Treatment of Electroplating Solution by Oxidation Reduction Method
Introduction
Electroplating solutions are used to deposit a metal coating onto a substrate to prevent corrosion, increase wear resistance, or provide decorative finishes. These solutions can contain a variety of dissolved and suspended materials such as metal ions, acids/bases, and organic contaminants. As electroplating processes are typically used for production of numerous items, large quantities of electrolytes can be generated with potential to cause environmental pollution. All of these components must be removed from electroplating solutions before being discharged into the environment or reused. Oxidation-reduction (redox) treatment is a method used to reduce the concentration of dissolved and suspended contaminants in electroplating solutions and is a cost-effective and reliable method for treating large volumes of wastewater.
Fundamentals of Redox Treatment
Oxidation-reduction reactions are redox reactions involving the transfer of electrons from one species to another. These reactions typically involve the transfer of electrons between oxidizing and reducing agents. An oxidizing agent is a chemical species that gains electrons, while a reducing agent is a species that loses electrons during the reaction. By carefully selecting the desired combination of oxidizing and reducing agents and controlling the pH, redox potential, and other reaction parameters, the concentration of contaminants in electroplating solutions can be reduced.
Types of Redox Treatment
There are several types of oxidation-reduction systems commonly used for treating electroplating solutions. These include the use of hydrogen peroxide, oxygen, ozone, and metal ions. Each of these systems has its own advantages and disadvantages depending upon the type of contaminants and reaction conditions present.
Hydrogen Peroxide
Hydrogen peroxide is a commonly used oxidant in redox treatment due to its ability to be used under mild conditions and its relatively mild toxicity. It is most effective in removing contaminants with low molecular weights such as organic acids and aldehydes. Hydrogen peroxide is typically added to the wastewater solution and reacts with contaminants via a free-radical oxidation mechanism, resulting in their mineralization to more stable products such as carbon dioxide and water.
Oxygen
Oxygen is the most commonly used oxidant for the removal of organic contaminants from electroplating solutions. It is usually added as either compressed or dissolved gas. When dissolved in water, it reacts with contaminants to form products such as carbon dioxide, water, and organic acids. Dissolved oxygen can also be used to oxidize metal ions in solution, although the rate of reaction is slow.
Ozone
Ozone is a highly reactive gas comprised of three oxygen atoms. It is effective in removing organic contaminants from electroplating solutions, due to its high oxidation potential. In addition, ozone can also be used to partially oxidize metal ions, although the rate of reaction is slower than other oxidizing agents.
Metal Ions
Metal ions such as ferric ions can be used as oxidizing agents to remove organic contaminants from electroplating solutions. When added to the wastewater, the metal ions react with the organic contaminants to form more stable products. This type of oxidation mechanism is less efficient than other oxidizing agents, however, and requires longer reaction times.
Conclusion
Oxidation-reduction treatment is a widely used method for reducing the concentration of dissolved and suspended contaminants in electroplating solutions. This method involves the addition of one or more oxidizing agents, such as hydrogen peroxide, oxygen, ozone, and metal ions, to react with the contaminants in the wastewater solution. This method is an effective and cost-efficient method for treating large volumes of wastewater, and due to its mild toxicity and mild reaction conditions, can be safely used for large-scale wastewater treatment projects.
