The Environmental Behaviors Of Lead And Cadmium As Influenced By Fe-Mn Composite Oxide From Different Facter

With the rapid development of industry and agriculture, the pollution caused by heavy metal has increasingly become one of the most enviroment threatening issues. Understanding the environmental behavior of heavy metals in soil is a fundamental scientific basis of soil heavy metal pollution control. The wide existence of Iron and Manganese oxides in soil, has important effect on their morpnology, transformation and (?)iological activity. Thus, the adsorbability of iron and manganese oxides for heavy metais aroused extensive attention internationally. So far, the adsorbability for heavy metals of two kinds of oxides has been researched all along. But the problem that Iron and manganese oxides usually coexist in soil and form mixed state of iron and manganese oxides which act with Pb and Cd has not been put into the systematic research. So it is very necessary to explore the mechanism of mutual action between Iron/manganese oxides and Pb/Cd, which is significant to illustrate the morphology, transformation and biological activity of Pb and Cd in soil, and also has great theoretical and practical value for control and recovery of heavy metal pollution.This paper studied adsorption and desorption characteristics of Fe-Mn composite oxides for Pb2+ and Cd2+ by isothermal adsorption/desorption experiment, and exposed the thermodynamics and dynamics characteristics under the effect from Solution pH, ionic strength, temperature,etc. Moreover, the competitive adsorbability of Pb2+,Cd2+ on the Fe-Mn mixed oxides was discussed,which provided scientific evidence for further recognition of the action of Pb2+ and Cd2+ and restoration of soil polluted by them. The major research results are listed as follows:Fe-Mn composite oxide has a strong adsorbability for Pb2+,Cd+,which could maximally reach the ratio of 97.56% and 85.23%, respectively at the 0.2mmol.L-1 of Pb2+,Cd2+ initial concentration. Absorption process could be best fitted by Langmuir equation that precedes Freundlich. The maximum absorption capacity were 587.78mg.g-1 and 163.93mg.g-1respectively. Compared to the absorption ratio, the desorption ratio of Pb2+,Cd2+ are 18.69% and 10.70% less. It is obvious that Fe-Mn composite oxide has stronger adsorbability for Pb2+ and Cd2+ that can not be desorpted.The adsorbability of Fe-Mn composite oxides for the Pb2+ and Cd2+ was affected by pH, ion strength and temperature. Evidence showed that the max adsorption capacity and constant K were both increased by raising pH at the range of 4.0 to 8.0, under the fixed ion strength and temperature. At pH=8.0, adsorption capacity were 258.34 mg.g-1 and 119.058 mg.g"1, while they were 222.964 mg.g-1 and 95.338 mg.g-1 at pH=4.0. Adsorption mechanism were mainly coordinated adsorption and ion exchange adsorption, where the former one are relatively stronger.When ion strength changed from zero to 0.01mol.L-1, adsorption capacity of Fe-Mn composite oxides to the Pb2+,Cd2+ were increased, but from 0.01mol.L-1 to 0.1Omol.L-1, adsorption capacity for Pb2+ decreased and increaseed for Cd2+. The thermodynamic experiment showed that with increased temperature, adsorption of the Fe-Mn composite oxides to Pb2+ was reduced, at temperature of 298.2 and 308.2 K.,△Gmθ(standard free energy change)<0,△Hθ(enthalpy change)<0,△Sθ(entropy change) < 0. It was an exothermic reaction and lower temperature was advantageous to adsorption reaction. On the other hand, On the other hand, adsorption of the Fe-Mn composite oxides to Cd2+ was increased as temperature increased. At the same level of temperature, this reaction was endothermic because△Gmθ> 0,△Hθ< 0,△Sθ< 0, higher temperature was favorable to adsorption reaction. Adsorption thermodynamic of the Fe-Mn composite oxides to Pb2+ and Cd2+ has some differences. The reasons are:Firstly, Pb2+ and Cd2+ have different hydrolysis capability, of which Pb2+ is stronger than Cd2+, i.e., more easily coordinated with adsorbent; Secondly, the difference of electron shell structure, whereas Pb2+ has six layers while Cd2+ has four, affected the deformation cf Cd2+ and Pb2+. So adsorption capacity of Fe-Mn composite oxides to Pb2+ is greater than the Cd2+, the same as adsorption force and enthalpy.Experiments showed that adsorption rate of Fe-Mn composite oxides to Pb2+,Cd2+ could be up to 90% of the balanced amount within the initial 60 minutes. Compared to the adsorption process, desorption was lagged. The desorption rate of Pb2+ achieved 90% of the max amount within the initial 120 minutes, where Cd2+ used 480min. the adsorption process could be best fitted with two steps Lagergren kinetic equation. Adsorption constant k could be increased by raising initial concentrations. Constant k was increased from 0.0106g.mg-1.min-1 to 0.0192g.mg-1.min-1 as initial Pb2+ concentration increased from 0.2 mmol.L-1 to 0.8 mmol.L-1, meanwhile, it was increased from 0.0110g.mg-1.min-1 to 0.0258g.mg-1.min-1 as initial Cd2+ concentration increased from 0.2 mmol.L-1 to 0.8 mmol.L-1. It verified that adsorption capacity could be improved as increase of initial concentration.Competitive ion also influences the adsorbability. As shown by research, the adsorption capacity in the competitive system is always lower than that of the non-competitive one, whenever change the initial concentration of Pb2+ at fixed Cd2+ concentration or reversely. This result demonstrated that if there existed competitive ion, which partly occupied the adsorptive point location, the adsorption capacity was lower. Under the same PH and temperature, the maximum adsorption capacity are respectively 476.19mg.g-1 and 102.658 mg.g-1 for Pb2+ and Cd2+. But when they coexisted, the max amount for Pb2+ decreased to 256.41 mg.g-1 and 74.074 mg.g-1 for Cd2+. The Fe-Mn mixed oxides are inclined to adsorb Pb2+ first, and the adsorbability is higher for Pb2+ than Cd2+.