Removal of Indigo Dye by CaCO₃/Ca(OH)₂Composites and Resource Recovery

Indigo dye wastewater from blue denim-manufacturing industries poses a significant threat to the environment if not adequately treated before discharge to water bodies. Indigo dye residues in the process effluent have complex molecular structures relatively stable to light and heat and resistant to biodegradation, making affordable and straightforward removal difficult. We demonstrate indigo dye removal using a low-cost, nontoxic, biodegradable, readily regenerated common adsorbent made of the CaCO3/Ca(OH)2 composite. The optimized catalyst loadings at 20 g/L for sulfur and non-sulfur indigo dye mixtures (MIX-ID) affirm 13% chemical oxygen demand (COD), 68.5% dissolved organic matter (as UVA254), and 98.4% color (indigo) removal. However, the non-sulfur indigo dye was tested with 10 g/L of the adsorbent, giving 45.4% COD, 74.6% dissolved organic matter (as UVA254), and 99% color (indigo) removal. The removal of indigo dye was compared to ozone- and peroxone-based advanced oxidation processes. Ozonation was ineffective at oxidizing the effluent, and even though the peroxone process was able to remove the color, the process increased COD. A regeneration pathway was proposed to recycle the used adsorbent and allow for resource recovery by separating pure indigo dye and reusable effluent for the textile process. The catalyst was reused three times to remove the dye from textile effluents without any loss of efficiency.