Paper production is a messy business. As mill workers process 100 million tons of pulp each year, dioxins and other troublesome compounds escape into the environment, where they linger in natural waters and harm fish (SN: 11/4/95, p. 295).
Now, chemists have developed a method that they say could help the paper industry clean up its act.
Wood pulp contains cellulose, the raw material of paper, and lignin, a natural gluelike polymer that gives wood much of its integrity. Generally, manufacturers remove the lignin by oxidizing it with chlorine dioxide–or, in some countries, chlorine. These reactions break down lignin but produce chlorinated chemical pollutants.
This chlorinated waste is a big problem, says Craig L. Hill of Emory University in Atlanta. Yet less worrisome oxidizing agents, such as oxygen, don’t work well because they tend to destroy cellulose as well as lignin. Catalysts that combine metal atoms with organic components improve these reactions, but the catalysts break down easily, says Ira A. Weinstock, a researcher at the U.S. Department of Agriculture’s Forest Products Laboratory in Madison, Wis. This makes processes using these catalysts inefficient and costly.
In the Nov. 8 Nature, Hill, Weinstock, and several collaborators report a fresh way to eliminate lignin from paper pulp.
The researchers found that an ensemble of several types of reusable, inorganic, oxygen- and metal-containing catalysts–called polyoxometalate anions (POMs)–could delignify pulp in water without breaking down cellulose. In the first step, one of the POMs oxidizes lignin, making it soluble in the water. By itself, this primary POM would produce a very acidic solution that damages the cellulose, but the other POMs buffer the solution, keeping it at water’s neutral pH.
In the second step, the team added oxygen to the reaction mix. The primary POM catalyzed the further breakdown of the soluble lignin fragments until just carbon dioxide and water remained. The primary POM returns to its original state and can be used again.
The new technique incorporates “clever chemistry,” comments Terry Collins of Carnegie Mellon University in Pittsburgh. However, the process is inefficient and requires a large amount of POMs, he says. Moreover, the POMs currently used by Hill and Weinstock contain heavy metals that could contaminate a mill in an accidental spill or release, Collins says.
Hill and Weinstock counter that studies of their POMs indicate that they would pose little threat of heavy metal contamination.
Bertil Stromberg is optimistic. He is director of technology for Andritz-Ahlstrom in Glens Falls, N.Y., a company that makes equipment for the pulp-and-paper industry and is a member of an industrial consortium supporting the research.
Since the cost of running the new process is on par with conventional delignification processes, an increase in its efficiency could make it financially attractive, he says. With this and other improvements, Stromberg suggests a POM-based technology could be ready for commercialization in 5 years or so.
