While chlorine dioxide has chlorine in its name, its chemistry is radically different from that of chlorine. The way it works is almost magical. It has to do with the way electrons interact with one another. As we all learned in high school chemistry, we can mix two compounds and create a third that bears little resemblance to its parents.
Regarding bleaching, chlorine dioxide and chlorine -- because of their fundamentally different chemistries -- react in distinct ways with organic compounds, and as a result generate very different by-products. It's this difference that explains the superior environmental performance of chlorine dioxide in papermaking.
On a Technical Note
Technically speaking, both chlorine and chlorine dioxide are oxidizing agents -- electron receivers. Chlorine has the capacity to take in two electrons, whereas chlorine dioxide can absorb five.
This property, along with the complex but well known ways chlorine combines with lignin (the cellular adhesive in wood tissue), explains the basic difference between the two compounds.
In the chlorine-based bleaching process, about 10 percent of the chlorine combines directly with lignin which has "aromatic" components. Aromatic compounds have atoms arranged in rings, and they may have other atoms, such as chlorine, attached to these rings. Within the group of chlorinated aromatics, which can be toxic to some organisms, are the infamous dioxins.
Chlorine dioxide's behavior as a bleaching agent is quite dissimilar. Instead of combining with the aromatic rings, chlorine dioxide breaks these rings apart. In addition, as the use of chlorine dioxide increases, the generation of chlorinated organics falls dramatically.
Chlorine dioxide's chemistry also explains why it is such an effective oxidant, or bleaching agent. It's 2.5 times more powerful than chlorine gas, and also much more selective. Chlorine dioxide attacks the lignin, but does not react with the desired cellulose in wood tissue. It's cellulose -- the tree's fiber -- that provides the strength in the final paper products.
It is these advantages that make chlorine dioxide the preferred environmental standard for eliminating toxic substances in mill waste water.