Natural coagulants from chestnut shells: A sustainable approach for textile wastewater treatment

The textile industry contributes to 2-10% of global greenhouse gas emissions, water extraction, and biodiversity loss, consuming 93 billion cubic meters of water annually with low reuse rates. Coagulation/flocculation is commonly used for industrial wastewater treatment, typically using conventional coagulants. Recently, interest in natural alternatives, particularly tannin-based coagulants, has grown. Given Portugal's role as a major chestnut producer and textile exporter, this study developed chestnut shell-based coagulants for textile wastewater treatment. Ethanolamine (ETA) and diethanolamine (DEA) were tested in the Mannich reaction, but only ETA produced a coagulant with a positive zeta potential and higher charge density. Five coagulants (CE_1-CE_5) were synthesized using different ETA/tannin and formaldehyde (FA)/tannin ratios. Lower ETA concentrations (7.5 mol L^-1) produced the coagulant with the highest charge density and zeta potential. Further testing of FA/tannin ratios (3.5, 6, and 7.5) showed that lower FA levels reduced color removal efficiency and increased toxicity. Comparing synthetic and real textile effluent performance, natural coagulants showed superior color removal, while FeCl₃ was more effective for organic matter and nitrogen removal. All coagulants removed phosphorus, with CE_2 achieving nearly 70% removal. Toxicity tests revealed that only CE_5 inhibited V. fischeri bacteria by over 70%. Formaldehyde leaching into treated water was minimal (0.17-0.3 mg L^-1), below WHO limits (2.6 mg L^-1), but concentrations in sludge were higher, especially in CE_3 and Tanfloc. These findings highlight chestnut shells as a promising source for producing natural, effective coagulants.