Optimization of Chestnut Shell Extract as a Natural Coagulant for Color Removal in Synthetic Wastewater

The growing pressure on water resources and the demand for sustainable wastewater treatment have driven interest in low-cost, renewable coagulants for industrial effluent remediation. In this context, chestnut shells (Castanea sativa) were explored as a potential source of natural coagulants through tannin extraction, followed by cationic modification using the Mannich reaction. A Box–Behnken Design was employed to assess the effects of chestnut shell mass, extraction pH, and extraction time on methylene blue decolorization in synthetic wastewater. The quadratic response surface model was not statistically significant, indicating that the tested variables did not collectively explain the observed variation within the investigated range. Consequently, the results were interpreted as exploratory observations within the investigated experimental domain rather than as evidence of optimized or predictive operating conditions. Under the tested conditions, methylene blue removal efficiencies ranged from 20 to 26%, and increasing the coagulant dosage improved decolorization, likely through polymeric bridging. Furthermore, this study demonstrated practical regional industrial integration in Northern Portugal by upcycling agricultural waste to mitigate the impact of textile effluents. This engineering approach aligns with the United Nations Sustainable Development Goals (SDG 6, 9, and 12), suggesting a promising pathway for sustainable industrial scale-up.