Study On Chemical Modification Of Orange Peel And Its Adsorption Behavior For Heavy Metal Ions

Nowadays heavy metals are among the most important pollutants in source and treated water and are becoming a severe public health problem. These metal ions can be harmful to aquatic life, and water contaminated by toxic metal ions remains a serious public health problem for human health. Consequently, it is urgent to find new technologies for removing heavy metal ions from wastewater. So, biosorption can be a promising alternative method to treat industrial effluents, mainly because of its low cost, high metal binding capacity, high efficiency in dilute effluents and environmental friendly.In this paper, a series of biosorbents were prepared using orange peel from agriculture residues by chemical modification and adsorption properties of Cu2+,Pb2+,Cd2+,Zn2+and Ni2+ on these biosorbents in aqueous solution were studied. The structure and morphology of orange peel and modified orange peel were characterized by lots of instruments, such as BET-N2 adsorption, infrared spectroscopy and scanning electron microscopy technique. Also the physical and chemical characteristics such as surface area, surface charge, were examined. The modifying mechanisms of orange peel were discussed, and the reaction conditions of modification were optimized. By using batch adsorption techniques, the influence of pH, adsorbent dosage, concentration and temperature on adsorption effect was investigated. In addition,the adsorption thermodynamics and the adsorption kinetics were studied systematically. Furthermore,the technologies of desorption and reuse for the modified orange peel and technological applications for the wastewater were also tested. By the column adsorption experiments, the figures of breakthrough curve were determined in different experimental conditions, such as flow rate of solution, initial concentration of adsorbent and the column height to inspect on the adsorption effects.The chemical modification of orange peel are in the methods of saponification(SOP), saponification-crosslinking(SCOP) and graft copolymerization(OPAA) with sodium hydroxide, sodium hydroxide and calcium chloride and methyl acrylate in this paper. When orange peel were modified by sodium hydroxide, sodium hydroxide and calcium chloride and methyl acrylate, the modification conditions, such as material ratio,reaction temperature and reaction time,must be controlled strictly,which could affect the adsorption capacity and the yield of the modified orange peel.The changes of physical structure and chemical properties of series of modified orange peel were characterized by using a variety of analytical instruments and methods. The results of scanning electronic microscope showed that the SOP and SCOP become rough porous in structural morphology, surface area was increased. The determination results of zeta potential and total negative charge show that the surface of modified orange peel has more negative charge than raw orange peel, which facilitates the electrostatic interaction with cations.For FT-IR spectrum of the SOP and SCOP, results indicated that the methyl esters were hydrolyzed with NaOH and ester group was converted to carboxylate ions, which improved the number of active functional groups. IR analysis of OPAA shows that polyacrylic acid was grafted to the main components of the orange peel.After the graft copolymerization of orange peel with methyl acrylate, the appearance of morphology was smooth and specific surface area decreased as a result of grafting polyacrylic acid. But due to the number of carboxyl functional groups increased, at the same pH value, the zeta potentials of OPAA are more negative than those of OP and total negative charge of OPAA is more than that of OP, which indicate that the amount of-COOH of OPAA is more than that of OP because of grafting with PMA on the OP.The adsorption process is a function of pH, metal ion concentration, time, temperature and the adsorbent dose. The optimum pH value was found out 5.0～6.0.The adsorption percentage increased with the increase of adsorbent dose.It was found that the adsorption effects of SOP and SCOP to Cu2+,Pb2+,Cd2+,Zn2+ and Ni2+ is better when adsorbent dose was 4g/L.In all other adsorption processes, adsorption capacity decreases with the adsorption temperature increasing and enthalpy, Gibbs free energy and entropy change were negtive except Cd2+ adsorption on SCOP. In the adsorption process,the enthalpy change(ΔH) was negative, which demonstrated that the adsorption process was of exothermic process.The free energy change(ΔG) was negative, which indicated the adsorption process was spontaneous.The entropy change (ΔS) was also negative because of the increasing of order caused by the new ion-bonds and the decreasing of disorder in aqueous solutions. The experimental adsorption data for modified orange peel were fitted to the Langmuir adsorption model.From Langmuir isotherm, the adsorption capacity for heavy metal ions on the modified orange peel increased compared with the OP. Especially for OPAA, the highest metal uptake for Cu2+,Pb2+,Cd2+, Zn2+and Ni2+ were 289.0,492.6,293.3,393.7 and 162.6mg/g, which is respectively about 6.5,4.3,4.6,18.5 and 16.5 time higher than that of the unmodified biomass and also much higher than some other biosorbents reported in literatures. It was found that the adsorption of heavy metals on SOP and SCOP was a fast adsorption process that could be reached equilibrium in 20 minutes. The adsorption process was mainly fitted the pseudo-second-order equation. The removal rate of heavy metals on OPAA was rapid in the first 30 min, and leveled off after 2h. The biosorption kinetics followed the pseudo-second order model and intra-particle diffusion model.In addition,the intra-particle diffusion was the main rate-controlling step.The existence of common cations such as K+, Na+, Ca2+, Mg2+ and other heavy metal ions in water interfered the adsorption of modified orange peel, but had no definite pattern. The loaded biosorbent was regenerated using 0.1 mol/L HCl solution and could be used repeatedly for four times with little loss of adsorption capacity.At dynamic process of SCOP, the figure of breakthrough curve was strongly dependent on experimental conditions, such as flow rate of solution, initial concentration of adsorbent and the column height. Higher flow rate, bigger initial concentration of adsorbent and shorter column height will quicken the breakthrough of column. A mutual chromatographic separation test was carried out by using SOP, SCOP and OPAA between Pb2+ and Zn2+, i.e.,a small amount of Pb2+ was separated from a large excess of Zn2+ using a column packed with the modified orange peel.These results suggest that effective mutual separation and pre-concentration of Pb2+ away from Zn2+ using modified orange peel can be satisfactorily achieved.The practical treatments were studied by using modified orange peel to dispose of electroplating wastewater. The results clearly indicate that the proposed method is prospective for the removal of heavy metal ions from wastewater.Different experimental approaches were applied to show mechanistic aspects, especially the role of calcium, sodium and hydrogen present in the biosorbents in ion exchange mechanism.In this paper, the direction of research conforms to China's current ecological protection, recycling economy and environmental policy. Based on the findings above, a new high-performance, low-cost approach and a new way of thinking will be provide to treat the heavy metals contamination by using orange peel discarded.