MIT Energy Laboratory researchers are using the tools of molecular engineering to meet a range of environmental challenges, including the cleanup of industrial waste streams and aquifers.
One of today's leading environmental challenges is cleaning aqueous industrial waste streams laden with hydrocarbons and other organic contaminants. Conventional solvents can capture these contaminants, but there are drawbacks. For example, such solvents are usually oily; and while oil and water do not generally mix, a little of the solvent will go into solution, contaminating the water it is supposed to be cleaning.
For the past five years, T. Alan Hatton, the Ralph Landau Professor of Chemical Engineering Practice in the Department of Chemical Engineering, and coworkers have been examining ways to make and use solvents that would solve such problems. One focus of their work is micelles. These structures are made of long polymeric molecules whose two ends have differing properties: one end "hates" water and the other end "loves" it.
When this molecule is in water, the water-hating end wants to separate from the water, while the water-loving end wants to stay in solution. As a compromise, the water-hating ends of 30 or 40 molecules come together to form a core, leaving the water-loving ends dangling on the outside. The resulting structure-the micelle-serves as an excellent water-based solvent. The "corona" of water-loving ends surrounds the compact water-hating core and keeps the micelle in suspension. But the core is still accessible to organic contaminants in the water and will readily absorb large amounts of them.
Using micelles for cleaning is not a new idea. Common household detergents contain micelles, which dramatically increase the capacity of the washwater to pick up organic contaminants. Micelles could likewise be used to remove organic contaminants from industrial waste streams.
But there is a problem. If the micellar solution and the waste stream come into direct contact, they will mix. How does one then separate the "loaded" micelles from the mixture so as to remove the contaminant and regenerate the micelles? Filtering is feasible but inefficient as it removes the bulk solvent (water) from the dilute contaminant. (One researcher likens the process of filtering out the micelles to going fishing by emptying all the water out of the lake and then picking up the fish.)
As an alternative, Professor Hatton and Patricia Hurter (MIT PhD '92) developed a technique in which the fresh micellar solution is pushed through hollow tubes whose walls are made of membranes that serve as ultrafine filters. Similar hollow-fiber units are used for kidney dialysis.
In the MIT technique, the contaminated stream flows in the opposite direction in the space surrounding the hollow fibers. The relatively large micelles cannot flow through the walls of the tubes, but the organic contaminants can-and will-to join the cores of the micelles. The result is a clean stream of treated water and a highly concentrated solution of contaminant-bearing micelles. Experiments have confirmed that the contaminants will move through the walls of the tubes and into the micelles. In one series of tests, a 2 percent micelle solution removed 80 percent of the contaminant toluene from the feed stream.
Professor Hatton and graduate students Gerard Prioleau and Randi Mosler of chemical engineering are performing further experiments on the technique as well as fundamental studies on the structure and behavior of micelles. They are also looking at options for cleaning up and recycling the micelles. For example, they are working with a micelle made of a new class of polymers that responds to pH. By manipulating the pH of the loaded micellar solution, the researchers can disrupt the structure of the micelle so that it can no longer accommodate the contaminant, which falls out of solution. By then adjusting the pH back to its original value, they can reform the micelles for reuse.
The initial work was supported by the MIT Sea Grant College Program and the US Department of Energy. The work on the pH-sensitive micelles is being funded by the National Science Foundation.
Another aqueous stream that often requires cleaning is groundwater. In polluted aquifers, contaminants are present both in the water and on the soil or permeable rock through which the water flows. One cleanup method involves injecting clean water down a well, forcing it through the aquifer to wash the contaminants off the soil, and then bringing the contaminant-bearing water back up via a second well.
This process, however, typically requires large quantities of water because the contaminants tend to cling to the soil. The process would be more efficient if the contaminants came off the soil more quickly-a change that could be achieved by adding micelles to the cleanup water. But there are several problems with this. For example, while a micelle's overall structure is generally stable, individual molecules are constantly moving into and out of the structure. Once in the aquifer, those individual molecules will be drawn by their oil-like tails to adsorb on the soil, adding to the contamination problem rather than alleviating it. Also, if the micellar solution becomes too dilute, the micelles will simply break apart.
Professor Hatton, St. Laurent Professor of Chemical Engineering Robert E. Cohen and Dr. Colleen A. Vandevoorde (MIT PhD '96) are therefore looking at a new type of molecule. The "star polymer" consists of a central core from which "arms" radiate. Certain parts of each arm are water-hating while other parts are water-loving. Like the micelle, the star polymer will both stay in solution and pick up contaminants. But unlike the micelle, the star polymer will not fall apart because it is a single molecule, with its arms chemically bound.
The MIT team found that one type of star polymer, the sulfonated polystyrene star, is easily synthesized and has a high capacity for taking up contaminants such as toluene and naphthalene. Work must continue on characterizing the nature and behavior of these stars and examining the economics of their use. The research was supported by the Charles E. Reed Initiatives Fund, the Emissions Reduction Research Center and the Northeast Hazardous Substances Research Center.
A version of this article appeared in MIT Tech Talk on October 30, 1996.