Factors affecting electrolytic treatment of wastewater containing Direct Red 83, a copper-complexed azo dye

Dyes are the first industrial contaminant to be recognized in wastewater due to their high visibility at very small concentrations, but they can be resistant to biological treatment because of their synthetic origin and complex aromatic molecular structures. Increasingly more stringent legislation regarding the removal of dyes from industrial effluents is motivating research into non-biological ways to treat these compounds.; The research described in this dissertation was performed to determine if electrolytic treatment is applicable to the removal of color from wastewater streams containing a copper-complexed direct dye, Direct Red 83. Once the applicability of electrolytic treatment was established, then factors that affect the rate of color removal and design of a practical process, such as voltage required, current density, electrode materials, electrolyte composition, and dye structure were explored in more detail.; The impact of the amount of chloride ion in the supporting electrolyte and the impact of dye structure were investigated in particular. The rate of dye decolorization was found to increase with increasing chloride concentration to a point where the rate of electron transfer became controlling. This increase in rate occurred because chloride not only promotes current flow for direct oxidation, but reacts to form oxidized chlorine species responsible for indirect oxidation of the dye. The rate no longer increased with chloride concentration at 0.04 N sodium chloride (with 0.01 N sodium sulfate) for a 10 μM solution of Direct Red 83, treated at 5 mA/cm2 current density and ∼2.7–2.8 V. The first order rate constant was about 2.7 × 10−4sec −1, if the samples were allowed to complete the indirect chemical reactions after collection compared to ∼4 × 10−5 sec−1, when no chloride ion was present in the system.; Dye structure had an impact on decolorization rate as well. The presence of copper in the dye slowed the rate and protected the dye somewhat from continuation of indirect oxidation after cessation of current flow, while methoxylation of one of the hydroxy groups adjacent to the azo bond reduced the rate even more than copper complexation, but did not protect the azo bond from continued indirect oxidation.