Kinetic simulating of Cr(VI) removal by the waste Chlorella vulgaris biomass

• Biosorption and bioreduction of Cr(VI) on waste C. vulgaris biomass was investigated.
• The maximal biosorption capacity of waste C. vulgaris biomass for total chromium was 43.3 mg/g.
• The mechanism involved in Cr(VI) removal was indirect reduction.
• The proposed kinetic model well-fitted Cr(VI) removal behavior at various pHs and temperatures.

Waste microalgae biomass, one of the most abundant residues from biodiesel production, can be used as a low-cost biosorbent for heavy metal removal. The ability and mechanism of Cr(VI) removal by lipid-extraction residue of Chlorella vulgaris were investigated in this study. The removal of Cr(VI) increased as the pH decreased from 4.0 to 0.5, or temperature increased from 15 °C to 45 °C. The Sips isotherm well-fitted the experiment data, and the maximal biosorption capacity of waste C. vulgaris biomass for total chromium was 43.3 mg/g at pH 1.5 and a temperature of 25 °C. X-ray photoelectron spectroscopy revealed that the majority of Cr(VI) bound on the biomass was reduced to Cr(III) with its subsequent partial biosorption. The results of Fourier Transform Infrared Spectrometer study indicated that both carboxyl and amino groups on the biomass were the main binding sites for Cr(VI) biosorption, while carbohydrate was mainly responsible for reduction of Cr(VI). Finally, the proposed kinetic model based on the indirect reduction mechanism well described the Cr(VI) removal behavior at various pHs (0.5–2) and temperatures (15–45 °C).