| With the fast development of modern industry, heavy metal contaminant becomes more severe. How to elimnate 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 (OP) and spruce bark were chosen as the raw material to prepare modified orange peel biosorbents and modified spruce bark biosorbents through chemical modification. The adsorption behaviors for heavy metal ions by these biosorbents were investigated. The characterizations of biosorbents were determined by COD, Zeta potential, SEM, XPS, FTIR and so on. The effects of various parameters, including solution pH, temperature, solid/liquid ratio, adsorption time and metal ion concentration on the adsorption process were investigated by using batch adsorption techniques. The adsorption thermodynamic, adsorption kinetics and adsorption isotherms of each metal ion by each adsorbent were analyzed systematically and the values of adsorption heat, maximum adsorption capacity and average adsorption energy were calculated. The separation of lead/zinc and lead/nicole under low pH values was conducted by column adsorption experiments. Furthermore, the technology of desorption and reuse for the modified orange peel and competitive adsorption behavior of binary metal ion system were also tested. The adsorption mechanisms were discussed through instrumental identification.The main components of orange peel and spruce bark are pectine, cellulose, hemi-cellulose and lignin, which have ability to bind heavy metal ions, but show lower adsorption capacity. By chemically modification, four kinds of orange peel biosorbents were prepared:MOP, XOP, MgOP and KOP which were modified by mercapto-acetic acid, carbon disulfide, magnesium chloride and potassium chloride, respectively and three kinds of spruce bark biosorbents were prepared: FB, AB and CB, which were modified by formaldehyde, dilute sulfuric acid and concentrated sulfuric acid, respectively.The adsorption behaviors of modified orange peel biosorbents for the removal of Cu2+, Cd2+, Pb2+, Zn2+and Ni2+were investigated, and similar results were obtained:best equilibrium adsorption pH range was5.0-5.5; adsorption process was exothermic; fast adsorption rate was noticed that adsorption equilibrium could be reached within20minutes. All the adsorption processes could be well described by the pseudo-second-order equation. The calculated maximum adsorption capacities for Cu2+, Cd2+,Pb2+, Zn2+and Ni2+by MOP, XOP, MgOP and KOP were higher than orinigal orange peel, which confirm the chemical modification processes successfully improve the adsorption ability of OP through adding functional groups or making more active sites exposure on the adsorbent surface. Metal loaded adsorbents could be regenerated using0.1mol/L HC1solution and repeatedly for ten times with little loss of adsorption capacity. The results of column adsorption experiments suggested that effective mutual separation of Pb2+away from Zn2+can be realized by MOP and XOP, and mutual separation of Pb2+away from Ni2+by MgOP and KOP could be satisfactorily achieved. Based on the analysis of the change of solution pH and movement of typical adsorption peaks in FTIR spectrum before and after heavy metal adsorption, the adsorption mechanism of modified orange peel biosorbents was mainly ion exchange and little complexation between heavy metal ions and functional groups (-OH,-COOH and sulfur bearing groups) on the surface of biosorbents.The competitive adsorption behavior for binary metal ion solutions by MgOP and KOP were discussed. The effects of the existence of common cations including K+, Na+, Ca2+and Mg2+on the adsorption of Cu2+, Pb2+and Ni2+by MgOP and KOP were negligible, but high concentrations of Ca2+and Mg2+will interfere the adsorption of target metal ions. MgOP and KOP were used to test the competitive adsorption behavior of two binary systems:Pb/Cu and Pb/Ni. And the results showed that the effects of Pb2+ions on the uptake capacities of Cu2+and Ni2+ions were antagonistic, whereas the effect of the coexistence of Cu2+or Ni2+ions in the binary mixture on the adsorption of Pb2+was very complicated and could be considered to be a combination of antagonistic and synergistic depending on the metal ion concentration in the mixtures.The adsorption behavior for the removal of Cr(Ⅵ) from aqueous solutions by modified spruce bark biosorbents were investigated. Spruce bark biosorbents has higher relative lignin content and more porous morphology, which plays a very important role in the adsorption of Cr(Ⅵ). The results of thr modified spruce bark biosorbents for the removal of Cr(VI) indicated that:lower pH was benefical for the adsorption of Cr(Ⅵ) by FB, AB and CB; adsorption process was accompanied by reduction reaction from Cr(Ⅵ) to Cr(Ⅲ); the lower the initial solution pH, the shorter the time to reach equilibrium, the speed order of the three spruce biosorbents was CB>AB>FB; the adsorption process was endothermic. Under different temperatures, all the adsorption processes could be well described by the pseudo-second-order equation. Fitting results of intra-particle diffusion model indicated that the adsorption process was controlled by both film and intraparticle diffusion. By analyzing the adsorption isotherms, the adsoprtion of Cr(Ⅵ) on FB, AB and CB fitted Langmuir model better. The lower the initial solution pH, the higher the adsorption capacity. When the initial pH was1, the maximum adsorption capacties of FB, AB and CB for the adsorption Cr(Ⅵ) were735,483and444mg/g, which were much higher than the reported adsorption capacities in literatures. The obtained results give a answer that there is no need to use traditional formaldehyde for modification method.The Cr(Ⅵ) adsorption processes by three kinds of spruce bark biosorbents were supposed to be typical adsorption-coupled reduction mechanism. During adsorption, Cr(Ⅵ) was first be adsorbed as anoin ions onto the surface of bark biosorbents, then was reduced to Cr(Ⅲ) by electron donor groups on the lignin and lignin derivatives, after that, the Cr(Ⅲ) was adsorbed onto the biosorbent surface through surface complexation with hydroxy, carboxyl and methoxy.This paper provides important foundmental date for the effective reuse of watste orange peel and spruce bark and their applications for heavy meatl ion wastewater treatment. |
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