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Process Analysis of Biosorption Kinetics of Heavy Metal Ions

Liyan Liu, Xingang Li, and Jinsheng Sun. School of Chemical Engineering and Technology, National Engineering Research Center for Distillation Technology, Tianjin University,P.R. China, Tianjin, China

The model of absorption kinetics can predict the reaction rate of reaching the equilibrium and pseudo-equilibrium, and obtain the rate constant and the reaction order, then further investigate the reaction mechanism. In the process of biosorption of heavy metal ions, many researchers directly assume the adsorption already reaches equilibrium, then use isothermal equilibrium equation instead of kinetics equation due to the complicated absorption mechanism of heavy metal ions. In most cases the errors caused by this assumption have not too much influence on the results. But this assumption can cause big influence on the results if the adsorption time is the determining factor of the whole process. Compared to the classic kinetics equations such as the four kinetics equations. These four equations were derived from Elovich equation and Ritchie rate equation; they are Lagergren Pseudo-one-order equation, Ritchie two-order equation, modified Ritchie two-order equation and Elovich equation. The adsorption equation proposed by Elovich was applied to the liquid adsorption process, and was liked by most researchers. Taylor and his collaborators successfully applied Elovich equation to Zn2+ adsorption kinetics by the soil, the experimental accuracy was up to 93%. Linear Elovich equation is usually applied to solid adsorption. Carter etc. successfully applied Elovich equation to Zn2+ adsorption by the soil particle, Juang and Chen also successfully applied this equation to Co2+ANi2+ACu2+ and adsorptions by impregnated resin. The above equations are applied widely in the heavy metal adsorption, but they are not always accurate because of not considering the influence of solution continuous concentration. We know the adsorption rate is closely related to not only adsorption activity, but also the concentration of heavy metal ions, therefore the influence of solution continuous concentration should be considered in order to get better kinetics equations. Based on the analysis of biosorption of heavy metal ions, this work simplified the conditions to build a kinetics differential equation to describe this process. This equation was solved to get the pseudo-one order biosorption equation as n=1. Least Squares Method was used to nonlinear curve fitting of the experimental data for Cu2+, Zn2+ and Cd2+ ions adsorption by Bacillus subtilis. The theoretical modeling equation in the defined system was obtained and compared with the experimental data. The results showed that the theoretical modeling equation fitted well with the experimental data, which means this model can be used to describe the heavy metal ions biosorption well.

Key words: Heavy metal ions, Bacillus subtilis, adsorption kinetics, Pseudo-one-order kinetics equation.