Deep eutectic solvent-assisted extraction of chitosan for polyethylene microplastic coagulation

Microplastic pollution has emerged as an important environmental challenge. Among existing remediation strategies, coagulation-flocculation offers a cost-effective and scalable approach. However, the reliance on conventional chemical coagulants raises concerns regarding sludge generation and secondary environmental impacts. Chitosan has therefore attracted interest as a bio-based coagulant, but the environmental advantages of its use are closely dependent on its extraction process. This study investigates the use of deep eutectic solvents (DESs) as alternative extractants for chitosan production from whiteleg shrimp (Litopenaeus vannamei) shells. Among the studied choline chloride-based DESs (glycerol, malonic acid, glucose and urea), choline chloride-malonic acid (CCMA) produced the highest chitosan yield (19.09 ± 0.22%) and solubility (90.68 ± 0.93%). FTIR, XRD and SEM analyses confirmed the formation of chitosan with semicrystalline characteristics, while SEM analysis revealed a porous, honeycomb-like morphology in the DES-extracted chitosan. Process optimisation using one-variable-at-a-time (OVAT) and response surface methodology (RSM) identified optimal conditions at 93.91 °C, 8.15 h and a L. vannamei:DES ratio of 0.042 g/mL, corresponding to 21:500, yielding a degree of deacetylation of 89.05% with good model fit (R² = 0.9521). The optimised CCMA-derived chitosan was subsequently evaluated for PE coagulation in a controlled deionised-water suspension. After correction against a no-coagulant control, CCMA-CS achieved maximum reductions of 58.10 ± 0.63% in turbidity and 63.55 ± 0.91% in TSS at 50 mg/L. These findings demonstrate that CCMA-assisted extraction can produce chitosan that possesses coagulation performance comparable to that of commercial chitosan under the conditions investigated, although further validation under representative wastewater conditions is required.