Synergistic effects in ferric coagulation–flocculation of simulated vegetable oil refinery wastewater

Ferric chloride coagulation–flocculation was systematically investigated using synthetic vegetable oil refinery wastewaters of increasing compositional complexity to elucidate the influence of multi-component matrices on colloidal destabilization mechanisms. Single-, binary-, and ternary systems containing inorganic phosphorus, palmitic acid, and phosphatidylcholine were employed as well-defined model effluents. Single-component systems exhibited narrow and distinct coagulation windows, whereas mixed matrices displayed broader operational ranges and pronounced synergistic effects. In the binary inorganic phosphorus–palmitic acid system, optimal clarification was achieved at pH 5.0 with a Fe/P molar ratio of 2, yielding >99% inorganic phosphorus removal, approximately 88% total organic carbon (TOC) removal, and near-complete turbidity reduction. The ternary system showed a comparable optimal pH range (5.0–5.5) while enabling simultaneous removal of >99% turbidity and inorganic/organic phosphorus, along with >92% TOC and chemical oxygen demand (COD), indicating enhanced robustness relative to simpler matrices. Polymer screening revealed a strong dependence of flocculation performance on wastewater composition. Non-ionic polymers performed best in single-component organic and inorganic systems, whereas medium-charge cationic flocculants were most effective in complex matrices. Accordingly, incorporation of a medium-charge cationic polymer in the ternary system enabled a reduction in ferric dosage (Fe/P = 1.5) while maintaining high clarification efficiency, with polymer doses between 50 and 125 mg L⁻1 providing the greatest benefit. Distinct removal mechanisms can be hypothesized. Palmitic acid removal in single-component systems was primarily governed by charge neutralization, while the presence of inorganic phosphorus promoted iron-mediated hydrolysis of organic matter followed by sweep flocculation. Increasing Fe/P ratios progressively shifted inorganic phosphorus removal from adsorption-dominated to precipitation-controlled pathways, whereas phosphatidylcholine removal was consistently dominated by electrostatic adsorption. Overall, these findings demonstrate that wastewater compositional complexity fundamentally alters ferric coagulation mechanisms and that targeted polymer selection can effectively exploit interfacial synergies in multicomponent contaminant systems.