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Heavy Metals Removal from Wastewater by Magnetic Field-Magnetotactic Bacteria Technology

Huiping Song1, Xingang LI2, Jinsheng Sun2, and Yanhong Wang2. (1) School of Chemical Engineering and Technology, Tianjin University, No.92 Weijin Road, Nankai district, Tianjin, 300072, China, (2) School of Chemical Engineering and Technology, National Engineering Research Center for Distillation Technology, Tianjin University,P.R. China, Tianjin, China

Plating, electron devices and other industries frequently generates large quantity of wastewater containing high levels of toxic heavy metal ions, such as copper, cadmium, nickel and lead, which are drastically harmful to aquatic and terrestrial life. Although traditional removal methods, such as chemical precipitation, solvent extraction and ion-exchange from wastewater, have been helpful to relief the water shortage, yet they become less effective under the metal ion concentration of 100 mg l-1, which involve high capital and operational costs as well as the generation of secondary wastes. Recent studies suggest that many algae, yeasts, bacteria and fungi are capable of concentrating metal species from dilute aqueous solutions and accumulating them within their cell structure. Compared with the conventional methods, this biosorption process is more economical, efficient and environmentally friendly. However, the biosorption technology needs a bridge to connect the idea and the practical difficulty to separate the microorganisms loading metal ions from the aqueous solution. Newly-developed immobilization cell technology seems theoretically a good option. Nevertheless the float, expand, conglutination of the immobilized cells and the high mass transfer resistance of the oxygen and substrate severely restricted its mass application.

The magnetotactic bacteria (MTB), discovered by Blakemore in 1975, becomes a feasible alternative. MTB can synthesize unique intracellular structures called magnetosomes (MS) by uptaking iron (III) ions from culture medium. This characteristic makes them possible to navigate along geomagnetic or applied magnetic field lines. Previous literatures are limited on the biosorption conditions of heavy metal ions on MTB and the efficiency of separation the MTB adsorbing metal ions, which will be crucially helpful to develop a practical process design for removal and recovery metals using MTB as sorbents.

In this paper, the effect of performance parameters, such as the initial pH, temperature, biomass concentration and adsorption duration, are investigated in the biosorption Cu (II) ions by MTB firstly. The "real sorbent" experiment shows that there is no significant sorption when MTB is absent in the solution, indicating that MTB is the only sorbent in our experiments. Each experiment above is repeated three times and the following data are given as average. The concentration of Cu (II) in solution is determined using HITACHI 180-80 Atomic Absorption Spectrophotometer. The experimental results indicate that pH value and concentration of biomass exerted important influence on the sorption process, and the optimum scope were 1~9 and 2.0~5.0 g l-1, respectively. No significant effect of temperature has been observed in the discussed range, and the adsorption could be finished within 5 min.

It is important for design purposes to get the fundamental knowledge of adsorption equilibrium and adsorption kinetics. The Langmuir, Freundlich equilibrium models and the pseudo second-order kinetic model are chosen as candidates for isothermal and kinetic adsorption behavior of Cu (II) ions, and then the model parameters are fixed. The adsorption equilibrium data of Cu (II) ions in optimum conditions fit well to both the Langmuir and Freundlich models with high correlation coefficient of 0.9996 and 0.9428. And the second-order kinetic model is very suitable for the experimental kinetic data with a high correlation coefficient of 1.0 at optimum conditions. The maximum biosorption capacity of MTB is determined as 24.3013 mg g-1 (wet-weight basis), and the rate constant 0.9549 g mg-1 min-1.

Then, separation of metal-loaded MTB from the solution is studied using separators with strings by applying a high gradient magnetic field, and the focusing is the intensity of magnetic field and the place of wire casing. The separation efficiency is very good when magnetic field intensity is 100 Gauss, and no distinct improvement with higher magnetic field intensity. Moreover, the separation efficiency is better when metal wires are vertical than parallel with magnetic force line. The experiment result shows that Cu (II) ions concentration decrease from 100 mg l-1 initially to less than 5 µg l-1 at optimum conditions. At the same time, the wires loading MTB are observed by Scanning Electron Microscope (SEM). The diameter of wires after loading MTB increases from 67 to 73 µm.

This "magnetic field-magnetotactic bacteria technology" shows great application potential in the area of wastewater treatment for many advantages, such as high efficiency, low power, low cost and no secondary pollution, and so on.

 

Keywords: biosorption, magnetotactic bacteria, Cu (II) ions, adsorption isotherms, adsorption kinetics, magnetic separation