Biosorption of chromium using factorial experimental design

An experimental design technique has been used to investigate the biosorption of chromium from water solutions, simulating typical tanning effluents. The United States Environmental Protection Agency (EPA) usually allows solutions containing heavy metals to be discharged if the concentration is less than 5.0 mg/L. The removal of Cr³⁺ and Cr⁶⁺ was studied, separately, using the factorial design 2³. The three factors considered were pH, temperature, and metal concentration at two markedly different levels: Cr³⁺, pH (2.0 and 6.0), T (29 and 55°C), and metal concentration (10 and 1200 mg/L); Cr⁶⁺, pH (1.0 and 3.0), T (29 and 55°C), and metal concentration (10 and 1200 mg/L). Experiments were carried out in a batch type reactor system with 0.2 g of biosorbent (Sargassum sp.), and 50 mL of Cr³⁺ or Cr⁶⁺ solutions. The efficiency of chromium removal during an exposition time of 6 h was then evaluated. The results were analyzed statistically using the Student's t-test, analysis of variance, F-test, and lack of fit to define the most important process variables affecting the metal removal efficiency. The most significant effect regarding Cr³⁺ uptake was ascribed to interaction between metal concentration and pH. For Cr⁶⁺, the most significant effect was ascribed to metal concentration.