Humic substances half life

If released to water, 1,3-DNB and 1,3,5-TNB may be subject to direct photolysis when exposed to sunlight because both compounds can absorb light at wavelengths greater than 290 nm (ERA 1976 Mill and Mabey 1985). However, no data were located regarding the photolysis of 1,3,5-TNB in water. The photolytic half-life of 1,3-DNB in pure water was calculated to be 23 days (Simmons and Zepp 1986). A three-to four-fold increase in the rate of photoreaction of 1,3-DNB was observed in ambient waters containing natural humic substances or in distilled water containing dissolved humic materials compared to reaction without humic substances (Simmons and Zepp 1986). This enhancement of the reaction rate has been attributed to catalysis of the photoreaction by photosensitization effects of humic substances. 

Apparent first-order rate constant phototransformation al X > 285 nm, k = (1.38 0.12) x 102 h 1 in purified water, and k = (1.68 0.12) x 10 2 h 1 in Capot river water with t,/2 40 h (Zamy et al. 2004) Oxidation half-life ranged from t,/2 5 h of midday sunlight during summer to t,/2 = 12 h during winter estimated from kinetic data for oxygenation reactions photosensitized by humic substances in water exposed to sunlight (Zepp et al. 1981) ... 

A pseudo first-order reaction with a half-life of 2 h was deduced for the photooxidation kinetics of anthracene in aqueous media under simulated solar radiation conditions [11]. Bertilsson and Anneli report that photochemical degradation of anthracene and phenanthrene in water proceeds rapidly (half-lives 1 and 20.4 h, respectively) while that of naphthalene (half-life > 100 h) does not [12], In the presence of humic substances, the anthracene degradation rate was unaffected while that of phenanthrene was slowed down considerably. Differences in the spectral absorbance of anthracene and... 

Acifluorfen (5-(2-chloro-o , a, a ,-trifluoro-p-tolyl)-2-nitrobenzoic acid) (IV) mainly undergoes photodecarboxylation upon irradiation at 254 or 313 nm. This compound was exposed to solar light, first in the presence of humic acids (lOmglr1), and then dissolved in a natural water sampled in an artificial lake [68]. No effect of humic acids was observed. The lack of a photo-inductive effect of humic substances can be explained by the fact that acifluorfen bears electron-withdrawing substituents rendering it poorly oxi-dizable by reactive triplet states. In contrast, the half-life of IV was found to decrease by a third in the natural water. This photo-inductive effect might be due to the photogeneration of hydroxyl radicals by chromophores other than humic components of DOM nitrate and nitrite ions are probable candidates. 

PROBABLE FATE photolysis no direct photolysis, half-life from surface waters 3500 hr, indirect photolysis is too slow to be important, photodegradation by hydroxyl radicals will occur with a half-life of 23.8 hrs oxidation not an important process, photooxidation half-life in air 4.7 days-46.6 days hydrolysis too slow to be important under natural conditions, first-order hydrolytic half-life 1163 days volatilization possible, but not important sorption sorption onto particles and biota and complexation with humic substances principal transport mechanism, little adsorption to soil or sediment is expected to occur biological processes bioaccumulation, biodegradation, and biotransformation by many organisms (including humans) are very significant fates... 

Chiron et al. (1995) investigated the photodegradation of bentazone (20 xg/L) in distilled water and Ebro River water using an xenon arc irradiatioa In distilled water, bentazone completely disappeared after 16 hours of irradiation. Photodegradation appeared to follow pseudo-first-order kinetics with a half-life of about 2.5 hours. The presence of humic substances (4 mg/L) increased the rate of photodegradation and the disappearance of bentazone was achieved in 8 hours. No significant breakdown photoproducts were identified. 

A major problem concerns extraction techniques. Humic substances are extremely complex polymers, but they are also relatively inert. They are a major fraction of the soil organic matter but they are retained vdth a half-life of at least 1000 years Hov/ever treatment v ith even mild extract-... 

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