4,4'-Dichloroazobenzene

Using a structure estimation method based on molecular connectivity indices(1), the Koc for 4,4'-dichloroazobenzene can be estimated to be 5100(SRC). According to a classification scheme(2), this estimated Koc value suggests that 4,4'-dichloroazobenzene is expected to be immobile in soil.

4,4'-Dichloroazobenzene's production and use in the dye industry(1) may result in its release to the environment through various waste streams(SRC). Formation of asymmetric and symmetric azobenzenes (e.g. 4,4'-dichloroazobenzene) from substituted aniline pairs in soil or peroxidase reaction mixture may also occur(2).

An estimated BCF of 10 was calculated for 4,4'-dichloroazobenzene(SRC), using an estimated log Kow of 5.4(1,SRC) and a regression-derived equation(2). According to a classification scheme(3), this BCF suggests the potential for bioconcentration in aquatic organisms is low.

The Henry's Law constant for 4,4'-dichloroazobenzene is estimated as 8.1X10-6 atm-cu m/mole(SRC) using a fragment constant estimation method(1). This Henry's Law constant indicates that 4,4'-dichloroazobenzene is expected to volatilize from water surfaces(2). Based on this Henry's Law constant, the volatilization half-life from a model river (1 m deep, flowing 1 m/sec, wind velocity of 3 m/sec)(2) is estimated as 3.5 days(SRC). The volatilization half-life from a model lake (1 m deep, flowing 0.05 m/sec, wind velocity of 0.5 m/sec)(2) is estimated as 44 days(SRC). However, this model underestimates the volatilization half-life of 4,4'-dichloroazobenzene since it does not take into account the effects of adsorption. This is apparent from the results of two EXAMS model runs, one in which the effect of adsorption was considered (half life = 13 months in a model pond) and one in which adsorption was ignored (half life = 21 days in a model pond)(5). 4,4'-Dichloroazobenzene's Henry's Law constant(1) indicates that volatilization from moist soil surfaces may occur(SRC). 4,4'-Dichloroazobenzene is not expected to volatilize from dry soil surfaces(SRC) based upon an estimated) vapor pressure of 6.6X10-5 mm Hg(SRC), determined from a fragment constant method(3).

The rate constant for the vapor-phase reaction of 4,4'-dichloroazobenzene with photochemically-produced hydroxyl radicals has been estimated as 1.1X10-12 cu cm/molecule-sec at 25 °C(SRC) using a structure estimation method(1). This corresponds to an atmospheric half-life of about 15 days(SRC) at an atmospheric concentration of 5X10+5 hydroxyl radicals per cu cm(1). 4,4'-Dichloroazobenzene is not expected to undergo hydrolysis in the environment due to the lack of hydrolyzable functional groups(2).

4,4'-Dichloroazobenzene's production and use in the dye industry may result in its release to the environment through various waste streams. If released to air, an estimated vapor pressure of 6.6X10-5 mm Hg at 25 °C indicates 4,4'-dichloroazobenzene will exist in both the vapor and particulate phases in the ambient atmosphere. Vapor-phase 4,4'-dichloroazobenzene will be degraded in the atmosphere by reaction with photochemically-produced hydroxyl radicals; the half-life for this reaction in air is estimated to be 15 days. Particulate-phase 4,4'-dichloroazobenzene will be removed from the atmosphere by wet and dry deposition. If released to soil, 4,4'-dichloroazobenzene is expected to be immobile, based upon an estimated Koc of 5100. 4,4'-Dichloroazobenzene will not ionize at environmentally relevant pHs. Volatilization from moist soil surfaces is expected to be somewhat of an important fate process based upon an estimated Henry's Law constant of 8.1X10-6 atm-cu m/mole. Hydrolysis is not expected to occur due to the lack of hydrolyzable functional groups. If released into water, 4,4'-dichloroazobenzene is expected to adsorb to suspended solids and sediment in water, based upon the estimated Koc. 4,4'-Dichloroazobenzene was slowly degraded in combined anaerobic sediment-water microcosms. Volatilization from water surfaces is expected to be an important fate process based upon its estimated Henry's Law constant. Estimated volatilization half-lives for a model river and model lake are 5.4 and 44 days, respectively. However, volatilization from water surfaces is expected to be attenuated by adsorption to suspended solids and sediment in the water column. An estimated BCF of 10 suggests the potential for bioconcentration in aquatic organisms is low. Occupational exposure to 4,4'-dichloroazobenzene may occur through inhalation and dermal contact with this compound at workplaces where it is produced or used. (SRC)