Adsorption Of Heavy Metal Ions From Aqueous Solution Onto Hydrilla Verticillata

As more and more attention is paid to environment, it is important to remove pollutants in aqueous solution. Biosorption is a process that utilizes inexpensive active/dead biomass to sequester heavy metals and is particularly useful for the removal of contaminants from industrial effluents. Compared with conventional methods such as ion exchange and precipitation with lime, the biosorption process offers the advantages of low operating cost, minimization of the volume of chemical and biological sludge to be disposed of, high efficiency in detoxifying very dilute effluents. These advantages have served as the primary incentives for developing full-scale biosorption processes to clean up heavy-metal pollution.Polysaccharides of algal cell wall can provide sites to adsorb heavy metal. Cheap and rich source algaes showed strong adsorptivity to heavy metals. In this study, biosorption of Pb2+, Cd2+, Zn2+, Cu2+and Ni2+by Hydrilla verticillata were investigated as a function of environment effects, biosorption thermodynamics, biosorption kinetics and biosorption mechanism. Adsorption equilibriums were described by Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm model.The environment effects results were as follows:Maximum biosorption capacities were obtained at pH4.0for Pb2+, Maximum biosorption capacities were obtained at pH5.0for Cd2+、Zn2+、Cu2+and Ni2+. The higher biosorption efficiency and biosorption capacities were obtained at biosorbent dosage2g/L for Pb2+, Cd2+Zn2+, Cu2+and Ni2+. The biosorption of Pb2+, Cd2+, Zn2+, Cu2+and Ni2+by Hydrilla verticillata were rapid biosorption processes. The biosorption of Zn2+by Hydrilla verticillata attained equilibrium after20min. The Pb2+、Cu2+and Ni2+biosorption capacities onto Hydrilla verticillata increased by increasing contact time up to30min, the biosorption capacities obtained steady value after30min. The biosorption capacity was maximum capacity’s95%within60min. The biosorption capacities of Pb2+、Cd2+、Zn2+、Cu2+and Ni2+by Hydrilla verticillata decreased with the rise in temperature. The results indicated that these biosorption processes wer exothermic in nature.The Sorption isotherms of Pb2+, Cd2+, Zn2+, Cu2+and Ni2+followed well Langmiur, Freundlich and D-R equation. The biosorption efficiency in the order of Pb2+> Cd2+> Zn2+> Cu2+> Ni2+. The mean free energy of adsorption E(kJ/mol) in the order of Pb2+> Cd2+> Zn2+, Cu2+, Ni2+. The mean free energy of adsorption E>8kJ/mol for all biosorption processes, it indicated that the biosorption of Pb2+, Cd2+, Zn2+, Cu2+and Ni2+by Hydrilla verticillata may be carried out via chemical ion-exchange mechanism. The negative ΔG0values showed that the biosorption of Pb2+, Cd2+, Zn2+, Cu2+and Ni2+by Hydrilla verticillata were feasible and spontaneous. The negative ΔH0values depicted exothermic nature of the adsorption. The negative ΔS0values revealed that the orderliness of the adsorbed system were higher than that in the solution phase before adsorption. The biosorption kinetics followed the pseudo-second order model.In order to investigate the mechanisms of heavy metal biosorption by Hydrilla verticillata, the amounts of metal ions of biosorbent before and after biosorption were measured by using Inductively Coupled plasma (ICP) Atomic Emission Spectrometer. The results showed the amounts of biosorbed Pb2+, Cd2+, Zn2+, Cu2+and Ni2+on Hydrilla verticillata increased as well as light metal ions (K+、Mg2+and Na+) decreased on Hydrilla verticillata during biosorption experiment. It indicated that ions exchange mechanism would be involved. Fresh and the Pb2+, Cd2+, Zn2+, Cu2+and Ni2+loaded Hydrilla verticillata were analyzed by SEM equipped with EDX. The results showed that a remarkable amount of Pb2+, Cd2+, Zn2+, Cu2+and Ni2+were adsorbed by Hydrilla verticillata. To compare the change of Hydrilla verticillata’s Fourier transform infrared spectrum before and after biosorption, the peaks of C-O group and Sulfate group were shifted towards low wave numbers. It showed that the sulfate group and C-O group involved in the biosorption process. Desorption experiments showed that heavy metal ions can be recovered by EDTA.The study indicated that Hydrilla verticillata may be used as an inexpensive, effective and easily obtained biosorbent for the removal of Pb2+, Cd2+, Zn2+, Cu2+and Ni2+.