Sodium hypochlorite

Endpoint summary

Additional information

Degradation

Hypochlorite is a highly reactive compound, which reacts rapidly in soil and in the sewer with organic matter. In water, there is an equilibrium between hypochlorous acid and the hypochlorite anion at environmental pH.

Biodegradation

Inorganic substances cannot be tested for (ready) biodegradability.

Abiotic Degradation

Hypochlorite decay model in the environment

The kinetic model of Vandepitte and Schowanek shows that hypochlorite is degraded during transport in the sewer within the first minutes. The abundance of reaction partners allows a very quick reaction. For a hypochlorite concentration of 75 mg/L (expressed as free available chlorine, FAC) a concentration at the end of the sewer below 1e-32 µg/L was calculated. The drop of FAC occurs in parallel with a sharp increase of the chloramine concentration, which can be explained by the high availability of ammonia in the sewer. Chloramine further reacts as an oxidant during further transport in the sewer, the STP and in the surface water (river). The extensive degradation of chloramine in the activated sludge can be explained by the presence of reduced organic material. Chloramine is calculated to fall below a concentration of 5e-10 µg/L in the river.

Photolysis in water

In the pH range relevant for environmental biota, the relevant species are hypochlorous acid and hypochlorite anion. Hypochlorite is very sensitive to light. The half life of a 10 - 15 % available chlorine solution at 25 °C is 220 days. The influence of light causes a 3 – 4 fold reduction of this half-life. Direct sunlight causes decomposition and results in the formation of chlorate, chloride and oxygen.

3 ClO- => ClO3- + 2 Cl- (1)

2 ClO- => 2 Cl- + O2(2)

The photolysis half-life of aqueous chlorine (initial concentrations in the range 13-18 mg TRC / L) in clear sky, summer noon sunlit (47°N) water of pH 8 is 12 min when measured at the surface. It increases with decreasing pH due to the decreasing ratio of OCl^-/HOCl to 60 min at pH 5. The pseudo-first-order rate constant for the photolysis of HOCl becomes 2 x 10^-4s^-1and that of OCl^-1.2 x 10^-3s^-1The variation of the rate of photolysis with depth was calculated for water columns exhibiting different light absorption coefficients by taking into account that, for both HOCl and OCl^-, the most effective wavelength for photolysis in sunlight is approx. 330 nm. These results show that in water treatment, chlorine photolysis should be minimized whenever possible by operating at low pH, sun shielding or night-time addition of chlorine or avoiding storage in shallow reservoirs reservoirs (Nowell & hoigné, 1992).

On u.v. (255 nm) irradiation both HOCl and OCl- photolyze at comparable rates and slowly enough that chlorine depletion will not occur during the time of irradiation typical in u.v. disinfection.

Photolysis can also contribute to the depletion of chlorine in atmospheric waters whenever chlorine is formed by (slow) ozonation of chloride.

Soil

In soil, free active chlorine reacts rapidly with organic matter. The ultimate fate of hypochlorite in soil is its reduction to chloride.

Atmospheric degradation

At environmental pH values (6.5-8.5) half of the hypochlorite is in the undissociated form of hypochlorous acid and half is dissociated to the hypochlorite anion. Only the hypochlorous acid fraction is volatile, but the amount of hypochlorous acid that could volatilise from water into air is expected to be very low. The calculated half life (Atkinson calculation) for hypochlorous acid in the atmosphere is 2750 hours, but there are indications that the half life is shorter (only a couple of hours in cloudwater, according to Nowell & Hoigné, 1992)

Photolysis of hypochlorous acid generates atomic chlorine and hydroxyl radicals OH°:

HOCl + hv => HO° + Cl°(3)

Thus, hypochlorous acid could participate in the atmospheric chlorine reaction routes

Summary of environmental degradation

In water, in the sewer and during sewage treatment, the degradation of hypochlorite is modelled by Vandepitte and Schowanek and the concentration is calculated to drop down to “zero” within a few minutes after release into the sewer. In soil, free active chlorine reacts rapidly with organic matter. The ultimate fate of hypochlorite in soil is its reduction to chloride. In the atmosphere, hypochlorous acid degrades photolytically to atomic chlorine and hydroxyl radicals OH° with a calculated half life (Atkinson calculation) of 2750 hours, but there are indications that the half life is much shorter (only a few hours).

Accumulation

Hypochlorite does not bioaccumulate or bioconcentrate due to its high water solubility and high reactivity.

The concentration of hypochlorite in the environment, is modelled by Vandepitte and Schowanek and is estimated to drop down to “zero” within the first minutes after release in the sewer.

Distribution

The following processes are involved in the distribution of hypochlorite in the environment.

• Fraction of substance in air associated with aerosol

• Partitioning between air and water

• Partitioning between solids and water in soil, sediment and suspended matter

Adsorption to aerosol particles

The fraction of substance associated with aerosol particles can be estimated on the basis of the vapour pressure of the substance

Fass_aer= CON_jungex SURF_aer / VP + CON_jungex SURF_aer

Fass_aer= fraction of the substance associated with aerosol particles

CON_junge= constant of Junge equation [Pa x m]

SURF_aer= surface area of aerosol particles [m2 x m3] According to Guidance R.16.4.3.1 as a default the product of CON_jungex SURF_aeris set to10^-4Pa.

VP = Vapour pressure of hypochlorous acid [Pa] = 2500

This results in Fassaer = 4.0 x 10^-7.

Thus, most atmospheric hypochlorous acid is not associated with atmospheric aerosols.

Volatilisation from water

At environmental pH values (6.5-8.5) half of the hypochlorite is in the undissociated form of hypochlorous acid and half is dissociated to the hypochlorite anion. Only the hypochlorous acid fraction is volatile. The measured Henry’s Law constant for hypochlorous acid of 0.097 Pa m³ mol^-1indicates that volatilisation from surface water is not expected to be an important process.

Adsorption onto / desorption from soils

As hypochlorite is a very strong oxidising substance, an adsorption/desorption test is technically not feasible. Hypochlorite would react with organic substance present in the test system and degrades to chloride within minutes. The adsorption coefficient Koc can only be calculated applying QSAR:

An hypothetical Koc can be calculated from Kow through different linear regression equations reported un Guidance (R.7.1.15.3). It can also be calculated by KOCWIN that delivers 2 figures

- using Molecular Connectivity Indices: log Koc = 1.12 (Koc = 13.22 L/kg)

- using regression equation: logKoc = 0.8679 logKow - 0.0004 = -2.9686 (Koc = 0.001075 L/kg)

Hypochlorite as an inorganic substance with an infinite water solubility and very low partitioning coefficients should be considered to be mobile in soil and sediment.

Summary of environmental distribution

The adsorption of hypochlorous acid to aerosol particles, the volatilisation from water into air and the adsorption of hypochlorite onto soil are very low. Thus, hypochlorite remains in the aqueous phase where it degrades very rapidly to chloride.