-life for this process is approximately 7 days. This relatively short half-life indicates that trichloroethylene is not a persistent atmospheric compound. Most trichloroethylene deposited in surface waters or on soil surfaces volatilizes into the atmosphere, although its high mobility in soil may result in substantial percolation to subsurface regions before volatilization can occur. In these subsurface environments, trichloroethylene is only slowly degraded and may be relatively persistent. 

It is also possible that COS might be a precursor for CS2 in soil and vegetation chemical analogies have been proposed.12 The reverse reaction, CS2—>COS, occurs in our oxidative atmosphere via hydroxyl radical and other gas phase oxidants. 

Endrin ketone may react with photochemically generated hydroxyl radicals in the atmosphere, with an estimated half-life of 1.5 days (SRC 1995a). Available estimated physical/chemical properties of endrin ketone indicate that this compound will not volatilize from water however, significant bioconcentration in aquatic organisms may occur. In soils and sediments, endrin ketone is predicted to be virtually immobile however, detection of endrin ketone in groundwater and leachate samples at some hazardous waste sites suggests limited mobility of endrin ketone in certain soils (HazDat 1996). No other information could be found in the available literature on the environmental fate of endrin ketone in water, sediment, or soil. 

Apart from the economic significance of such loss there are potentially adverse effects on the environment arising from acidification of rain and soil. Ammonia may react with hydroxyl radicals in the atmosphere to produce NOx contributing to the acidification of rain (4). Wet and dry deposition of NH3/NH4+ inevitably contributes to soil acidification through their subsequent nitrification. This effect can be accentuated in woodland by absorption of aerosols containing NH4+ within the canopy followed by transport to the soil in stem flow (5). In more extreme cases, NH3 emission from feedlots, pig and poultry... 

Bromomethane in air is quite stable, undergoing breakdown by reaction with hydroxyl radicals with a half-life of about 11 months. Bromomethane in other media (water, soil) volatilizes sufficiently rapidly that breakdown in these media (via hydrolysis or reaction with organic components) is usually minor. 

Environmental Fate. The fate of bromomethane in the environment is dominated by rapid evaporation into air, where it is quite stable (EPA 1986b). The rates of volatilization from soil and water have been studied and are known with reasonable precision (although such rates are typically site-specific) (Jury et al. 1984 Lyman et al. 1982). The rates of breakdown by hydrolysis, reaction with hydroxyl radical, and direct photolysis in the stratosphere have also been estimated (Castro and Belser 1981 Davis et al. 1976 Robbins 1976). Further studies to improve the accuracy of available rate constants for these processes would be helpful, but do not appear to be essential in understanding the basic behavior of bromomethane in the environment. 

CASRN 61-82-5 molecular formula C2H4N4 FW 84.08 Soil When radiolabeled amitrole-5- C was incubated in a Hagerstown silty clay loam, 50 and 70% of the applied amount evolved as C02 after 3 and 20 d, respectively. In autoclaved soil, however, no C02 was detected. The chemical degradation in soil was probably via hydroxyl radicals (Kaufman et al, 1968). 

All cresol isomers can be rapidly removed from environmental media. The dominant removal mechanism in air appears to be oxidation by hydroxyl radical during the day and nitrate radical at night, with half-lives on the order of a day. In water under aerobic conditions, biodegradation will be the dominant removal mechanism half-lives will be on the order of a day to a week. Under anaerobic conditions, biodegradation should still be important, but half-lives should be on the order of weeks to months. In soil under aerobic conditions, biodegradation is also important, but half-lives are less certain, although probably on the order of a week or less. 

Little is known concerning the chemistry of nickel in the atmosphere. The probable species present in the atmosphere include soil minerals, nickel oxide, and nickel sulfate (Schmidt and Andren 1980). In aerobic waters at environmental pHs, the predominant form of nickel is the hexahydrate Ni(H20)g ion (Richter and Theis 1980). Complexes with naturally occurring anions, such as OH, SO/, and Cf, are formed to a small degree. Complexes with hydroxyl radicals are more stable than those with sulfate, which in turn are more stable than those with chloride. Ni(OH)2° becomes the dominant species above pH 9.5. In anaerobic systems, nickel sulfide forms if sulfur is present, and this limits the solubility of nickel. In soil, the most important sinks for nickel, other than soil minerals, are amorphous oxides of iron and manganese. The mobility of nickel in soil is site specific pH is the primary factor affecting leachability. Mobility increases at low pH. At one well-studied site, the sulfate concentration and the... 

Except for occupational atmospheres, no information was found in the available literature on concentrations of HDI or HDI prepolymers in air, water, soil, or sediment. Because of the relatively rapid reaction of HDI with hydroxyl radicals in the atmosphere and its high reactivity with water, significant environmental concentrations of HDI are not expected to occur except near emission sources. 

Photolysis degraded photolytically on soil thin films, t,/2 = 13-57 d in artificial sunlight (Tomlin 1994). Oxidation photooxidation t,/2 = 4.2 h in air, based on an estimated rate constant for the vapor-phase reaction with photochemically produced hydroxyl radicals in the atmosphere (Atkinson 1985 quoted, Howard 1991). Hydrolysis neutral hydrolysis rate constant k < 1.5 x lO 5 h 1 with a calculated t,/2 > 700 d in neutral solution and with faster hydrolysis rates in acidic and basic solutions to be expected (Ellington et al. 1987, 1988 quoted, Howard 1991). 

Oxidation rate constant k, for gas-phase second order rate constants, koH for reaction with OH radical, kNQ3 with N03 radical and kQ3 with 03 or as indicated, data at other temperatures see reference photooxidation t,/2 = 1-9.5 h, based on an estimated rate constant for vapor-phase reaction with hydroxyl radical in air (Atkinson 1987 quoted, Howard et al. 1991) t,/2 = 1.7-12 d in soil for pH 1-10 with little change in rate between pH 4.4-10 (Lemley et al. 1988 quoted, Howard 1991)... 

Soil systems present even greater challenges. In these systems, hydro-phobic pollutants can be sorbed to soil in sites that have low accessibility for the hydroxyl radical. Dissolved iron is not likely to enter into hydrophobic soil pores and, therefore, pollutants in these sites can be extremely difficult to degrade. 

Aqueous Fe2+ and many of its coordination complexes serve as excellent catalysts for the formation of hydroxyl radical from hydrogen peroxide. Iron oxyhydroxides have also been found to catalyze the formation of hydroxyl radical [45], although at a much slower rate than dissolved iron. Consequently, a number of researchers have investigated the potential for using soil minerals as catalyst to avoid the need for the addition of soluble iron to the system. 

---