An environmentally benign technique for removing and recovering metals from waste water that is being developed by LBL scientists could be used to clean up abandoned open-pit mines such as the Berkeley Pit in Butte, Montana.
Richard H. Fish, a chemist with the Energy and Environment Division, led the research into chemically modifying tiny polystyrene beads with a special type of organic molecule called a ligand so that they recognize and tightly bind to certain metal ions. The beads are then removed from water and the metals are recovered from the beads.
Located in an ore-rich section of southwestern Montana, the Berkeley Pit is a vast open-pit mine that was closed in the 1980s and has been slotted by the U.S. Department of Energy as a "demonstration site" for environmental reclamation. Following the shutdown of mining operations, the Berkeley Pit was filled with some 17 billion gallons of water. Suspended in this water are substantial concentrations of various types of metal ions, including iron, copper, zinc, manganese, magnesium, and aluminum.
"Recovering the metals in the Berkeley Pit could be worth billions of dollars," says Fish. "Even if the recovery of the metals were not in itself economically viable, it would help defray the costs of cleaning up the pit."
To recover metal ions from waste water, Fish took what is called a "biomimetic approach," that is, he mimicked natural ligands that selectively sequester metals. By modifying commercial polystyrene beads with these biomimics, Fish and his colleagues created a cage-like molecular complex that will recognize and encapsulate specific metal ions.
The original complex that Fish and his group synthesized on the beads, called a "catechol ligand," did not work on metals in aqueous solution. However, by adding a sulfonic acid group, the researchers not only made their catechol ligand work in an aqueous solution, they also increased its ability to capture and remove metal ions by a factor of ten.
"Making and treating the beads is a relatively easy and inexpensive chemical procedure," says Fish. "The beads have a long life and can be reused after recovery of their metals."
The ability of Fish's treated beads to selectively react with specific types of metal ions depends upon the acidity of the water. At the highest acidity, a pH of 2.5 or less, the beads primarily capture iron, which is by far the biggest concentration of metal ions in the Berkeley Pit.
"It is important that ferric ions be removed first, because otherwise they will compete and interfere with the recovery of other metals of value," says Fish.
At a pH of 3.0, after the iron has been removed, the beads favor the recovery of mercury. Though not present at the Berkeley Pit, mercury is an environmentally important contaminant present at other DOE demonstration sites. At a pH of 5.0, the beads begin to select for a range of metals including copper, nickel, zinc, and manganese. As the water becomes more basic, Fish says the beads can even be used to remove radioisotopes.
"The more selectivity we have the better off we are because it circumvents the chemistry of competition that complicates most recovery programs," says Fish. "If we could provide the same level of recognition in our beads that biological molecules display we would have the ultimate in selectivity."
Working with Fish on this project have been postdoctoral fellows Songping Huang and Wei Li, and undergraduates Katherine Franz of Wellesley College, Dana Miggins of Jackson State University, and Mercedes Coughlin of UC Berkeley. Working with him as a consultant on the project was Robert Albright.