Surface complexation mechanism and modeling in Cr(III) biosorption by a microalgal isolate, Chlorella miniata

The mechanism involved in the removal of Cr(III) by a green microalgal isolate, Chlorella miniata, was examined based on a series of batch experiments and microscopic analyses, and a mathematical model was proposed. Results showed that Cr(III) biosorption increased with the increase of pH from 2.0 to 4.5, and no significant changes in biosorption outside this pH range. Langmuir isotherm indicated that the maximum Cr(III) sorption capacity of Chlorella miniata was 14.17, 28.72, and 41.12 mg g^-1 biomass at pH 3.0, 4.0, and 4.5, respectively. Results from desorption studies, SEM (scanning electron microscopy), TEM (transmission electron microscopy), and EDX (energy-dispersive X-ray spectroscope) analyses confirmed that surface complexation was the main process involved in Cr(III) biosorption. Potentiometric titration revealed that carboxyl (p K_{a 1} = 4.10), phosphonate (p K_{a 2} = 6.36) and amine (p K_{a 3} = 8.47) functional groups on the surface of Chlorella miniata were the possible sites for Cr uptake, and their average amounts were 0.53, 0.39, and 0.36 mmol g^-1 biomass, respectively. A surface complexation model further indicated that carboxyl group played the main role in Cr(III) complexation, with a binding constant of K₁₁ = 1.87 × 10^-4 and K₁₂ = 6.11 × 10^-4 for Cr³⁺ and Cr(OH)²⁺, respectively. This model also suggested that the hydroxy species was more easily to complex with the cell surface of Chlorella miniata. © 2006 Elsevier Inc. All rights reserved.