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Chitosan Beads to Remove Heavy Metals from Wastewater

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Chitosan Beads to Remove Heavy metals from Wasterwater

Gregory L. Rorrer
J. Douglas Way
Oregon State University
 
A process for removing heavy metals from waste- water is being developed using 
chitosan beads, a selective, high-capacity adsorbent.  The process minimizes 
fouling and generates a concentrated heavy-metal stream that can be processed to 
recover the heavy metals.
 
Introduction
Heavy-metal pollution of groundwater is a pervasive and extremely serious 
environmental problem.  The cleanup costs for groundwater pollution have been very 
high and are likely to increase. Most processes for removing heavy metals from 
groundwater can be classified as  pump and treat processes, where the water is 
pumped out of an aquifer or well, treated, and returned to the aquifer or sent to 
municipal waste treatment.  While effective, these processes generally do not allow 
the potentially valuable heavy metals to be recycled or recovered. 
 
Concept Description
A new process for removing heavy metals from wastewater is being developed using a 
selective, high-capacity adsorbent.  The process minimizes fouling and generates 
a concentrated heavy-metal stream that can be processed to recover the heavy metals.
As the figure shows, the continuous-flow contacting unit is a fluidized bed 
containing magnetic biopolymer adsorbent beads immobilized in magnetic field that is 
externally applied.  The dilute aqueous waste stream first passes through the 
adsorber bed, and heavy-metal ions chelate onto the biopolymer bead in concentrated 
form.  By periodically releasing the magnetic field, the heavy-metal-laden beads
pass down into the desorber bed.  By lowering the pH of the desorbent stream, 
heavy-metal ions are recovered in concentrated form, and the beads are regenerated.
Both the biopolymer adsorbent and the magnetically stabilized fluidized bed (MSFB) 
have several novel features.  The adsorbent biopolymer is made from chitosan, 
a natural and abundant biopolymer of glucosamine, an amino sugar, with a very high 
affinity for non-alkali metals.  Protan, Inc., (Redmond, WA) presently manufactures 
chitosan from shellfish wastes, ensuring a steady supply of the adsorbent raw 
material. Chito-san s current price is ~$7/lb.  
The MSFB has several advantages over conventional fluidized and packed beds, 
particularly for treating high-volume, dilute, aqueous waste streams. The MSFB uses 
an externally applied, axially aligned magnetic field to independently control the 
magnetic particles  movement in the bed and to maintain constant solid movement 
through the bed even when the beads are increasing in density because of adsorption 
of heavy metals.  Even at high liquid velocities, the magnetically stabilized beads 
resist the inertia of the fluid and remain in the bed.  Also, the stabilized 
particles in the magnetic field minimize damage of the potentially fragile chitosan 
adsorbent beads from hydrodynamic shear forces and particle collisions.  
 
Economics and Market Potential
Chitosan has a high ion-exchange site density (200 meq/g) comparable to many 
commercial ion-exchange resins.  Therefore, high degrees of concentration of the 
metal ion species are possible, which may make recovery of the metals economically 
feasible. The United States imports over 70% of  strategic metals  such as cobalt, 
chromium, niobium, and the platinum group metals, which are used in aerospace and 
military applications such as jet engines.  For example, an estimated $1 billion 
of nonradioactive platinum group metals (rhodium-103 and palladium-106) could be 
removed from nuclear waste.  
 
Key Experimental Results
Our research project has three major experimental objectives:
      to synthesize magnetic, porous chitosan beads that have been chemically 
modified for high metal adsorption capacity and stability in aggressive chemical 
environments
      to construct an MSFB to provide continuous contact of the chitosan beads and 
the aqueous waste stream
      to define the operating parameters neded to remove low-concentration heavy 
metals (10 to 500 ppm) in the aqueous waste stream using the chitosan beads suspended 
in the MSFB. The chitosan is cast into highly porous beads of about 1 to 5 mm and 
chemically crosslinked to improve both stability in chemical environments and 
heavy-metal adsorption capacity.  During the casting process, magnetite powder 
(~5 to 10 wt%) is added to the beads, so that they are susceptible to an externally 
applied magnetic field.  The chelation is reversible, and heavy metals can be 
desorbed from the bead by reducing the pH of the surrounding liquid to about 2.
Target parameters for our porous-magnetic chitosan beads are as follows:  
size (0.1 to 5 mm), surface area (150 m2/g), adsorption capacity 
(e.g., 1 g Hg/g-chitosan), and magnetite loading (2 to 5 wt%). We are using a 
bench-scale MSFB is in our laboratory.  The bed is 2.5 cm in diameter and 60 cm in
length (300 mL working volume).  The liquid flow rate into the bed ranges 
from 5 to 20 mL/min. Experiments are under way to determine the extent of metal-ion 
removal as a function of bead parameters and process parameters.
 
Future Development Needs
We will continue to focus on obtaining design correlations during further testing 
with our bench-scale MSFB.  The bench-scale tests will consist of hydrodynamic 
and mass transfer rate experiments to demonstrate that we can operate the MSFB over 
a wide range of the Reynold s number (Re) using magnetic stabilization and to show 
its efficiency for removing heavy metals from dilute solutions.  
When we obtain a correlation describing the influence of Re on the mass transfer 
coefficient, we can perform a preliminary economic analysis to compare our concept 
with existing  pump and treat  processes to determine the economic advantages of 
our process. For more information about this concept, contact
Gregory L. Rorrer
(503) 737-3370
J. Douglas Way
(503) 737-3406
Department of Chemical
  Engineering
Oregon State University
103 Gleeson Hall
Corvallis, OR  97331-2702
(503) 737-3462 (fax)
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Last updated April 12, 1999

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