(572e) Heavy Metals Removal from Wastewater by Magnetic Field-Magnetotactic Bacteria Technology

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 Blakemorein 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