Removal of chromium using rhamnolipid biosurfactant, ferrous sulfate and cationic tannin-based flocculant in a dissolved air floating system using a central composite design (CCD)

This research studied the wastewater chromium removal efficiency by dissolved air flotation (DAF) using a rhamnolipid (RL) biosurfactant as a collector. An experimental flotation DAF apparatus with 6 vessels of 2 L containing a coupled saturator injecting compressed air at 5.88 kPa in the vessels was used. The total RL concentration of the broth resulting from fermentation was 9 ± 1.0 g/L. This broth was used in nature in the DAF experiments. A central composite design (CCD) was used to optimize removal of Cr(VI) and total Cr with regards to two independent variables, pH (3.17–8.83) and iron concentration of the medium (0–225.0 mg/L), with a three assays performed at the conditions of the central point of the design. The experimental conditions for DAF were an initial hexavalent Cr concentration of 100 mg/L; RL broth volume of 500 mL; saturated with oxygen water volume of 200 mL; and a rapid mixing time of 6 min through stirring at 120 rpm. The results showed that under acidic pH conditions and with high iron concentrations, both the Cr(VI) and total Cr removal rates were highest. The optimal removal region determination was at a pH of 3.5 and iron concentration of 180 mg/L. Subsequently, cationic tannin-based flocculant was also evaluated as a collector, and ferrous sulfate was used as a coagulant during Cr(III) removal. The best Cr(III) removal percentage was obtained at cationic polymer concentrations of 300 mg/L with Cr(III) removal of 50.8% and a pH of 5.5.