Bioremoval and Bioreduction of Chromium (VI) by the Green Microalga, Chlorella vulgaris Beij., Isolated from Laguna de Bay, Philippines

A strain of the green microalga, Chlorella vulgaris Beij., isolated from West Bay, Laguna de Bay, Philippines was tested for its ability to biologically remove and reduce chromium (VI) at varying concentrations (0.1, 1.0, 3.0, 5.0, and 7.0 mg/L) in BG-11 medium without EDTA. The growth of the microalga was significantly inhibited at concentrations higher than 0.1 mg/L with a computed EC 50 of 1.76 mg/L after 12 days of cultivation. The bioremoval of the Cr from the medium increased with increasing metal concentration up to 1.0 mg/L then started to decline. The cells of C. vulgaris bioremoved 62% (4.70 µg 100/mL) of the metal from the medium with 0.1 mg/L Cr (VI) while 0.83% (6.08 µg 100/mL) of the metal was bioremoved from the medium with 7.0 mg/L Cr (VI). A higher proportion (>80%) of Cr bioremoved was bioabsorbed inside the cells than bioadsorbed on the cell walls in all concentrations. The total Cr bioreduction also increased with increasing metal concentration but started to level off after 3.0 mg L -1 concentration. The microalga bioreduced a total of 57% (4.30 µg 100/mL) of the metal in the medium with 0.1 mg L Cr (VI) while a total of 5% (36.41 µg 100 mL) was bioreduced in the medium with 7.0 mg/L Cr (VI) . A higher proportion (>67%) of the bioreduced Cr was observed inside the cells for the media with Cr (VI) concentrations below 1.0 mg/L while higher proportion was observed in the culture medium for the treatments with higher concentrations. In terms of the amount of Cr (VI) bioremoved and bioreduced per mg biomass (DW), the cells that grew on the medium with 7.0 mg/L Cr (VI) exhibited the highest values.