Chia mucilage as a green biocoagulant for fertilizer wastewater: Process optimization, coagulation mechanism, and...

Fertilizer industries generate effluents with high nutrient and metal loads, requiring effective and sustainable treatment. This study investigated chia mucilage (CM) as a natural biocoagulant for fertilizer-industry wastewater, aiming to replace conventional aluminum sulfate (AS) and support circular-bioeconomy strategies. A 2³ factorial design was applied to evaluate the effects of CM dose (0.05-0.50 g/100 mL), contact time (15-45 min), and pH (3-7) on turbidity, color, chemical oxygen demand (COD), conductivity, and total solids. The optimal condition (0.05 g/100 mL, 15 min, and pH 7) achieved removals of 96.2 % of turbidity, 99.3 % of color, and 85.7 % of COD. Specifically, CM significantly outperformed AS, achieving complete removal of Fe, greater than 92 % removal of P, and greater than 80 % removal of Cu. In contrast, AS achieved 93.30 % removal of Fe, 37.56 % removal of P, and 72.09 % removal of Cu, while maintaining a stable, near-neutral pH. Zeta potential, SEM, DSC, and FTIR analyses indicated that the coagulation-flocculation mechanism was governed mainly by adsorption and polymer bridging mediated by negatively charged carboxylic and hydroxyl groups. SEM and DSC further confirmed the formation of denser and more thermally stable flocs with chia mucilage. Phytotoxicity assays with Lactuca sativa showed that the undiluted treated effluent remained inhibitory; however, toxicity decreased sharply at dilutions of ≥ 75 %, indicating partial detoxification. Overall, these findings highlight chia mucilage as a biodegradable, low-cost, and efficient alternative for treating and valorizing fertilizer effluents in sustainable industrial contexts.