As discussed in Chapter 10, pollutant remediation methods are quite varied. Each one has its own merits and challenges. For example, one of the most popular remediation methods, incineration, consists of the elevation of temperature of organic waste in air until combustion occurs. Unfortunately, this is not feasible for many on-site applications, can produce harmful gaseous side-products, requires the generation and handling of high temperatures, and gives rise to non-favorable public reception. As another example, chlorination (bleaching) is used for the discoloration of unwanted colored materials but it may also produce toxic by-products that require further treatment.

These problems can be overcome in many cases through the use of hydrogen peroxide, which is ca pable of self-decomposing to produce a powerful oxidizer intermediate: the hydroxyl radical, *OH (see: Advanced Oxidation Processes, AOP in Section 10.1).

As discussed in Section 10.1 of the companion book, hydrogen peroxide is paradoxically used both as an oxidizing and as a reducing agent, depending on the redox potential of the pollutant to be treated. H2O2 can also yield *OH radicals in the presence of UV light, although it weakly absorbs solar radiation and thus radical formation by this process is slow. The reaction between Fe2+ and H202 (Fenton's reaction) is given by

Another photoassisted process involves the generation of H202 in aqueous solutions at the surface of an illuminated semiconductor (e.g., ZnO), which can in turn form Fenton's reagent with artificially added Fe2+. Interestingly, H202 can also be generated naturally by active species (mainly radicals) resulting from the photosensitization of humic acids.

The following experiments demonstrate how a surrogate pollutant or hazardous waste (e.g., the organic dye rhodamine B) can be mineralized (i.e., converted into innocuous species like C02 and H20) upon oxidation in the presence of H202 under different conditions. We selected this dye due to its resistance to degradation by UV light. The mixture of Fe2+ + H202 (Fenton's reagent) is highly oxidizing, and so is H202 when used in conjunction with UV light. In summary, students will be able to compare (either by naked eye observations or by instrumental measurements) the effect of the following substances or conditions: Fe2+ alone, H202 alone,

H202 + Mn2+, H202 + Fe2+ (Fenton's reagent), sunlight, and H202 + sunlight (see Ibanez, 2003). Mn2+ was selected for one of the experiments due to its ability to decompose H202.

0 0

Post a comment