Study On Removal Of Heavy Metal Ions In Water By Crab Shell Powder

With the development of industrialization, surface water had been severely polluted by heavy metal.Traditional methods to solve this problem have been unsuitable in many conditions. Biosorption as a novel technology has caused more and more attentions from scientists and engineers around the world. Crab shell(CS) is easy to be obtained as the solid waste with very low cost to use high-efficiently as the bio-adsorbent to remove heavy metals from water. Crab shell with a large specific area and porous microstructure, is mainly composed of mineral substance CaCO3, organic chitin, protein and so on, which makes it possible for excellent bio-adsorbent. This paper has explored the sorption behavior of CS for Pb2+、Zn2+、Cd2+ and other heavy metal ion in water under different conditions. The sorption isotherm, kinetics and thermodynamics of CS to these ions has been carefully studied for optimizing the sorption experiments. The sorption mechanisms for different systems were also discussed through the CS particles before and after the sorption.In the system of Cd(Ⅱ) onto CS, experimental data have been perfectly fitted to Langmuir and Dubinin-Radushkevich models, while the uptake kinetics are well described using the pseudo-second-order model. Effects of experimental conditions on the uptake isotherms and kinetics are discussed in detail. Uptake thermodynamic study has indicated that the Cd(Ⅱ) uptake onto crab shell is an endothermic and spontaneous process,where △H° and △S° is 18.05kJ mol-1、67.99 J/mol,respectively. For further understanding the process, the possible mechanisms of Cd(Ⅱ) uptake onto crab shell have been deduced based on experimental results. Desorption experiment manifests that 93.7% of adsorbed Cd(Ⅱ) could be recovered with 0.1 M HNO3 solution. Besides, the inhibition effects of other metal co-ions on the Cd(Ⅱ) uptake on the crab shell are also discussed..Removals of Pb(Ⅱ) and Zn(Ⅱ) ions from water using crab (Chinese hairy) shell particles as biosorbent have been compared in this study. Uptake equilibriums for two ions well described by Langmuir isotherm revealed that crab shell possessed higher uptake capacity for Pb(Ⅱ) (709 mg/g) than that for Zn(Ⅱ) (117 mg/g). Kinetics data for two metals uptake were successfully modeled using the pseudo-second-order model, where the initial uptake rate of Pb(Ⅱ) was much faster than that of Zn(Ⅱ). Dubinin-Radushkevick modeling and thermodynamic parameters hinted different uptake mechanisms of Pb(Ⅱ) and Zn(Ⅱ) removal by crab shell, attested by FTIR, XRD, FESEM analysis. Pb(Ⅱ) ion was removed mainly through the chemical reaction, while the uptake of Zn(II) ion onto crab shell was attributed to the chelation and coordination interactions. The polluted river water and laboratory wastewater both satisfied the standards for drinking and irrigation/fishery water, respectively, after being treated with crab shell particles.Finally, sorption behaviors of CS in the ten binary systems composed of five heavy metal ions including Pb(Ⅱ), Cd(Ⅱ), Zn(Ⅱ), Cu(Ⅱ) and Ni(Ⅱ), have been studied in detail. Adsorption kinetics of each metal ion was explored through examining the concentration of metal ion in sorption system which varied with the sorption time. Results have indicated that Pb(Ⅱ) or Cu(Ⅱ) could affect severely the sorption amount and initial sorption rate of other metal ion onto CS, while the sorption behaviour of metal ions were slightly restrained by Ni(Ⅱ).