Bleaching Agents: Types, Mechanism, Application

Bleaching agents are the chemicals that lighten or whiten substances by dissolving colored substances or by changing its light-absorbing properties. Bleaching agents are commonly used in the paper and pulp industry, the textile industry, and the commercial and household laundering. The bleaching reactions usually involve oxidative or reductive processes that degrade the color systems. These processes may involve the destruction or the alteration of the chromophoric groups in the substrate as well as breaking down the color bodies into smaller, more soluble units that are easier to remove during the bleaching process. Some of the examples of bleaching agents include hydrogen peroxide, chlorine, and sodium hypochlorite.

Bleaching Agents
Bleaching Agents

Types of Bleaching Agents

Bleaching agents can be broadly categorized into two main groups. They are as follows:

  1. Oxidizing Bleaching Agents
  2. Reducing Bleaching Agents

The brief description of these groups of bleaching agents is given below:

Oxidizing Bleaching Agents

Oxidizing bleaching agents are the substances that donate oxygen atoms or accept electrons during the chemical reaction, leading to the removal or alteration of the color compounds. During the bleaching process, these bleaching agents oxidize the pigments and stains by breaking them down and by reducing or eliminating the color effectively. Some of the common examples of oxidizing bleaching agents include hydrogen peroxide and chlorine-based compounds like chlorine gas and sodium hypochlorite.

Hydrogen Peroxide

Chemical Formula: H2O2

Physical Properties: Colorless liquid resembling water. Soluble in water.

Applications: Used in textile bleaching, hair bleaching and wound disinfection.

Environmental Consideration: Decomposes into water and oxygen, minimizing the environmental impacts. Considered as a more eco-friendly bleaching option.

Safety Precautions: As higher concentrations can be corrosive, it should be handled with care. Commonly used in its diluted forms.

Sodium Hypochlorite

Chemical Formula: NaClO

Physical Properties: A pale yellowish-green liquid. Soluble in water.

Applications: Used in household bleach for laundry and disinfection. Used in the treatment of drinking water.

Environmental Considerations: Disposal should be done carefully.

Safety Precautions: Corrosive in nature, thus, it requires careful handling.

Chlorine gas

Chemical Formula: Cl2

Physical Properties: Greenish-yellow color, Pungent odor. Heavier than air

Applications: Used in water treatment for disinfection. Used in the paper and pulp industry for bleaching.

Environmental Considerations: Requires careful consideration during its disposal. Contribute to the formation of chlorinated organic compounds in water.

Reducing Bleaching Agents

Reducing bleaching agents are the substances that act as the electron donors during a chemical reaction, leading to the alteration or the removal of the color compounds. Unlike oxidizing agents, reducing agents add electrons to the chromophores which results in a reduction of color intensity. Some of the common examples of reducing bleaching agents include sodium dithionite and sodium bisulfite. During the bleaching process, these bleaching agents achieve their bleaching effect by adding electrons to color compounds, resulting in a reduction of color intensity or the removal of color altogether.

Sodium dithionite

Chemical Formula: Na2S2O4

Common Name: Sodium dithionite, also known as sodium hydrosulfite

Physical Properties: White, crystalline powder. Soluble in water

Applications: Used in textile industries for bleaching and color removal. Used in the pulp and paper industry as a bleaching agent

Decomposition: Decomposes in the presence of heat and in alkaline conditions. Releases sulfur dioxide and water.

Environmental Considerations: Biodegradable and less environmentally harmful than other bleaching agents

Safety Precautions: It can release irritating sulfur dioxide, thus, should be handled carefully.

Sodium Bisulfite

Chemical Formula: NaHSO3

Physical Properties: White crystalline powder. Soluble in water

Applications: Used in the food industry for preserving freshness and preventing discoloration. Used in the textile industry for the bleaching process and for color removal.

Environmental Considerations: Biodegradable under certain conditions. Less harmful than other bleaching agents.

Safety Precautions: There may be potential release of sulfur dioxide, so, it should be handled with care.

Mechanisms of Bleaching

The mechanisms of bleaching involve the alteration or the removal of the colour compounds. They differ between the oxidizing and the reducing agents, which is described below separately:

Oxidizing Bleaching Agents

The oxidizing bleaching agents involve the following processes:

  • Oxidation Reaction:

The oxidizing agents like hydrogen peroxide, sodium hypochlorite, etc. facilitate the oxidation reactions. They accept electrons or donate oxygen by breaking down the colour compounds through chemical reactions.

  • Chromophore Alteration:

These oxidizing agents modify the chemical structure of the colour-producing groups i.e. chromophores by introducing oxygen. This modification diminishes their capacity to absorb the light, which ultimately reduces the colour intensity.

Reducing Bleaching Agents

The reducing bleaching agents involve the following processes:

  • Reduction Reaction:

The reducing agents like sodium dithionite or sodium bisulfite facilitate the reduction reactions by donating the electrons. This process breaks down the color compounds in such a way that reduces their color intensity.

  • Electron Addition:

The reducing agent adds electrons and modifies the colour-producing groups i.e. chromophores. This process breaks down the structure of chromophore, resulting in a reduction of colour intensity.

