Photochemical discoloration, paper

Industrial wastewater may be colored and color removal may be required to comply with regulation. Coque et al. describe how various discoloration methods were studied with standard azo dyes solutions and an industrial wastewater from the paper industry as model systems. The liquids were recirculated between the photochemical discoloration reactor and an at-line spectrophotometer (Suprasil quartz cell, path length 2 or 10mm, resp.). A discoloration yield x was calculated in CIE L a b space, which permitted monitoring the process and determining completion in an easily implemented manner. 

Influence of the Hemicellulose Fraction on Thermal and Photochemical Discoloration of Paper... 

Figure 3. Postcolor number of filter papers saturated to a level of 3% add-on with arabinogalactan. Key (thermal discoloration) , 50% rh and 90 °C , 0% rh and 90 °C and (photochemical discoloration) , daylight fluorescent lamps A, BLB fluorescent lamps V, Fade-ometer.

The first law of photochemistry, named the Grotthus-Drapper Principle, states that for a photochemical reaction to occur, the first event must be the absorption of light by some component of the system. The second law of photochemistry, named the Stark-Einstein Principle, states that a molecule can only absorb one quantum of radiation. The absorbed energy in the resultant excited molecule may be dissipated by either photophysical or photochemical processes. It is the latter of these that eventually changes the chemical and mechanical properties of the substance (26,27). Thus, the reactions based on the absorption of radiation by the chemical components of modern papers are of prime importance in discoloration. 

Principally, the photostabilization of modem paper involves the retardation or elimination of the various photophysical and photochemical processes that take place during photoinduced discoloration. This goal may be accomplished by the following three stabilizing systems. 

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