Response surface methodology optimisation of catanionic surfactant mixtures in de-inking packaging plastic waste

Printing inks impede the quality and economic feasibility of packaging plastic recycling. Conventionally, different single-surfactant systems have been explored for plastic de-inking. Despite the better detergency potential, surfactant mixtures remain unexplored. This study investigates catanionic surfactant mixtures, specifically hexadecyltrimethylammonium bromide (CTAB) - sodium dodecyl sulfate (SDS) systems. A response surface methodology approach was employed to optimise key parameters: temperature (25-55 °°C), pH (10-12), and surfactant concentration (1-25 times the critical micelle concentration, CMC). Experimental results indicated that cationic-rich mixtures (Mix 2:1) achieved up to 100 % de-inking efficiency, comparable to CTAB, while anionic-rich mixtures (Mix 1:2) performed poorly. Statistically, pH was the most significant factor, with strong quadratic and interaction effects. Kinetic modelling demonstrates de-inking follows pseudo-first-order, with CTAB exhibiting a higher rate constant (k = 0.0356 min^-1) than Mix 2:1 (k = 0.0222 min^-1). Surfactant solution pre-heating improved efficiency by 34.49 %. While Mix 2:1 shows promising synergy in de-inking, the study recommends CTAB, optimised at 25°C, pH 12, and 16.5× CMC, as a viable candidate for industrial-scale de-inking. Future directions in plastic deinking should explore bio-based surfactants, solid-liquid ratio optimisation and wastewater treatment. This work advances circular economy initiatives by improving recyclate quality and reducing plastic waste contamination.