Chitosan Beads to Remove Heavy Metals from Wastewater
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) Return to Index
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Last updated April 12, 1999
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