Optimisation and economic evaluation of the supercritical carbon dioxide extraction of waxes from waste date palm (Phoenix dactylifera) leaves

Wax extraction has been achieved from an abundant and sustainable waste residue, date palm leaves (or leaflets). Supercritical carbon dioxide extraction was utilised as a clean and efficient extraction technology in this process. Extraction conditions including pressure and temperature have been optimised using response surface methodology (via a 2 × 2 factorial experimental design). The highest yield obtained was 3.49% at 400 bar and 100 °C, which is significantly higher than other reported agricultural residues. The wax obtained at optimum condition (400 bar, 100 °C) exhibited a comparable melting point (78 °C) to carnauba wax, making it a suitable alternative or replacement for this overexploited commercial plant wax. The supercritical process required significantly shorter extraction times (with a 97% isolated wax yield after 120 min), as compared to soxhlet systems which typically required extraction times of up to 5 h to obtain comparable yields. Supercritical carbon dioxide was utilised to tailor the extraction process and obtain waxes with different textural properties, chemical composition and melting profile (35–78 °C). An economic study for the date palm leaf wax extraction estimated a lowest cost of manufacture (COM) at €3.78 kg−1 wax. The low cost of the manufacturing, combined with high wax recovery, the thermal properties of the extract and abundance of this waste resource, makes date palm leaf an attractive feedstock for the natural wax industry.