The free activated chloride is a mixture of Cl2, HOCl, and OCl- where the distribution of the three components is mainly defined by the pH. At pH 2, Cl2 is 50% and HOCl is 50%, where HOCl increases to 100% by pH 6; then decreases to 50% at pH 7.4, at which OCl- becomes 50%; and above pH 9, only OCl- is left.
[pKa (Cl2/HOCl) = 2; pKa (HOCl/OCl-) = 7.4
That is theoretical, but when experimenting electrochemical removal of micro-pollutants in the presence of chloride ions, the pH in the anode is low and can be lower than 2.
Can we account the removal of micro-pollutants to free active chlorine? or will this be depleted because Cl2 will escape as a gas resulting in the dominance of OH radicals instead? Or will this be accounted for with regards to the oxidation power causing the removal of organic matter?
Cl2 is not so insoluble in water that it cannot contribute to degradation of micropollutants.
There are many different micropollutants which likely have very different reactivity towards Cl2(aq) so the actual removal properly varies a lot between the different structures while most organic molecules (except halogenated) reacts at an almost similar rate.
If you have the setup it would be easy to take a sample from the anode and quantify the chlorine concentration and pH.
Another elegant experiment would be to recreate the chlorine dose and pH in a batch experiment there you determine the kinetic of degradation of micropollutants without the possible influence of HO·.
Dear Ali Farhat,
How well chlorine works in water, depends on what the pH level is. At a pH of 8.0, chlorine is only 20% effective! That means green and cloudy water and dangerous bacteria. The higher the pH, the less effective chlorine is. At a pH between 7.4 – 7.6, chlorine works best.
Chlorine is a gas. It does not have a pH value. However, if the chlorine gas is dissolved in water, the following occurs:
Cl2 + H2O → HOCl + HCl
hypochlorous acid (HOCl) and hydrochloric acid are formed. Obviously these are acid solutions, and the pH will be below 7.00. The actual figure will depend on the concentrration.
At pH 0, Chlorine in water is present as Chlorine gas – Cl2. With rising pH, Chlorine reacts with water to form Hypochloric acid - HOCl. In the range of pH 2-6, this is the prominent compound. At higher pH values, Hypochloric acid is neutralized to form its corresponding salt ion, the Hypochlorite ion – ClO-. At approx. pH 9, the neutralisation is complete. The disinfection activity of Hypochlorite is only 1% of that of the Hypochloric acid. As a result of its reaction with water, Chlorine remains in solution for several hours, depending on temperature, circulation etc.
Regards,
Prem Baboo
I think there is a misunderstanding about chlorine solubility. Even chlorine is a gas at room temperature the solubility is not insignificant. In fact it is about 6-7 g/L at room temperature. Therefor you are going to have a significant concentration at the anode.
For comparison disinfection of wastewater is done with concentrations of hypochlor of a few mg/L and many micropollutants are converted in this process.
http://www.engineeringtoolbox.com/gases-solubility-water-d_1148.html
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