The present study was carried out to optimize the various conditions for biosorption of malachite green (MG) ions onto Scenedesmus quadricauda and Chlorella vulgaris biomass by applying Response Surface Method (RSM). The effects of process parameters such as initial MG concentration (2, 6 and 10 mg/L), initial solution pH (3, 4.5 and 6), algae amount (40, 80 and 120 mg/L), and contact time (10, 50 and 90 min) on the dye biosorption were analyzed using three-level Box-Behnken design. Biosorption of dye from aqueous solution was optimized in a batch mode. The analysis of variance (ANOVA) for removal efficiency of MG ions for S. quadricauda and C. vulgaris was used in order to validity a model. Then, the second-order polynomial analysis and quadratic model was employed to find out the relationship between removal efficiency of MG and four independent variables. In the quadratic models, the regression equation obtained after the ANOVA indicated the correlation coefficient (R2) as 0.9918 and 0.9936 for MG removal of S. quadricauda and C. vulgaris, respectively. Values of insignificant lack-of-fit of 0.6744 and 0.7171 obtained for S. quadricauda and C. vulgaris, respectively. P-values of both models for MG removal by algae were < 0.0001. Probability > F less than 0.05 indicated model are significant. Also, the minimum value of standard error design 0.42 around the central point indicates that present model can be used to conduct the design space. Thus, the high R2 value, insignificant lack-of-fit p-value, significant models P-value, and minimum value of standard error design indicating its high adequacy and validity of models in predicting the MG removal percentage by S. quadricauda and C. vulgaris. Therefore, these models were used for further analysis. The contour plots show the type of interaction between the tested variables and hence allow us to obtain the optimum conditions. On the basis, the MG removal efficiency onto these algae was increased with a decrease in initial MG concentration at 2–10 mg/L, and an increase in initial solution pH 3 to 6, biomass amount nearly 40 to 100 mg/L, and contact time of reaction at 10-90 min. In the optimum experimental conditions, the results showed that maximum dye removal efficiency of C. vulgaris biomass (91.2%) is more than S. quadricauda biomass (72.0%) for biosorption of MG ions from aqueous suspensions. In this conditions, various kinetic models of pseudo-first order, pseudo-second order and intra-particle diffusion were examined, and pseudo-second order could much better explain the sorption kinetics of dye than other models (R2>0.99). The FT-IR spectroscopy method was used to obtain information on the nature of possible interactions between the functional groups of algal biomass and MG ions. These observations indicated that several functional groups especially carboxyl, hydroxyl and amine groups on the surface of the S. quadricauda and C. vulgaris biomass are responsible for binding of MG ions in the biosorption process. Furthermore, different removal efficiency of MG onto S. quadricauda and C. vulgaris biomass can be attributed to different interaction between dye molecule and algal biomass. These findings revealed that these algae are suitable biomaterial for MG removal from wastewaters.
Keywords: Biosorption; Scenedesmus quadricauda; Chlorella vulgaris; Response surface methodology; Malachite green