The authors reply to Soltzberg and Brown.

The experiment with methyl red is also interesting as it may be with any of the many water-dissolved organic wastes that can be degraded by TiO₂ photocatalysis. The advantages of using methyl red instead of p-nitrophenol, such as its availability in any academic lab and excellent results with the experiment, are to be considered. However, one of the most interesting aspects of our article is the use of real wastes, in this case from a previous student experiment (the determination of the Freundlich adsorption isotherm of p-nitrophenol on activated carbon). Additionally, aqueous p-nitrophenol solutions are not as hazardous as the solid compound, but of course if no wastes are available (unusual in a lab) the solid compound must be used, prepared by the lab trainer or by the students.

We are now repeating a similar experiment with marine science and chemical engineering students, but using aqueous wastes from the HPLC determination of formaldehyde in veterinarian wastes. HPLC wastes are quite common in almost any lab. These contain acetonitrile, low formaldehyde concentrations, and DNPH (dinitrophenylhydrazine). The degradation of a 100 ppm total organic carbon (TOC) solution is monitored by TOC, HPLC, and UV–vis measurements. Also, sunlight and a UV-lamp are used for comparison. Results are also quite interesting since notable degradations are obtained in 30 min for DNPH and longer times for acetonitrile. Additionally, after 1 h of degradation a non-identified HPLC peak corresponding to an intermediate compound appears. This is quite interesting because three aspects of waste treatment are pointed out:

1. The importance of using different analytical techniques
2. The possible appearance of intermediates that may be even more toxic than the parent compound
3. The need of combining different treatment methods since quite often the complete elimination of all the contaminants is not achieved

Consequently, other interesting analyses to be added to this kind of experiment, in general to any waste degradation experiment, are toxicity determinations such as those with the marine bacteria Vibrio fisheri, which is no more complicated than a luminescence measurement. We have observed in other wastes that even after about 80% TOC reductions, toxicity was increased, quite probably due to the formation of more toxic intermediates.