Explain the bleaching action of ozone

Ozone is a powerful oxidizing agent, and its strong reactivity makes it both highly effective and challenging to use for bleaching wood pulp. It readily reacts with unsaturated sites, including those in aromatic rings, enabling pulp to be bleached without chlorine-based chemicals, thus supporting totally chlorine-free (TCF) bleaching processes.

Physical Properties of Ozone

• Ozone is blue in color.
• It is slightly soluble in water but highly soluble in non-polar solvents such as carbon tetrachloride and fluorocarbons, where it forms a deep-blue solution.
• It has a sharp, distinctive odor and can be detected even at concentrations as low as 0.01 ppm in air.

Bleaching and Oxidizing Action

Because of its ability to release nascent oxygen, ozone acts as both a mild bleaching agent and a sterilizing agent. Its rapid oxidizing action makes it effective in decolorizing vegetable dyes and pigments.

Reaction:

Vegetable colouring matter + Ozone → Oxidized coloured matter (Colourless)

For example, ivory, litmus, and delicate fabrics can be bleached effectively with ozone.

Important Considerations

• Ozone is toxic even at low concentrations and explosive at high levels; hence, its concentration in air must be continuously monitored.
• Factors such as temperature, pH, pulp consistency, and mixing directly influence the efficiency and selectivity of ozone bleaching.

Note: The bleaching action of ozone is based on oxidation, whereas that of sulphur dioxide is based on reduction. Sulphur dioxide provides only temporary bleaching, while ozone’s nascent oxygen ensures a permanent bleaching effect.