Optimization of rice husk ash, Moringa oleifera powder, biochar, and okra seed powder for turbidity and color removal in the Turag and Buriganga Rivers using response surface methodology

• Optimized natural coagulants for polluted river water treatment. • Response Surface Methodology improved coagulant performance predictions. • RHA and MOP showed the highest color and turbidity removal efficiency. • Dosage and treatment duration significantly influenced water quality. • Eco-friendly alternative to conventional chemical coagulants. Water pollution from untreated industrial and municipal effluents poses a growing concern in developing countries, particularly in major rivers such as the Turag and Buriganga in Bangladesh. This study evaluates the use of natural coagulants, including Rice Husk Ash (RHA), Moringa oleifera powder (MOP), Biochar (BC), and Okra Seed Powder (OSP), to remove turbidity and color from polluted waters. Using Response Surface Methodology (RSM), optimal coagulant dosages and post-filtration durations were identified. The study also assessed adsorption kinetics using the pseudo-second-order kinetic model. Results revealed that RHA and MOP achieved superior removal efficiencies compared to BC and OSP. For the Turag River, RHA removed 67.38 % of color and 55.75 % of turbidity at an optimal dosage of 20 mg/L and a 2-hour post-filtration duration. MOP achieved 66.26 % turbidity removal and 55.85 % color removal under the same conditions. In contrast, the Buriganga River required higher dosages and longer post-filtration durations due to its higher pollutant load. RHA achieved 61.79 % color removal and 33.87 % turbidity removal at 40 mg/L and 4 h. BC and OSP performed modestly, with maximum color removal efficiencies of 38.39 % and 33.78 %, respectively, under similar conditions. The pseudo-second-order kinetic model (R² > 0.9) confirmed chemisorption as the dominant mechanism. RSM regression models highlighted significant interaction effects between dosage and duration, emphasizing the importance of source-specific treatment strategies. These findings establish natural coagulants as cost-effective, sustainable alternatives to conventional treatment methods. Further research should focus on scalability, long-term impacts, and integration with other technologies to enhance performance in real-world applications.