Hydroquinone

Phototransformation in water

Description of key information

Based on an oxidation rate constant for the reaction of hydroquinones with alkylperoxy radicals (RO2•) of  1*10^6 (M*sec)^(-1) and an average environmental concentration of RO2• near surface of 1*10^(-9) M, the half-life of hydroquinone in sunlight exposed natural waters is calculated to be 12 minutes. 
Considering turbidity and decreasing light intensities in deeper water layers,  a 100-times lower concentration of  [RO2•] is assumed, leading to a worst-case estimate for the half-life of hydroquinone in surface waters of 20 hours based on photooxidation with RO2• alone (leaving behind other oxidative degradation events as well as biodegradation). 
This value is confirmed by the OECD SIDS document (2002) on hydroquinone, which states a half-life in surface water of 20 hours.

Key value for chemical safety assessment

Half-life in water:

20 h

Additional information

Key study on indirect phototransformation (Mill et al.): When natural organics present in many water bodies (e.g. humic acids) undergo photolysis, they generate free radicals such as alkylperoxy (RO2•) or hydroxyl (HO•) and other oxidants such as singlet oxygen, and these oxidants contribute to the transformation of some synthetic chemicals in water (one kind of indirect photolysis).

Chemical oxidation kinetics are outlined by Mill and Mabey (1985). Concluding from the kinetic equations, the half-life of a compound in water with regard to chemical oxidation is determined by the sum of all photo-oxidation processes occurring in parallel. The half-life with regard to each single contributing process is determined by the concentration of oxidant species in water (assumed to be constant during observation phase) as well as the specific rate constant for the reaction of this oxidant species (e.g. alkylperoxy radicals, RO2•) with the specific target compound class (e.g. hydroquinones). In conclusion, not the slowest process is determining kinetics of photodegradation; rather, the slowest process is contributing least and degradation kinetics of a compound will be largely determined by the fastest process. Thus, as a prerequisite for calculation of half-lives, besides the rate constant for hydroquinone the concentration of the oxidant(s) present in natural waters must be known.

Using cumene and pyridine as probes, Mill et al. (1980) could demonstrate that 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 determined in pure waters with defined radical species, the average concentrations of RO2• and HO• in natural waters were estimated to be about 10^(-9) and 10^(-17) mole per liter, respectively. The authors concluded that the concentration of RO2• is large enough that, for some classes of reactive chemicals, oxidation can be an important process in natural waters. The estimated value for [RO2•] in sunlight exposed natural waters was 5*10^(-9) M on average. From the fact that this concentration was determined using experiments based on two different probes (namely cumene and pyridine) and results being in good agreement the authors concluded this value to be precise within a factor of 5. Further, the authors stated this concentration to be valid for filtered water in thin layers, only. As natural waters will contain particulates, and depths of more than 1 or two meters may be relevant, this concentration must be considered as upper bound.

Thus, 3 calculations are performed,

- (1) one assuming a concentration of [RO2•] in sunlight exposed natural waters of 1*10^(-9) M (surface region)

- (2) one assuming a 10-times lower concentration of [RO2•] in sunlight exposed natural waters of 1*10^(-10) M (deeper layers, considering particulate matter)

- (3) one assuming a 100-times lower concentration of [RO2•] in sunlight exposed natural waters of 1*10^(-11) M (deep layers, considering particulate matter)

While HO• radicals will also contribute to photooxidation of hydroquinone, due to the much lower concentration in natural waters this is not considered here (conservative assessment).

With an oxidation rate constant for the reaction of hydroquinones with [RO2•] of 1*10^6 (M*sec)^(-1), the following half-lives result according to (1) and (2) using equations as given by Mill and Mabey (1985):

Half-life (1): 11.55 minutes;

Half-life (2): 1.9 hours;

Half-life (3): 19.25 hours, equivalent to 1.9 days (10h-day)

As a very conservative estimate, a half-life of 20 hours will be assumed as a worst-case estimate for the half-life of hydroquinone in surface waters based on photooxidation with RO2• alone (leaving behind other oxidative degradation events as well as biodegradation). This value will be used for risk assessment of hydroquinone.

In a reliable supporting study (Tissot et al, 1985), direct phototransformation (photooxidation) of hydroquinone is demonstrated in water in the presence of oxygen at 295 +/- 5 nm over 22 hours. The time course of degradation was monitored by determination of pH and HPLC analysis. To evaluate aquatic toxicity of the mixture of reaction products, acute toxicity (24 hours EC50) to Daphnia magna was determined at 0, 0.5, 4, and 22 hours.

Hydroquinone exhibits a relevant absorption in the spectral range of natural sunlight:

_{ᵋ= 1100 mol^(-1) l cm^(-1) at 300 nm; with a Quantum yield in aerated solution of 0.03. The reaction quantum yield is independent of the concentration in the concentration range between 10^(-4) to 2*10^(-2) mol/L (concentration applied in this test: 6.1*10^(-4) M (67.1 mg/L)).}

Hydroquinone is photo-oxidized at 300 nm (High pressure Hg lamp, 200 W, monochromator) in aqueous solution (no buffers used) forming p-benzoquinone, trihydroxybenzene and finally hydroxy-p-benzoquinone as products. The EC50 (24 h; Daphnia magna) increases due to phototransformation from initially (no irradiation) 0.15 mg/L to a final (after 22 h of irradiation: 80% of hydroquinone phototransformed) of 0.5 mg/L by a factor of 3.3. This means a reduction in acute Daphnia toxicity by a factor of 3.3 due to photooxidation events (direct phototransformation).

In conclusion, direct phototransformation (photooxidation) of hydroquinone in the spectral range of natural sunlight (300 nm) was completed by 80% within 22 hours and leads to transformation products of considerable less ecotoxicity (by a factor of 3.3 in terms of acute immobilization of Daphnia magna) than the parent hydroquinone.

The work of Perbet et al. (1979) formed the mechanistic basis for the study by Tissot et al. (1985) outlined above:

Direct phototransformation induced photooxidation events were mechanistically analyzed for diphenols including hydroquinone. On irradiation of a dilute aqueous solution of hydroquinone with UV-light (253.7 nm) hydroquinone is transformed into hydroxy-p-benzoquinone as result of photolysis and photo-oxidation reactions. Intermediates identified were semiquinonic radicals, p-benzoquinone, and 1,2,4-trihydroxybenzene.

A further supporting study (Knoevenagel and Himmelreich, 1976) demonstrates the potential of UV-light to photooxidize hydroquinone to CO2 in aqueous solution. Direct photooxidation of hydroquinone to CO2 by UV light at a temperature between 90 and 95°C is reported:

25% of theoretical CO2: after 10.3 h;

50% of theoretical CO2: after 22.9 h;

75% of theoretical CO2: after 43.7 h;

The authors point out that the degradation velocity depended on the height of the temperature. They further state that the degradation also occurred if the light of the UV lamp was filtered through Duran-glass, and it occurred in sunlight or diffuse daylight but with a lower degradation velocity.