Bleaching of wood pulp
The primary product of wood pulp is paper, for which whiteness (similar to, but distinct from brightness) is an important characteristic.
Because the ISO standard only measures a narrow range of blue light, it is not directly comparable to human vision of whiteness or brightness.
Therefore, the objective of bleaching mechanical pulp (also referred to as brightening) is to remove only the chromophores (color-causing groups).
These conditions allow alkaline peroxide to selectively oxidize non-aromatic conjugated groups responsible for absorbing visible light.
The decomposition of hydrogen peroxide is catalyzed by transition metals, and iron, manganese and copper are of particular importance in pulp bleaching.
The goal in bleaching chemical pulps is to remove essentially all of the residual lignin, hence the process is often referred to as delignification.
[12] All bleaching agents used to delignify chemical pulp, with the exception of sodium dithionite, break lignin down into smaller, oxygen-containing molecules.
[13] Oxygen exists as a ground-state triplet, which is relatively unreactive and needs free radicals or very electron-rich substrates such as deprotonated lignin phenolic groups.
The production of these phenoxide groups requires that delignification with oxygen be carried out under very basic conditions (pH > 12).
Transition-metal compounds, particularly those of iron, manganese and copper, which have multiple oxidation states, facilitate many radical reactions and impact oxygen delignification.
Oxygen-based radicals, especially hydroxyl radicals, HO•, can oxidize hydroxyl groups in the cellulose chains to ketones, and under the strongly basic conditions used in oxygen delignification, these compounds undergo reverse aldol reactions, leading to cleavage of cellulose chains.
Magnesium salts are added to oxygen delignification to help preserve the cellulose chains,[16] but mechanism of this protection has not been confirmed.
[19] Ozone is a very powerful oxidizing agent, and the biggest challenge in using it to bleach wood pulp is to get sufficient selectivity so that the desirable cellulose is not degraded.
In the 1990s ozone was touted as good reagent to allow pulp to be bleached without any chlorine-containing chemicals (totally chlorine-free, TCF).
[22] A negative impact of chelating agents, as DTPA, is their toxicity for the activated sludges in the treatment of kraft pulping effluent.
[27] The bleaching of chemical pulps has the potential to cause significant environmental damage, primarily through the release of organic materials into waterways.
An increased public awareness of environmental issues from the 1970s and 1980s, as evidenced by the formation of organizations like Greenpeace, influenced the pulping industry and governments to address the release of these materials into the environment.
Dioxins are highly toxic, and health effects on humans include reproductive, developmental, immune and hormonal problems.
[31] Most TCF pulp is produced in Sweden and Finland for sale in Germany,[31] all markets with a high level of environmental awareness.
[33] ECF bleaching can substantially reduce but not fully eliminate chlorinated organic compounds, including dioxins, from effluent.
Within the EU, the average chlorinated organic compound emissions for ECF plants is 0.15 kg per tonne.
