Many of the presentations relied on the use of various kinetic studies to evaluate the treatment methodologies. The rate of oxidation of the trace contaminant was used to judge the effectiveness of the technique. The rate (conversion per unit time) of any type of photoreaction depends upon the rate of light absorption by the photoactive species or ''chromophore,'' Ia, and the quantum efficiency of the reaction, (Equation 1).
The rate of light absorption is affected by the spectral overlap between the light source and the spectrum of the chromophore. The average light absorption rate in a system is proportional to the incident light intensity and can be reduced by light attenuation within the system. In the various water treatment systems discussed in the workshop, ozone and hydrogen peroxide absorb UV radiation strongly in the far UV region (< 280 nm), but only very weakly in the middle (280 to 320 nm) and near UV (320 to 390 nm) region. Ground-level solar radiation includes ultraviolet radiation only in the middle and near ultraviolet, with the cutoff at about 295 nm. Thus, at least under "one-sun" irradiations, light absorption and contaminant oxidation rates induced by photolysis of ozone and hydrogen peroxide by solar radiation are generally considered to be too low for practical usage in treatment processes. (However, it should be noted here that solar ozone photolysis is the key natural process in initiating organic oxidations in the troposphere.) On the other hand, commonly used photocatalysts absorb strongly in the near UV, and thus rapidly absorb sunlight as well as radiation from black lights or other near UV sources. Photocatalytic systems thus appear to be better suited for solar applications.
As discussed by William Glaze, Gary Peyton, and Jack Zeff, UV/ozone and UV/hydrogen peroxide treatment systems have maximized light absorption rates by using lamps that emit far UV light, e.g., low pressure mercury arc lamps. Apparently, concentrated solar radiation has not been employed in conjunction with ozone and/or hydrogen peroxide, although it seems that it may be worthwhile to take a closer look at this option.
Commonly used photocatalysts such as titanium dioxide (anatase) absorb strongly in the near UV region. Hussain Al-Ekabi reported the successful use of near UV lamps to activate photocatalytic contaminant removal, whereas Craig Turchi, Mark Mehos, and Jim Pacheco all described efforts to use concentrated solar radiation in heterogeneous photocatalytic treatment systems (more details below).
The quantum yield for reaction is an important parameter that describes the fraction of absorbed radiation that results in reaction. Despite this fact, as emphasized by David Ollis, all too often information about quantum yields is omitted from treatment studies, making it very difficult to compare results from one system to another. Glaze pointed out that, when the contaminant is very dilute, frequently the case in the types of treatment situations considered at the workshop, the quantum yield and rate are usually directly proportional to contaminant concentration; that is, a pseudo-first order rate expression applies. As the contaminant concentration increases, however, the quantum yield and rate become independent of contaminant concentration. Similarly, in the case of photocatalysis, the rate becomes independent of concentration with increasing contaminant concentration, as clearly demonstrated by Pacheco's data for photocatalytic systems at the Sandia National Laboratories.
The quantum yield is affected by "primary" photoreactions, which directly ensue from an excited state, as well as by secondary reactions, such as free radical chain processes, recombinations, and scavenging reactions that follow the primary photoreaction. The discussions by Glaze and Peyton demonstrated that useful information about the secondary reactions in