| With the fast development of modern industry, heavy metal contaminent becomes more severe. How to enlimate the damage and effectively recover heavy metals is the outstanding problem of current environmental protection work. Biosorption as a novel heavy metal removal technology has many advantages, including abundant resource, low cost, high metal binding capacity, environmental friendly and without second pollution, which make biosorption have a wide application prospect.In this paper, orange peel was chosen as the raw material to prepare two biosorbents MOP and XOP through chemical modification by using mercapto-acetic acid and carbon disulfide, respectively. These two biosorbents were used to adsorb Cu2+, Cd2+, Pb2+, Zn2+ and Ni2+ from aqueous solutions. COD, Zeta potential, SEM and FTIR were used to characterize the biosorbents. The effects of various parameters, including solution pH, temperature, time and metal ion concentration on the biosorption process were investigated by using batch adsorption techniques. The adsorption thermodynamic, adsorption kinetics and adsorption isotherms of these five individual metals were analyzed systematically. The separation of lead and zinc under low pH values was conducted by column adsorption experiments. Furthermore, the technology of desorption and reuse for the modified orange peel and practical applications for the real wastewater were also tested. Also, the adsorption mechanism was discussed.The results of characterization showed that after chemical modification, MOP and XOP had lower COD, more negative charges, more porous morphology and more funcational groups on surface. These results all led to better heavy metal adsorption ability for both biosorbents.Solution pH had great effect on the adsorption efficiencies of Cu2+ Cd2+, Pb2+, Zn2+ and Ni2+ on biosorbents and the optimum experimental pH was 5.0-5.5. Under the temperature range from 30℃to 60℃, the adsorption effciencies of the five metal ions decreased with the increase of temperature, this indicated that the adsorption processes were exothermic. Metal loaded biosorbents were be regenerated using 0.1 mol/L HCl solution and could be used repeatedly for ten times with little loss of adsorption capacity. The results of column adsorption experiments suggested that effective mutual separation and pre-concentration of Pb2+ away from Zn2+ using modified orange peel could be satisfactorily achieved.It was found that all the adsorption of Cu2+, Cd2+,Pb2+,Zn2+ and Ni2+ on MOP and XOP was a fast adsorption process that could be reached equilibrium within 20 minutes. The adsorption processes could be well described by the pseudo-second-order equation.By analyzing the adsorption isotherms, the adsoprtion of Cu2+, Pb2+, Zn2+ and Ni2+ on MOP fitted both Langmuir and Freundlich model, but Cd2+ adsorption only fitted Langmuir model. The maximum adsorption capacities for Cu2+, Cd2+, Pb2+, Zn2+ and Ni2+ on MOP were 70.67,136.05, 162.87,64.14 and 33.94 mg/g, respectively, higher than raw orange peel. For XOP, the adsorption of Cu2+, Cd2+, Pb2+, Zn2+ and Ni2+ fitted Langmuir monolayer adsorption model and the maximum adsorption capacity were 77.58,156.99,223.31,92.68 and 61.80 mg/g, respectively, which were about 1.75,1.38,3.64,4.36 and 6.29 times higher than that of the unmodified orange peel, also higher than corrsponding data of MOP.Based on the anaysis of the change of solution pH and movement of typical adsorption peaks in FTIR spectrum before and after heavy metal adsorption, the adsorptio mechanism involved both ion exchange and complexation between heavy metal ions and functional groups (-OH,-COOH and C-S) on the surface of biosorbents. |
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