Synergistic effects of binary surfactant mixtures in the removal of Cr(VI) from its aqueous solution by foam fractionation

Foam fractionation is an adsorptive bubble separation process and it has been regarded as a cost-effective unit operation of wastewater treatment. The objective of this work was to investigate the synergistic effects of binary surfactant mixtures in the removal of chromium ion Cr(VI) from its aqueous solution by foam fractionation. In order to reduce the dosage of chemical surfactant, a binary surfactant mixture was prepared by mixing sapindus saponin (SS) and cetyl trimethyl ammonium bromide (CTAB). First, the optimum mass ratio of SS to CTAB was determined by measuring surface activity and foam properties of the mixture solution. Moreover, the experimental results indicated that the attachment of Cr(VI) on the gas-liquid interface contributed to improving foam drainage, resulting in a low liquid holdup. Under the suitable operation conditions of foam fractionation of Cr (VI) concentration 10 mg/L, binary surfactant concentration 0.17 g/L (mass ratio of SS to CTAB was 1:1 (w/w)), pH 5.0 and volumetric air flow rate 150 mL/min, the removal percentage and enrichment ratio of Cr(VI) reached 94.05% and 48.15, respectively. The CTAB concentration in the binary surfactant mixture was as low as 0.085 g/L. The enrichment ratio of Cr(VI) was increased by 38.6%, but its removal percentage was only decreased by 1.6% compared to the case that CTAB only was used under the same operating conditions.