 To sum up, the ultimate goal in both of the cases is to alter the chemical structure of the colour compounds, making them less noticeable or eliminating colour effectively. The selection between the oxidizing and the reducing agents is determined by the nature of the material undergoing treatment and the desired result.

Applications of Bleaching Agents in Industries

Bleaching agents are widely used in the textile industry, the paper and pulp industry and in water treatment. Their applications are described below in brief:

Textile Industry

Bleaching agents play a very crucial role in textile industry. They help to remove the impurities, stains, and natural color from fibers, yarns, and fabrics, thus play a pivotal role in enhancing the quality and appearance of fabrics. Bleaching agents are also used to prepare textiles from dyeing or printing processes, which ensures that the fabric is a neutral color and is ready to absorb the desired dyes or pigments. Ultimately, the strategic use of bleaching agents in the textile industry contributes to the production of brighter, cleaner and high-quality fabrics.

Paper and Pulp Industry

Bleaching agents play a pivotal role in the paper and pulp industry. In this industry, bleaching agents are used to whiten and brighten the pulp, which is the raw material for the production of paper. Bleaching agents also help to remove lignin i.e. a natural substance that gives a colour for the wood. By removing lignin, bleaching agents help to improve the clarity and the brightness of the pulp which results in the production of high-quality white paper.

The paper and pulp industry also utilizes bleaching agents to enhance the brightness of recycled papers. Overall, the use of bleaching agents in the paper and pulp industry contributes to the production of high-quality pulp with desired level of whiteness and purity.

Water Treatment

Bleaching agents play a vital role in water treatment which ensures public safety. They are primarily used to disinfect and purify water by killing or inactivating harmful microorganisms, bacteria and viruses. The chlorine-based components like chlorine gas or sodium hypochlorite are commonly used for water disinfection. Chlorination is the key process which involves the addition of chlorine in required amount in water. This results to the chemical reaction that disrupts the cellular structure of the microorganisms and makes them harmless. Overall, bleaching agents help to ensure that the water is safe for drinking and for other uses, as they eliminate the potential pathogens and contaminants.

Environmental Impacts of Bleaching Agents and Safety Measures

The environmental impacts of the bleaching agents and the safety measures should be taken into consideration in various industries. Its brief description is given below:

Environmental Impacts

  • Chemical Release:

Bleaching agents especially chlorine-based compounds, can release harmful chemicals during the bleaching process that can be hazardous to human and animal health when released into the air or water bodies. Chlorine has a high toxicity level that damages the aquatic lives. It can also alter the pH balance of water, making it unfit for the aquatic creatures.

  • Biodegradability:

The biodegradability of bleaching agents is a factor evaluating their environment impact. The agents that break down into non-toxic components are considered as environmentally friendly.

  • Global Warming:

It can also contribute to global warming by releasing chlorofluorocarbons (CFCs) into the atmosphere.

  • Pollution:

Bleaching agents can be hazardous to the environment when used in excess, resulting to water and soil contamination as well as causing air pollution. It may cause potential risks to the natural ecosystem.

  • Energy Consumption:

Some of the bleaching processes may involve high energy consumption, thus increasing the overall carbon footprint of the manufacturing process.

Safety Measures

The following safety measures should be taken into considerations while using the bleaching agents:

  • Toxicity:

Some of the bleaching agents such as chlorine gas are highly toxic and can cause potential risks to human health. Thus, the strict safety protocols, proper handling, and protective equipment are required to minimize the risk of exposure.

  • Corrosiveness:

Some of the bleaching agents can be corrosive which may cause damage to skin, eyes, or equipment. Thus, personal protective equipment (PPE) is required to prevent from accidents.

  • Storage and Handling:

The improper storage and handling of the concentrated bleaching agents can lead to accidents. Therefore, those substances should be stored and transported following the industry guidelines to mitigate potential risks.

  • Worker Exposure:

The workers in the industries are more likely to face risks. So, they should be provided with proper training and proper ventilation and safety equipment’s must be used in the working areas to safeguard workers health.

Future Trends and Developments

The recent global bleaching agent market is experiencing several emerging trends. Firstly, there is a growing demand for natural and eco-friendly bleaching agents which is driven by the increasing of consumer awareness about the environmental impact of chemicals. Additionally, there is a rising demand of the bleaching agents derived from the natural sources due to the popularity of clean labelled products.

Another trend is the increasing use of bleaching agents in the health care sector for the applications such as sterilization and disinfection. Furthermore, there is an implementation of several strategies to minimize the wastes generated during the bleaching process, addressing the environmental concerns and contributing to a circular economy. Apart from this, technological advancements, such as the development of new and efficient bleaching agents, are expected to create significant opportunities in the market.

These future trends and developments aim to promote a more sustainable and environment friendly approach to bleaching processes across various industries. Overall, these trends are fundamentally reshaping the dynamics of the global bleaching agent market.

References

About Author

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Kabita Sharma

Kabita Sharma is a graduate student from the central department of chemistry, Tribhuvan University. She has been actively involved in research related to natural products, computational chemistry, and nanochemistry. She is currently working on enzyme assay, molecular docking, and molecular dynamic simulation.

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