Polychromatic actinometry with filter solutions

In the domain of photochemical processes for water treatment much of the technology has been developed using u.v.-radiation in order to degrade the microbiological and chemical contaminants in drinking water (Legrini et al., 1993). With regard to the removal of microbiological pollutants, the u.v.-disinfection of water is discussed as a promising alternative to the use of chemical disinfectants (Von Sonntag and Schuchmann, 1992). In addition, a number of papers have reported on advanced techniques for the oxidation of organic pollutants by the combination of u.v.-light and oxidants such as O3 and/or H2O2 (Von Sonntag et al., 1993; Paillard et al., 1992; Hessler et al., 1993). These advanced oxidation processes (AOP) are particularly oriented towards commercial application and have stimulated interest in the design of efficient reactors and construction of new light sources. Thus, the formulation of the photokinetic rates of a photochemical process is one of the most difficult parts of reactor design and involves determination of the absorbed light intensity causing chemical conversion, e.g. the generation of highly reactive hydroxyl radicals. Actinometric measurements may often cause problems due to the polychromatic emission of medium-pressure Hg-arcs, which are used in oxidative degradation procedures. Referring to polychromatic actinometry, this paper deals with the practical use of filter solutions as an appropriate method for approaching monochromatic conditions with polychromatic radiation. The experiments were carried out with the annular photoreactor of a pilot plant for drinking water treatment (Fig. 1). For evaluation of the u.v.-irradiance of the medium-pressure Hg-arc (Fig. 2) in the 240–470 nm range, the azobenzene photoisomerization was used as a chemical actinometer (Gauglitz and Hubig, 1985; Gauglitz and Hubig, 1981; Kuhn et al., 1989) and filter solutions were taken to isolate certain parts of the radiation spectrum. The general problem of preparing solution filters is essentially an empirical one of combining inorganic salts and organic, mainly aromatic, compounds in appropriate solvents like water. The selection requirements for choosing a proper solution filter are as follows: • — specific transmittance to obtain wavelength bands or strong emission lines, which are within the range of the absorption of pollutants;• — removal of unwanted spectral bands for wavelength selective actinometry;• — photochemical stability and no dark reactions. With respect to the determination of light fluxes over the entire irradiated volume, which could be carried out inside the irradiation unit as internal actinometry, experiments with liquid filters are not practicable because filter and actinometric solutions must be separated (Table 2). Thus, only external actinometry was possible and cuvettes containing a filter and the actinometer solution were placed outside the vessel (Fig. 3). As a consequence, the reactor geometry must be taken into account by theoretical equations concerning the radiation field (Alfano et al., 1986). The band pass filters, which were used for actinometric measurements, are composed of various filter components (Table 1) and gave a light distribution enriched with narrow bands of radiation [Fig. 4(a) and (b)]. The spectral overlapping of distinct absorption bands of the azobenzene actinometer (Fig. 5) with non-attenuated wavelengths of the arc-spectrum [Fig. 6(a)–(c)] is required for obtaining selectivity in actinometric measurements. However, the calculation of intensities by photokinetic rate equations is limited to monochromatic light and influences of slightly polychromatic irradiation on fundamental parameters, such as quantum yields, still exist. These difficulties could be overcome if the photokinetic quantities, particularly the molar absorptivities of trans- and cis-azobenzene and thus the pseudo quantum yield, were approximated by weighted averages. Irradiation intensities in the range of 290–360, 335–400 and 375–470 nm could be measured with the filters I–III by assuming that the absorption of the actinometer solutions is negligible. Measurement of the light intensity in the u.v.-C region could be achieved by using the filter solutions IV and V with different transmission characteristics (Fig. 7). In the case when total absorption of the actinometer is assumed, a linear relationship between ΔE and the irradiation time is obtained for each filter respectively. The intensity of the irradiation source can finally be calculated by the difference of the slopes. In general, radiation powers of light sources measured by external actinometry must be related to the total intensity at the outside wall of the inner quartz tube of the u.v.-arc. Experimental quantities, which decrease the u.v.-radiation in the actinometric solution like the transmittance of the filter solutions and the cooling water must therefore be considered.