Production, Characterization and Performance Evaluation of Modified Crab Shell Chitosan for Sustainable Coagulation of Abattoir Wastewater

Abattoir wastewater is among the most heavily polluted agro-industrial effluents, and conventional coagulants such as alum and ferric chloride raise concerns regarding residual metal toxicity and non-biodegradable sludge. This study developed a chitosan-based coagulant derived from crab shell waste and cross-linked with epichlorohydrin (CL-CS) for sustainable abattoir wastewater treatment. Extraction was optimised using a Box-Behnken design, achieving a degree of deacetylation of 88.2% at 50% NaOH and 120 °C. FTIR, XRD, TGA, BET and zeta potential analyses confirmed successful deacetylation and cross-linking, with CL-CS exhibiting reduced crystallinity (38.2%), higher surface area (24.3 m²/g), improved thermal stability (Tpeak = 315 °C) and stronger cationic charge (+41.7 mV at pH 6) than native chitosan. Coagulation was optimised using a central composite design, yielding 92.7% turbidity removal and 78.8% COD removal at a 60 mg/L dose, pH 4.0, 250 rpm rapid mixing, 40 rpm slow mixing and 60 min settling time (R² = 0.985). Coagulation followed second-order kinetics (K = 5.84 × 10⁻⁴ NTU⁻¹ min⁻¹), with a floc fractal dimension of 2.34, indicating dense, polymer-bridged aggregates. CL-CS outperformed alum and ferric chloride in turbidity, COD, TSS and oil/grease removal, while producing the lowest sludge volume (41.8 mL/L), leaving no residual metal contamination and achieving the lowest unit treatment cost ($0.079/m³). Pearson correlation confirmed the degree of deacetylation and zeta potential as the strongest predictors of performance (r > 0.91). These findings indicate that CL-CS is a promising low-cost and environmentally sustainable alternative to conventional coagulants for abattoir wastewater treatment.