Kinetics of the reaction between hydrogen peroxide and hypochlorite

In desalination by RO or MSF distillation, brackish water or seawater is chlorinated to prevent biofouling of the equipment. Residual chlorine in discharged brine is a potential pollutant affecting the environment. Dechlorination prior to discharge is recommended. On the other hand, potable water is chlorinated to eradicate harmful microbes. For certain purposes potable water needs to be freed of its residual chlorine. As means of dechlorination, the reaction between hypochlorite and hydrogen peroxide was investigated. The two agents react quantitatively and stoichiometrically according to: ClO- + H2O2 = Cl- + H2O + O-2. The reaction is completed within 10-15 min. In the present investigation a simple gasometric technique is used to study the kinetics of the reaction in the temperature range 15-35 degrees C both in potable as well as in seawater. In both media the reaction is second order. In the presence of one of the two components in excess, the reaction is first order with respect to the other constituent. In potable water the reaction has a constant activation energy of 11.8 kcal/mole. In seawater, on the other hand, activation energies ranging between 3.0 and 13.4kcal/mole are obtained, depending on the ClO- content of the medium. Cost estimate of dechlorination with H2O2, of seawater containing 0.25 ppm residual chlorine resulting from the operation of surface condensers of 10 steam turbines and 16 MSF distillers in the Umm Al Nar power and desalination plant (Abu Dhabi, UAE) was carried out. The figure obtained, US$ 0.065/100 m(3), is much lower than those involving treatment with SO2, a holding surface pond or adsorption on active carbon.