Effect of bromide on the chlorination of a polyamide membrane

In this work, the effects of bromide on the chlorination of NF90 polyamide membranes have been investigated by a comparison between chlorination with and without bromide ions. The experimental results showed that chlorination in the presence of bromide ions (from 1 to 200 ppm Br−) makes the membrane simultaneously chlorinated and brominated. Increasing bromide concentration enhanced bromine uptake of the membrane, and shifted the halogenated form of the membrane from chlorinated to mixed and then to fully brominated. Chlorination under the condition of 100 ppm-hr Cl2 increased the ratio of bromine to chlorine content from 0.13 at 1 ppm bromide concentration to 252 at 100 ppm bromide concentration. The bromine, oxidized from bromide ions by the addition of chlorine, outcompeted the chlorine during chlorination. Furthermore, chlorination of the membrane with 100 ppm bromide ions caused more bromination than chlorination with 100 ppm bromine did. Once the chlorine bound to the membrane, the chlorine could not oxidize bromide to bromine. The flux decline of membranes exposed to chlorine with bromide was greater than that of membranes chlorinated without bromide, especially at lower pH. Chlorination of NF90 polyamide membranes with bromide can cause more severe degradation of the membranes.

► Bromide is a natural constituent of water that is often present at ppm concentration.
► Effect of bromide on the chlorination was investigated.
► Chlorination oxidized bromide ions to bromine.
► Bromine converted by the bromide ions outcompeted the chorine.
► Presence of bromide ions caused more severe degradation of a polyamide membrane.