Hello. I conducted batch leaching tests with samples of construction and demolition waste with adhered paints to assess the influence of the paint on the release of contaminants in the leachate. I observed that in all samples with water-based enamel paints, the pH of the leachate was significantly lower compared to the samples painted with acrylic paints. What could explain this decrease in pH?
Water-Based Enamel Paints typically contain alkyd resins, which are modified with fatty acids and other organic components. During the leaching process, these components can hydrolyze and release organic acids into the leachate, thereby lowering the pH. Alkyd resins, when exposed to water, tend to break down into smaller organic acids such as acetic acid and formic acid, which are weak acids but sufficient to reduce the pH of the leachate....in other words: The fatty acids released during hydrolysis are weak acids, such as acetic acid (CH3COOH) and formic acid (HCOOH). These acids dissociate in water, releasing hydrogen ions (H+), which lower the pH of the leachate....
And the difference with Acrylic paints are... They are composed primarily of acrylic resins, which are more stable and less prone to hydrolysis compared to alkyd resins. Acrylic resins form a stable polymer network that does not readily degrade in water, leading to a less acidic leachate. The additives in acrylic paints, such as surfactants and coalescing agents, are typically neutral or slightly alkaline, which can help to maintain a higher pH.
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