Green Soapnut Surfactant for the Separation of Fine Particles from Aqueous Solutions using Colloidal Gas Aphrons

Abstract

Fine particles resist sedimentation owing to low mass and settling velocity. Colloidal gas aphrons (CGAs) are surfactant-stabilised microbubbles (10–100 µm) whose multilayer shell offers a high interfacial area for particle attachment and flotation. This study assesses soapnut (Sapindus), a biodegradable plant surfactant, for generating CGAs to separate kaolinite in semi-batch flotation.

Kaolinite was the model particle. Turbidity (0–2990 FTU) was calibrated against concentration (0–1 wt%) to obtain residual concentration and removal efficiency. CGAs were generated from a 5 g/L soapnut solution (pH 5.3) at 2000 rpm and fed at 20 mL/min into a column charged with 1 g kaolinite dispersed in 200, 400 or 800 mL water. A fourth run used the 1 g/400 mL feed, identical to one baseline dilution, with pH raised to 10.26 using NaOH, isolating surface charge at constant dilution.

Removal was fastest in the most dilute feed, reaching 78.6% at 5 min (1 g/800 mL) against 47.4% and 32.6% for the 200 and 400 mL feeds. These early peaks decayed: the 800 mL run fell to 53.4% by 60 min, and the 200 mL run from 67.1% at 10 min to 58.8% at 30 min, consistent with detachment as aphron shells drained. At matched dilution and matched time, alkaline conditioning slowed capture (49.1% versus 60.2% at 30 min), yet removal rose monotonically to 56.5% at 60 min and was still increasing. Time-matched at 60 min, it exceeded the best neutral-pH condition (56.5% versus 53.4%). Post-run column pH (5.28–5.48) indicates the alkaline excursion decayed under acidic CGA feed.

Soapnut CGAs removed up to 79% of fine kaolinite. Dilution maximises early removal but that gain decays, whereas alkaline conditioning slows kinetics yet gives a stable, still-rising recovery overtaking neutral pH by 60 min. Retention time is as critical a design variable as dilution.