Study On Mechanism And Application For Hexavalent Chromium Removal By Chlorella Sp.

The toxicity and mobility of Cr(Ⅵ) makes it crucial to decrease the heavy mental from industrial waste streams. Algae biomass, as a sustainable, cost-effective and efficient solution, shows the potential to detoxify Cr(Ⅵ) into the less hazardous Cr(Ⅲ). In this work, two widely available microalgae, Chlorella vulgaris and Chlorella pyrenoidsa, have been selected for the Cr(Ⅵ) removal. The application and mechanism for Cr(Ⅵ) removal from aqueous solution by both living and nonliving Chlorella sp. are characterized and discussed extensively.The mechanism of Cr(Ⅵ) detoxification by nonliving Chlorella sp. is first analyzed using FTIR, XPS and EDX analysis. The results show that:(1) Cr(Ⅵ) adsorption can be carried out through surface complexation, and -NH2,-COOH are found to be the main functional groups involved. Cr(Ⅵ) adsorption increases with the increase of protein content in microalgae biomass; (2) Cr(VI) anions can be reduced into Cr(Ⅲ) cations mainly by secondary alcohol groups on biosorbents; (3) Alkaline earth metals are responsible for the ion-exchange mechanism for Cr(Ⅲ) cations binding.The investigation on effects of pH, temperature, initial Cr(Ⅵ) concentration and algal dosage indicates that:(1) Low pH accelerates the removal of Cr(Ⅵ). The optimal pH for Cr adsorption, and for both Cr(Ⅵ) reduction and removal is found at 2.0 and 1.0, respectively; (2) High temperature is beneficial to Cr adsorption, Cr(Ⅵ) reduction and removal; (3) The removal percentage of Cr(Ⅵ) decreases with increasing initial Cr(Ⅵ) concentration, whereas the amount of Cr adsorbed and Cr(Ⅵ) reduced by unit weight of sorbent follows the opposite tendency; (4) The removal percentage of Cr(Ⅵ) increases with increment in algal dosage, however, the specific removal capacity decreases with the increase of algal dosage. When the operating conditions are controlled:Cr(Ⅵ) of 50 mg/L, algae dosage of lg/L, pH 2.0 at 45℃, the Cr adsorption, the Cr(Ⅵ) reduction, the Cr(Ⅵ) removal are found to be 10.8 mg/g, 25 mg/g,58.5% for C. vulgaris and 13.5 mg/g,26 mg/g,60% for C. pyrenoidsa, respectively. The living microalgae is supposed to be suitable for treatment of low concentration of Cr(Ⅵ) due to the toxicity of Cr(Ⅵ). The investigation on Cr(Ⅵ) uptake as well as Cr resistance of Chlorella vulgar is demonstrates that:(1) Cr has a negative effect on cell growth and chlorophyll production; (2) The Cr uptake amount increases with the increment in Cr(Ⅵ) concentration at the cost of Cr(Ⅵ) removal; (3) The bioaccumulation and resistance of Cr are proposed to attributed to the mechanism including Cr accumulation in chloroplast, the induction of vacuoles, and the expel of Cr outside of cell.In summary, the study on the mechanism and application for Cr(Ⅵ) removal from aqueous solution by living and nonliving Chlorella sp. is carried out, which has laid theoretical basis for further studies.