<![CDATA[Water treatment plants (WTP) in Indonesia generally use a physical-chemical process that generates a massive volume of sludge as waste. This sludge is typically discharged into the river without sufficient treatment due to land availability and/or treatment cost constraints. Aim: This study aimed to recycle the alum sludge from a drinking water treatment plant as an adsorbent for removing dye from wastewater. Methodology and results: Alum sludge was activated by several methods including calcination at 400oC and 600oC (AS-CAL400 and AS-CAL600), gelation (AS-GEL), synthesis of TiO2-alum sludge composites (TiO2@AS), and synthesis of ZnCl2-alum sludge composites (ZnCl2@AS). The adsorbents were characterized by SEM-EDS, XRF, BET, and pHPZC. The adsorption assays were carried out in a batch system and used synthetic wastewater containing Rhodamine-B dye as a model contaminant. The result shows that the highest dye removal of up to 87% was obtained using ZnCl2@AS adsorbent, contact time of 2 hours, the adsorbent concentration of 3 g/L, initial dye concentration of 50 mg/L, and pH 2. Conclusion, significance and impact study: The conclusion shows that recycled alum sludge, especially when activated with ZnCl₂, is highly effective for removing Rhodamine B dye from wastewater. It achieved 87% removal efficiency and 26 mg/g adsorption capacity under optimal conditions. The adsorbent has a rough surface with a 128.60 m²/g area, and its adsorption behavior follows the Freundlich isotherm and pseudo-second-order kinetics. Further research including long-term use, and application in real dye wastewater should be conducted to evaluate the feasibility of this adsorbent.]]>
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